A SHORT ACCOUNT OF THE Nature and Uſe of Spectacles.

MY Deſign in the following Tract is to explain the Structure of the Eye, and the Nature of Viſion, ſo far as is neceſſary to underſtand the Uſe and Ad⯑vantage of Spectacles. The Method of chuſing the beſt Sort of Glaſſes, and ſuch as are ſuitable to the different Defects of the Eye in different Perſons; and to recommend a Kind of Glaſs, for this Purpoſe, much pre⯑ferable to any that has been hitherto made uſe of.

The Nature of Viſion, and Uſe of Spec⯑tacles, will be beſt explain'd, by the com⯑mon Experiment of the dark Room, or as it is uſually called the Camera Obſcura, which is this. In any Room that is pretty much darken'd, let an Hole be made in the Win⯑dow-Shutter, [4]in which Hole place a convex Lens, or round globular Glaſs; then hold⯑ing a Sheet of white Paper within the Room, at a proper Diſtance from the Hole, (which you will find by Tryals) you will ſee on the Paper the Pictures of all Objects before the Hole, both in their juſt Propor⯑tions, and in their proper Colours, only they will be upſide down. If the Paper remain in the ſame Place, and in the Room of the aforeſaid convex Glaſs, or Lens, one that is flatter be uſed, the Picture upon the white Paper will then be confuſed; but by placing a convex Spectacle-Glaſs before the Lens of a due Convexity, the Image on the Paper will again be render'd diſtinct. If a more con⯑vex Lens than the firſt be uſed, the Image on the Paper will again be confus'd, but may be render'd diſtinct, by placing in this caſe a concave Spectacle-Glaſs before the Lens.

This Experiment exactly repreſents the Manner of Viſion by the Eye; for the Eye is of a globular or convex Form, reſembling the Lens plac'd in the Hole in the Window-Shutter, the outſide Coat of which is call'd the Cornea, and gives the convex Shape to the aqueous or watery Humour contain'd with⯑in it: On the Bottom of the Eye is expanded a fine Membrane call'd the Retina, which [5]reſembles the white Paper mention'd in the Experiment; on this Membrane the Pictures of all external Objects are form'd. When the Pictures are form'd diſtinct on the Reti⯑na, then the Object appears clear and di⯑ſtinct; if the Pictures on the Retina are con⯑fuſed, then the Object appears hazy and confuſed. This Account of the Eye, and the Cauſe of Viſion, is farther confirm'd by theſe Arguments; that Anatomiſts, when they have taken off from the Bottom of the Eye, that outward and thickeſt Coat call'd the Dura Mater, can then ſee the Pictures of Objects lively painted on the Retina. And theſe Pictures propagated by Motion along the Fibres of the optick Nerves into the Brain, are the Cauſe of Viſion. For, ac⯑cording as theſe Pictures are perfect or im⯑perfect, the Object is ſeen perfectly or im⯑perfectly. If the Eye be ting'd with any Colour, (as in the Diſeaſe of the Jaundice) ſo as to tinge the Pictures in the Bottom of the Eye with that Colour, then all Objects appear ting'd with the ſame Colour.

From hence we ſhall eaſily ſee the Uſe of Spectacles; for the Humours of the Eye de⯑cay by old Age, ſo as by ſhrinking to make the Cornea grow flatter than before, in which Caſe the Light will not be refracted [6]enough, and for want of a ſufficient Re⯑fraction will not converge to the Bottom of the Eye, but to ſome Place beyond it; and by Conſequence will paint in the Bottom of the Eye a confuſed Picture, and according to the Indiſtinctneſs of this Picture the Ob⯑ject will appear confuſed, juſt as the Image on the Paper became confuſed, when a Lens flatter than juſt, was ſubſtituted in the Room of the right one.

This is the reaſon of the Decay of Sight in old Men, and ſhews why their Sight is mended by Spectacles. For the convex Glaſſes ſupply the Defect of Plumpneſs in the Eye; and by increaſing the Refractions make the Rays converge ſooner, ſo as to convene diſtinctly at the Bottom of the Eye, if the Glaſs has a due Degree of Convexity, exactly in the ſame manner as the Defect of the too flat Lens was corrected, and the Picture render'd diſtinct, by placing a Spec⯑tacle-Glaſs of due Convexity before it.

And the contrary happens in ſhort-ſighted Men, whoſe Eyes are too plump; for the Refractions being now too great, the Rays converge and convene in the Eyes before they come at the Bottom; and therefore the Picture made in the Bottom, and the Vi⯑ſion cauſed thereby, will not be diſtinct, un⯑leſs [7]the Plumpneſs of the Eye be taken off, and the Refraction diminiſh'd by a concave Glaſs of a due Concavity, juſt as the Con⯑fuſion of the Image occaſion'd by the Uſe of too convex a Lens was remedied, by placing a Spectacle-Glaſs of a due Concavity before it.

This in general is the Manner in which the Defects of the Eye are remedied by the uſe of Glaſſes. But theſe Defects are neither the ſame in different Perſons of the ſame Age, nor in the ſame Perſons at different Ages. The Defects of the Eye are partly natural, and partly ariſe from Habits of looking either at very remote, or very near Objects; in the former Caſe we flatten the Eye a little, in the latter we draw it up into a more convex Form to ſee diſtinctly; and by Habit the Eye will acquire and retain that Form, and will have a fix'd Shape, either too flat, or too globular for Objects at a mode⯑rate Diſtance. We ſee this confirm'd by every Day's Experience, for Engravers, Watchmakers, Chaſers, &c. by frequently forcing their Eyes into too convex a Form, to ſee minute Objects very near; at laſt have their Eyes fix'd in this Form, and become ſhort-ſighted, eſpecially if they are uſed to theſe Buſineſſes very young, while the Coats [8]of the Eye are tender and pliable. On the contrary, Sailors and Huſbandmen, by look⯑ing out for diſtant Objects, are rarely or ever ſhort-ſighted, and want the Aſſiſtance of convex Glaſſes very ſoon, becauſe the Habit of flattening the Eye to ſee remote Objects, at laſt fixes it in this Form.

From hence we may ſee that the Defects of all perſons Eyes will likely alter, accord⯑ing as they accuſtom themſelves to ſuch Ha⯑bits as may either increaſe or diminiſh theſe Defects. Beſides this, Age itſelf alters the Form of the Eye, making the Eyes of moſt Perſons grow flatter, eſpecially thoſe who ſee well at a moderate Diſtance, when young Perſons, naturally ſhort-ſighted, ſeldom find any conſiderable Alteration in the Eye; but when they do alter, I have obſerved that they oftener grow more ſhort-ſighted than otherwiſe.

Since therefore the Defects of the Eye, and the Glaſſes proper to remedy theſe De⯑fects vary ſo much, both in different Per⯑ſons and different Ages, I ſhall now lay down ſome Rules, by which every one may chuſe not only good Glaſſes in their Kind; but ſuch too, as are beſt ſuited to the Form and Shape of their own Eye. Every good Eye has one certain Diſtance (or nearly ſo) [9]of ſeeing a common Print to read with Eaſe: When the Eye begins to grow too flat, you will find yourſelf obliged to hold your Book at a greater Diſtance; you may obſerve alſo, that if you read, write, or work any long Time, a ſort of Dizineſs like a Cobweb will appear before your Eyes, or if you read by a Candle you find it neceſſary to hold the Paper behind it.

Theſe are Signs of the want of Spectacles, and whenever this appears to be the Caſe, no Perſon ſhould be without them; for your Endeavours to ſee what you cannot without great Difficulty, ſtrains and weakens the Eye, and by this you receive more Injury in a Month or two, than perhaps you other⯑wiſe would do in as many Years, with the Aſſiſtance of ſuch Glaſſes, as would ſupply that Defect, and render the Viſion diſtinct and eaſy.

The next Thing then to be conſidered is, what Degree of Convexity is ſufficient to ſupply this Defect. In order to make a proper Choice, hold a ſmall Print at the Di⯑ſtance, at which you was uſed to read di⯑ſtinctly, when your Eyes were good, which with moſt People is about nine or ten Inch⯑es, then chuſe a Pair of Spectacles of ſuch a Degree of Convexity as render the Letters [10]as plain as they uſed to appear before your Sight was defective; if you chuſe them too young, i. e. not convex enough, they will ſcarce remedy the Defect of the Eye; and you will not ſee diſtinctly, unleſs the Print is ſo far off, that the Letters will appear too ſmall to be read. If you chuſe them too old, that is too convex, you will then be obliged to flatten the Eye to compenſate the over great Convexity, and thereby be in danger of increaſing that Defect, which Age natu⯑rally brings on.

But in many curious Pieces of Workman⯑ſhip, there is an abſolute Neceſſity of view⯑ing the Object under a larger Angle, that the moſt minute Parts may be obſerv'd. In this caſe, Glaſſes of a larger Convexity may be uſed, and if the Work be brought nearer to the Eye, its Form will not be much al⯑tered, but the ſhorter time theſe Objects are view'd the better.

For ſhort-ſighted Perſons, the beſt way is to look through a concave Lens at ſome di⯑ſtant Object, and the leaſt Concave of all the Glaſſes, through which you can ſee it di⯑ſtinctly, is the beſt. I divide all my Glaſſes into 20 Degrees, mark'd 1, 2, 3, and ſo on. When a Perſon has fitted himſelf, I ſet down his Name, with the Number of the [11]Glaſs choſen, which prevents any more Trouble of Tryal, or even going for it; but ſuch Perſon may be ſupplied by ſending a Servant or Letter, unleſs any material Al⯑teration of the Eye ſhould happen.

A Perſon whoſe Reſidence is in the Coun⯑try, and has no Opportunity of making that Tryal, may nearly as well be fitted by the following Method; viz. Let the Perſon take a common Print, and move it to the Eye, till he ſees diſtinctly, then meaſure the ex⯑act Diſtance from the Eye to the Paper: By ſending an Account of that Diſtance to any Perſon in the Trade, they may be fitted to a ſufficient Degree of Exactneſs.

I come now to point out the Defects of the Glaſſes themſelves, and the way of diſ⯑covering theſe Defects. Now theſe are ei⯑ther a falſe Figure, Veins in the Glaſs, or the Colour of it. And firſt for the Figure. Take any common Print, lay it on a Table, then hold the Spectacles in your Hand at a good Diſtance from the Eye, and looking through them, remove them gradually from the Paper, till the Spectacles are at the focal Diſtance from it, which in a convex Glaſs is ſomewhat nearer to the Print, than the Point at which it begins to appear conſuſed. If the Spectacles are ſalſe, the Object through [12]the Middle of the Glaſs will appear regular and diſtinct, but through the Edges diſtorted and confuſed. Thus if you view any croſs Lines, which form ſmall Squares through a falſe ground Glaſs, the Squares in the Mid⯑dle will appear with ſtrait Sides, diſtinct, and all of a Size; but thoſe near the Edges, with crooked Sides, of a different Bigneſs, and confuſed; ſo that the whole Appear⯑ance will neither be like the Original, nor all Parts of it diſtinct at the ſame Diſtance. If you view it through a true Glaſs, it will be perfectly like the original Figure, only bigger, and diſtinct in every Part. And as you gradually remove the Glaſs beyond the Focus, all Parts of the Object will become indiſtinct together.

To diſcover the Veins in convex Glaſſes, place a Candle from you about five or ſix Yards, then looking through the Glaſs move it from your Eye, till you fill it full of Light, and you will then diſcover every Vein and Speck in it. Theſe Veins always diſtort Objects, and in the Object Glaſſes of Teleſcopes and Microſcopes are eſpecially miſchievous, ſince all theſe Errors are much magnified by the Eye-Glaſs. Specks differ from Veins; for Specks are only ſmall o⯑pake Spots, which do not diſtort the Ob⯑ject, [13]but only intercept an inconſiderable Part of the Light coming from it.

For the Colour of Glaſs, the whiteſt has been generally made uſe of for Spectacles, but I am inclin'd on many Accounts to pre⯑fer another Sort before it, or any other Ma⯑terial that has been made uſe of for this Pur⯑poſe: For the common white Glaſs gives an offenſive gla [...]ing Light, very painful and prejudicial to the Eyes; inſomuch, that ſome adviſe green or blue Glaſs, tho' it tinge every Object with its own Colour, ra⯑ther than this white Sort. That which I would recommend, is of a finer Compoſition than any commonly uſed; its Colour is the fineſt of any, and being a little ting'd with Blue, it takes off the glaring Light from the Paper, and renders every Object ſo eaſy and pleaſant, that the tendereſt Eye, may thro' it view any thing intently, without Pain. But, beſides this, there are many other Reaſons for rejecting the common white Glaſs, for 'tis of a ſofter Body than any other, and will not receive ſo true a Figure in the poliſhing, as a Glaſs of a harder Nature will, neither will the Poliſh itſelf be ſo fine and beautiful, nor of ſo long Continuance; for if it lies by any conſide⯑rable Time, it will change to a reddiſh Co⯑lour, [14]which is owing to the Nature of the Materials it is compoſed of; beſides, this Sort of Glaſs is generally found to be very full both of Specks and Veins; inſomuch, that one can hardly ſee any Spectacles made of this Sort of Glaſs that are free from it. That which I would here recommend is of an harder Nature, ſuch as will receive a true Figure, and will take a very fine and high Poliſh, is perfectly tranſparent, and the freeſt of any from Veins and Specks. I have always found this much preferable to any other Sort, both in Teleſcopes and Mi⯑croſcopes, where the leaſt Faults are diſ⯑cernable, eſpecially in the Object-Glaſs, where every Error in the Image, they form, is greatly magnified by the Eye-Glaſs.

Brazil Pebbles, and Rock Cryſtal Stones are much cried up by ſome for Spectacles; they are indeed very hard, and therefore will take a true Figure and a good Poliſh; but, beſides a glaring Whiteneſs, all theſe Stones have a Grain much like the Iſland CryſtalSee Sir Iſaac Newton's Optics, Book 3. Quere 25.; and not only a Grain, but like that too, a double Refraction, ſo that all Ob⯑jects appear double: Thus inſtead of one Letter there will appear two, one over the other. What Confuſion this muſt occaſion [15]in the Print of a Book may be eaſily gueſs'd at; and tho' this is ſometimes not great, ſo that the Letters are not much confuſed, yet it will occaſion an Hazineſs and Indi⯑ſtinctneſs, very like that of ſome Copper-Plate Prints, which have been moved a little on the Plate while they were printing off. If any one has a Mind to be fully ſa⯑tisfied of the Fitneſs of theſe Bodies for Spectacles, let him try to uſe them for the Object-Glaſs of a Teleſcope. Every one converſant in Opticks knows, the greateſt Exactneſs in Figure, the moſt perfect Free⯑dom from Veins, and the cleareſt Sub⯑ſtances are always requiſite to make a good Object-Glaſs in a long Teleſcope. But, if theſe Subſtances are ſo far from being pre⯑ferable, that they are the worſt of all others for that Purpoſe, ſo that it is ſcarce poſſible to make any Teleſcope at all with them; they muſt, of Courſe, be the worſt of all others for Spectacles too, tho' their Faults are not ſo eaſily diſcern'd in ſo ſimple an Inſtrument.