AN ESSAY TOWARDS A New Theory of Viſion.
[]I. MY Deſign is to ſhew the Manner, where⯑in we perceive by Sight the Diſtance, Magnitude, and Situation of Objects. Alſo to conſider the Difference there is betwixt the Ideas of Sight and Touch, and whether there be any Idea common to both Senſes.
II. It is, I think, agreed by all, that Diſtance, of it ſelf and immediately, cannot be ſeen: For Diſtance being a Line directed end-wiſe to the Eye, it projects only one Point in the Fund of the Eye, which Point remains invariably the ſame, whether the Diſtance be longer or ſhorter.
[154] III. I find it alſo acknowledged, that the Eſti⯑mate we make of the Diſtance of Objects conſide⯑derably remote, is rather an Act of Judgment grounded on Experience, than of Senſe. For Example, when I perceive a great Number of in⯑termediate Objects, ſuch as Houſes, Fields, Ri⯑vers, and the like, which I have experienced to take up a conſiderable Space, I thence form a Judg⯑ment or Concluſion, that the Object I ſee beyond them is at a great Diſtance. Again, when an Ob⯑ject appears faint and ſmall, which at a near Di⯑ſtance I have experienced to make a vigorous and large Appearance, I inſtantly conclude it to be far off: And this, 'tis evident is the reſult of Experi⯑ence; without which, from the faintneſs and lit⯑tleneſs I ſhould not have inferred any thing con⯑cerning the Diſtance of Objects.
IV. But when an Object is placed at ſo near a Diſtance, as that the Interval between the Eyes bears any ſenſible Proportion to it, the Opinion of ſpeculative Men is, that the two Optic Axes (the Fancy that we ſee only with one Eye at once being exploded) concurring at the Object do there make an Angle, by means of which, according as it is greater or leſſer, the Object is perceived to be nearer or farther off †.
V. Betwixt which, and the foregoing manner of eſtimating Diſtance, there is this remarkable Diffe⯑rence: That, whereas there was no apparent, ne⯑ceſſary Connexion between ſmall Diſtance and a large and ſtrong Appearance, or between great Diſtance and little and faint Appearance, there ap⯑pears a very neceſſary Connexion between an ob⯑tuſe [155] Angle and near Diſtance, and an acute Angle and farther Diſtance. It does not in the leaſt de⯑pend upon Experience, but may be evidently known by any one before he had experienced it, that the nearer the Concurrence of the Optic Axes, the greater the Angle, and the remoter their Con⯑currence is, the leſſer will be the Angle compre⯑hended by them.
VI. There is another way mentioned by Optic Writers, whereby they will have us judge of thoſe Diſtances, in reſpect of which the Breadth of the Pupil hath any ſenſible bigneſs: And that is the greater or leſſer Divergency of the Rays, which iſſuing from the viſible Point, do fall on the Pupil: That Point being judged neareſt, which is ſeen by moſt diverging Rays; and that remoter, which is ſeen by leſs diverging Rays: And ſo on, the ap⯑parent Diſtance ſtill increaſing, as the Divergency of the Rays decreaſes, till at length it becomes in⯑finite, when the Rays that fall on the Pupil are to Senſe Parallel. And after this manner it is ſaid we perceive Diſtance when we look only with one Eye.
VII. In this Caſe alſo, 'tis plain we are not be⯑holding to Experience: It being a certain, neceſſa⯑ry Truth, that the nearer the direct Rays falling on the Eye approach to Paralleliſm, the farther off is the Point of their Interſection, or the viſible Point from whence they flow.
VIII. Now though the Accounts here given of perceiving near Diſtance by Sight are receiv'd for true, and accordingly made uſe of in determining the apparent places of Objects, they do neverthe⯑leſs [156] ſeem very unſatisfactory: And that for theſe following Reaſons.
IX. It is evident that when the Mind perceives any Idea, not immediately and of it ſelf, it muſt be by the means of ſome other Idea: Thus, for Inſtance, the Paſſions which are in the Mind of a⯑nother, are of themſelves to me inviſible. I may nevertheleſs perceive them by Sight, though not immediately, yet by means of the Colours they produce in the Countenance. We often ſee Shame or Fear in the Looks of a Man, by perceiving the Changes of his Countenance to Red or Pale.
X. Moreover it is evident that no Idea, which is not it ſelf perceived, can be the means of per⯑ceiving any other Idea. If I do not perceive the Redneſs or Paleneſs of a Man's Face themſelves, it is impoſſible I ſhould perceive by them the Paſſions which are in his Mind.
XI. Now from SECT. II. it is plain that Di⯑ſtance is in its own nature imperceptible, and yet it is perceived by Sight. It remains, therefore, that it be brought into view by means of ſome o⯑ther Idea, that is it ſelf immediately perceived in the Act of Viſion.
XII. But thoſe Lines and Angles, by means whereof ſome Men pretend to explain the Percep⯑tion of Diſtance, are themſelves not at all perceiv⯑ed, nor are they in truth ever thought of by thoſe unskilful in Optics. I appeal to any one's Expe⯑rience, whether upon Sight of an Object, he com⯑putes its Diſtance by the bigneſs of the Angle, made by the meeting of the two Optic Axes? Or whe⯑ther he ever thinks of the greater or leſſer Diver⯑gency [157] of the Rays, which arrive from any Point to his Pupil? Every one is himſelf the beſt judge of what he perceives, and what not. In vain ſhall any Man tell me, that I perceive certain Lines and Angles which introduce into my Mind the various Ideas of Diſtance, ſo long as I my ſelf am conſci⯑ous of no ſuch thing.
XIII. Since therefore thoſe Angles and Lines are not themſelves perceived by Sight, it follows from SECT. X. that the Mind does not by them judge of the Diſtance of Objects.
XIV. The Truth of this Aſſertion will be, yet, farther evident to any one that conſiders thoſe Lines and Angles have no real Exiſtence in Nature, being only an Hypotheſis fram'd by the Mathema⯑ticians, and by them introduced into Optics, that they might treat of that Science in a Geometrical way.
XV. The laſt Reaſon I ſhall give for rejecting that Doctrine, is, that tho' we ſhould grant the real Exiſtence of thoſe Optic Angles, &c. and that it was poſſible for the Mind to perceive them; yet theſe Principles wou'd not be found ſufficient to explain the Phoenomena of Diſtance, as ſhall be ſhewn hereafter.
XVI. Now, it being already ſhewn that Diſtance is ſuggeſted to the Mind, by the Mediation of ſome other Idea which is it ſelf perceived in the Act of Seeing, it remains that we inquire what Ideas, or Senſations there be that attend Viſion, unto which we may ſuppoſe the Ideas of Diſtance are connected, and by which they are introduced into the Mind. And Firſt, It is certain by Expe⯑rience, [158] that when we look at a near Object with both Eyes, according as it approaches, or recedes from us, we alter the Diſpoſition of our Eyes, by leſſening or widening the Interval between the Pu⯑pils. This Diſpoſition or Turn of the Eyes is at⯑tended with a Senſation, which ſeems to me to be that which in this Caſe brings the Idea of greater or leſſer Diſtance into the Mind.
XVII. Not that there is any natural or neceſſary Connexion between the Senſation we perceive by the Turn of the Eyes, and greater or leſſer Di⯑ſtance; but becauſe the Mind has by conſtant Ex⯑perience found the different Senſations correſpond⯑ing to the different Diſpoſitions of the Eyes, to be attended each with a different Degree of Di⯑ſtance in the Object; There has grown an Habi⯑tual or Cuſtomary Connexion between thoſe two ſorts of Ideas, ſo that the Mind no ſooner perceives the Senſation ariſing from the different Turn it gives the Eyes, in order to bring the Pupils near⯑er, or farther aſunder, but it withal perceives the different Idea of Diſtance which was wont to be connected with that Senſation: Juſt as upon hear⯑ing a certain Sound, the Idea is immediately ſug⯑geſted to the Underſtanding, which Cuſtom had united with it.
XVIII. Nor do I ſee, how I can eaſily be miſta⯑ken in this Matter. I know evidently that Di⯑ſtance is not perceived of it ſelf. That by conſe⯑quence, it muſt be perceived by means of ſome o⯑ther Idea which is immediately perceived, and varies with the different Degrees of Diſtance. I know alſo that the Senſation ariſing from the Turn of the Eyes is of it ſelf immediately perceived, and various Degrees thereof are connected with [159] different Diſtances, which never fail to accompany them into my Mind, when I view an Object diſtinctly with both Eyes, whoſe Diſtance is ſo ſmall that in reſpect of it the Interval between the Eyes has any conſiderable Magnitude.
XIX. I know it is a received Opinion, that by altering the Diſpoſition of the Eyes, the Mind perceives whether the Angle of the Optic Axes, or the lateral Angles comprehended between the Interval of the Eyes and the Optic Axes, are made greater or leſſer; and that accordingly by a kind of Natural Geometry, it judges the Point of their Interſection to be nearer, or farther off. But that this is not true, I am convinced by my own Ex⯑perience, ſince I am not conſcious, that I make any ſuch uſe of the Perception I have by the Turn of my Eyes. And for me to make thoſe Judgments, and draw thoſe Concluſions from it, without know⯑ing that I do ſo, ſeems altogether incomprehen⯑ſible.
XX. From all which it follows, that the Judg⯑ment we make of the Diſtance of an Object, view⯑ed with both Eyes, is entirely the Reſult of Ex⯑perience. If we had not conſtantly found certain Senſations ariſing from the various Diſpoſition of the Eyes, attended with certain Degrees of Diſ⯑tance, we ſhou'd never make thoſe ſudden Judg⯑ments from them, concerning the Diſtance of Ob⯑jects; no more than we wou'd pretend to judge of a Man's Thoughts by his pronouncing Words we had never heard before.
XXI. Secondly, An Object placed at a certain Diſtance from the Eye, to which the breadth of the Pupil bears a conſiderable Proportion, being made [160] to approach, is ſeen more confuſedly: And the nearer it is brought, the more confuſed Appear⯑ance it makes. And this being found conſtantly to be ſo, there ariſeth in the Mind an Habitual Con⯑nexion between the ſeveral Degrees of Confuſion and Diſtance; the greater Confuſion ſtill imploying the leſſer Diſtance, and the leſſer Confuſion, the greater Diſtance of the Object.
XXII. This confuſed Appearance of the Ob⯑ject doth therefore ſeem to be the Medium, where⯑by the Mind judgeth of Diſtance in thoſe Caſes, wherein the moſt approved Writers of Optics will have it judge by the different Divergency, with which the Rays flowing from the Radiating Point fall on the Pupil. No Man, I believe, will pre⯑tend to ſee or feel thoſe imaginary Angles, that the Rays are ſuppoſed to form according to their various Inclinations on his Eye. But he cannot chooſe Seeing whether the Object appear more or leſs confuſed. It is therefore a manifeſt Conſe⯑quence from what has been demonſtrated, that in⯑ſtead of the greater, or leſſer Divergency of the Rays, the Mind makes uſe of the greater or leſſer Confuſedneſs of the Appearance, thereby to deter⯑mine the apparent Place of an Object.
XXIII. Nor doth it avail to ſay, there is not any neceſſary Connexion between confuſed Viſion, and Diſtance, great or ſmall. For I ask any Man, what neceſſary Connexion he ſees between the Red⯑neſs of a Bluſh and Shame? And yet no ſooner ſhall he behold that Colour to ariſe in the Face of another, but it brings into his Mind the Idea of that Paſſion which hath been obſerved to accompa⯑ny it.
[161] XXIV. What ſeems to have miſled the Writers of Optics in this Matter is, that they imagine Men judge of Diſtance, as they do of a Concluſion in Mathematics; betwixt which and the Premiſes it is indeed abſolutely requiſite there be an apparent, neceſſary Connexion: But it is far otherwiſe, in the ſudden Judgments Men make of Diſtance. We are not to think, that Brutes and Children, or even grown reaſonable Men, whenever they perceive an Object to approach, or depart from them, do it by virtue of Geometry and Demonſtration.
XXV. That one Idea may ſuggeſt another to the Mind, it will ſuffice that they have been ob⯑ſerved to go together, without any Demonſtration of the Neceſſity of their Coexiſtence, or without ſo much as knowing what it is that makes them ſo to coexiſt. Of this there are innumerable Inſtances, of which no one can be ignorant.
XXVI. Thus, greater Confuſion having been conſtantly attended with nearer Diſtance, no ſooner is the former Idea perceived, but it ſuggeſts the latter to our Thoughts. And if it had been the ordinary Courſe of Nature, that the farther off an Object were placed, the more confuſed it ſhou'd appear, it is certain, the very ſame Perception, that now makes us think an Object approaches, would then have made us to imagine it went far⯑ther off. That Perception, abſtracting from Cuſtom and Experience, being equally fitted to produce the Idea of great Diſtance, or ſmall Diſtance, or no Diſtance at all.
XXVII. Thirdly, An Object being placed at the Diſtance above ſpecified, and brought nearer to the Eye, we may nevertheleſs prevent, at leaſt for ſome time, the Appearance's growing more [162] confuſed, by ſtraining the Eye. In which Caſe, that Senſation ſupplys the place of confuſed Viſion, in aiding the Mind to judge of the Diſtance of the Object; it being eſteemed ſo much the nearer, by how much the Effort or Straining of the Eye in order to diſtinct Viſion is greater.
XXVIII. I have here ſet down thoſe Senſations or Ideas, that ſeem to be the conſtant and general Occaſions of introducing into the Mind the different Ideas of near Diſtance. It is true in moſt Caſes, that divers other Circumſtances contribute to frame our Idea of Diſtance, to wit, the particular Num⯑ber, Size, Kind, &c. of the things ſeen. Concern⯑ing which, as well as all other the forementioned Occaſions which ſuggeſt Diſtance, I ſhall only ob⯑ſerve, they have none of them, in their own Na⯑ture, any Relation or Connexion with it: Nor is it poſſible, they ſhou'd ever ſignify the various De⯑grees thereof, otherwiſe than as by Experience they have been found to be connected with them.
XXIX. I ſhall proceed upon theſe Principles to account for a Phaenomenon, which has hitherto ſtrangely puzzled the Writers of Optics, and is ſo far from being accounted for by any of their The⯑ories of Viſion, that it is, by their own Confeſſion, plainly repugnant to them: And of Conſequence, if nothing elſe cou'd be objected, were alone ſuf⯑ficient to bring their Credit in Queſtion. The whole Difficulty I ſhall lay before you in the Words of the Learned Dr. Barrow, with which he concludes his Optic Lectures.
Haec ſunt, quae circa pantem Opticae praecipue Mathematicam dicenda mihi ſuggeſſit meditatio. Circa reliquas, (quae [...] ſunt, adeoque ſaepiuſcule pro certis principiis plauſibiles conjectu⯑ras [163] venditare neceſſum habent) nihil fere quic⯑quam admodum veriſimile ſuccurrit, a pervulga⯑tis (ab iis, inquam, quae Keplerus, Scheinerus, Carteſius, & poſt illos alii tradiderunt) alienum aut diverſum. Atqui tacere malo, quam toties oblatam cramben reponere. Proinde receptui ca⯑no; nec ita tamen ut prorsùs diſcedam antea⯑quam improbam quandam difficultatem (pro ſin⯑ceritate quam & vobis & veritati debeo minime diſſimulandam) in medium protulero, quae doctri⯑nae noſtrae, hactenus inculcatae, ſe objicit adver⯑ſam, ab ea ſaltem nullam admittit ſolutionem. Illa, breviter, talis eſt: Lenti vel Speculo cavo EBF exponatur punctum viſi⯑bile
A, ita Diſtans ut Radii ex A manantes ex inflexione ver⯑ſus axem A B cogantur. Sit⯑que radiationis Limes (ſeu punc⯑ti A imago, qualem ſupra paſ⯑ſim ſtatuimus) punctum Z. In⯑ter hoc autem & inflectentis ver⯑ticem B uſpiam poſitus concipi⯑atur Oculus. Quaeri jam po⯑teſt ubi loci debeat punctum A apparere? Retrorſum ad punc⯑tum Z videri non ſert Natura (cum omnis impreſſio ſenſum af⯑ficiens proveniat a partibus A) ac experientia reclamat. No⯑ſtris autem e placitis conſequi videtur, ipſum ad partes anti⯑cas apparens ab intervallo lon⯑giſſime diſſito, (quod & maxi⯑mum ſenſibile quodvis Intervallum quodammodo exſuperet) apparere. Cum enim quo Radiis mi⯑nus divergentibus attingitur Objectum, eo (ſe⯑cluſis utique praenotionibus & praejudiciis) longius abeſſe ſentiatur; et quod Parallelos ad Oculum [164] Radios projicit, remotiſſime poſitum aeſtimetur. Exigere Ratio videtur ut quod convergentibus ra⯑diis apprehenditur, adhuc magis, ſi fieri poſſet, quoad apparentiam elongetur. Quin & circa Caſum hunc generatim inquiri poſſit, quidnam omnino ſit, quod apparentem puncti A locum de⯑terminet, faciatque quod conſtanti ratione nunc propius, nunc remotius appareat? Cui itidem dubio, nihil quicquam ex hactenus dictorum A⯑nalogia, reſponderi poſſe videtur, niſi debere punctum A perpetuo longiſſime ſemotum videri. Verum experientia ſecus atteſtatur, illud pro di⯑verſa Oculi inter puncta B, Z, poſitione varie diſtans; nunquam fere (ſi unquam) longinquius ipſo A libere ſpectato, ſubinde vero multo pro⯑pinquius adparere; quinimo, quo oculum appel⯑lentes radii magis convergunt eo ſpeciem Objecti propius accedere. Nempe, ſi puncto B admoveatur Oculus, ſuo (ad lentem) fere nativo in loco con⯑ſpicitur punctum A (vel aeque diſtans, ad Spe⯑culum;) ad O reductus oculus ejuſce ſpeciem ap⯑propinquantem cernit; ad P adhuc vicinius ip⯑ſum exiſtimat; ac ita ſenſim, donec alicubi tan⯑dem, velut ad Q, conſtituto oculo objectum ſumme proquinquum apparens, in meram confuſionem inci⯑piat evaneſcere. Quae ſane cuncta rationibus atque decretis noſtris repugnare videntur, aut cum iis ſaltem parum amice conſpirant. Neque no⯑ſtram tantum ſententiam pulſat hoc experimentum; at ex aequo caeteras quas norim omnes, veterem imprimis ac vulgatam noſtrae prae reliquis affinem ita convellere videtur, ut ejus vi coactus doctiſ⯑ſimus A. Tacquetus iſti principio (cui pene ſoli totam in aedificaverat Catoptricam ſuam) ceu in⯑fido ac inconſtanti renunciarit, adeoque ſuam ip⯑ſe doctrinam labefactarit; id tamen, opinor, mi⯑nime facturus, ſi rem totam inſpexiſſet penitius, [165] atque difficultatis fundum attigiſſet. Apud me vero non ita pollet haec, nec eouſque praepollebit ulla difficultas, ut ab iis, quae manifeſte rationi conſentanea video, diſcedam; praeſertim quum ut hic accidit, ejuſmodi difficultas in ſingularis cu⯑iuſpiam casûs diſparitate fundetur. Nimirum in praeſente caſu peculiare quiddam, naturae ſub⯑tilitati involutum, deliteſcit, aegre fortaſſis, niſi perfectius explorato videndi modo, detegendum. Circa quod nil, fateor, hactenus excogitare potui, quod adblandiretur animo meo, nedum plane ſa⯑tisfaceret. Vobis itaque nodum hunc, utinam feli⯑ciore conatu, reſolvendum committo.
In Engliſh as follows.
XXX. The ancient and receiv'd Principle, which Dr. Barrow here mentions as the main Foundation of Tacquet's Catoptrics, is that every viſible Point ſeen by Reflection from a Speculum, ſhall appear placed at the Interſection of the reflected Ray, and the Perpendicular of Incidence. Which Inter⯑ſection in the preſent Caſe, happening to be behind the Eye, it greatly ſhakes the Authority of that Principle, where on the aforementioned Author proceeds throughout his whole Catoptrics, in de⯑termining the apparent Place of Objects ſeen by Reflexion from any kind of Speculum.
XXXI. Let us now ſee how this Phaenomenon agrees with our Tenets. The Eye the nearer it is placed to the Point B in the foregoing Figures, the more diſtinct is the Appearance of the Object; but as it recedes to O, the Appearance grows more Confuſed; and at P it ſees the Object yet more [169] Confuſed; and ſo on till the Eye being brought back to Z ſees the Object in the greateſt Confuſi⯑on of all. Wherefore by SECT. XXI. the Object ſhou'd ſeem to approach the Eye gradually, as it recedes from the Point B, that is at O it ſhou'd (in Conſequence of the Principle I have laid down in the aforeſaid Section) ſeem nearer than it did at B, and at P nearer than at O, and at Q nearer than at P; and ſo on, till it quite vaniſhes at Z. Which is the very matter of Fact, as any one that pleaſes may eaſily ſatisfy himſelf by Experiment.
XXXII. This Caſe is much the ſame, as if we ſhou'd ſuppoſe an Engliſhman to meet a Foreigner, who uſed the ſame Words with the Engliſh, but in a direct contrary Signification. The Engliſhman wou'd not fail to make a wrong Judgment of the Ideas annexed to thoſe Sounds, in the Mind of him that uſed them. Juſt ſo, in the preſent Caſe the Object ſpeaks (if I may ſo ſay) with Words that the Eye is well acquainted with, that is, Confuſions of Appearance; but whereas heretofore the greater Confuſions were always wont to ſignify nearer Diſtances, they have in this Caſe a direct, contrary Signification, being connected with the greater Diſtances. Whence it follows, that the Eye muſt unavoidably be miſtaken, ſince it will take the Confuſions in the Senſe it has been uſed to, which is directly oppoſed to the True.
XXXIII. This Phaenomenon as it entirely ſub⯑verts the Opinion of thoſe, who will have us judge of Diſtance by Lines and Angles, on which Sup⯑poſition it is altogether inexplicable, ſo it ſeems to me no ſmall Confirmation of the Truth of that Principle whereby it is explain'd. But in order to a more full Explication of this Point, and to ſhew how far the Hypotheſis of the Mind's judg⯑ing [170] by the various Divergency of Rays, may be of uſe in determining the apparent Place of an Object, it will be neceſſary to premiſe ſome few Things, which are already well known to thoſe who have any Skill in Dioptrics.
XXXIV. Firſt, Any radiating Point is then diſtinctly ſeen when the Rays proceeding from it are, by the refractive Power of the Cryſtalline, ac⯑curately reunited in the Retina or Fund of the Eye: But if they are reunited, either before they arrive at the Retina, or after they have paſt it, then there is confuſed Viſion.
XXXV. Secondly, Suppoſe in the adjacent Fi⯑gures NP repreſent an Eye duly framed, and re⯑taining
[171] its natural Figure. In Fig. 1. the Rays falling nearly Parallel on the Eye, are by the Cry⯑ſtalline AB refracted, ſo as their Focus or Point of Union F falls exactly on the Retina: But if the Rays fall ſenſibly diverging on the Eye, as in Fig. 2. then their Focus falls beyond the Retina: Or if the Rays are made to converge by the Lens QS, before they come at the Eye, as in Fig. 3. their Focus F will fall before the Retina. In which two laſt Caſes, it is evident from the fore⯑going Section, that the Appearance of the Point Z is confuſed. And by how much the greater is the Convergency, or Divergency of the Rays fall⯑ing on the Pupil, by ſo much the farther will the Point of their Reunion be from the Retina, either before or behind it, and conſequently the Point Z will appear, by ſo much the more confuſed. And this by the bye, may ſhew us the Difference be⯑tween confuſed, and faint Viſion. Confuſed Viſion is, when the Rays proceeding from each diſtinct Point of the Object are not accurately recollected in one correſponding Point on the Retina, but take up ſome Space thereon: So that Rays from dif⯑ferent Points become mixed, and confuſed together. This is oppoſed to a diſtinct Viſion, and attends near Objects. Faint Viſion is, when by reaſon of the Diſtance of the Object or Groſſneſs of the in⯑terjacent Medium few Rays arrive from the Object to the Eye. This is oppoſed to vigorous or clear Viſion, and attends remote Objects. But to re⯑turn.
XXXVI. The Eye, or (to ſpeak truly) the Mind perceiving only the Confuſion it ſelf, with⯑out ever conſidering the Cauſe from which it pro⯑ceeds, doth conſtantly annex the ſame Degree of Diſtance to the ſame Degree of Confuſion. [172] Whether that Confuſion be occaſioned by converging, or by diverging Rays, it matters not. Whence it follows, that the Eye viewing the Object Z through the Glaſs QS (which by Refraction cauſeth the Rays ZQ, ZS, &c. to converge) ſhou'd judge it to be at ſuch a Nearneſs, at which if it were placed, it wou'd radiate on the Eye with Rays diverging to that Degree, as wou'd produce the ſame Confuſion, which is now pro⯑duced by Converging Rays, i. e. wou'd cover a Portion of the Retina equal to DC. vid. Fig. 3. ſupra. But then this muſt be underſtood (to uſe Dr. Barrow's Phraſe) ſecluſis praenotionibus & praeju⯑diciis, in caſe we abſtract from all other Circum⯑ſtances of Viſion, ſuch as the Figure, Size, Faint⯑neſs, &c. of the viſible Objects; all which do or⯑dinarily concur to form our Idea of Diſtance, the Mind having by frequent Experience obſerved their ſeveral Sorts or Degrees, to be connected with va⯑rious Diſtances.
XXXVII. It plainly follows from what hath been ſaid, that a Perſon perfectly Purblind (i. e. that cou'd not ſee an Object diſtinctly, but when placed cloſe to his Eye) wou'd not make the ſame wrong Judgment that others do, in the foremen⯑tioned Caſe. For, to him, greater Confuſions con⯑ſtantly ſuggeſting greater Diſtances, he muſt, as he recedes from the Glaſs, and the Object grows more Confuſed, judge it to be at a farther Diſtance con⯑trary to what they do, who have had the Percep⯑tion of the Objects growing more confuſed, con⯑nected with the Idea of Approach.
XXXVIII. Hence alſo it doth appear, there may be good uſe of Computation by Lines and Angles in Optics; not that the Mind judgeth of Diſtance immediately by them, but becauſe it judg⯑eth [173] by ſomewhat which is connected with them, and to the Determination whereof they may be ſubſervient. Thus the Mind judging of the Di⯑ſtance of an Object, by the Confuſedneſs of its Ap⯑pearance, and this Confuſedneſs being greater or leſſer to the naked Eye, according as the Object is ſeen by Rays more or leſs diverging, it follows, that a Man may make uſe of the Divergency of the Rays in computing the apparent Diſtance, though not for its own ſake, yet on account of the Confuſion with which it is connected. But, ſo it is, the Confuſion it ſelf is intirely neglected by Mathematicians, as having no neceſſary Relation with Diſtance, ſuch as the greater or leſſer Angles of Divergency are conceived to have. And theſe (eſpecially for that they fall under Mathematical Computation) are alone regarded, in determin⯑ing the apparent Places of Objects, as though they were the ſole and immediate Cauſe of the Judgments the Mind makes of Diſtance. Whereas, in Truth, they ſhou'd not at all be regarded in themſelves, or any otherwiſe, than as they are ſuppoſed to be the Cauſe of Confuſed Viſion.
XXXIX. The not conſidering of this has been a fundamental and perplexing Overſight. For Proof whereof, we need go no farther than the Caſe be⯑fore us. It having been obſerved, that the moſt diverging Rays brought into the Mind the Idea of neareſt Diſtance, and that ſtill, as the Divergency decreaſed, the Diſtance increaſed: and it being thought, the Connexion between the various De⯑grees of Divergency and Diſtance, was immediate, this naturally leads one to conclude, from an ill grounded Analogy, that converging Rays ſhall make an Object appear at an immenſe Diſtance: And that, as the Convergency increaſes, the Di⯑ſtance (if it were poſſible) ſhou'd do ſo likewiſe. [174] That this was the Cauſe of Dr. Barrow's Miſtake, is evident from his own Words which we have quoted. Whereas had the learned Doctor obſerv'd, that diverging and converging Rays, how oppoſite ſoever they may ſeem, do nevertheleſs agree in producing the ſame Effect, to wit, Confuſedneſs of Viſion, greater Degrees whereof are produced in⯑differently, either as the Divergency or Conver⯑gency of the Rays increaſeth. And that it is by this Effect, which is the ſame in both, that either the Divergency or Convergency is perceived by the Eye; I ſay had he but conſider'd this, it is certain he would have made a quite contrary Judg⯑ment, and rightly concluded, that thoſe Rays which fall on the Eye with greater Degrees of Convergency ſhou'd make the Object from whence they proceed, appear by ſo much the nearer. But it is plain, it was impoſſible for any Man to attain to a right Notion of this Matter, ſo long as he had regard only to Lines and Angles, and did not apprehend the true Nature of Viſion, and how far it was of Mathematical Conſideration.
XL. Before we diſmiſs this Subject, it is fit we take notice of a Query relating thereto, propoſed by the ingenious Mr. Molyneux, in his Treatiſe of Dioptrics *, where ſpeaking of this Difficulty, he has theſe Words: ‘And ſo he (i. e. Dr. Barrow) leaves this Difficulty to the Solution of others, which I (after ſo great an Example) ſhall do like⯑wiſe; but with the Reſolution of the ſame ad⯑mirable Author of not quitting the evident Doc⯑trine which we have before laid down, for de⯑termining the Locus Objecti, on account of being preſs'd by one Difficulty, which ſeems inexpli⯑cable till a more intimate Knowledge of the Vi⯑ſive [175] Faculty be obtained by Mortals. In the mean time, I propoſe it to the Conſideration of the Ingenious, Whether the Locus Apparens of an Object placed as in this 9th Section, be not as much before the Eye, as the diſtinct Baſe is be⯑hind the Eye?’ To which Query we may ven⯑ture to anſwer in the Negative. For in the pre⯑ſent Caſe, the Rule for determining the Diſtance of the diſtinct Baſe, or reſpective Focus from the Glaſs is this: As the Difference between the Diſtance of the Object and Focus is to the Focus or Focal Length, ſo the Diſtance of the Object from the Glaſs is to the Diſtance of the reſpective Focus or diſtinct Baſe from the Glaſs *. Let us now ſuppoſe the Object to be placed at the Diſtance of the Focal Length, and one half of the Focal Length from the Glaſs, and the Eye cloſe to the Glaſs, hence it will follow by the Rule, that the Diſtance of the diſtinct Baſe behind the Eye is double the true Diſtance of the Object before the Eye. If therefore Mr. Molyneux's Conjecture held good, it wou'd follow that the Eye ſhou'd ſee the Object, twice as far off as it really is; and in other Caſes at three or four times its due Diſtance, or more. But this manifeſtly contradicts Experience, the Object never appear⯑ing, at fartheſt, beyond its due Diſtance. What ever therefore is built on this Suppoſition (vid. Corol. 1. Prop. 57. ibid.) comes to the Ground along with it.
XLI. From what hath been premis'd, it is a manifeſt Conſequence, that a Man born blind, be⯑ing made to ſee, wou'd, at firſt, have no Idea of Diſtance by Sight; The Sun and Stars, the remo⯑teſt Objects as well as the nearer wou'd all ſeem to be in his Eye, or rather in his Mind. The Ob⯑jects [176] intromitted by Sight, wou'd ſeem to him (as in truth they are) no other than a new Set of Thoughts or Senſations, each whereof is as near to him, as the Perceptions of Pain or Pleaſure, or the moſt inward Paſſions of his Soul. For our judg⯑ing Objects perceiv'd by Sight to be at any Diſ⯑tance, or without the Mind, is (vid. SECT. XXVIII.) intirely the Effect of Experience, which one in thoſe Circumſtances cou'd not yet have attained to.
XLII. It is indeed otherwiſe upon the common Suppoſition, that Men judge of Diſtance by the Angle of the Optic Axes, juſt as one in the Dark, or a Blind-man by the Angle comprehended by two Sticks, one whereof he held in each Hand. For if this were true, it wou'd follow that one blind from his Birth being made to ſee, ſhou'd ſtand in need of no new Experience, in order to perceive Diſtance by Sight. But that this is falſe, has, I think, been ſufficiently demonſtrated.
XLIII. And perhaps upon a ſtrict Inquiry, we ſhall not find that even thoſe, who from their Birth have grown up in a continu'd Habit of Seeing, are irrecoverably prejudiced on the other ſide, to wit, in thinking what they ſee to be at a Diſtance from them. For at this time it ſeems agreed on all hands, by thoſe who have had any Thoughts of that Matter, that Colours, which are the proper and immediate Object of Sight, are not without the Mind. But then it will be ſaid, by Sight we have alſo the Ideas of Extenſion, and Figure, and Mo⯑tion; all which may well be thought without, and at ſome Diſtance from the Mind, though Colour ſhou'd not. In anſwer to this, I appeal to any Man's Experience, whether the viſible Extenſion of any Object doth not appear as near to him, as [177] the Colour of that Object; Nay, whether they do not both ſeem to be in the very ſame Place. Is not the Extenſion we ſee Coloured, and is it poſſi⯑ble for us, ſo much as in Thought, to ſeparate and abſtract Colour from Extenſion? Now, where the Extenſion is, there ſurely is the Figure, and there the Motion too. I ſpeak of thoſe which are per⯑ceived by Sight.
XLIV. But for a fuller Explication of this Point, and to ſhew that the immediate Objects of Sight are not ſo much as the Ideas or Reſemblan⯑ces of things placed at a Diſtance, it is requiſite that we look nearer into the Matter, and carefully obſerve what is meant in common Diſcourſe, when one ſays, that which he ſees is at a Diſtance from him. Suppoſe, for Example, that looking at the Moon I ſhou'd ſay it were fifty or ſixty Semidia⯑meters of the Earth diſtant from me. Let us ſee what Moon this is ſpoken of: It is plain it cannot be the viſible Moon, or any thing like the viſible Moon, or that which I ſee, which is only a round, luminous Plain, of about thirty viſible Points in Diameter. For in caſe I am carried from the place where I ſtand directly towards the Moon, it is mani⯑feſt the Object varies, ſtill as I go on; and by the time that I am advanced fifty or ſixty Semidiameters of the Earth, I ſhall be ſo far from being near a ſmall, round, luminous Flat, that I ſhall perceive nothing like it; this Object having long ſince diſappeared, and if I wou'd recover it, it muſt be by going back to the Earth from whence I ſet out. Again, ſup⯑poſe I perceive by Sight the faint and obſcure Idea of ſomething, which I doubt whether it be a Man, or a Tree, or a Tower, but judge it to be at the Diſtance of about a Mile. 'Tis plain I cannot mean, that what I ſee is a Mile off, or that it is the Image or Likeneſs of any thing which is a Mile off, ſince [178] that every Step I take towards it, the Appearance alters, and from being obſcure, ſmall, and faint, grows clear, large and vigorous. And when I come to the Mile's end, that which I ſaw firſt is quite loſt, neither do I find any thing in the like⯑neſs of it.
XLV. In theſe and the like Inſtances, the truth of the Matter ſtands thus: Having of a long time experienced certain Ideas, perceivable by Touch, as Diſtance, tangible Figure, and Solidity, to have been connected with certain Ideas of Sight, I do upon perceiving theſe Ideas of Sight, forth⯑with conclude what Tangible Ideas are, by the wonted ordinary courſe of Nature, like to follow. Looking at an Object I perceive a certain viſible Figure and Colour, with ſome degree of Faintneſs and other Circumſtances, which from what I have formerly obſerved, determine me to think, that if I advance forward ſo many Paces or Miles, I ſhall be affected with ſuch and ſuch Ideas of Touch: So that in truth and ſtrictneſs of Speech, I neither ſee Diſtance it ſelf, nor any thing that I take to be at a Diſtance. I ſay, neither Diſtance, nor things placed at a Diſtance are themſelves, or their Ideas, truly perceived by Sight. This I am perſuaded of, as to what concerns my ſelf; and I believe whoever will look narrowly into his own Thoughts, and examine what he means by ſaying, he ſees this or that thing at a Diſtance, will agree with me, that what he ſees only ſuggeſts to his Underſtanding, that after having paſſed a certain Diſtance, to be meaſured by the Motion of his Body, which is perceivable by Touch, he ſhall come to perceive ſuch and ſuch tangible Ideas which have been uſu⯑ally connected with ſuch and ſuch viſible Ideas. But that one might be deceived by theſe ſuggeſti⯑ons of Senſe, and that there is no neceſſary Con⯑nexion [179] between viſible and tangible Ideas ſuggeſted by them, we need go no farther than the next Looking-glaſs or Picture to be convinced. Note, that when I ſpeak of Tangible Ideas, I take the word Idea for any the immediate Object of Senſe, or Underſtanding, in which large Signification it is commonly uſed by the Moderns.
XLVI. From what we have ſhewn it is a mani⯑feſt Conſequence, that the Ideas of Space, Outneſs, and Things placed at a Diſtance, are not, ſtrictly ſpeaking, the Object of Sight; they are not other⯑wiſe perceived by the Eye than by the Ear. Sit⯑ting in my Study I hear a Coach drive along the Street; I look through the Caſement and ſee it; I walk out and enter into it; thus, common Speech wou'd incline one to think, I heard, ſaw, and touch'd the ſame thing, to wit, the Coach. It is nevertheleſs certain, the Ideas intromitted by each Senſe are widely different, and diſtinct from each other; but having been obſerved conſtantly to go together, they are ſpoken of as one and the ſame thing. By the variation of the Noiſe I perceive the different Diſtances of the Coach, and know that it approaches before I look out. Thus by the Ear I perceive Diſtance, juſt after the ſame manner as I do by the Eye.
XLVII. I do not nevertheleſs ſay, I hear Diſ⯑tance in like manner as I ſay that I ſee it, the Ide⯑as perceived by Hearing not being ſo apt to be confounded with the Ideas of Touch, as thoſe of Sight are; ſo likewiſe a Man is eaſily convinced that Bodies and external Things are not properly the Object of Hearing, but only Sounds, by the Mediation whereof the Idea of this or that Body, or Diſtance is ſuggeſted to his Thoughts. But then one is with more difficulty brought to diſcern [180] the Difference there is betwixt the Ideas of Sight and Touch: Though it be certain, a Man no more ſees or feels the ſame thing, than he hears and feels the ſame thing.
XLVIII. One Reaſon of which ſeems to be this. It is thought a great Abſurdity to imagine, that one and the ſame thing ſhou'd have any more than one Extenſion, and one Figure. But the Extenſion and Figure of a Body, being let into the Mind two ways, and that indifferently, either by Sight or Touch, it ſeems to follow that we ſee the ſame Extenſion, and the ſame Figure which we feel.
XLIX. But if we take a cloſe and accurate View of Things, it muſt be acknowledged that we never ſee and feel one and the ſame Object. That which is ſeen is one thing, and that which is felt is ano⯑ther; if the viſible Figure and Extenſion be not the ſame with the tangible Figure and Extenſion, we are not to infer that one and the ſame thing has divers Extenſions. The true Conſequence is, that the Objects of Sight and Touch are two diſ⯑tinct things. It may perhaps require ſome Thought rightly to conceive this Diſtinction. And the Difficulty ſeems not a little increaſed, becauſe the Combination of Viſible Ideas hath conſtantly the ſame Name, as the Combination of Tangible Ideas wherewith it is connected: Which doth of neceſſi⯑ty ariſe from the uſe and end of Language.
L. In order therefore to treat accurately and unconfuſedly of Viſion, we muſt bear in mind that there are two ſorts of Objects apprehended by the Eye, the one primarily and immediately, the other ſecondarily and by Intervention of the former. [181] Thoſe of the firſt ſort neither are, nor appear to be without the Mind, or at any Diſtance off; they may indeed grow greater, or ſmaller, more con⯑fuſed, or more clear, or more faint, but they do not, cannot appraoch or recede from us. When⯑ever we ſay an Object is at a Diſtance, whenever we ſay it draws near, or goes farther off, we muſt always mean it of the latter ſort, which properly belong to the Touch, and are not ſo truly per⯑ceived, as ſuggeſted by the Eye in like manner as Thoughts by the Ear.
LI. No ſooner do we hear the Words of a fa⯑miliar Language pronounced in our Ears, but the Ideas correſponding thereto preſent themſelves to our Minds; in the very ſame inſtant the Sound and the Meaning enter the Underſtanding: So cloſely are they united, that it is not in our Power to keep out the one, except we exclude the other alſo. We even act in all reſpects as if we heard the very Thoughts themſelves. So likewiſe the ſecondary Objects, or thoſe which are only ſug⯑geſted by Sight, do often more ſtrongly affect us, and are more regarded than the proper Objects of that Senſe; along with which they enter into the Mind, and with which they have a far more ſtrict Connexion, than Ideas have with Words. Hence it is, we find it ſo difficult to diſcriminate between the immediate and mediate Objects of Sight, and are ſo prone to attribute to the former, what be⯑longs only to the latter. They are, as it were, moſt cloſely twiſted, blended, and incorporated together. And the Prejudice is confirmed and ri⯑veted in our Thoughts by a long tract of Time, by the uſe of Language, and want of Reflexion. However, I believe any one that ſhall attentively conſider what we have already ſaid, and ſhall ſay upon this Subject before we have done, (eſpecially [182] if he purſue it in his own Thoughts) may be able to deliver himſelf from that Prejudice. Sure I am 'tis worth ſome Attention, to whoever wou'd un⯑derſtand the true nature of Viſion.
LII. I have now done with Diſtance, and pro⯑ceed to ſhew how it is, that we perceive by Sight the Magnitude of Objects. It is the Opinion of ſome that we do it by Angles, or by Angles in con⯑junction with Diſtance; but neither Angles, nor Diſtance being perceivable by Sight, and the things we ſee being in truth at no Diſtance from us, it follows, that as we have ſhewn Lines and Angles not to be the Medium, the Mind makes uſe of in apprehending the Apparent Place, ſo neither are they the Medium whereby it apprehends the Ap⯑parent Magnitude of Objects.
LIII. It is well known that the ſame Extenſion at a near Diſtance ſhall ſubtend a greater Angle, and at a farther Diſtance, a leſſer Angle. And by this Principle (we are told) the Mind eſtimates the Magnitude of an Object comparing the Angle un⯑der which it is ſeen with its Diſtance, and thence inferring the Magnitude thereof. What inclines Men to this Miſtake (beſide the Humour of mak⯑ing one ſee by Geometry is, that the ſame Percep⯑tions or Ideas which ſuggeſt Diſtance, do alſo ſug⯑geſt Magnitude. But if we examine it, we ſhall find they ſuggeſt the latter, as immediately as the former. I ſay, they do not firſt ſuggeſt Diſtance, and then leave it to the Judgment to uſe that as a Medium, whereby to collect the Magnitude; but they have as cloſe, and immediate a Connexion with the Magnitude, as with the Diſtance; and ſuggeſt Magnitude as independently of Diſtance, as they do Diſtance independently of Magnitude. All [183] which will be evident to whoever conſiders what hath been already ſaid, and what follows.
LIV. It hath been ſhewn, there are two ſorts of Objects apprehended by Sight; each whereof hath its diſtinct Magnitude, or Extenſion. The one, properly Tangible, i. e. to be perceived and mea⯑ſured by Touch, and not immediately falling un⯑der the Senſe of ſeeing: The other, properly and immediately Viſible, by Mediation of which the former is brought in View. Each of theſe Mag⯑nitudes are greater or leſſer, according as they con⯑tain in them more or fewer Points, they being made up of Points or Minimums. For, whatever may be ſaid of Extenſion in Abſtract, it is certain ſenſible Extenſion is not infinitely Diviſible. There is a Minimum Tangibile, and a Minimum Viſibile, be⯑yond which Senſe cannot perceive. This every one's Experience will inform him.
LV. The Magnitude of the Object which exiſts without the Mind, and is at a Diſtance, continues always invariably the ſame: But the Viſible Object ſtill changing as you approach to, or recede from the Tangible Object, it hath no fixed and deter⯑minate Greatneſs. Whenever therefore, we ſpeak of the Magnitude of any thing, for Inſtance a Tree or a Houſe, we muſt mean the Tangible Magni⯑tude, otherwiſe there can be nothing ſteady and free from Ambiguity ſpoken of it. But though the Tangible and Viſible Magnitude in truth be⯑long to two diſtinct Objects: I ſhall nevertheleſs (eſpecially ſince thoſe Objects are called by the ſame Name, and are obſerved to coexiſt) to avoid tediouſneſs and ſingularity of Speech, ſometimes ſpeak of them, as belonging to one and the ſame thing.
[184] LVI. Now in order to diſcover by what means, the Magnitude of Tangible Objects is perceived by Sight; I need only reflect on what paſſes in my own Mind, and obſerve what thoſe things be, which introduce the Ideas of greater or leſſer into my Thoughts, when I look on any Object. And theſe I find to be, Firſt, the Magnitude or Exten⯑ſion of the Viſible Object, which being immediate⯑ly perceived by ſight, is connected with that o⯑ther which is Tangible, and placed at a Diſtance. Secondly, The Confuſion or Diſtinctneſs. And Thirdly, the Vigorouſneſs or Faintneſs of the afore⯑ſaid Viſible Appearance. Caeteris paribus, by how much the greater or leſſer, the Viſible Object is, by ſo much the greater or leſſer, do I conclude the Tangible Object to be. But, be the Idea imme⯑diately perceived by Sight never ſo large, yet if it be withal Confuſed, I judge the Magnitude of the thing to be but ſmall. If it be Diſtinct and Clear, I judge it greater. And if it be Faint, I appre⯑hend it to be yet greater. What is here meant, by Confuſion and Faintneſs, hath been explained in SECT. XXXV.
LVII. Moreover the Judgments we make of Greatneſs do, in like manner as thoſe of Diſtance, depend on the Diſpoſition of the Eye, alſo on the Figure, Number and Situation of Objects and o⯑ther Circumſtances that have been obſerv'd to at⯑tend great, or ſmall Tangible Magnitudes. Thus, for Inſtance, the very ſame Quantity of Viſible Ex⯑tenſion, which in the Figure of a Tower, doth ſuggeſt the Idea of great Magnitude, ſhall, in the Figure of a Man ſuggeſt the Idea of much ſmaller Magnitude. That this is owing to the Experi⯑ence we have had of the uſual Bigneſs of a Tower and a Man, no one, I ſuppoſe, need be told.
LVIII. It is alſo evident, that Confuſion or Faintneſs, have no more a neceſſary Connexion [185] with little or great Magnitude, than they have with little or great Diſtance. As they ſuggeſt the latter, ſo they ſuggeſt the former to our Minds. And by Conſequence, if it were not for Experi⯑ence, we ſhou'd no more judge a faint or confuſed Appearance to be connected with great or little Magnitude, than we ſhou'd that it was connected with great or little Diſtance.
LIX. Nor will it be found, that great or ſmall Viſible Magnitude hath any neceſſary Relation to great or ſmall Tangible Magnitude: So that the one may certainly be infer'd from the other. But, before we come to the Proof of this, it is fit we conſider the Difference there is betwixt the Extenſion and Figure which is the proper Object of Touch, and that other which is termed Viſible; and how the former is principally, though not im⯑mediately taken notice of, when we look at any Object. This has been before mentioned, but we ſhall here inquire into the Cauſe thereof. We regard the Objects that environ us, in proportion as they are adapted to benefit or injure our own Bodies, and thereby produce in our Minds the Senſations of Pleaſure or Pain. Now Bodies ope⯑rating on our Organs, by an immediate Applica⯑tion, and the Hurt or Advantage ariſing there⯑from, depending altogether on the Tangible, and not at all on the Viſible, Qualities of any Object: This is a plain Reaſon, why thoſe ſhou'd be re⯑garded by us much more than theſe; and for this End, the Viſive Senſe ſeems to have been beſtow⯑ed on Animals, to wit, that by the Perception of Viſible Ideas (which in themſelves are not capable of affecting, or any wiſe altering the Frame of their Bodies) they may be able to foreſee (from the Experience they have had, what Tangible Ideas are connected with ſuch, and ſuch Viſible Ideas) [186] the Damage or Benefit which is like to enſue, up⯑on the Application of their own Bodies to this or that Body which is at a Diſtance. Which Fore⯑ſight, how neceſſary it is to the preſervation of an Animal, every one's Experience can inform him. Hence it is, that when we look at an Object, the Tangible Figure and Extenſion thereof are princi⯑pally attended to; whilſt there is ſmall heed taken of the Viſible Figure and Magnitude, which, though more immediately perceived, do leſs con⯑cern us, and are not fitted to pruduce any Altera⯑tion in our Bodies.
LX. That the Matter of Fact is true, will be evident to any one, who conſiders that a Man placed at Ten Foot Diſtance, is thought as great, as if he were placed at the Diſtance only of Five Foot; which is true, not with Relation to the Vi⯑ſible, but Tangible Greatneſs of the Object. The Viſible Magnitude being far greater, at one Station, than it is at the other.
LXI. Inches, Feet, &c. are ſettled, ſtated Lengths, whereby we meaſure Objects, and eſti⯑mate their Magnitude, we ſay, for Example, an Object appears to be ſix Inches, or Six Foot long. Now, that this cannot be meant of Viſible Inches, &c. is evident, becauſe a Viſible Inch is it ſelf no conſtant, determinate Magnitude, and cannot there⯑fore ſerve to mark out, and determine the Magni⯑tude of any other thing. Take an Inch mark'd upon a Ruler; view it, ſucceſſively, at the diſtance of half a Foot, a Foot, a Foot and a Half, &c. from the Eye: At each of which, and at all the intermediate Diſtances, the Inch ſhall have a diffe⯑rent Viſible Extenſion, i. e. there ſhall be more or fewer Points diſcerned in it. Now I ask which of all theſe various Extenſions, is that ſtated, deter⯑minate [187] one, that is agreed on, for a common Mea⯑ſure of other Magnitudes? No Reaſon can be aſſign⯑ed, why we ſhou'd pitch on one, more than ano⯑ther: And except there be ſome invariable, deter⯑minate Extenſion fixed on to be marked by the Word Inch, it is plain, it can be uſed to little Pur⯑poſe; and to ſay, a Thing contains this or that Number of Inches, ſhall imply no more than that it is extended, without bringing any particular Idea of that Extenſion into the Mind. Farther, an Inch and a Foot, from different Diſtances, ſhall both exhibit the ſame Viſible Magnitude, and yet at the ſame time, you ſhall ſay, that one ſeems ſe⯑veral times greater than the other. From all which it is manifeſt, that the Judgments we make of the Magnitude of Objects by Sight, are altogether in reference to their Tangible Extenſion. When⯑ever we ſay an Object is Great, or Small, of this or that determinate Meaſure, I ſay, it muſt be meant of the Tangible, and not the Viſible Exten⯑ſion, which, though immediately perceived, is ne⯑vertheleſs little taken notice of
LXII. Now, that there is no neceſſary Con⯑nexion, between theſe two Diſtinct Extenſions is evident from hence: Becauſe our Eyes might have been framed in ſuch a manner, as to be able to ſee nothing but what were leſs than the Minimum Tan⯑gibile. In which Caſe, it is not impoſſible we might have perceived all the immediate Objects of Sight, the very ſame that we do now: But unto thoſe Viſible Appearances, there wou'd not be connected thoſe different Tangible Magnitudes, that are now. Which ſhews, the Judgments we make of the Mag⯑nitude of Things placed at a diſtance, from the va⯑rious Greatneſs of the Immediate Objects of Sight, do not ariſe from any Eſſential or Neceſſary, but only [188] a Cuſtomary Tye, which has been obſerv'd between them.
LXIII. Moreover, it is not only certain, that any Idea of Sight might not have been connected with this or that Idea of Touch, which we now obſerve to accompany it: But alſo, that the great⯑er Viſible Magnitudes might have been connected with, and introduced into our Minds leſſer Tangi⯑ble Magnitudes, and the leſſer Viſible Magnitudes greater Tangible Magnitudes. Nay, that it actu⯑ally is ſo, we have daily Experience; that Object which makes a ſtrong and large Appearance, not ſeeming near ſo great as another, the Viſible Mag⯑nitude whereof is much leſs, but more faint, and the Appearance upper, or which is the ſame thing painted lower on the Retina, which Faintneſs and Situation ſuggeſt both greater Magnitude and greater Diſtance.
LXIV. From which, and from SECT. LVII. and LVIII. it is manifeſt, that as we do not per⯑ceive the Magnitudes of Objects immediately by Sight, ſo neither do we perceive them, by the Me⯑diation of any thing which has a neceſſary Connex⯑ion with them. Thoſe Ideas that now ſuggeſt un⯑to us the various Magnitudes of External Objects, before we touch them, might poſſibly have ſuggeſt⯑ed no ſuch thing: Or they might have ſignified them, in a direct contrary manner, ſo that the very ſame Ideas, on the Perception whereof we judge an Object to be Small, might as well have ſerv'd to make us conclude it Great. Thoſe Ideas being in their own Nature equally fitted to bring into our Minds the Idea of Small or Great, or no Size at all of outward Objects; juſt as the Words of any Language are in their own Nature indifferent to ſignify this or that thing, or nothing at all.
[189] LXV. As we ſee Diſtance, ſo we ſee Magnitude. And we ſee both, in the ſame way that we ſee Shame or Anger in Looks of a Man. Thoſe Paſ⯑ſions are themſelves Inviſible, they are nevertheleſs let in by the Eye along with Colours and Alterati⯑ons of Countenance, which are the immediate Ob⯑ject of Viſion: And which ſignify them for no o⯑ther Reaſon, than barely becauſe they have been obſerved to accompany them. Without which Experience, we ſhou'd no more have taken Bluſh⯑ing for a Sign of Shame, than of Gladneſs
LXVI. We are nevertheleſs exceeding prone to imagine thoſe things, which are perceived only by the Mediation of others, to be themſelves the im⯑mediate Objects of Sight; or, at leaſt, to have in their own Nature a Fitneſs to be ſuggeſted by them, before ever they had been experienced to coexiſt with them. From which Prejudice every one, perhaps, will not find it eaſy to emancipate himſelf, by any the cleareſt Convictions of Reaſon. And there are ſome Grounds to think, that if there was one only invariable and univerſal Language in the World, and that Men were born with the Fa⯑culty of ſpeaking it, it wou'd be the Opinion of many, that the Ideas in other Mens Minds were properly perceived by the Ear, or had at leaſt a neceſſary and inſeparable Tye with the Sounds that were affixed to them. All which ſeems to a⯑riſe from want of a due Application of our diſcern⯑ing Faculty, thereby to diſcriminate between the Ideas that are in our Underſtandings, and conſi⯑der them apart from each other; which wou'd preſerve us from confounding thoſe that are diffe⯑rent, and make us fee what Ideas do, and what do not include or imply this or that other Idea.
[190] LXVII. There is a Celebrated Phaenomenon, the Solution whereof I ſhall attempt to give, by the Principles that have been laid down, in refe⯑rence to the manner wherein we apprehend by Sight the Magnitude of Objects. The apparent Magnitude of the Moon when placed in the Hori⯑zon, is much greater than when it is in the Meri⯑dian. Though the Angle under which the Di⯑ameter of the Moon is ſeen, be not obſerved grea⯑ter in the former Caſe, than in the latter: And the Horizontal Moon doth not conſtantly appear of the ſame Bigneſs, but at ſome times ſeemeth far greater than at others.
LXVIII. Now in order to explain the Rea⯑ſon of the Moon's appearing greater than ordinary in the Horizon, it muſt be obſerved, that the Par⯑ticles which compoſe our Atmoſphere intercept the Rays of Light proceeding from any Object to the Eye; and by how much the greater is the Portion of Atmoſphere, interjacent between the Object and the Eye, by ſo much the more are the Rays intercepted; and by conſequence, the Ap⯑pearance of the Object rendered more Faint, every Object appearing more Vigorous or more Faint, in Proportion as it ſendeth more or fewer Rays, into the Eye Now, between the Eye and the Moon, when ſituated in the Horizon, there lies a far greater Quan⯑tity of Atmoſphere, than there does when the Moon is in the Meridian. Whence it comes to paſs, that the Appearance of the Horizontal Moon is fainter, and therefore by SECT. LVI. it ſhou'd be thought bigger in that Situation, than in the Meridian, or in any other Elevation above the Horizon.
LXIX. Farther, the Air being variouſly im⯑pregnated, ſometimes more and ſometimes leſs [191] with Vapours and Exhalations fitted to retund and intercept the Rays of Light, it follows, that the Appearance of the Horizontal Moon hath not al⯑ways an equal Faintneſs, and by Conſequence, that Luminary, tho' in the very ſame Situation, is at one time judged greater than at another.
LXX. That we have here given the true Ac⯑count of the Phaenomena of the Horizontal Moon, will, I ſuppoſe, be farther evident to any one from the following Conſiderations. Firſt, It is plain, that which in this Caſe ſuggeſts the Idea of greater Magnitude, muſt be ſomething which is it ſelf per⯑ceived; for, that which is unperceived cannot ſug⯑geſt to our Perception any other thing. Secondly, It muſt be ſomething that does not conſtantly re⯑main the ſame, but is ſubject to ſome Change or Variation, ſince the Appearance of the Horizontal Moon varies, being at one time greater than at another. And yet, Thirdly, It cannot be the viſible Figure or Magnitude, ſince that remains the ſame, or is rather leſſer, by how much the Moon is near⯑er to the Horizon. It remains therefore, that the true Cauſe is that Affection or Alteration of the Viſible Appearance, which proceeds from the greater Paucity of Rays arriving at the Eye, and which I term Faintneſs: Since this anſwers all the forementioned Conditions, and I am not conſcious of any other Perception that doth.
LXXI. Add to this, that in miſty Weather it is a common Obſervation, that the Appearance of the Horizontal Moon is far larger than uſual, which greatly conſpires with, and ſtrengthens our Opinion. Neither wou'd it prove, in the leaſt, Irreconcilable with what we have ſaid, if the Ho⯑rizontal Moon ſhou'd chance ſometimes to ſeem enlarged beyond its uſual Extent, even in more [192] Screne Weather. For we muſt not only have re⯑gard to the Miſt, which happens to be in the place where we ſtand; we ought alſo to take into our Thoughts, the whole Sum of Vapours and Exha⯑lations, which lie betwixt the Eye and the Moon: All which cooperating to render the Appearance of the Moon more Faint, and thereby increaſe its Magnitude, it may chance to appear greater than it uſually does, even in the Horizontal Poſition, at a time when, though there be no extraordinary Fog or Hazineſs, juſt in the place where we ſtand; yet, the Air between the Eye and the Moon, taken altogether, may be loaded with a greater quantity of interſperſed Vapours and Ex⯑halations, than at other times.
LXXII. It may be objected, that in Conſe⯑quence of our Principles, the Interpoſition of a Body in ſome degree Opaque, which may inter⯑cept a great Part of the Rays of Light, ſhou'd ren⯑der the Appearance of the Moon in the Meridian as large, as when it is viewed in the Horizon. To which I anſwer, it is not Faintneſs any how appli⯑ed, that ſuggeſts greater Magnitude, there being no neceſſary, but only an experimental Connexion between thoſe two things: It follows, that the Faintneſs, which enlarges the Appearance, muſt be applied in ſuch Sort, and with ſuch Circumſtan⯑ces, as have been obſerved to attend the Viſion of great Magnitudes. When from a Diſtance we be⯑hold great Objects, the Particles of the intermedi⯑ate Air and Vapours, which are themſelves unper⯑ceivable, do interrupt the Rays of Light, and thereby render the Appearance leſs ſtrong and vivid; now, Faintneſs of Appearance cauſed in this Sort, hath been experienced to coexiſt with great Magnitude. But when it is cauſed by the Interpoſition of an opaque ſenſible Body, this Cir⯑cumſtance [193] alters the Caſe, ſo that a faint Appear⯑ance this way cauſed, doth not ſuggeſt greater Magnitude, becauſe it hath not been experienced to coexiſt with it.
LXXIII. Faintneſs, as well as all other Ideas or Perceptions which ſuggeſt Magnitude or Diſtance, doth it in the ſame way that Words ſuggeſt the Notions to which they are annexed. Now it is known, a Word pronounced with certain Circum⯑ſtances, or in a certain Context with other Words, hath not always the ſame Import and Signification that it hath when pronounced in ſome other Cir⯑cumſtances, or different Context of Words. The very ſame viſible Appearance as to Faintneſs and all other reſpects, if placed on high, ſhall not ſug⯑geſt the ſame Magnitude that it would if it were ſeen at an equal Diſtance, on a level with the Eye. The Reaſon whereof is, that we are rarely ac⯑cuſtomed to view Objects at a great Height; our Concerns lie among things ſituated rather before than above us; and accordingly our Eyes are not placed on the top of our Heads, but in ſuch a Po⯑ſition, as is moſt convenient for us to ſee diſtant Objects ſtanding in our way, and this Situation of them being a Circumſtance, which uſually attends the Viſion of diſtant Objects, we may from hence account for (what is commonly obſerved) an Ob⯑ject's appearing of different Magnitude, even with reſpect to its Horizontal Extenſion, on the top of a Steeple, for example, an hundred Feet high to one ſtanding below, from what it would if placed at an hundred Feet diſtance on a level with his Eye. For it hath been ſhewn, that the Judgment we make on the Magnitude of a thing, depends not on the viſible Appearance alone, but alſo on divers other Circumſtances, any one of which being omit⯑ted or varied may ſuffice to make ſome alteration in [194] our Judgment. Hence, the Circumſtance of view⯑ing a diſtant object in ſuch a Situation as is uſual, and ſuits with the ordinary Poſture of the Head and Eyes being omitted, and inſtead thereof a diffe⯑rent Situation of the Object, which requires a diffe⯑rent Poſture of the Head taking place, it is not to be wondered at, if the Magnitude be judged diffe⯑rent; but it will be demanded, why an high Object ſhoul'd conſtantly appear leſs than an equidiſtant low Object of the ſame Dimenſions, for ſo it is ob⯑ſerved to be; it may indeed be granted that the va⯑riation of ſome Circumſtances may vary the Judg⯑ment, made on the Magnitude of High Objects, which we are leſs uſed to look at: But it does not hence appear, why they ſhou'd be judged leſs rather than greater? I anſwer, that in caſe the Magnitude of diſtant Objects was ſuggeſted by the Extent of their viſible Appearance alone, and thought Proportional thereto, it is certain they wou'd then be judged much leſs than now they ſeem to be, Vide SECT. LXXIX. But ſeveral Circumſtances concurring to form the Judgment we make on the Magnitude of diſtant Objects, by means of which they appear far larger than others, whoſe viſible Appearance hath an equal or even greater Extenſion; it follows, that upon the Change or Omiſſion of any of thoſe Circumſtances, which are wont to attend the Viſion of diſtant Ob⯑jects, and ſo come to influence the Judgments made on their Magnitude, they ſhall proportionably ap⯑pear leſs than otherwiſe they would. For any of thoſe things that cauſed an Object to be thought greater, than in proportion to its viſible Extenſi⯑on, being either omitted or applied without the uſual Circumſtances, the Judgment depends more intirely on the viſible Extenſion, and conſequently the Object muſt be judged leſs. Thus in the pre⯑ſent Caſe, the Situation of the thing ſeen being dif⯑ferent [195] from what it uſually is in thoſe Objects we have occaſion to view, and whoſe Magnitude we obſerve, it follows, that the very ſame Object, be⯑ing an hundred Feet high, ſhou'd ſeem leſs than if it was an hundred Feet off on (or nearly on) a level with the Eye. What has been here ſet forth, ſeems to me to have no ſmall ſhare in contributing to magnify the Appearance of the horizontal Moon, and deſerves not to be paſſed over in the Explicati⯑on of it.
LXXIV. If we attentively conſider the Phaeno⯑menon before us, we ſhall find the not diſcerning between the mediate and immediate Objects of Sight, to be the chief Cauſe of the Difficulty that occurs in the Explication of it. The Magnitude of the viſible Moon, or that which is the proper and immediate Object of Viſion, is no greater when the Moon is in the Horizon, than when it is in the Meridian. How comes it therefore, to ſeem greater in one Situation than the other? What is it can put this Cheat on the Underſtanding? It has no other Perception of the Moon, than what it gets by Sight: And that which is ſeen, is of the ſame Extent, I ſay, the viſible Appearance hath the ſame, or rather a leſs Magnitude when the Moon is viewed in the Horizontal, than when in the Meri⯑dional Poſition: And yet it is eſteemed greater in the former than in the latter. Herein conſiſts the difficulty, which doth vaniſh and admit of a moſt eaſy Solution, if we conſider that as the viſible Moon is not greater in the Horizon than in the Meridian, ſo neither is it thought to be ſo. It hath been already ſhewn, that in any act of Viſion, the viſible Object abſolutely, or in it ſelf, is little taken notice of, the Mind ſtill carrying its View from that to ſome tangible Ideas, which have been obſerved to be connected with it, and by that means come [196] to be ſuggeſted by it. So that when a thing is ſaid to appear great or ſmall, or whatever Eſtimate be made of the Magnitude of any thing, this is meant not of the viſible, but of the tangible Object. This duly conſidered, it will be no hard matter to re⯑concile the ſeeming Contradiction there is, that the Moon ſhou'd appear of a different Bigneſs, the vi⯑ſible Magnitude thereof remaining ſtill the ſame. For by SECT. LVI. the very ſame viſible Exten⯑ſion, with a different Faintneſs, ſhall ſuggeſt a dif⯑ferent tangible Extenſion. When therefore the Ho⯑rizontal Moon is ſaid to appear greater than the Meridional Moon, this muſt be underſtood not of a greater viſible Extenſion, but of a greater tangible or real Extenſion, which by reaſon of the more than ordinary Faintneſs of the viſible Appearance, is ſuggeſted to the Mind along with it.
LXXV. Many Attempts have been made by Learned Men, to account for this Appearance. Gaſſendus, Deſcartes, Hobbes, and ſeveral others, have emplowed their Thoughts on that Subject; but how fruitleſs and unſatisfactory their Endeavours have been, is ſufficiently ſhewn in The Philoſophical Tranſactions *, where you may ſee their ſeveral O⯑pinions at large ſet forth and confuted, not with⯑out ſome Surpriſe at the groſs Blunders that inge⯑nious Men have been forced into, by endeavouring to reconcile this Appearance with the ordinary Principles of Optics. Since the Writing of which, there hath been publiſhed in the Tranſactions † another Paper relating to the ſame Affair, by the celebrated Dr. Wallis, wherein he attempts to ac⯑count for that Phaenomenon, which, though it ſeems not to contain any thing new, or different from [197] what had been ſaid before by others, I ſhall never⯑theleſs conſider in this place.
LXXVI. His Opinion, in ſhort, is this; We judge not of the Magnitude of an Object by the viſual Angle alone, but by the viſual Angle in conjunction with the Diſtance. Hence, though the Angle remain the ſame, or even become leſs, yet if withal the Diſtance ſeem to have been increaſed, the Object ſhall appear greater. Now, one way whereby we eſtimate the Diſtance of any thing, is by the Number and Extent of the intermediate Objects: When therefore the Moon is ſeen in the Horizon, the Variety of Fields, Houſes, &c. to⯑gether with the large Proſpect of the wide extend⯑ed Land or Sea, that lies between the Eye and the utmoſt Limb of the Horizon, ſuggeſt unto the Mind the Idea of greater Diſtance, and conſequent⯑ly magnify the Appearance. And this, according to Dr. Wallis, is the true Account of the ex⯑traordinary Largeneſs attributed by the Mind to the Horizontal Moon, at a time when the Angle ſubtended by its Diameter, is not one jot greater than it uſed to be.
LXXVII. With reference to this Opinion, not to repeat what hath been already ſaid concerning Diſtance, I ſhall only obſerve, Firſt, That if the Proſpect of interjacent Objects be that which ſug⯑geſts the Idea of farther Diſtance, and this Idea of farther Diſtance be the Cauſe that brings into the Mind the Idea of greater Magnitude, it ſhou'd hence follow, that if one looked at the Horizontal Moon from behind a Wall, it would appear no bigger than ordinary. For in that Caſe, the Wall interpoſing cuts off all that Proſpect of Sea and Land, &c. which might otherwiſe increaſe the ap⯑parent Diſtance, and thereby the apparent Magni⯑tude [198] of the Moon. Nor will it ſuffice to ſay, the Memory doth even then ſuggeſt all that Extent of Land, &c. which lies within the Horizon; which Suggeſtion occaſions a ſudden Judgment of Senſe, that the Moon is farther off and larger than uſual. For ask any Man, who from ſuch a Station behold⯑ing the Horizontal Moon, ſhall think her greater than uſual, whether he hath at that time in his Mind any Idea of the Intermediate Objects, or long Tract of Land that lies between his Eye and the extreme Edge of the Horizon? And whether it be that Idea which is the Cauſe of his making the aforementioned Judgment? He will, I ſuppoſe, reply in the Negative, and declare the Horizontal Moon ſhall appear greater than the Meridional, though he never thinks of all or any of thoſe things that lie between him and it. Secondly, It ſeems impoſſible by this Hypotheſis, to account for the Moon's appearing in the very ſame Situation, at one time greater than at another; which neverthe⯑leſs has been ſhewn to be very agreeable to the Principles we have laid down, and receives a moſt eaſy and natural Explication from them. For the further clearing up of this Point, it is to be ob⯑ſerved that what we immediately and properly ſee are only Lights and Colours in ſundry Situations and Shades, and Degrees of Faintneſs and Clear⯑neſs, Confuſion and Diſtinctneſs. All which viſi⯑ble Objects are only in the Mind; nor do they ſuggeſt ought external, whether Diſtance or Mag⯑nitude, otherwiſe than by habitual Connexion as Words do Things. We are alſo to remark, that, beſide the Straining of the Eyes, and beſide the vivid and faint, the diſtinct and confuſed Appear⯑ances (which bearing ſome Proportion to Lines and Angles, have been ſubſtituted inſtead of them, in the foregoing Part of this Treatiſe) there are other means which ſuggeſt both Diſtance and Mag⯑nitude; [199] particularly, the Situation of viſible Points, or Objects, as upper or lower; the former ſug⯑geſting a farther Diſtance and greater Magnitude, the latter a nearer Diſtance and leſſer Magnitude: All which is an Effect only of Cuſtom and Experi⯑ence; there being really nothing intermediate in the Line of Diſtance, between the Uppermoſt and Lowermoſt, which are both Aequidiſtant, or ra⯑ther at no Diſtance from the Eye, as there is alſo nothing in Upper or Lower, which by neceſſary Connexion ſhou'd ſuggeſt greater or leſſer Magni⯑tude. Now, as theſe cuſtomary, experimental means of ſuggeſting Diſtance, do likewiſe ſuggeſt Magnitude, ſo they ſuggeſt the one as immediately as the other. I ſay, they do not (Vide SECT. LIII.) firſt ſuggeſt Diſtance, and then leave the Mind from thence to infer or compute Magnitude, but ſuggeſt Magnitude as immediately and directly as they ſuggeſt Diſtance.
LXXVIII. This Phaenomenon of the Horizon⯑tal Moon is a clear Inſtance of the inſufficiency of Lines and Angles, for explaining the way wherein the Mind perceives, and eſtimates the Magnitude of outward Objects. There is nevertheleſs a uſe of Computation by them, in order to determine the apparent Magnitude of things, ſo far as they have a Connexion with, and are proportional to thoſe other Ideas, or Perceptions which are the true and immediate Occaſions that ſuggeſt to the Mind the apparent Magnitude of Things. But this in general may, I think, be obſerved concern⯑ing Mathematical Computation in Optics: That it can never be very preciſe and exact, ſince the Judg⯑ments we make of the Magnitude of External Things do often depend on ſeveral Circumſtances, which are not proportionable to, or capable of be⯑ing defined by Lines and Angles.
[200] LXXIX. From what has been ſaid, we may ſafely deduce this Conſequence, to wit, that a Man born blind, and made to ſee, wou'd, at firſt opening of his Eyes make a very different Judg⯑ment of the Magnitude of Objects intromitted by them, from what others do. He wou'd not con⯑ſider the Ideas of Sight, with reference to, or as having any Connexion with the Ideas of Touch: His View of them being intirely terminated within themſelves, he can no otherwiſe judge them Great or Small, than as they contain a greater or leſſer Number of viſible Points. Now, it being certain that any viſible Point can cover or exclude from View, only one other viſible Point, it follows, that whatever Object intercepts the View of another, hath an equal Number of viſible Points with it; and conſequently they ſhall both be thought by him to have the ſame Magnitude. Hence it is evi⯑dent, one in thoſe Circumſtances would judge his Thumb, with which he might hide a Tower, or hinder its being ſeen, equal to that Tower, or his Hand, the Interpoſition whereof might conceal the Firmament from his View, equal to the Firma⯑ment: How great an Inequality ſoever there may, in our Apprehenſions, ſeem to be betwixt thoſe two things, becauſe of the cuſtomary and cloſe Connexion that has grown up in our Minds be⯑tween the Objects of Sight and Touch, whereby the very different and diſtinct Ideas of thoſe two Senſes, are ſo blended and confounded together, as to be miſtaken for one and the ſame thing; out of which Prejudice we cannot eaſily extricate our ſelves.
LXXX. For the better explaining the Nature of Viſion, and ſetting the manner wherein we per⯑ceive Magnitudes in a due Light, I ſhall proceed [201] to make ſome Obſervations concerning Matters re⯑lating thereto, whereof the want of Reflexion, and duly ſeparating between tangible and viſible Ideas, is apt to create in us miſtaken and confuſed Notions. And Firſt, I ſhall obſerve that the Minimum Viſibile is exactly equal in all Beings what⯑ſoever, that are endowed with the viſive Faculty. No exquiſite Formation of the Eye, no peculiar Sharpneſs of Sight can make it leſs in one Creature than in another; for it not being diſtinguiſhable into Parts, nor in any wiſe conſiſting of them, it muſt neceſſarily be the ſame to all. For ſuppoſe it otherwiſe, and that the Minimum Viſibile of a Mite, for Inſtance, be leſs than the Minimum Viſi⯑bile of a Man; the latter therefore may by De⯑traction of ſome part be made equal to the former: It doth therefore conſiſt of Parts, which is incon⯑ſiſtent with the Notion of a Minimum Viſibile, or Point.
LXXXI. It will perhaps be objected that the Minimum Viſibile of a Man doth really, and in it ſelf contain Parts whereby it ſurpaſſes that of a Mite, though they are not perceivable by the Man. To which I anſwer, the Minimum Viſibile having (in like manner as all other the proper and imme⯑diate Objects of Sight) been ſhewn not to have any Exiſtence without the Mind of him who ſees it, it follows there cannot be any part of it that is not actually perceived, and therefore viſible. Now for any Object to contain ſeveral diſtinct viſi⯑ble Parts, and at the ſame time to be a Minimum Viſibile, is a manifeſt Contradiction.
LXXXII. Of theſe viſible Points we ſee at all times an equal Number. It is every whit as great when our View is contracted and bounded by near Objects, as when it is extended to larger and re⯑moter. [202] For it being impoſſible that one Minimum Viſibile ſhould obſcure, or keep out of Sight more than one other, it is a plain Conſequence, that when my View is on all ſides bounded by the Walls of my Study, I ſee juſt as many viſible points as I cou'd, in caſe that by the removal of the Study-walls, and all other Obſtructions, I had a full Proſ⯑pect of the circumjacent Fields, Mountains, Sea, and open Firmament; for ſo long as I am ſhut up within the Walls, by their Interpoſition, every Point of the external Objects is covered from my View: But each Point that is ſeen being able to cover or exclude from Sight, one only other cor⯑reſponding Point, it follows, that whilſt my Sight is confined to thoſe narrow Walls, I ſee as many Points, or Minima Viſibilia, as I ſhould were thoſe Walls away, by looking on all the external Ob⯑jects, whoſe Proſpect is intercepted by them. Whenever therefore we are ſaid to have a greater Proſpect at one time than another, this muſt be un⯑derſtood with relation not to the proper and im⯑mediate, but the ſecondary and mediate Objects of Viſion, which, as hath been ſhewn, properly be⯑long to the Touch.
LXXXIII. The viſive Faculty conſidered, with reference to its immediate Objects, may be found to labour of two Defects, Firſt, In reſpect of the Extent or Number of viſible Points that are at once perceivable by it, which is narrow and limit⯑ed to a certain Degree. It can take in at one View but a certain determinate Number of Mini⯑ma Viſibilia, beyond which it cannot extend its Proſpect. Secondly, Our Sight is defective in that its View is not only narrow, but alſo for the moſt part confuſed; of thoſe things that we take in at one Proſpect, we can ſee but a few at once clearly and unconfuſedly; and the more we fix our Sight [203] on any one Object, by ſo much the Darker and more Indiſtinct ſhall the reſt appear.
LXXXIV. Correſponding to theſe two Defects of Sight, we may imagine as many Perfections, to wit, 1ſt. That of comprehending in one View a greater Number of viſible Points. 2dly. Of being able to view them all equally and at once, with the utmoſt Clearneſs and Diſtinction. That thoſe Perfections are not actually in ſome Intelligences of a different Order and Capacity from ours, it is impoſſible for us to know.
LXXXV. In neither of thoſe two Ways do Mi⯑croſcopes contribute to the improvement of Sight; for when we look through a Microſcope, we nei⯑ther ſee more viſible Points, nor are the collateral Points more diſtinct than when we look with the naked Eye, at Objects placed in a due Diſtance. A Microſcope brings us as it were into a new World: It preſents us with a new Scene of viſible Objects, quite different from what we behold with the naked Eye. But herein conſiſts the moſt re⯑markable Difference, to wit, that whereas the Ob⯑jects perceived by the Eye alone, have a certain Connexion with tangible Objects, whereby we are taught to foreſee what will enſue upon the Ap⯑proach or Application of diſtant Objects to the Parts of our own Body, which much conduceth to its Preſervation; there is not the like Connex⯑ion between things tangible and thoſe viſible Ob⯑jects, that are perceived by help of a fine Micro⯑ſcope.
LXXXVI. Hence it is evident, that were our Eyes turned into the Nature of Microſcopes, we ſhou'd not be much benefited by the Change; we ſhou'd be deprived of the forementioned Advan⯑tage [204] we at preſent receive by the viſive Faculty; and have left us only the empty Amuſement of Seeing, without any other benefit ariſing from it. But in that Caſe, it will perhaps be ſaid, our Sight wou'd be endued with a far greater Sharp⯑neſs and Penetration than it now hath. But I wou'd fain know wherein conſiſts that Sharpneſs, which is eſteemed ſo great an Excellency of Sight. It is certain from what we have already ſhewn, that the Minimum Viſibile is never greater or leſſer, but in all Caſes conſtantly the ſame: And in the Caſe of Microſcopical Eyes, I ſee only this Difference, to wit, that upon the ceaſing of a certain obſerva⯑ble Connexion betwixt the divers Perceptions of Sight and Touch, which before enabled us to re⯑gulate our Actions by the Eye, it wou'd now be rendered utterly unſerviceable to that Purpoſe.
LXXXVII. Upon the whole, it ſeems that if we conſider the Uſe and End of Sight, together with the preſent State and Circumſtances of our Being, we ſhall not find any great Cauſe to com⯑plain of any Defect or Imperfection in it, or eaſily conceive how it cou'd be mended. With ſuch ad⯑mirable Wiſdom is that Faculty contrived, both for the Pleaſure and Convenience of Life.
LXXXVIII. Having finiſhed what I intended to ſay, concerning the Diſtance and Magnitude of Objects, I come now to treat of the Manner, where⯑in the Mind perceives by Sight their Situation. Among the Diſcoveries of the laſt Age, it is re⯑puted none of the leaſt, that the Manner of Viſion hath been more clearly explained, than ever it had been before. There is, at this Day, no one Igno⯑rant, that the Pictures of external Objects are painted on the Retina, or Fund of the Eye. That we can ſee nothing which is not ſo painted: And [205] that, according as the Picture is more Diſtinct or Confuſed, ſo alſo is the Perception we have of the Object: But then in this Explication of Viſion, there occurs one mighty Difficulty. The Objects are painted in an inverted Order on the Bottom of the Eye: The upper part of any Object being painted on the lower part of the Eye, and the lower part of the Object on the upper part of the Eye: And ſo alſo as to Right and Left. Since therefore the Pictures are thus inverted, it is demanded how it comes to paſs, that we ſee the Objects erect and in their natural Poſture?
LXXXIX. In anſwer to this Difficulty, we are told, that the Mind perceiving an Impulſe of a Ray of Light, on the upper part of the Eye, con⯑ſiders this Ray as coming in a direct Line, from the lower part of the Object; and in like manner tracing the Ray that ſtrikes on the lower part of the Eye, it is directed to the upper part of the Object. Thus in the adjacent Figure C the lower Point of the Object A B C is projected on c the upper part of the Eye. So likewiſe, the higheſt
Point A is projected on a the loweſt part of the Eye, which makes the Repreſentation c b a in⯑verted: But the Mind conſidering the Stroke that is made on c as coming in the ſtraight Line C c from the lower end of the Object; and the Stroke or Impulſe on a, as coming in the Line [206] A a from the upper End of the Object, is directed to make a right Judgment of the Situation of the Object A B C, notwithſtanding the Picture of it is inverted. This is illuſtrated by conceiving a blind Man, who holding in his Hands two Sticks that croſs each other, doth with them touch the extre⯑mities of an Object, placed in a perpendicular Si⯑tuation. It is certain, this Man will judge that to be the upper part of the Object, which he touches with the Stick held in the undermoſt Hand, and that to be the lower part of the Object, which he touches with the Stick in his uppermoſt Hand. This is the common Explication of the erect Ap⯑pearance of Objects, which is generally received and acquieſced in, being (as Mr. Molyneux tells us *) allowed by all Men as Satisfactory.
XC. But this account to me does not ſeem in any degree True. Did I perceive thoſe Impulſes, Decuſſations, and Directions of the Rays of Light, in like manner as hath been ſet forth, then, indeed, it wou'd not at firſt view be altogether void of Probability. And there might be ſome Pretence for the Compariſon of the Blind-Man and his croſs Sticks. But the Caſe is far otherwiſe. I know very well that I perceive no ſuch thing. And of Conſequence, I cannot thereby make an Eſtimate of the Situation of Objects. I appeal to any one's Experience, whether he be conſcious to himſelf, that he thinks on the Interſection made by the Ra⯑dious Pencils, or purſues the Impulſes they give in right Lines, whenever he perceives by Sight the Poſition of any Object? To me it ſeems evi⯑dent, that Croſſing and Tracing of the Rays, is never thought on by Children, Idiots, or in truth by any other, ſave only thoſe who have applyed [207] themſelves to the Study of Optics. And for the Mind to judge of the Situation of Objects by thoſe things, without perceiving them, or to perceive them without knowing it, is equally beyond my Comprehenſion. Add to this, that the explaining the manner of Viſion by the Example of croſs Sticks, and hunting for the Object along the Axes of the Radious Pencils, doth ſuppoſe the proper Objects of Sight to be perceived at a Diſtance from us, contrary to what hath been demonſtrated.
XCI. It remains, therefore, that we look for ſome other Explication of this Difficulty: And I believe it not impoſſible to find one, provided we examine it to the Bottom, and carefully diſtin⯑guiſh between the Ideas of Sight and Touch; which cannot be too oft inculcated in treating of Viſion: But more eſpecially throughout the conſi⯑deration of this Affair, we ought to carry that Diſtinction in our Thoughts: For that from want of a right Underſtanding thereof, the Difficulty of explaining Erect Viſion ſeems chiefly to ariſe.
XCII. In order to diſentangle our Minds, from whatever Prejudices we may entertain with relation to the Subject in hand, nothing ſeems more appo⯑ſite, than the taking into our Thoughts the Caſe of one born Blind, and afterwards, when grown up, made to ſee. And though perhaps, it may not be an eaſy Task to diveſt our ſelves intirely of the Experience received from Sight, ſo as to be able to put our Thoughts exactly in the Poſture of ſuch a one's; we muſt, nevertheleſs as far as poſſible, endeavour to frame true Conceptions, of what might reaſonably be ſuppoſed to paſs in his Mind.
[208] XCIII. It is certain, that a Man actually Blind, and who had continued ſo from his Birth, wou'd by the ſenſe of Feeling attain to have Ideas of Upper and Lower. By the Motion of his Hand he might diſcern the Situation of any Tangible Object placed within his Reach. That part on which he felt himſelf ſupported, or towards which he perceived his Body to gravitate, he wou'd term Lower, and the contrary to this Upper; and ac⯑cordingly denominate whatſoever Objects he touch⯑ed.
XCIV. But then, whatever Judgments he makes concerning the Situation of Objects, are confined to thoſe only that are perceivable by Touch. All thoſe things that are Intangible, and of a ſpiritual Nature, his Thoughts and Deſires, his Paſſions, and in general all the Modifications of his Soul, to theſe he wou'd never apply the Terms Upper and Lower, except only in a Metaphorical Senſe. He may, perhaps, by way of Alluſion, ſpeak of high or low Thoughts: But thoſe Terms in their pro⯑per Signification, wou'd never be applyed to any thing, that was not conceived to exiſt without the Mind. For a Man born Blind, and remaining in the ſame State, could mean nothing elſe by the Words Higher and Lower, than a greater or leſſer Diſtance from the Earth: Which Diſtance he wou'd meaſure by the Motion or Application of his Hand, or ſome other part of his Body. It is, therefore, evident, that all thoſe things which, in reſpect of each other, wou'd by him be thought Higher or Lower, muſt be ſuch as were conceived to exiſt without his Mind, in the ambient Space.
XCV. Whence it plainly follows, that ſuch a one, if we ſuppoſe him made to ſee, wou'd not at [209] firſt Sight think, that any thing he ſaw was High or Low, Erect or Inverted; for it hath been al⯑ready demonſtrated in SECT. XLI. that he wou'd not think the Things he perceived by Sight to be at any Diſtance from him, or without his Mind. The Objects to which he had hitherto been uſed to apply the Terms Up and Down, High and Low, were ſuch only as affected, or were ſome way per⯑ceived by his Touch: But the proper Objects of Viſion make a new Set of Ideas, perfectly diſtinct and different from the former, and which can in no ſort make themſelves perceived by Touch. There is, therefore, nothing at all that cou'd induce him to think thoſe Terms applicable to them: Nor wou'd he ever think it, till ſuch time as he had obſerved their Connexion with Tangible Objects, and the ſame Prejudices began to inſinuate it ſelf into his Underſtanding, which from their Infancy had grown up in the Underſtandings of other Men.
XCVI. To ſet this Matter in a clearer Light, I ſhall make uſe of an Example. Suppoſe the above-mentioned blind Perſon, by his Touch, perceives a Man to ſtand Erect. Let us inquire into the manner of this. By the application of his Hand to the ſeveral Parts of a Humane Body, he had perceived different Tangible Ideas, which being collected into ſundry complex ones have diſtinct Names annexed to them. Thus one Combination of a certain Tangible Figure, Bulk, and Conſiſt⯑ency of Parts is called the Head, another the Hand, a Third the Foot, and ſo of the reſt: All which Complex Ideas cou'd, in his Underſtanding, be made up only of Ideas perceivable by Touch. He had alſo by his Touch obtained an Idea of Earth or Ground, towards which he perceives the Parts of his Body to have a natural Tendency. Now, by Erect nothing more being meant, than [210] that perpendicular Poſition of a Man, wherein his Feet are neareſt to the Earth: If the blind Perſon by moving his Hand, over the Parts of the Man who ſtands before him, perceives the tangible Ideas that compoſe the Head, to be fartheſt from, and thoſe that compoſe the Feet to be neareſt to, that other Combination of Tangible Ideas which he calls Earth: He will denominate that Man Erect. But if we ſuppoſe him on a ſudden to receive his Sight, and that he behold a Man ſtanding before him, it is evident, in that Caſe, he wou'd neither judge the Man he ſees to be Erect nor Inverted; for he never having known thoſe Terms applied to any other, ſave Tangible Things, or which exiſted in the Space without him, and what he ſees neither being Tangible, nor perceived as exiſting without, he cou'd not know that in propriety of Language they were applicable to it.
XCVII. Afterwards, when upon turning his Head or Eyes up and down to the right and left, he ſhall obſerve the viſible Objects to change, and ſhall alſo attain to know, that they are called by the ſame Names, and connected with the Objects perceived by Touch; then, indeed, he will come to ſpeak of them and their Situation, in the ſame Terms that he has been uſed to apply to Tangible Things: And thoſe that he perceives by turning up his Eyes, he will call Upper, and thoſe that by turning down his Eyes, he will call Lower.
XCVIII. And this ſeems to me the true Rea⯑ſon why he ſhou'd think thoſe Objects uppermoſt that are painted on the Lower part of his Eye: For, by turning the Eye up they ſhall be diſtinct⯑ly ſeen; as likewiſe thoſe that are painted on the higheſt part of the Eye ſhall be diſtinctly ſeen, by turning the Eye down, and are for that Reaſon [211] eſteemed loweſt: For we have ſhewn that to the immediate Objects of Sight, conſidered in them⯑ſelves, he wou'd not attribute the Terms High and Low. It muſt therefore be on account of ſome Circumſtances, which are obſerved to attend them: And theſe, it is plain, are the Actions of turning the Eye up and down, which ſuggeſt a very obvi⯑ous Reaſon, why the Mind ſhou'd denominate the Objects of Sight accordingly High or Low. And without this Motion of the Eye, this turning it up and down in order to diſcern different Objects, doubtleſs Erect, Inverſe, and other the like Terms relating to the Poſition of Tangible Objects, wou'd never have been transferred, or in any degree ap⯑prehended to belong to the Ideas of Sight: The meer Act of Seeing including nothing in it to that Purpoſe; whereas the different Situations of the Eye naturally direct the Mind to make a ſuitable Judgment of the Situation of Objects intromitted by it.
XCIX. Farther, when he has by Experience learned the Connexion there is between the ſeveral Ideas of Sight and Touch, he will be able, by the Perception he has of the Situation of Viſible Things in reſpect of one another, to make a ſudden and true Eſtimate of the Situation of Outward, Tan⯑gible Things correſponding to them. And thus it is, he ſhall perceive by Sight the Situation of Ex⯑ternal Objects, which do not properly fall under that Senſe.
C. I know we are very prone to think, that if juſt made to ſee, we ſhou'd judge of the Situation of Viſible Things as we do now: But, we are alſo as prone to think, that at firſt Sight, we ſhou'd in the ſame way apprehend the Diſtance and Magnitude of Objects, as we do now: Which hath been ſhewn [212] to be a falſe and groundleſs Perſuaſion. And for the like Reaſons, the ſame Cenſure may be paſt on the poſitive Aſſurance, that moſt Men, before they have thought ſufficiently of the Matter, might have of their being able to determine by the Eye at firſt view, whether Objects were Erect or Inverſe.
CI. It will, perhaps be objected to our Opini⯑on, that a Man, for Inſtance, being thought Erect when his Feet are next the Earth, and Inverted when his Head is next the Earth, it doth hence follow, that by the meer act of Viſion, without a⯑ny Experience or altering the Situation of the Eye, we ſhou'd have determined whether he were Erect or Inverted: For both the Earth it ſelf, and the Limbs of the Man who ſtands thereon, being e⯑qually perceived by Sight, one cannot chooſe ſee⯑ing, what part of the Man is neareſt the Earth, and what part fartheſt from it, i. e. whether he be Erect or Inverted.
CII. To which I anſwer, the Ideas which con⯑ſtitute the Tangible Earth and Man, are intirely different from thoſe which conſtitute the Viſible Earth and Man. Nor was it poſſible, by virtue of the Viſive Faculty alone, without ſuperadding any Experience of Touch, or altering the Poſition of the Eye, ever to have known, or ſo much as ſuſpected, there had been any Relation or Connex⯑ion between them: Hence, a Man at firſt view wou'd not denominate any thing he ſaw Earth, or Head, or Foot; and conſequently, he cou'd not tell by the meer act of Viſion, whether the Head or Feet were neareſt the Earth: Nor, in⯑deed, wou'd we have thereby any thought of Earth or Man, Erect or Inverſe, at all: Which will be made yet more evident, if we nicely obſerve, and [213] make a particular Compariſon between the Ideas of both Senſes.
CIII. That which I ſee is only variety of Light and Colours. That which I feel is Hard or Soft, Hot or Cold, Rough or Smooth. What Simili⯑tude, what Connexion have thoſe Ideas with theſe? Or how is it poſſible, that any one ſhou'd ſee Rea⯑ſon, to give one and the ſame Name to Combinati⯑ons of Ideas ſo very different, before he had expe⯑rienced their Coexiſtence? We do not find there is any neceſſary Connexion betwixt this or that Tan⯑gible Quality, and any Colour whatſoever. And we may ſometimes perceive Colours, where there is nothing to be felt. All which doth make it ma⯑nifeſt, that no Man at firſt receiving of his Sight, wou'd know there was any Agreement between this or that particular Object of his Sight, and any Object of Touch he had been already acquainted with: The Colours therefore of the Head, wou'd to him no more ſuggeſt the Idea of Head, than they wou'd the Idea of Foot.
CIV. Farther, we have at large ſhewn (vid. SECT. LXIII and LXIV.) there is no dicoverable, neceſ⯑ſary Connexion, between any given Viſible Magni⯑tude, and any one particular Tangible Magnitude; but that it is intirely the reſult of Cuſtom and Ex⯑perience, and depends on foreign and accidental Circumſtances, that we can by the Perception of Viſible Extenſion inform our ſelves, what may be the Extenſion of any Tangible Object connected with it. Hence it is certain that neither the Viſi⯑ble Magnitude of Head or Foot, wou'd bring a⯑long with them into the Mind, at firſt opening of the Eyes, the reſpective Tangible Magnitudes of thoſe Parts.
[214] CV. By the foregoing Section, it is plain the Viſible Figure of any Part of the Body hath no ne⯑ceſſary Connexion with the Tangible Figure there⯑of, ſo as at Firſt Sight to ſuggeſt it to the Mind: For Figure is the Termination of Magnitude, whence it follows, that no Viſible Magnitude, hav⯑ing in its own Nature an aptneſs to ſuggeſt any one particular Tangible Magnitude, ſo neither can any Viſible Figure be inſeparably connected with its correſponding Tangible Figure: So as of it ſelf and in a way prior to Experience, it might ſug⯑geſt it to the Underſtanding. This will be farther evident, if we conſider that what ſeems ſmooth and round to the Touch, may to Sight, if viewed through a Microſcope, ſeem quite otherwiſe.
CVI. From all which laid together and duly conſidered, we may clearly deduce this Inference. In the firſt act of Viſion, no Idea entering by the Eye, wou'd have a perceivable Connexion with the Ideas to which the Names Earth, Man, Head, Foot, &c. were annexed in the Underſtanding of a Perſon Blind from his Birth; ſo as in any ſort to introduce them into his Mind, or make themſelves be called by the ſame Names, and reputed the ſame Things with them, as afterwards they come to be.
CVII. There doth, nevertheleſs, remain one Difficulty, which perhaps may ſeem to preſs hard on our Opinion, and deſerve not to be paſſed over: For though it be granted that neither the Colour, Size, nor Figure of the viſible Feet have any ne⯑ceſſary Connexion with the Ideas that compoſe the Tangible Feet, ſo as to bring them at firſt ſight in⯑to my Mind, or make me in danger of confounding them before I had been uſed to, and for ſome time experienced their Connexion: Yet thus much ſeems [215] undeniable, namely, that the Number of the viſi⯑ble Feet, being the ſame with that of the Tangible Feet, I may from hence without any Experience of Sight, reaſonably conclude, that they repreſent or are connected with the Feet rather than the Head. I ſay, it ſeems the Idea of two viſible Feet will ſooner ſuggeſt to the Mind, the Idea of two tangible Feet than of one Head; ſo that the blind Man upon firſt Reception of the viſive Faculty might know, which were the Feet or Two, and which the Head or One.
CVIII. In order to get clear of this ſeeming Difficulty, we need only obſerve, that Diverſity of viſible Objects doth not neceſſarily infer diverſi⯑ty of tangible Objects correſponding to them. A Picture painted with great variety of Colours af⯑fects the Touch in one uniform manner; it is there⯑fore evident, that I do not by any neceſſary Con⯑ſecution, independent of Experience, judge of the number of things Tangible, from the Number of things Viſible. I ſhou'd not therefore at firſt open⯑ing my Eyes conclude, that becauſe I ſee two I ſhall feel two. How, therefore can I, before Ex⯑perience teaches me, know that the viſible Legs, becauſe two, are connected with the tangible Legs, or the viſible Head, becauſe one is connected with the tangible Head? The truth is, the things I ſee are ſo very different and heterogeneous from the things I feel, that the Perception of the one wou'd never have ſuggeſted the other to my thoughts, or enabled me to paſs the leaſt Judgment thereon, un⯑til I had experienced their Connexion.
CIX. But for a fuller Illuſtration of this Matter, it ought to be conſidered that Number (however ſome may reckon it amongſt the Primary Qualities) is nothing fixed and ſettled, really exiſting in things [216] themſelves. It is intirely the Creature of the Mind, conſidering, either an Idea by it ſelf, or any Com⯑bination of Ideas to which it gives one Name, and ſo makes it paſs for an Unite. According as the Mind variouſly combines its Ideas, the Unite varies; and as the Unite, ſo the Number, which is only a Collection of Unites, doth alſo vary. We call a Window one, a Chimney one, and yet a Houſe in which there are many Windows, and many Chim⯑neys, hath an equal right to be called one, and ma⯑ny Houſes go to the making of one City. In theſe and the like Inſtances, it is evident the Unite con⯑ſtantly relates to the particular Draughts the Mind makes of its Ideas, to which it affixes Names, and wherein it includes more or leſs, as beſt ſuits its own Ends and Purpoſes. Whatever therefore the Mind conſiders as one, that is an Unite. Every Combination of Ideas is conſidered as one thing by the Mind, and in token thereof is marked by one Name. Now, this Naming and Combining to⯑gether of Ideas is perfectly arbitrary, and done by the Mind in ſuch ſort, as Experience ſhews it to be moſt convenient: Without which, our Ideas had never been collected into ſuch ſundry diſtinct Combinations as they now are.
CX. Hence it follows, that a Man born Blind, and afterwards, when grown up, made to ſee, wou'd not in the firſt act of Viſion, parcel out the Ideas of Sight, into the ſame diſtinct Collections that o⯑thers do, who have experienced which do regular⯑ly coexiſt and are proper to be bundled up toge⯑ther under one Name. He wou'd not, for Exam⯑ple, make into one complex Idea, and thereby eſteem, and unite all thoſe particular Ideas, which conſtitute the viſible Head or Foot. For there can be no Reaſon aſſigned why he ſhou'd do ſo, barely upon his ſeeing a Man ſtand upright before him: [217] There croud into his Mind the Ideas which com⯑poſe the viſible Man, in company with all the o⯑ther Ideas of Sight perceiv'd at the ſame time: But all theſe Ideas offer'd at once to his View, he wou'd not diſtribute into ſundry diſtinct Combinations, till ſuch as by obſerving the Motion of the Parts of the Man and other Experiences, he comes to know, which are to be ſeparated, and which to be collected together.
CXI. From what hath been premiſed, it is plain the Objects of Sight and Touch make, if I may ſo ſay, two Sets of Ideas, which are widely different from each other. To Objects of either kind, we indifferently attribute the Terms high and low, right and left, and ſuch like, denoting the Poſition or Situation of things: But then we muſt well ob⯑ſerve that the Poſition of any Object is determined with reſpect only to Objects of the ſame Senſe. We ſay any Object of Touch is high or low; ac⯑cording as it is more or leſs diſtant from the tangi⯑ble Earth: And in like manner we denominate any Object of Sight high or low, in Proportion as it is more or leſs diſtant from the viſible Earth: But to define the Situation of viſible Things, with re⯑lation to the Diſtance they bear from any tangible Thing, or vice verſa, this were abſurd and perfect⯑ly untintelligible. For all viſible things are equal⯑ly in the Mind, and take up no part of the exter⯑nal Space: And conſequently are equidiſtant from any tangible thing, which exiſts without the Mind.
CXII. Or rather to ſpeak truly, the proper Ob⯑jects of Sight are at no Diſtance, neither near nor far from any tangible Thing. For if we in⯑quire narrowly into the Matter we ſhall find that thoſe things only are compared together in reſpect of Diſtance, which exiſt after the ſame manner, or [218] appertain unto the ſame Senſe. For by the Di⯑ſtance between any two Points, nothing more is meant than the Number of intermediate Points: If the given Points are viſible, the Diſtance between them is marked out by the Number of the interja⯑cent viſible Points: If they are tangible, the Di⯑ſtance between them is a Line conſiſting of tangi⯑ble Points; but if they are one Tangible, and the other Viſible, the Diſtance between them doth nei⯑ther conſiſt of Points perceivable by Sight nor by Touch, i. e. it is utterly inconceivable. This, per⯑haps, will not find an eaſy Admiſſion into all Mens Underſtanding: However, I ſhould gladly be in⯑formed whether it be not true, by any one who will be at the pains to reflect a little, and apply it home to his Thoughts.
CXIII. The not obſerving what has been deli⯑vered in the two laſt Sections, ſeems to have occa⯑ſioned no ſmall part of the Difficulty that occurs in the Buſineſs of Erect Appearances. The Head, which is painted neareſt the Earth, ſeems to be fartheſt from it; and on the other hand, the Feet, which are painted fartheſt from the Earth, are thought neareſt to it. Herein lies the Difficulty, which vaniſhes if we expreſs the thing more clear⯑ly and free from Ambiguity, thus: How comes it that, to the Eye, the viſible Head which is near⯑eſt the tangible Earth, ſeems fartheſt from the Earth, and the viſible Feet, which are fartheſt from the tangible Earth, ſeem neareſt the Earth? The Queſtion being thus propoſed, who ſees not the Difficulty is founded on a Suppoſition, that the Eye, or viſive Faculty, or rather the Soul by means thereof, ſhou'd judge of the Situation of vi⯑ſible Objects, with reference to their Diſtance from the tangible Earth? Whereas it is evident the tan⯑gible Earth is not perceived by Sight: And it hath [219] been ſhewn in the two laſt preceding Sections, that the Location of Viſible Objects is determined only by the Diſtance they bear from one another; and that it is Nonſenſe to talk of Diſtance, far or near, between a viſible and tangible Thing.
CXIV. If we confine our Thoughts to the proper Objects of Sight, the whole is plain and eaſy. The Head is painted fartheſt from, and the Feet near⯑eſt to the viſible Earth; and ſo they appear to be. What is there ſtrange or unaccountable in this? Let us ſuppoſe the Pictures in the Fund of the Eye, to be the immediate Objects of the Sight. The Conſequence is, that things ſhou'd appear in the ſame Poſture they are painted in; and is it not ſo? The Head which is ſeen, ſeems fartheſt from the Earth which is ſeen; and the Feet, which are ſeen, ſeem neareſt to the Earth which is ſeen; and juſt ſo they are painted.
CXV. But, ſay you, the Picture of the Man is inverted, and yet the Appearance is Erect: I ask, what mean you by the Picture of the Man, or, which is the ſame thing, the viſible Man's being inverted? You tell me it is inverted, becauſe the Heels are uppermoſt, and the Head undermoſt? Explain me this. You ſay, that by the Head's being under⯑moſt, you mean that it is neareſt to the Earth; and by the Heels being uppermoſt, that they are fartheſt from the Earth. I ask again, what Earth you mean? You cannot mean the Earth that is painted on the Eye, or the viſible Earth: For the Picture of the Head is fartheſt from the Picture of the Earth, and the Picture of the Feet neareſt to the Picture of the Earth; and accordingly the viſible Head is fartheſt from the viſible Earth, and the viſible Feet neareſt to it. It remains, therefore, that you mean the tangible Earth, and ſo determine the Situation of viſible things with reſpect to tangible Things; contrary to what [220] hath been demonſtrated in SECT. CXI. and CXII. The two diſtinct Provinces of Sight and Touch ſhou'd be conſidered apart, and as if their Objects had no Intercourſe, no manner of Relation to one another, in point of Diſtance or Poſition.
CXVI. Farther, what greatly contributes to make us miſtake in this Matter is, that when we think of the Pictures in the Fund of the Eye, we imagine our ſelves looking on the Fund of another's Eye, or another looking on the Fund of our own Eye, and beholding the Pictures painted thereon. Suppoſe two Eyes A and B: A from ſome diſtance looking on the Pictures in B ſees them inverted, and for that reaſon concludes they are inverted in B: But this is wrong. There are projected in little on the Bottom of A, the Images of the Pictures of, ſuppoſe Man, Earth, &c. which are painted on B. And beſides theſe, the Eye B it ſelf, and the Ob⯑jects which environ it, together with another Earth, are projected in a larger Size on A. Now, by the Eye A, theſe larger Images are deemed the true Objects, and the leſſer only Pictures in miniature. And it is with reſpect to thoſe greater Images, that it determines the Situation of the ſmaller Ima⯑ges: So that comparing the little Man with the great Earth, A judges him inverted, or that the Feet are fartheſt from, and the Head neareſt to the great Earth. Whereas, if A compare the lit⯑tle Man with the little Earth, then he will appear Erect, i. e. his Head ſhall ſeem fartheſt from, and his Feet neareſt to the little Earth. But we muſt conſider that B does not ſee two Earths as A does: It ſees only what is repreſented by the little Pic⯑tures in A, and conſequently ſhall judge the Man Erect: For, in truth, the Man in B is not invert⯑ed, for there the Feet are next the Earth; but it is the Repreſentation of it in A which is inverted, [221] for there the Head of the Repreſentation of the Picture of the Man in B is next the Earth, and the Feet fartheſt from the Earth, meaning the Earth which is without the Repreſentation of the Pictures in B. For if you take the little Images of the Pic⯑tures in B, and conſider them by themſelves, and with reſpect only to one another, they are all Erect and in their natural Poſture.
CXVII. Farther, there lies a Miſtake in our i⯑magining that the Pictures of external Objects are painted on the Bottom of the Eye. It hath been ſhewn, there is no reſemblance between the Ideas of Sight, and things Tangible. It hath likewiſe been demonſtrated, that the proper Objects of Sight do not exiſt without the Mind. Whence it clearly follows, that the Pictures painted on the Bottom of the Eye, are not the Pictures of external Objects. Let any one conſult his own Thoughts, and then ſay what Affinity, what Likeneſs there is between that certain Variety and Diſpoſition of Colours, which conſtitute the viſible Man, or Picture of a Man, and that other Combination of far different Ideas, ſenſible by Touch, which compoſe the tangible Man. But if this be the Caſe, how come they to be accounted Pictures or Images, ſince that ſup⯑poſes them to copy or repreſent ſome Originals or other?
CXVIII. To which I anſwer: In the foremen⯑tioned Inſtance, the Eye A takes the little Images, included within the Repreſentation of the other Eye B, to be Pictures or Copies, whereof the Ar⯑chetypes are not things exiſting without, but the larger Pictures projected on its own Fund: and which by A are not thought Pictures, but the O⯑riginals, or true Things themſelves. Though if we ſuppoſe a third Eye C, from a due Diſtance to [222] behold the Fund of A, then indeed the Things projected thereon, ſhall, to C, ſeem Pictures or Images, in the ſame Senſe that thoſe projected on B do to A.
CXIX. Rightly to conceive this Point, we muſt carefully diſtinguiſh between the Ideas of Sight and Touch, between the viſible and tangible Eye; for certainly on the tangible Eye, nothing either is or ſeems to be painted. Again, the viſible Eye, as well as all other viſible Objects, hath been ſhewn to exiſt only in the Mind, which perceiving its own Ideas, and comparing them together, calls ſome Pictures in reſpect of others. What hath been ſaid, being rightly comprehended and laid together, doth, I think, afford a full and genuine Explication of the erect Appearance of Objects; which Phaenomenon, I muſt confeſs, I do not ſee how it can be explain⯑ed by any Theories of Viſion hitherto made pub⯑lick.
CXX. In treating of theſe things, the uſe of Language is apt to occaſion ſome Obſcurity and Confuſion, and create in us wrong Ideas: For Language being accommodated to the Common Notions and Prejudices of Men, it is ſcarce poſſi⯑ble to deliver the naked and preciſe Truth, with⯑out great Circumlocution, Impropriety, and (to an unwary Reader) ſeeming Contradictions; I do, therefore, once for all deſire whoever ſhall think it worth his while to underſtand what I have written concerning Viſion, that he would not ſtick in this or that Phraſe, or manner of Expreſſion, but can⯑didly collect my Meaning from the whole Sum and Tenor of my Diſcourſe, and laying aſide the Words as much as poſſible, conſider the bare Notions themſelves, and then judge whether they are agree⯑able to Truth and his own Experience, or no.
[223] CXXI. We have ſhewn the way wherein the Mind by mediation of viſible Ideas doth perceive or apprehend the Diſtance, Magnitude, and Situ⯑ation of tangible Objects. I come now to inquire more particularly concerning the Difference be⯑tween the Ideas of Sight and Touch, which are call'd by the ſame Names, and ſee whether there be any Idea common to both Senſes. From what we have at large ſet forth and demonſtrated in the fore⯑going parts of this Treatiſe, it is plain there is no one ſelf ſame numerical Extenſion, perceived both by Sight and Touch; but that the particular Fi⯑gures and Extenſions perceived by Sight, however they may be called by the ſame Names, and reput⯑ed the ſame Things, with thoſe perceived by Touch, are nevertheleſs different, and have an Ex⯑iſtence diſtinct and ſeparate from them: So that the Queſtion is not now concerning the ſame nume⯑rical Ideas, but whether there be any one and the ſame ſort or Species of Ideas equally perceiveable to both Senſes? Or, in other Words, whether Ex⯑tenſion, Figure, and Motion perceived by Sight, are not ſpecifically diſtinct from Extenſion, Figure and Motion perceived by Touch?
CXXII. But before I come more particularly to diſcuſs this Matter, I find it proper to conſider Extenſion in Abſtract: For of this there is much talk, and I apt to think, that when Men ſpeak of Exten⯑ſion, as being an Idea common to Two Senſes, it is with a ſecret Suppoſition, that we can ſingle out Extenſion from all other tangible and viſible Qua⯑lities, and form thereof an Abſtract Idea, which Idea they will have common both to Sight and Touch. We are therefore to underſtand by Ex⯑tenſion in Abſtract, an Idea of Extenſion; for in⯑ſtance, a Line or Surface, intirely ſtript of all o⯑ther [224] ſenſible Qualities and Circumſtances that might determine it to any particular Exiſtence; it is nei⯑ther black nor white, nor red, nor hath it any Co⯑lour at all, or any tangible Quality whatſoever, and conſequently it is of no finite determinate Magni⯑tude: For that which bounds or diſtinguiſhes one Extenſion from another, is ſome Quality or Cir⯑cumſtance wherein they diſagree.
CXXIII. Now I do not find that I can perceive, imagine, or any wiſe frame in my Mind ſuch an ab⯑ſtract Idea, as is here ſpoken of. A Line or Sur⯑face, which is neither black, nor white, nor blue, nor yellow, &c. nor long, nor ſhort, nor rough, nor ſmooth, nor ſquare, nor round, &c. is per⯑fectly incomprehenſible. This I am ſure of as to my ſelf; how far the Faculties of other Men may reach, they beſt can tell.
CXXIV. It is commonly ſaid, that the Object of Geometry is abſtract Extenſion; but Geometry contemplates Figures: Now, Figure is the Ter⯑mination of Magnitude, but we have ſhewn that Extenſion in Abſtract hath no finite determinate Magnitude, whence it clearly follows that it can have no Figure, and conſequently is not the Object of Geometry. It is indeed a Tenet as well of the modern as of the ancient Philoſophers, that all ge⯑neral Truths are concerning univerſal abſtract Ideas; without which, we are told, there cou'd be no Science, no Demonſtration of any general Propoſi⯑tion in Geometry. But it were no hard matter, did I think it neceſſary to my preſent Purpoſe, to ſhew that Propoſitions and Demonſtrations in Geometry might be Univerſal, though they who make them, never think of abſtract general Ideas of Triangles or Circles.
[225] CXXV. After reiterated endeavours to appre⯑hend the general Idea of a Triangle, I have found it altogether incomprehenſible. And ſurely if any one were able to introduce that Idea into my Mind, it muſt be the Author of the Eſſay concerning Hu⯑mane Underſtanding; He, who has ſo far diſtinguiſh⯑ed himſelf from the generality of Writers, by the Clearneſs and Significancy of what he ſays. Let us therefore ſee how this celebrated Author deſcribes the general, or abſtract Idea of a Triangle. ‘It muſt be (ſays he) neither Oblique, nor Rectan⯑gular, neither Equilateral, Equicrural, nor Sca⯑lenum; but all and none of theſe at once. In effect it is ſomewhat imperfect that cannot exiſt; an Idea, wherein ſome Parts of ſeveral different and inconſiſtent Ideas are put together.’ Eſſay on Hum. Underſtanding. B. iv. C. 7. S. 9. This is the Idea, which he thinks needful, for the Enlarge⯑ment of knowledge, which is the Subject of Ma⯑thematical Demonſtration, and without which we could never come to know any general Propoſition concerning Triangles. That Author acknow⯑ledges it doth ‘require ſome Pains and Skill to form this general Idea of a Triangle.’ Ibid. But had he called to mind what he ſays in another place, to wit, ‘That Ideas of mixed Modes wherein any inconſiſtent Ideas are put together, cannot ſo much as exiſt in the Mind, i. e. be conceived.’ Vid. B. iii. C. 10. S. 33. Ibid. I ſay, had this oc⯑curred to his Thoughts, it is not improbable he would have owed it above all the Pains and Skill he was maſter of, to form the above-mentioned I⯑dea of a Triangle, which is made up of manifeſt, ſtaring Contradictions. That a Man who thought ſo much, and laid ſo great a ſtreſs on clear and de⯑terminate Ideas, ſhou'd nevertheleſs talk at this rate, ſeems very ſurpriſing. But the wonder will [226] leſſen if it be conſidered, that the Source whence this Opinion flows, is the prolific Womb which has brought forth innumerable Errors and Difficul⯑ties, in all parts of Philoſophy, and in all the Sciences: But this Matter, taken in its full Ex⯑tent, were a Subject too vaſt and comprehenſive to be inſiſted on in this place. And ſo much for Extenſion in Abſtract.
CXXVI. ſome, perhaps, may think pure Space, Vacuum, or Trine Dimenſion to be equally the Object of Sight and Touch: But though we have a very great Propenſion, to think the Ideas of Outneſs and Space to be the immediate Ob⯑ject of Sight; yet if I miſtake not, in the forego⯑ing Parts of this Eſſay, That hath been clearly demonſtrated to be a meer Deluſion, ariſing from the quick and ſudden ſuggeſtion of Fancy, which ſo cloſely connects the Idea of Diſtance with thoſe of Sight, that we are apt to think it is it ſelf a proper and immediate Object of that Senſe, till Reaſon corrects the Miſtake.
CXXVII. It having been shewn, that there are no Abſtract Ideas of Figure, and that it is impoſſi⯑ble for us, by any Preciſion of Thought, to frame an Idea of Extenſion ſeparate from all other Viſi⯑ble and Tangible Qualities, which ſhall be com⯑mon both to Sight and Touch: The Queſtion now remaining is, whether the particular Extenſions, Figures and Motions perceived by Sight be of the ſame kind, with the particular Extenſions, Figures, and Motions perceived by Touch? In anſwer to which, I ſhall venture to lay down the following Propoſition: The Extenſion, Figures, and Motions, perceived by Sight are ſpecifically diſtinct from the Ide⯑as of Touch, called by the ſame Names, nor is there any ſuch thing as one Idea, or kind of Idea common to [227] both Senſes. This Propoſition may, without much Difficulty, be collected from what hath been ſaid in ſeveral Places of this Eſſay. But, becauſe it ſeems ſo remote from, and contrary to, the receiv⯑ed Notions and ſettled Opinion of Mankind, I ſhall attempt to demonſtrate it more particularly, and at large, by the following Arguments.
CXXVIII. When upon Perception of an Idea, I range it under this or that ſort; it is becauſe it is perceived after the ſame manner, or becauſe it has a Likeneſs or Conformity with, or affects me in the ſame way as the Ideas of the ſort I rank it under. In ſhort, it muſt not be intirely new, but have ſomething in it Old, and already perceived by me: It muſt, I ſay, have ſo much, at leaſt, in common with the Ideas I have before known and named, as to make me give it the ſame Name with them. But it has been, if I miſtake not, clearly made out, that a Man born Blind wou'd not at firſt reception of his Sight, think the Things he ſaw were of the ſame Nature with the Objects of Touch, or had any thing in common with them; but that they were a new Set of Ideas, perceived in a new manner, and intirely different from all he had ever perceived before: So that he wou'd not call them by the ſame Name, nor repute them to be of the ſame Sort, with any thing he had hitherto known.
CXXIX. Secondly, Light and Colours are al⯑lowed by all to conſtitute a ſort or Species intirely different from the Ideas of Touch: Nor will any Man, I preſume, ſay they can make themſelves perceived by that Senſe: But there is no other immediate Object of Sight, beſides Light and Co⯑lours. It is therefore a direct Conſequence, that there is no Idea common to both Senſes.
[228] CXXX. It is a prevailing Opinion, even a⯑mongſt thoſe who have thought and writ moſt ac⯑curately concerning our Ideas, and the Ways whereby they enter into the Underſtanding, that ſomething more is perceived by Sight, than barely Light and Colours with their Variations. Mr. Locke termeth Sight, ‘The moſt Comprehenſive of all our Senſes, conveying to our Minds the Ideas of Light and Colours, which are peculiar only to that Senſe; and alſo the far different Ideas of Space, Figure and Motion.’ Eſſay on Human Underſtand. B. ii. C. 9. S. 9. Space or Diſtance, we have ſhewn is no otherwiſe the Object of Sight than of Hearing. vid. SECT. XLVI. And as for Figure and Extenſion, I leave it to any one, that ſhall calmly attend to his own clear and diſtinct Ideas, to decide whether he has any Idea intro⯑mitted immediately and properly by Sight, ſave only Light and Colours: Or whether it be poſſible for him, to frame in his Mind a diſtinct Abſtract Idea of Viſible Extenſion, or Figure, excluſive of all Colour; and on the other hand, whether he can conceive Colour without Viſible Extenſion? For my own part, I muſt confeſs, I am not able to attain ſo great a nicety of Abſtraction; in a ſtrict Senſe, I ſee nothing but Light and Colours, with their ſeveral Shades and Variations. He who beſide theſe doth alſo perceive by Sight Ideas far different and diſtinct from them, hath that Faculty in a degree more perfect and comprehenſive than I can pretend to. It muſt be owned, that by the mediation of Light and Colours, other far different Ideas are ſuggeſted to my Mind: but ſo they are by Hearing, which beſide Sounds which are pecu⯑liar to that Senſe, doth by their mediation ſuggeſt not only Space, Figure and Motion, but alſo all other Ideas whatſoever that can be ſignified by Words.
[229] CXXXI. Thirdly, It is, I think, an Axiom univerſally received, that Quantities of the ſame kind may be added together, and make one intire Sum. Mathematicians add Lines together; but they do not add a Line to a Solid, or conceive it as making one Sum with a Surface: Theſe three kinds of Quantity being thought incapable of any ſuch mutual Addition, and conſequently of being compared together, in the ſeveral ways of Pro⯑portion, are by them eſteemed intirely Diſparate and Heterogeneous. Now let any one try in his Thoughts to add a Viſible Line or Surface to a Tangible Line or Surface, ſo as to conceive them making one continued Sum or Whole. He that can do this, may think them Homogeneous; but he that cannot muſt, by the foregoing Axiom, think them Heterogeneous: A Blue, and a Red Line I can conceive added together into one Sum, and making one continued Line; but to make, in my Thoughts, one continued Line of a Viſible and Tangible Line added together is, I find, a Task far more difficult, and even inſurmountable; and I leave it to the Reflexion and Experience of every particular Perſon to determine for himſelf.
CXXXII. A farther Confirmation of our Tenet may be drawn from the Solution of Mr. Molyneux's Problem, publiſhed by Mr. Locke in his Eſſay: Which I ſhall ſet down as it there lies, together with Mr. Locke's Opinion of it, ‘Suppoſe a Man born Blind, and now Adult, and taught by his Touch to diſtinguiſh between a Cube, and a Sphere of the ſame Metal, and nighly of the ſame Bigneſs, ſo as to tell, when he felt one and t'other, which is the Cube, and which the Sphere. Suppoſe then the Cube and Sphere placed on a Table, and the blind Man to be made to See: Quaere, Whether by his [230] Sight, before he touch'd them, he could now diſtin⯑guiſh, and tell, which is the Globe, which the Cube. To which the acute and judicious Propoſer an⯑ſwers: Not. For though he has obtained the Ex⯑perience of, how a Globe, how a Cube affects his Touch; yet he has not yet attained the Experience, that what affects his Touch ſo or ſo, muſt affect his Sight ſo or ſo: Or that a protuberant Angle in the Cube, that preſſed his Hand unequally, ſhall appear to his Eye, as it doth in the Cube. I agree with this thinking Gentleman, whom I am proud to call my Friend, in his Anſwer to this his Prob⯑lem; and am of opinion, that the blind Man, at firſt Sight would not be able with certainty to ſay, which was the Globe, which the Cube, whilſt he only ſaw them.’ Eſſay on Human Under⯑ſtanding. B. ii. C. 9. S. 8.
CXXXIII. Now, if a Square Surface perceived by Touch be of the ſame ſort with a Square Sur⯑face perceived by Sight; it is certain the blind Man here mentioned might know a Square Sur⯑face, as ſoon as he ſaw it: It is no more but in⯑troduced into his Mind, by a new Inlet, an Idea he has been already well acquainted with. Since therefore he is ſuppoſed to have known by his Touch, that a Cube is a Body terminated by Square Surfaces; and that a Sphere is not termi⯑nated by Square Surfaces: upon the ſuppoſition that a Viſible and Tangible Square differ only in numero, it follows, that he might know, by the unerring mark of the Square Surfaces, which was the Cube, and which not, while he only ſaw them. We muſt therefore allow, either that Viſible Ex⯑tenſion and Figures are ſpecifically diſtinct from Tangible Extenſion and Figures, or elſe, that the Solution of this Problem, given by thoſe two thoughtful and ingenious Men, is wrong.
[231] CXXXIV. Much more might be laid together in Proof of the Propoſition I have advanced: But what has been ſaid is, if I miſtake not, ſufficient to convince any one that ſhall yield a reaſonable At⯑tention: And, as for thoſe that will not be at the pains of a little Thought, no Multiplication of Words will ever ſuffice to make them underſtand the Truth, or rightly conceive my Meaning.
CXXXV. I cannot let go the above-mentioned Problem without ſome Reflexion on it. It hath been made evident, that a Man blind from his Birth wou'd not, at firſt Sight, denominate any thing he ſaw, by the Names he had been uſed to appropri⯑ate to Ideas of Touch, vid. SECT. CVI. Cube, Sphere, Table, are Words he has known applied to Things perceivable by Touch, but to Things perfectly Intangible he never knew them appli⯑ed. Thoſe Words in their wonted application, always marked out to his Mind Bodies, or ſo⯑lid Things which were perceived by the Reſiſtance they gave: But there is no Solidity, no Reſiſtance or Protruſion perceived by Sight. In ſhort, the Ideas of Sight are all new Perceptions, to which there be no Names annexed in his Mind; he can⯑not therefore underſtand what is ſaid to him con⯑cerning them: And to ask of the two Bodies he ſaw placed on the Table, which was the Sphere, which the Cube, were, to him, a Queſtion down⯑right bantering and unintelligible; nothing he ſees being able to ſuggeſt to his Thoughts, the Idea of Body, Diſtance, or in general, of any thing he had already known.
CXXXVI. It is a miſtake, to think the ſame thing affects both Sight and Touch. If the ſame Angle or Square, which is the Object of Touch, be alſo the Object of Viſion, what ſhou'd hinder [232] the blind Man, at firſt Sight, from knowing it? For though the Manner wherein it affects the Sight, be different from that wherein it affected his Touch; yet, there being, beſide this Manner or Circumſtance, which is new and unknown, the Angle or Figure, which is old and known, he can⯑not chooſe but diſcern it.
CXXXVII. Viſible Figure and Extenſion hav⯑ing been demonſtrated to be of a nature, intirely different and heterogeneous from tangible Figure and Extenſion, it remains that we inquire concern⯑ing Motion. Now that viſible Motion is not of the ſame ſort with tangible Motion, ſeems to need no farther Proof, it being an evident Corollary from what we have ſhewn concerning the Difference there is between viſible and tangible Extenſion: But for a more full and expreſs Proof hereof, we need only obſerve, that one who had not yet ex⯑perienced Viſion, wou'd not at firſt ſight know Motion. Whence it clearly follows, that Motion perceivable by Sight is of a ſort diſtinct from Mo⯑tion perceivable by Touch. The Antecedent I prove thus: By Touch he cou'd not perceive any Motion, but what was up or down, to the right or left, nearer or farther from him; beſides theſe, and their ſeveral Varieties or Complications, it is impoſſible he ſhou'd have any Idea of Motion. He wou'd not therefore think any thing to be Motion, or give the name Motion to any Idea, which he cou'd not range under ſome or other of thoſe par⯑ticular kinds thereof. But from SECT. XCV, it is plain that by the meer act of Viſion, he cou'd not know Motion upwards or downwards, to the right or left, or in any other poſſible Direction. From which I conclude, he wou'd not know Motion at all at firſt ſight. As for the Idea of Motion in Abſtract, I ſhall not waſte Paper about it, but [233] leave it to my Reader, to make the beſt he can on't. To me it is perfectly Unintelligible.
CXXXVIII. The Conſideration of Motion may furniſh a new Field for Inquiry: But ſince the Man⯑ner wherein the Mind apprehends by Sight, the Motion of Tangible Objects, with the various De⯑grees thereof, may be eaſily collected, from what hath been ſaid concerning the Manner, wherein that Senſe doth ſuggeſt their various Diſtances, Magnitudes and Situations, I ſhall not enlarge any farther on this Subject, but proceed to inquire what may be alledged with greateſt appearance of Reaſon, againſt the Propoſition we have ſhewn to be true: For where there is ſo much Prejudice to be encountered, a bare and naked Demonſtration of the Truth will ſcarce ſuffice. We muſt alſo ſatisfy the Scruples that Men may raiſe in favour of their preconceived Notions, ſhew whence the miſtake ariſes, how it came to ſpread, and carefully diſ⯑cloſe and root out thoſe falſe Perſuaſions, that an early Prejudice might have implanted in the Mind.
CXXXIX. Firſt, Therefore, it will be demand⯑ed, how viſible Extenſion and Figures come to be called by the ſame Name, with tangible Extenſion and Figures, if they are not of the ſame kind with them? It muſt be ſomething more than Humour or Accident, that cou'd occaſion a Cuſtom ſo con⯑ſtant and univerſal as this, which has obtained in all Ages and Nations of the World, and amongſt all Ranks of Men, the Learned as well as the Il⯑literate.
CXL. To which I anſwer, we can no more ar⯑gue a viſible and tangible Square to be of the ſame Species, from their being called by the ſame Name, [234] than we can, that a tangible Square and the Mo⯑noſyllable conſiſting of Six Letters, whereby it is marked, are of the ſame Species becauſe they are both called by the ſame Name. It is cuſtomary to call written Words, and the Things they ſignify, by the ſame Name: For Words not being regarded in their own Nature, or otherwiſe than as they are Marks of Things, it had been ſuperfluous, and beſide the deſign of Language, to have given them Names diſtinct from thoſe of the Things marked by them. The ſame Reaſon holds here alſo. Viſi⯑ble Figures are the Marks of tangible Figures, and from SECT. LIX. it is plain, that in themſelves they are little regarded, or upon any other Score than for their Connexion with tangible Figures, which by Nature they are ordained to ſignify. And becauſe this Language of Nature does not vary in different Ages or Nations, hence it is, that in all Times and Places, viſible Figures are called by the ſame Names, as the reſpective tangible Figures ſuggeſted by them, and not becauſe they are alike, or of the ſame ſort with them.
CXLI. But, ſay you, ſurely a tangible Square is liker to a viſible Square, than to a viſible Circle: It has four Angles, and as many Sides; ſo alſo has the viſible Square, but the viſible Circle has no ſuch thing, being bounded by one uniform Curve, without right Lines or Angles, which makes it un⯑fit to repreſent the tangible Square, but very fit to repreſent the tangible Circle. Whence it clearly follows, that viſible Figures are Patrons of, or of the ſame Species with the reſpective tangible Fi⯑gures repreſented by them; that they are like unto them, and of their own Nature fitted to repreſent them, as being of the ſame ſort; and that they are in no reſpect arbitrary Signs, as Words.
[235] CXLII. I anſwer, it muſt be acknowledged, the viſible Square is fitter than the viſible Circle, to repreſent the tangible Square, but then it is not be⯑cauſe it is liker, or more of a Species with it; but becauſe the viſible Square contains in it ſeveral diſtinct Parts, whereby to mark the ſeveral diſtinct, correſponding Parts of a tangible Square, whereas the viſible Circle doth not. The Square perceived by Touch, hath four diſtinct, equal Sides, ſo alſo hath it four diſtinct equal Angles. It is therefore neceſſary, that the viſible Figures which ſhall be moſt proper to mark it, contain four diſtinct equal Parts correſponding to the four Sides of the tangible Square; as likewiſe four other diſtinct and equal Parts, whereby to denote the four equal Angles of the tangible Square. And accordingly we ſee the viſible Figures contain in them diſtinct viſible Parts, anſwering to the diſtinct tangible Parts of the Fi⯑gures ſignified, or ſuggeſted by them.
CXLIII. But it will not hence follow, that any viſible Figure is like unto, or of the ſame Species with its correſponding tangible Figure, unleſs it be alſo ſhewen, that not only the Number, but alſo the Kind of the Parts be the ſame in both. To illuſtrate this, I obſerve that viſible Figures repre⯑ſent tangible Figures, much after the ſame manner that written Words do Sounds. Now, in this re⯑ſpect, Words are not arbitrary, it not being in⯑different, what written Word ſtands for any Sound: But it is requiſite, that each Word con⯑tain in it ſo many diſtinct Characters, as there are Variations in the Sound in ſtands for. Thus the ſingle Letter a is proper to mark one ſimple uni⯑form Sound; and the word Adultery is accommo⯑dated to repreſent the Sound annexed to it, in [236] the Formation whereof, there being eight different Colliſions, or Modifications of the Air by the Organs of Speech, each of which produces a diffe⯑rence of Sound, it was fit, the Word repreſenting it ſhou'd conſiſt of as many diſtinct Characters, thereby to mark each particular Difference or Part of the whole Sound: And yet no Body, I preſume will ſay, the ſingle Letter a, or the word Adultery are like unto, or of the ſame Species with the re⯑ſpective Sounds by them repreſented. It is indeed arbitrary that, in general, Letters of any Lan⯑guage repreſent Sounds at all; but when that is once agreed, it is not arbitrary what Combination of Letters ſhall repreſent this or that particular Sound. I leave this with the Reader to purſue, and apply it in his own Thoughts.
CXLIV. It muſt be confeſt that we are not ſo apt to confound other Signs with the Things ſigni⯑fied, or to think them of the ſame Species, as we are viſible and tangible Ideas. But a little Conſi⯑deration will ſhew us how this may be, without our ſuppoſing them of a like Nature. Theſe Signs are conſtant and univerſal, their Connexion with tangible Ideas has been learnt at our firſt Entrance into the World; and ever ſince, almoſt every Mo⯑ment of our Lives, it has been occurring to our Thoughts, and faſtening and ſtriking deeper on our Minds. When we obſerve that Signs are variable, and of Humane Inſtitution; when we remember, there was a time they were not connected in our Minds, with thoſe things they now ſo readily ſug⯑geſt; but that their Signification was learned by the ſlow Steps of Experience: This preſerves us from confounding them. But when we find the ſame Signs ſuggeſt the ſame Things all over the World; when we know they are not of Humane [237] Inſtitution, and cannot remember that we ever learned their Signification, but think that at firſt Sight they would have ſuggeſted to us the ſame Things they do now: All this perſuades us they are of the ſame Species as the Things reſpectively repreſented by them, and that it is by a natural Reſemblance they ſuggeſt them to our Minds.
CXLV. Add to this, that whenever we make a nice Survey of any Object, ſucceſſively directing the Optic Axis to each Point thereof; there are certain Lines and Figures deſcribed by the Motion of the Head or Eye, which being in truth perceiv⯑ed by Feeling, do nevertheleſs ſo mix themſelves as it were, with the Ideas of Sight, that we can ſcarce think but they appertain to that Senſe. A⯑gain, the Ideas of Sight enter into the Mind, ſeve⯑ral at once more diſtinct and unmingled, than is uſual in the other Senſes beſide the Touch. Sounds, for example, perceived at the ſame In⯑ſtant, are apt to coaleſce, if I may ſo ſay, into one Sound: But we can perceive at the ſame time great variety of viſible Objects, very ſeparate and diſtinct from each other. Now tangible Extenſi⯑on being made up of ſeveral Diſtinct coexiſtent parts, we may hence gather another Reaſon, that may diſpoſe us to imagine a Likeneſs or Analogy between the immediate Objects of Sight and Touch. But nothing, certainly, doth more contribute to blend and confound them together, than the ſtrict and cloſe Connexion they have with each other. We cannot open our Eyes, but the Ideas of Diſ⯑tance, Bodies, and tangible Figures are ſuggeſted by them. So ſwift and ſudden, and unperceiv'd is the Tranſition from viſible to tangible Ideas, that we can ſcarce forbear thinking them equally the im⯑mediate Object of Viſion.
[238] CXLVI. The Prejudice, which is grounded on theſe, and whatever other Cauſes may be aſſigned thereof, ſticks ſo faſt, that it is impoſſible with⯑out obſtinate Striving, and Labour of the Mind, to get intirely clear of it. But then the Reluctan⯑cy we find, in rejecting any Opinion, can be no Argument of its Truth, to whoever conſiders what has been already ſhewn, with regard to the Prejudices we entertain concerning the Diſtance, Magnitude, and Situation of Objects; Prejudices ſo familiar to our Minds, ſo confirmed and inve⯑terate, as they will hardly give way to the cleareſt Demonſtration.
CXLVII. Upon the whole, I think we may fairly conclude, that the proper Objects of Viſion conſtitute an Univerſal Language of the Author of Nature, whereby we are inſtructed how to regu⯑late our Actions, in order to attain thoſe things, that are neceſſary to the Preſervation and Well-being of our Bodies, as alſo to avoid whatever may be hurtful and deſtructive of them. It is by their Information that we are principally guided in all the Tranſactions and Concerns of Life. And the manner wherein they ſignify, and mark unto us the Objects which are at a Diſtance, is the ſame with that of Languages and Signs of Humane Appoint⯑ment, which do not ſuggeſt the things ſignified, by any likeneſs or Indentity of Nature, but only by an habitual Connexion, that Experience has made us to obſerve between them.
CXLVIII. Suppoſe one who had always conti⯑nued Blind, be told by his Guide, that after he has advanced ſo many Steps, he ſhall come to the Brink of a Precipice, or be ſtopt by a Wall; muſt not [239] this to him ſeem very admirable and ſurprizing? He cannot conceive how it is poſſible for Mortals to frame ſuch Predictions as theſe, which to him would ſeem as ſtrange and unaccountable, as Pro⯑pheſy doth to others. Even they who are bleſſed with the viſive Faculty, may (though familiarity make it leſs obſerved) find therein ſufficient Cauſe of Admiration. The wonderful Art and Contri⯑vance wherewith it is adjuſted to thoſe Ends and Purpoſes for which it was apparently deſigned, the vaſt Extent, Number, and Variety of Objects that are at once with ſo much eaſe, and quickneſs, and pleaſure ſuggeſted by it: All theſe afford Subject for much and pleaſing Speculation, and may, if a⯑ny thing, give us ſome glimmering, analogous Prae⯑notion of Things, which are placed beyond the certain Diſcovery and Comprehenſion of our pre⯑ſent State.
CXLIX. I do not deſign to trouble my ſelf with drawing Corollaries, from the Doctrine I have hi⯑therto laid down. If it bears the Teſt, others may, ſo far as they ſhall think convenient, employ their Thoughts in extending it farther, and applying it to whatever Purpoſes it may be ſubſervient to: Only, I cannot forbear making ſome Inquiry con⯑cerning the Object of Geometry, which the Subject we have been upon doth naturally lead one to. We have ſhewn there is no ſuch Idea as that of Exten⯑ſion in Abſtract, and that there are two kinds of ſenſible Extenſion and Figures, which are intirely diſtinct and heterogeneous from each other. Now, it is natural to inquire which of theſe is the Object of Geometry.
CL. Some things there are, which at firſt ſight incline one to think Geometry converſant about [240] Viſible Extenſion. The conſtant uſe of the Eyes, both in the practical and ſpeculative Parts of that Science doth very much induce us thereto. It would, without doubt, ſeem odd to a Mathemati⯑cian to go about to convince him, the Diagrams he ſaw upon Paper were not the Figures, or even the Likeneſs of the Figures, which make the Subject of the Demonſtration. The contrary being held an unqueſtionable Truth, not only by Mathematicians, but alſo by thoſe who apply themſelves more par⯑ticularly to the Study of Logick; I mean, who conſider the Nature of Science, Certainty and De⯑monſtration: It being by them aſſigned as one Rea⯑ſon, of the extraordinary Clearneſs and Evidence of Geometry, that in this Science the Reaſonings are free from thoſe Inconveniencies, which attend the uſe of arbitrary Signs, the very Ideas themſelves being copied out, and expoſed to view upon Paper. But, by the bye, how well this agrees with what they likewiſe aſſert of abſtract Ideas, being the Object of Geometrical Demonſtration, I leave to be conſidered.
CLI. To come to a Reſolution in this Point, we need only obſerve what hath been ſaid in SECT. LIX. LX. LXI. where it is ſhewn that viſible Ex⯑tenſions in themſelves are little regarded, and have no ſettled determinate Greatneſs, and that Men meaſure altogether, by the Application of Tangi⯑ble Extenſion to Tangible Extenſion. All which makes it evident, that Viſible Extenſion and Fi⯑gures are not the Object of Geometry.
CLII. It is therefore plain that Viſible Figures are of the ſame Uſe in Geometry, that Words are: And the one may as well be accounted the Object of that Science, as the other; neither of them be⯑ing [241] any otherwiſe concerned therein, than as they repreſent or ſuggeſt to the Mind the particular Tangible Figures connected with them. There is indeed this Difference between the Signification of Tangible Figures by Viſible Figures, and of Ideas by Words: That whereas the Latter is variable and uncertain, depending altogether on the Arbi⯑trary Appointment of Men, the former is fixed, and immutably the ſame in all Times and Places. A Viſible Square, for Inſtance, ſuggeſts to the Mind the ſame Tangible Figure in Europe, that it doth in America. Hence it is that the Voice of the Author of Nature, which ſpeaks to our Eyes, is not liable to that Miſinterpretation and Ambiguity, that Languages of Humane Contrivance are un⯑avoidably ſubject to.
CLIII. Though what has been ſaid may ſuffice to ſhew what ought to be determined, with rela⯑lation to the Object of Geometry; I ſhall never⯑theleſs, for the fuller illuſtration thereof, conſi⯑der the Caſe of an Intelligence, or unbodied Spi⯑rit, which is ſuppoſed to ſee perfectly well, i. e. to have a clear Perception of the proper and imme⯑diate Objects of Sight, but to have no Senſe of Touch. Whether there be any ſuch Being in Na⯑ture or no, is beſide my purpoſe to inquire. It ſufficeth, that the Suppoſition contains no Contra⯑diction in it. Let us now examine, what Profici⯑ency ſuch a one may be able to make in Geometry. Which Speculation will lead us more clearly to ſee, whether the Ideas of Sight can poſſibly be the Ob⯑ject of that Science.
CLIV. Firſt, then it is certain, the aforeſaid Intelligence could have no Idea of a Solid, or Quantity of three Dimenſions, which followeth [242] from its not having any Idea of Diſtance. We in⯑deed are prone to think, that we have by Sight the Ideas of Space and Solids, which ariſeth from our imagining that we do, ſtrictly ſpeaking, ſee Di⯑ſtance, and ſome parts of an Object at a greater diſtance than others, which hath been demonſtrat⯑ed to be the Effect of the Experience we have had, what Ideas of Touch are connected with ſuch and ſuch Ideas attending Viſion: But the Intelligence here ſpoken of is ſuppoſed to have no Experience of Touch. He wou'd not, therefore judge as we do, nor have any Idea of Diſtance, Outneſs, or Profundity, nor conſequently of Space or Body, either immediately or by Suggeſtion. Whence it is plain, he can have no Notion of thoſe Parts of Geometry, which relate to the Menſuration of Solids, and their Convex or Concave Surfaces, and contemplate the Properties of Lines generat⯑ed by the Section of a Solid. The conceiving of any part whereof, is beyond the reach of his Faculties.
CLV. Farther, he cannot comprehend the Manner wherein Geometers deſcribe a right Line or Circle; the Rule and Compaſs with their uſe, being things of which it is impoſſible he ſhould have any Notion: Nor is it an eaſier matter for him to conceive the placing of one Plain or Angle on another, in order to prove their Equality: Since that ſuppoſeth ſome Idea of Diſtance, or External Space. All which makes it evident, our pure Intelligence could never attain to know ſo much as the firſt Elements of plain Geometry. And perhaps, upon a nice Inquiry, it will be found, he cannot even have an Idea of plain Fi⯑gures any more than he can of Solids; ſince ſome Idea of Diſtance is neceſſary, to form the Idea of [243] a Geometrical Plain, as will appear to whoever ſhall reflect a little on it.
CLVI. All that is properly perceived by the viſive Faculty, amounts to no more than Colours with their Variations, and different Proportions of Light and Shade: But, the perpectual Mutabi⯑lity, and Fleetingneſs of thoſe immediate Objects of Sight, render them incapable of being manag⯑ed after the manner of Geometrical Figures; nor is it in any Degree uſeful that they ſhould. It is true, there are divers of them perceived at once; and more of ſome, and leſs of others: But accu⯑rately to compute their Magnitude, and aſſign pre⯑ciſe determinate Proportions, between Things ſo variable and inconſtant, if we ſuppoſe it poſſible to be done, muſt yet be a very trifling and inſig⯑nificant Labour.
CLVII. I muſt confeſs, it ſeems to be the O⯑pinion of ſome ingenious Men, that flat or plain Figures are immediate Objects of Sight, though they acknowledge Solids are not. And this O⯑pinion of theirs is grounded on what is obſerved in Painting, wherein (ſay they) the Ideas imme⯑diately imprinted on the Mind, are only of Plains variouſly coloured, which by a ſudden Act of the Judgment are changed into Solids: But, with a little Attention we ſhall find the Plains here men⯑tioned, as the immediate Objects of Sight, are not Viſible but Tangible Plains. For when we ſay that Pictures are Plains: we mean thereby, that they appear to the Touch ſmooth and uniform. But then this Smoothneſs and Uniformity, or, in other Words, this Plainneſs of the Picture, is not perceived immediately by Viſion: For it appear⯑eth to the Eye various and multiform.
[244] CLVIII. From all which we may conclude, that Plains are no more the immediate Object of Sight than Solids. What we ſtrictly ſee are not Solids, nor yet Plains variouſly coloured; they are only diverſity of Colours. And ſome of theſe ſuggeſt to the Mind Solids, and others plain Fi⯑gures; juſt as they have been experienced to be con⯑nected with the one, or the other: So that we ſee Plains, in the ſame way that we ſee Solids; both being equally ſuggeſted by the immediate Objects of Sight, which accordingly are themſelves de⯑nominated Plains and Solids: But though they are called by the ſame Names, with the Things mark⯑ed by them, they are nevertheleſs of a Nature in⯑tirely different, as hath been demonſtrated.
CLIX. What hath been ſaid is, if I miſtake not, ſufficient to decide the Queſtion we propoſe to examine, concerning the Ability of a pure Spi⯑rit, ſuch as we have deſcribed, to know Geometry: It is, indeed, no eaſy matter for us to enter pre⯑ciſely into the Thoughts of ſuch an Intelligence; becauſe we cannot, without great Pains, cleverly ſeparate and diſintangle in our Thoughts the pro⯑per Objects of Sight from thoſe of Touch which are connected with them. This, indeed, in a compleat Degree, ſeems ſcarce poſſible to be per⯑formed: Which will not ſeem ſtrange to us, if we conſider how hard it is, for any one to hear the Words of his Native Language pronounced in his Ears without underſtanding them. Though he endeavour to diſunite the meaning from the Sound, it will nevertheleſs intrude into his Thoughts, and he ſhall find it extreme difficult, if not impoſ⯑ſible, to put himſelf exactly in the Poſture of a Foreigner, that never learned the Language, ſo [245] as to be affected barely with the Sounds themſelves, and not perceive the Signification annexed to them By this time, I ſuppoſe, it is clear that neither Abſtract, not Viſible Extenſion makes the Object of Geometry; the not diſcerning of which may perhaps, have created ſome Difficulty and uſeleſs Labour in Mathematics.