An account of the mechanism of the eye: Wherein its power of refracting the rays of light, and causing them to converge at the retina, is consider'd: ... By John Taylor, ...

AN ACCOUNT OF THE Mechaniſm of the EYE.

AS I have propos'd to give ſome Account of the Mechaniſm of the EYE, I know no properer Way of beginning it, than by an Anatomical Diſſection of the EYE Itſelf; and after I have ſhewn ſome few general Diſeaſes that the particular Coats and Humors of the Eye are ſubject to, I ſhall proceed to ſhew how the ſaid Coats and Humors are inſtrumental to Viſion, and then by what particular Diſeaſes the Eye is render'd Totally Opake: And there I ſhall endeavour to aſcertain the true Place of a Cataract, and the Conſequence of a judicious or injudicious Removal of It.

The Globe of the Eye is moved by Six Muſcles, Four Streight, and Two Oblique, viz. Elevator; Depreſſor, Adductor, Abductor, Obliquus Superior, and Inferior: The Firſt of theſe ariſes out of the Bottom of the Orbit, and after it has aſcended and grown tendinous, is inſerted into the Upper Part of the Tunica Sclerotis, near the Cornea. The Depreſſor ariſes and is inſerted directly oppoſite to the Elevator. The Adductor ariſes from the Bottom of the Orbit, and is inſerted on the Side next the Noſe, into the Tunica Sclerotis. The Abductor ariſes and is inſerted directly oppoſite to the Adductor. The Superior Oblique Muſcle ariſes between the Elevator and Adductor, at the Bottom of the Orbit, thence aſcending by the Sutura Tranſverſalis becomes a Round Tendon, which paſſes thro' a little Annular Cartilage call'd Trochlea, (from its bearing the Form of a Pully) at the upper and inner Part of the Orbit near its Edge, is inſerted near the Bottom of the Eye. The Inferior Oblique Muſcle ariſes from the Os Maxillae Superioris, thence paſſing over the Depreſſor, is inſerted at the Bottom of the Eye near the Abductor. The Four Streight Muſcles are ſo fix'd to the Sclerotis, that they lift up the Eye, depreſs it, bring it towards the Noſe or from it, hold it faſt, turn it round, juſt as they act either ſeparately or conjointly.

The Superior Oblique Muſcle moves the Eye round towards the Noſe about its Axis, and pulls it upward and inward, and thereby directs the Pupil outward and downward; and thus the Eye is diſpos'd to ſee Things placed near it. The Inferior Oblique Muſcle turns the Eye towards the outward Corner, and alſo upwards; if theſe Two act together, they fix the Eye fierce and protuberant out of the Orbit: Moreover, another Uſe of theſe Muſcles is, to ſuſpend the Eye, and render it very fit to perform the Motions of the other Four. Between theſe Muſcles Fat is interſpers'd, which ſerves to lubricate and facilitate their Motion.

Next we come to the Coats and Humors of the Eye: The firſt is the Tunica Adnata, or Conjunctiva; it makes that which is call'd the White of the Eye, and is full of Blood-Veſſels; which are very viſible in an Ophthalmia: The Symptoms of this Diſeaſe are Heat, Pain, Pulſation, and an apparent Diſtention of theſe Blood-Veſſels; it may be occaſion'd externally, by any Thing forc'd againſt or coming into the Eye, irritating or compreſſing the Fibres of this Coat. If this Inflammation be only external, the Caſe ſymptomatical, and the Patient not worne out by other Diſeaſes, 'tis eaſily Curable: On the Contrary, if it becomes ſo great as to tumefie the Face, and the Humors very pungent and corroſive, we judge it difficult to cure. Its Cure depends on Bleeding, lenient Purgation, Bliſtring, Cupping, &c. together with the Aſſiſtance of a cooling Diet, topical Application, and proper Collyria.

The next Coat is the Sclerotis: It expandes itſelf nearly into a Globe, membranous and hard almoſt like Leather, and encompaſſes the whole Eye, growing gradually thinner, clearer, and prominent in the Fore-part, which Part of it is call'd Cornea, from its Tranſparency; they make together one firm Coat, for the Uſe of the other Coats and Humors. This Coat is ſometimes diſeas'd by a little White Speck of a Cartilaginous Subſtance, covering Part of, and ſometimes the whole Pupilla: If this Excreſcence is only ſuperficial, it may be remov'd by an Operation in Surgery call'd Leucotomy; but if the Cornea is throly affected, I believe it incurable. This Operation is perform'd in the following Manner, viz. I place the Patient in a proper Poſition, and fixing the Speculum Oculi invented for that Purpoſe in order to keep the Globe without Motion, pare off the Excreſcence with a ſmall curv'd Knife, leaving as few Inequalities as poſſible; and having prevented an Inflammation by proper Repellents, I blow a Powder into the Eye; which, aſſiſted by the Motion of the Eyelid, ſmooths off the Inequalities left by the Knife.

Next to this Coat lies the Tunica Choroides: 'Tis a Membrane of little Firmneſs, being much thinner than the former; 'tis ting'd by a dark Liquor ſeparated from the Blood-Veſſels, ſo that it appears of a ruſty dark Colour; this Coat has a Perforation in it, call'd Pupilla: The Part of this Coat which makes the Circumference of this Hole, and which lies upon the Side of the Cryſtalline Humor, is call'd Uvea, compos'd of circular and ſtreight Fibres; 'tis qualified by this Means to contract and dilate, as Light and Objects make different Impreſſions upon it. The Outſide of this laſt Membrane is call'd Iris, where appear different Colours: On the Inſide of it from its Circumference ariſe the Proceſſus Ciliares, made of ſhort Fibres like Lines drawn from the Circumference to the Center.

The laſt Coat is the Retina, ſo call'd from its Reſemblance to a Net; it covers the whole Fund of the Eye, and is nothing more than a fine Expanſion of the Medullary Fibres of the Optick Nerve, upon which the Rays of Light meeting in a Point, convey the Images of outward Objects to the Brain. The Diſeaſe of this Coat is call'd Gutta Serena; the Eye ſeems clear in Appearance, and there are no other Signs of Blindneſs except a preternatural Dilatation of the Pupilla. I ſhall have Occaſion to ſpeak more largely of this Diſeaſe by and by.

Between theſe Coats are contain'd Three Humors, the Aqueous, the Cryſtalline, and the Vitreous: The Firſt is call'd Aqueous becauſe it reſembles Water, being a thin tranſparent Fluid; it ſerves to extend the Cornea into an equal Convexity; beſides, it ſerves moreover for the Cryſtalline Humor to move forward or backward in, as Occaſion requires us to ſee Objects nearer or farther from us.

Next is the Cryſtalline: It's compounded of numberleſs ſpherical Segments, cloſely united and tranſparent; its Shape is a depreſt Spheriod, it's inclos'd in a fine Membrane call'd Aranea, and is much thicker than either the Aqueous or Vitreous: Its Diſeaſes and Uſes I ſhall ſpeak of in another Place.

The next is the Vitreous: It's very clear and tenacious, it lies behind the Cryſtalline, and fills the greateſt Part of the Eye; it's vaſcular on every Side, its Foreſide concave for the Cryſtalline Humor to lodge in, and Backſide convex; it keeps the Retina and Cryſtalline Humor at a proper Diſtance: This Humor is ſometimes ſo vitiated as to hinder or utterly deſtroy Sight, which Diſeaſe is often miſtook for a Gutta Serena.

I ſhall now proceed to ſpeak of the Parts of the Eye, as they are ſubſervient to Viſion.

The Uſe of the Muſcles in general is to direct the Globe of the Eye, and fix it in ſuch a Poſition, as that the Rays of Light reflected from any Object which we would have viſible to us, may paſs thro' the Pupilla and paint the Image of it on the Retina.

To form the Image of any Object, 'tis neceſſary that the Rays of Light coming from it ſhould converge and meet in a Foous: And we judge of the comparative Magnitude of all Bodies, by the Angle which is ſubtended by thoſe Bodies before the Eye.

To qualifie the Eye to perform this, Nature has given it a ſpherical Form, and compos'd it of tranſparent Goats and Humors; the latter to tranſmit the Rays coming from the luminous Points of Objects, and the former to refract them in ſuch a Degree as to bring them all together in a Point on the Retina.

All convex diaphanous Bodies refract the Rays of Light by Attraction, and the Ray bends towards that Part or the Body where there is the greateſt Quantity of Matter; this in convex Bodies is the Middle.

Therefore all Rays on whatever Parts of ſuch Bodies they fall, are refracted towards a Line drawn thro' the Center of thoſe Bodies, and made to converge behind them at a Diſtance proportionable to the greater or leſs Convexity of the Bodies themſelves: Thus when the Eye is too flat, as in old People, (from a Decay of the Humors, or a Shrinking of the Coats, or both) the converging Rays do not coaleſce at the Retina; but would paſs on if ſuffer'd, and not come to a Point but at ſome Diſtance beyond it: By this Means Viſion becomes weak and indiſtinct.

On the contrary, thoſe call'd Myopes, or Near-ſighted People, have their Eyes too gibbous, and the Convexity of them is ſo great, that the Rays meet and croſs one another before they arrive at the Retina. The former Eyes are help'd by Convex-Glaſſes, which put the Rays into a State of eaſie Refraction before they fall upon the Eye: The latter are help'd by Concave-Glaſſes, thro' which the Rays paſſing do diverge, and fall upon the Eye in ſuch a Manner, as to require a greater Degree of Refraction than they would in their natural Courſe: In the former Caſe the Eye is not help'd by the Object's appearing bigger, but more diſtinct; for very ſmall Objects are render'd diſtinctly viſible to ſuch an Eye by the Help of a Convex-Glaſs, whereas very large ones cannot be ſeen enough to be diſtinguiſh'd by the Naked Eye.

I don't propoſe to give any new Theory of Viſion, nor to be able to deſcribe the Manner of it better than 'tis already done by an eminent Author; for which Reaſon I ſhall make uſe of his own Words, ſo far as they are ſerviceable to my preſent Purpoſe, and conducive to the main End of this Treatiſe, which is to aſcertain the True PLACE of a Cataract.

All the Rays which come from one Point of an Object, are by the Cornea and Humors of the Eye united in a Point of the Retina, which is in a ſtreight Line, drawn from the ſame Point of the Object, thro' the Center of the Eye; and conſequently all the Rays, which come from all the Points of an Object, are united on the Retina, in the ſame Order and Proportion as the Points of the Objects are from whence thoſe Rays come.

Therefore the Interpoſition which theſe Rays make upon the Retina, muſt be the Image of the Object.

And thus Viſion in general is perform'd: But to know what the ſeveral Parts of the Globe contribute hereunto, it's needful to obſerve that the Cornea is more convex than any other Part of it; by which Means all the Rays are gather'd to paſs thro' the Pupilla, and none of them are loſt upon the Uvea.

The Aqueous Humor being thinneſt, and moſt liquid, eaſily changes its Figure, when either the Ligamentum Ciliare contracts, or both the Oblique Muſcles ſqueeze the Middle of the Bulb of the Eye, to render it oblong, when Objects are too near us.

The ſtreight Fibres of the Uvea dilate the Pupilla, when there are but few Rays of Light; and the circular Fibres contract it, when there are too many.

When the Pupilla is contracted, we ſee moſt diſtinctly; when it is dilated, we ſee moſt clearly.

The Glaſſy Humor keeps the Cryſtalline at ſuch a Diſtance from the Retina, as is neceſſary for uniting the Rays, which come from one Point of the Object, exactly in one Point of the Retina. The Impreſſion of the Object is made upon the Retina.

The Choroides is tinctur'd black, that the Rays of Light which paſs thro' the Retina may not be reflected back again to confuſe the Image of the Object.

Being diſtinct, Viſion conſiſts in the Union of all the Rays which come from one Point of an Object exactly in one Point of the Retina; and that the Rays which come from Objects at different Diſtances, are united at different Diſtances, behind the Cryſtalline Humor; they cannot both be exactly united upon the Retina, therefore the Eye cannot ſee equally diſtinct, at the ſame Time, Objects at different Diſtances.

The Optick Nerves pierce the Globe of the Eye, a little on the Inſide of the Optick Axes. Their external Coat, which is a Production of the Dura Mater, is continu'd to the Sclerotis, as their internal from the Pia Mater is to the Choroides; and their medullary Fibres paſſing thro' all, are expanded into the Retina, upon which the Images of Objects are painted.

The Center of this Expanſion is inſenſible, and all Rays which fall upon it are loſt; and conſequently that Point of the Object from which thoſe Rays come, is inviſible to the Eye: The Reaſon of which proceeds probably from the Blood-Veſſels, which enter with the Optick Nerve, and cover this Part of the Retina.

But whatſoever its Cauſe is, there is a manifeſt Advantage in the Optick Nerves being inſerted on the Inſide of the Optick Axes: For if they had pierc'd the Eye in the Axis, then the middle Point of every Object had been inviſible; and where all Things conduce to make us ſee beſt, there we had not ſeen at all.

We muſt likewiſe have loſt ſome Part of an Object, if the Optick Nerves had been placed on the Outſide of the Optick Axes: Becauſe an Object may be ſo placed, as that all the Rays which come from one Point, may fall upon the Outſide of both Eyes: But it is impoſſible they ſhould fall upon the Inſide of both Eyes; and therefore that Point which is loſt in one Eye, is viſible by the other.

This ſhall ſerve for the general Theory of VISION: I ſhall now come to ſpeak of a few Diſeaſes of the Eye, by which Viſion is render'd incompleat, or totally deſtroy'd: And firſt of the CATARACT.

Before I define a Cataract, 'twill be neceſſary to premiſe a few Things, in Order to clear the Way for my Definition: For ſince the Hypotheſis I go upon is New, and not yet clearly demonſtrated, it would be look'd upon as a ſtrange Peice of Preſumption to overlook in the Groſs all the different Objections which may be made againſt it, and ſatisfie myſelf with only a few Probabilities (collected out of my own little Experience and Obſervation) which may ſeem to make for it.

Objection I. 'Tis Demonſtrated by the Mathematicians, that neither Corpuſcles in the Aqueous or Cryſtalline Humor can be perceiv'd on the Retina; and 'tis ſaid by Phyſicians, that a Beginning of a Cataract is a Suffuſion of the Eye, when little Clouds, Motes, and Flies, ſeem to float about in the Air: Now if theſe Aſſertions are both true, one would be apt to conclude, that neither the Aqueous nor Cryſtalline Humor can be the Seat of a Cataract: The firſt is indiſputable, but the latter is not always True in Fact.

For I have known a good Number of Subject with real Confirm'd Cataracts, who never remember to have been thus affected at the Beginning of them.

And altho' ſome at the Beginning of Cataracts are thus affected, 'tis no Proof that the Diſeaſe may not be in one or the other of theſe Humors: For the ſame Cauſe which will produce a Cataract, may at the ſame Time the Cataract is forming affect alſo the Fund of the Eye; and by diſordering the Blood-Veſſels, here and there ſtop the Influx of the Animal Spirits into the little Filaments of the Optick Nerve which help to make the Retina. This will certainly produce them Clouds, Motes, &c.

And I have the more Reaſon to believe this, becauſe I have Couch'd ſeveral who after the Cataract has been well ſuppreſt, have remain'd blind with a Gutta Serena; which is, I ſuppoſe, a total Stoppage of the Influx of the Animal Spirits into the nervous Filaments afore-mention'd.

Objection II. 'Tis ſuppos'd, that without the Aſſiſtance of the Cryſtalline Humor there could be no Viſion; that is, except that Humor lay always ready to receive the Rays and refract them after they had paſs'd the Pupilla, they would ſtill proceed with ſo ſmall a Degree of Refraction that their Place of Converging would be beyond the Retina; and by Conſequence Viſion would be render'd very indiſtinct, or perhaps the Eye would only perceive a confuſs'd Glare of Light; if this Humor was taken away, and this could be demonſtrated, 'twould not deſtroy my Suppoſition: For I freely acknowledge, that after the Removal of a Cataract, (or the Cryſtalline Humor, which as I proceed I ſhall uſe ſynonymouſly) Viſion is never ſo perfectly perform'd as it was before the Eye was diſeas'd.

And if the Subjects I have hitherto perform'd the Operation upon were of Conſideration enough to have given me Pains beyond my Needle, I make no Doubt but I could have fix'd them with Glaſſes which would have ſtill aſſiſted their Sight, and made them ſome Amends for the Loſs of the aforeſaid Humor.

Objection III. If Viſion can be perform'd without this Humor, and the Maxim is true Quod Natura Nihil agit fruſtra, Why was it placed in the Eye?

I anſwer, 'Tis placed there to enable the Eye to ſee equally well at different Diſtances: For ſince the Rays proceeding from the luminous Points of nigh Objects diverge more than thoſe from more remote Objects, the Power of making them equally converge at the Retina is very probably lodg'd in the Cryſtalline Humor; which, according to the different Diſtances of Objects, approaches nigher to, or removes farther from the Retina, and perhaps too aſſumes a different Convexity.

And this I look upon to be unqueſtionable: Becauſe I have made it my conſtant Obſervation, That after a ſucceſsful Depreſſure of a Cataract, the Patient ſees beſt at a Diſtance ſomething greater than the Eye requires when all its Parts are in their natural Situation; and that if the Object is placed conſiderably further from, or nearer to the Eye, Viſion is render'd indiſtinct or confus'd.

Objection IV. But if the Cryſtalline Humor is depreſs'd, what becomes of it afterwards? And how is the Cavity fill'd up, which the ſaid Humor poſſeſs'd in its natural Situation?

Anſwer. When I paſs my Needle into the Eye, I preſs directly forward to the Cryſtalline Humor, and endeavouring to diſengage it from the Ligamentum Ciliare, I depreſs it to the Bottom of the Vitreous Humor; when the Pupilla immediately ſeems clear to me, and ſuch a Torrent of confus'd Light ruſhes into the Eye, as ſometimes affrights the Patient even more than the Puncture of the Needle.

Common Senſe tells us, That if a ſolid Body which is immers'd in a Fluid ſhould be remov'd from one Part of that Fluid into another, it makes no Alteration in the Bulk of the Whole; for whatever Place the Solid is remov'd to, it fills up but ſo much Space preciſely as it did in the Space which it was remov'd from. Immediately upon the Removal of the Cryſtalline Humor, the Aqueous turns in and fills up the Cavity.

N. B. From the different Specifick Gravity of the Cryſtalline Humor, which in all Probability is different in different Stages of the Diſtemper, we may account for what is call'd the Riſing again of the Cataract: As to what becomes of the Cryſtalline Humor after 'tis depreſs'd, 'tis not unlikely that being broke from the Veſſels which nouriſh'd it, it ſhrinks by Degrees, and as it were melts into the Vitreoeus.

Dr. Turner ſays, A Cataract is a Diſeaſe of the Eye, in which a Film or Skin growing before the Pupilla, like a Curtain drawn againſt a Hole letting the Light into a Room, hinders the Paſſage of the ſame, till it is depreſs'd by the Needle, as in Couching.

There is no one of my Profeſſion who pays a greater Veneration to the Authority of that worthy, learned, and ingenious Gentleman, than myſelf; I am beholden to him that I have a regular, methodical, and I may venture to ſay a certain Syſtem of Surgery in my Head and Hand; but as he himſelf (who had always more Buſineſs in other Parts of Surgery than a Man of his ſtudious Temper could well attend to) very rarely, if ever perform'd the Operation, I hope 'twill not be thought Preſumption if when I come to ſpeak of the Operation itſelf, I ſhould venture to give ſome Reaſons which may ſeem to thwart his Opinion, and eſtabliſh my own.

That there are ſuch Films as the Doctor mentions on this Side the Pupilla, and in the Aqueous Humor, I can readily grant, having met with ſeveral of them in the little Courſe of my Practice: Theſe the Practitioners are allow'd to call Suffuſions, Films, &c. but not Cataracts; for I am determin'd to call no Diſeaſe of the Eye by this latter Name, except that particular One which is to be remedied by the Uſe of the Needle: But the, Caſe as mention'd by the Doctor can receive no Benefit that I know of by the Operation, nor did I ever attempt to Couch a Patient (tho' I have had ſeveral offer'd me) when the Circumſtances were as before-mention'd: This I ſhall endeavour to demonſtrate, in deſcribing the Operation itſelf.

The French Authors, with Vaughion at the Head of them, imagine a Cataract to be a Film ſeparated from the Cryſtalline Humor, and floating in tine Aqueous; that all its Parts are exiſtent in its firſt Conformation, that it is a Cyſtis, and that Cyſtiſſes are never form'd de novo, but are only Membranes diſengag'd from the neighbouring Parts, which increaſing in Thickneſs receive Nouriſhment from the Parts to which they adhere.

I ſhall beg Leave to examine this Opinion a little, and however ſpecious and refin'd it may ſeem, I am afraid we ſhall find as little Reality in it, as there is in the ordinary Profeſſion of Friendſhip amongſt the Gentlemen of their Country.

Firſt they ſay, 'tis a Membrane diſengag'd from the Cryſtalline, and floating in the Aqueous; the firſt they don't offer to prove, and the latter is falſe in Fact: For if this pretended diſengag'd Membrane ſhould float in the Aqueous, the Patient would have at leaſt a Chance of ſeeing now and then a little, by its being remov'd wholy or in part from before the Pupilla; but ſuch a Removal we find never happens; and that after the Formation of the Cataract, the Eye continues in a conſtant equal Blindneſs, till the Cauſe is remov'd by the Needle: I muſt beg them then to fix their floating Iſland, before they pretend to make a rational Hypotheſis of it; and if they do, I hope they'll be ſo kind as to let us know to what Membranes they'll join it, and in what Manner it receives Nouriſhment from them, then we'll talk farther with them.

What have led the Frenchmen into the Talk of its being a Cyſtis, and that no Cyſtiſſes are form'd de novo, is the late Diſcoveries made by the Improvements of Microſcopes. They are wonderfully pleas'd with and fond of the Opinion, that every Thing which is proceeds originally from an Egg or Seed; but however true this may be of perfect Beings, I don't expect that the ſame will be aſſerted of the Fungus's of Trees, and other ſupernatural Excreſcences: I can be very well content to believe, that Sloughs and Mucus may be detach'd from ſeveral Membranes in the Body, and may afterwards increaſe by an Addition of the ſame Matter, and yet there may be no Cyſtis; but I ſhall prove preſently, that 'tis impoſſible a Cataract ſhould be in this Part of the Eye; and that will at once put an End to the Controverſie.

What has given me frequent Occſion o • reflecting lately, has been the odd Manne • of our Authors of General Theories; they undertake to deſcribe and define every particular Diſeaſe, with as much Freedom an • Poſitiveneſs, as if they had been acquainte • themſelves with a good Number of Patient labouring under ſuch Diſeaſe, and had mad • the Cauſes, Symptoms, Cure, &c. of it, thei • peculiar Study.

How juſt a Proceeding this is, will appea • by the following Definition of a Cataract in a late modern Writer of this Sort.

A Suffuſion, ſays he, is a thick Foulneſs o • Excreſcence, of the Tunica Cornea, Adnata or Aqueous Humor, which when confirm' • makes the Cataract.

According to this Account, any thing which thickens the Cornea, or makes the Aqueous Humor dark and cloudy, may be term'd a Cataract; for he ſeems to uſe the Word Cataract not as any particular Diſeaſe, but only as ſomething which ſhall darken and obſcure the Sight, by hindring the Tranſmiſſion of the Rays of Light. Thus the Albugo, Pterygium, Hyopyon, or the Viſcidity of the Aqueous Humor, frequently conſequent upon an Ophthalmia, may be juſtly deem'd a Cataract. But how this ſquares with the real Notion of this Diſeaſe, and its Cure, I ſhall leave my Readers after a few Pages to determine.

But paſſing over all the different Definitions and Surmiſes of others, I ſhall take the Liberty (for Reaſons which I ſhall give afterwards) to define it in the following Manner.

A Cataract is a Diſeaſe of the Cryſtalline Humor, whereby the ſaid Humor is totally or at leaſt one Side of it render'd opake: The Humor in this diſeas'd State reflect all or moſt of the Rays of Light, inſtead of tranſmitting them to the Retina; in Conſequence of which, the Humor appears whitiſh, bluiſh, yellowiſh, &c. and the Eye is blind.

CAUSES.

Any Blow or Shock, which may alter the particular Poſition of the Particles of which this Humor is compos'd: for to make a Body diaphanous, 'tis not only neceſſary that it ſhould be made of ſuch particular Particles, but that thoſe Particles ſhould be combin'd and united after a particular Manner. Thus Salts, wet Paper, or the Oculus Mundi Stone, grow opake by being dried; Horn, by being ſcrap'd; Glaſs, by being reduc'd to Powder, or otherwiſe flaw'd; Turpentine, by being ſtirr'd about with Water till they mix imperfectly.

The Matter which is ſeparated for the natural Nouriſhment of this Humor from the Blood, may from the too great Viſcidity of the Blood itſelf, either be ſeparated in too ſmall a Quantity, (the Glands and Strainers of theſe Parts being exceeding fine) and by this Atrophy the Humors may grow dry and opake, as Paper, Salts, Oculus Mundi Stone, &c. Or otherwiſe upon a Fever or Ophthalmia, the Veſſels leading to this Humor may be ſo enlarg'd, as to admit Particles of an improper Figure or Magnitude: The Heterogeneous Particles, (if I may ſo call them) inſtead of properly nouriſhing the Humor, may be lodg'd irregularly in its Pores, (as Water and Turpentine as above) and by deſtroying that Equality which we are told is neceſſary to Tranſparency, render the Humor opake.

It may be caus'd by violent Grief, which occaſions a more than natural Secretion of Tears from an adjacent Part; this may from a Continuance make ſo great a Revulſion, that the Cryſtalline Humor may probably grow dry, or ſuffer Diminution, for Want of its uſual Supplies. I have Reaſons to imagine this, becauſe in Eyes thus diſeas'd I have found this Humor much ſmaller than ordinary.

It may and often does proceed from an unskillful Treating of the Eye in an Ophthalmia, whilſt there is a more than ordinary Flux of Matter upon the Parts which ſeems to intreat a Paſſage thro' the Coats: Inſtead of alluring it forwards by kind Diſcutients, I have too frequently known it drove back by cold Repellents; and by lodging itſelf in the Coats and Humors of the Eye, at once deprive it of its Uſe and Beauty.

Dr. Pitcarn, in ſpeaking of a Cataract, ſays, Some aſcribe it to the thickning or coagulating of the Aqueous Humor, and others to that of the Cryſtalline: But theſe know Nothing of the Nature of the Eye; for Corpuſcles floating in the Aqueous or Cryſtalline Humo, can't be prerceiv'd on the Retina.

For once I'll run the Riſque of having my Underſtanding cenſur'd, and venture to ſay. That tho' the Corpuſcles themſelves are not ſeen being within the Eye, yet they can hinder the Eye from diſcerning every diſtinct Part of any Object which is without it: And theſe little Specks of Darkneſs, amidſt a Glare of Light, are miſtaken by the Patient for Something poſitive, when they are only Privations; and the Effect would be the ſame, if there were any Stoppages in the ſmall nervous Filaments which compoſe the Retina.

Thus the Doctor's Philoſophy is true, when he ſays they can't be ſeen, &c. But if he would pretend to inferr from thence, that they can be no Cauſes why the Eye can't ſee other Things, his Conſequence is falſe; and clearly demonſtrated to be ſo, by Mr. Cheſelden at the End of his ANATOMY.

DIAGNOSTICK.

'Tis known by a Diminution of Sight in the Patient, and by Something which appears like a Bluiſh Cloud behind the Pupilla, tho' the Colour of it often varies: I do not yet know whether there be any Reaſon given for the Variety of Colours, but I have obſerv'd that the Green and Yellow are generally in old Subjects.

PROGNOSTICK.

Not ſo rnuch the Age of the Perſon is to be conſider'd, as the Age and Ripeneſs of the Cataract: I have ſeen both very young and very old Subjects couch'd with Succeſs, tho' the beſt Prognoſtick is certain to be made in a middle Age and good Habit.

If a Surgeon underſtands the Operation, and is at all able to judge when the Cataract is ripe for it, he has a great deal of Reaſon to promiſe Succeſs.

If an uncommon Inflammation ſupervenes, which he can't foreſee, and ſometimes 'tis impoſſible for him to prevent, he is not to be blam'd if he miſſes of Succeſs, tho' he had ſeem'd pretty poſitive in promiſing it both to his Patient and himſelf.

Regimen before the Operation.

In Order to prevent an Ophthalmia, uſe proper Evacuations, as Bleeding, Lenient Purgatives, &c. a few Days before the Operation.

OPERATION.

The moſt temperate Seaſons are the beſt, for which Reaſon I make Choice of Spring or Autumn. 'Tis requiſite the Day ſhould be fine and clear, and the Light well choſen. I oblige the Patient to recline his Head backwards upon a Pillow, the Pillow is ſupported by the Back of a Chair of a proper Heighth; behind the Patient an Aſſiſtant is plac'd, to keep his Head without Motion; 'twill be neceſſary too for a Perſon on each Side, to ſecure the Hands of the Patient; for I have ſeen very ill Conſequences attend the Neglect of this: Having thus prepar'd the Patient, I place myſelf on a Stool, rais'd by Pillows according to the Heighth of the Subject, till I have a full Command of the Patient's EYE; then bidding him turn his Eye towards his Noſe, ſo that the Tunica Conjunctiva may be ſufficiently apparent, I plunge my Needle directly into the Vitreous Humor, thro' the Tunica Conjunctiva, about the Diſtance of the Tenth Part of an Inch from the Cornea; then thruſting it boldly to the upper Part of the Cryſtalline, gently turn the flat Part of my Needle on the Top of the Cataract; by which Means I have a greater Surface upon it, and the Cataract conſequently leſs liable to ſlip from under my Needle: I depreſs it very ſlowly, and place it as I think at the Bottom of the Vitreous Humor; I don't immediately remove my Needle off it, but continue it depreſs'd for about Half a Minute, for Reaſons which I ſhall give when I come to ſpeak of the Cauſes of a Cataract's riſing again: After this, I draw my Needle back about the Sixth Part of an Inch; and gently raiſing the Point, take it quite out of the EYE.

Regimen after the Operation.

Immediately after the Operation I apply a Repellant Compreſs, and order the Patient to be put to Bed, raiſing his Head ſomething higher than uſual; and after ſuffering of him to remain quiet a few Hours, I renew my dreſſing, and begin to treat him with a cool Regimen, proper Diet, &c. as in a ſymptomatical Fever: This I do to ſecond the Intention of the former Regimen; notwithſtanding which, an Ophthalmia in Spite of all I can do will ſometimes ſupervene, and then it muſt be treated as ſuch.

Theſe Ophthalmia's are ſometimes ſo violent as to become difficult of Cure, and often produce all the Effects, and perhaps too really bring that Diſtemper upon the Eye call'd Gutta Serena. This laſt mention'd Diſeaſe is ſeated on the Retina, and may be brought on by any Thing which will enlarge the Capillary Arteries of that Membrane; for they lying contiguous to the Nerves, and their Diameters being increas'd, the intermediate Nerves are ſo preſs'd betwixt them, that the Influx of the Animal Spirits is intirely prohibited, and all Commumication is cut off betwixt them and the Original of the Optick Nerves in the Brain, which is the Commune Senſorium: Thus tho' the Cataract continues well depreſs'd, and the Eye in all Appearance perfectly clear, yet the Patient will remain Blind, and 'tis too often beyond the Reach of Art to give him any Aſſiſtance.

However, I generally uſe all the Evacuations proper in a ſtubborn Ophthalmia, with Calomel, Purgatives, &c. and keep the Patient to an attenuating Diet; obliging him to abſtain from all viſcid Nutriments, and to uſe much Exerciſe: This Method I have known to meet with its deſir'd Succeſs.

There are many other Cauſes beſides this Common One, which may render the Operation unſucceſsful; firſt, the Riſing again of the Cataract.

The Cataract will ſometimes riſe again, even before the Needle is out of the Eye: There are many ſpecious Reaſons to be aſſign'd for it, tho' perhaps none of them true; however I ſhall mention what I have hitherto imagin'd, not without a Promiſe to correct any Thing I ſhall here ſay if I find it contradicted by future Practice.

The firſt Reaſon (which is perhaps more obvious to others than myſelf) is, That the Cryſtalline Humor being diſeas'd, is become ſpecifically lighter than the Vitreous; and that being diſengag'd from the Membranes to which it in ſome Meaſure adhered, and depreſs'd to the Bottom of the Vitreous, upon Removal of the Needle it aſcends by its own ſpecifick Levity, and places itſelf at or near the Place from whence it was remov'd.

The next is, That conſiſting of very glutinous Parts, and being join'd perhaps to the Uvea, the Needle only removes the main Body, which ſtill remains attach'd to the Uvea by certain ſmall elaſtick Filaments; which as ſoon as the Needle is remov'd contract themſelves, and again lift up the Cryſtalline Humor into its natural Poſition.

If it remains depreſs'd for ſome Days and afterwards riſes again, but being a ſecond or third Time depreſs'd it continues where you would place it, it may proceed from an increas'd Viſcidity in the Humor itſelf, or from a Rarefaction of that Fluid into which it's immers'd; both of which will perhaps be conſequent upon an Inflammation, perhaps from an Elaſticity in the Cornea, whereby the Aqueous Humor may be immediately thruſt into its Place by Removal of the Cryſtalline; which Elaſticity and Contraction of the Cornea, may poſſibly be too great to be overcome by the diffrent ſpecifick Gravity of the Cryſtalline.

But I muſt confeſs, if I was to give my Opinion of the general Cauſe of the Riſing again of the Cataract immediately upon its being depreſs'd, I ſhould imagine it to be the lifting up the Point of the Needle before I withdraw it very near back again to the Place from whence I remov'd the Cataract. For I imagine that the ſpecifick Gravity of the Cryſtalline Humor, (being very little different if at all from that of the Vitreous) would never be ſufficient to overcome the Reſiſtance of the ſaid Vitreous Humor, which is a very viſcid and tenacious Fluid, unleſs there was a Paſſage made for it to riſe again by that Motion of the Needle which I have juſt now expreſs'd; and therefore hold it neceſſary that after the Cataract is depreſs'd, the Needle ought to be kept ſome Time upon it, that the Fluid above it may have Time perfectly to reunite; ſo as becoming all over equally tenacious, it may be impower'd to keep the Cataract in the Poſition we would have it.

Another Cauſe of ill Succeſs is, the wounding the Uvea or Iris; the Conſequence of which will be an irregular Form of the Pupilla, ſome of which I have ſeen intirely clos'd: However if they are not ſo, there is always ſuch a Calloſity upon the wounded Membrane, as ſtops up the Pupilla in one Part, and leaves ſuch an Opening of it on the other, that by ſtopping up the Rays where they ſhould paſs, and ſuffering them to paſs where they ſhould not, the Eye is render'd intirely uſeleſs.

Theſe Parts too, when wounded, ſometimes ulcerate; by which Ulceration, the whole Coats and Humors of the Eye become one common Maſs of Pus, and the Eye intirely ſinks.

The Reaſon that a Perſon is oblig'd to wound the Uvea and Iris is, when he would endeavour to remove a Film which lies in the Aqueous Humor, or on this Side the Pupilla. And this, till I know better, I ſhall call an Injudicious Practice.

It often happens that the ſaid Coats are wounded thro' the Fault of the Patient, even when the Operation is the moſt judiciouſly performed; for he not havingCourage to keep his Eye in the Poſition he was directed, and by endeavouring to fly from the Pain which is given by the firſt Puncture of the Needle, which he imagines is only to be done by Motion, and not being able to move his Eye any farther towards the great Canthus, turns it back againſt the Point or Edge of the Needle; and by this Means, in Spite of all the Art or Addreſs of the Surgeon, will with that Motion wound the Coats above-mention'd: But this laſt Accident does not happen to a Man who knows what he is doing, perhaps above Once in Twenty Times.

Another Reaſon of ill Succeſs is, the Cataract proving Milky: To be able to determine mine whether 'tis ſo or not before I touch it with my Needle I have not yet learn'd, and ſhould be glad to be inform'd by any Body who has.

Another Cauſe of ill Succeſs, which is alſo ſometimes attributed to the Surgeon, may be a ſtrange Fondneſs of Sight, which engages the Patient after having been couch'd Three or Four Days to pull off the Dreſſings in the Abſence of the Surgeon; by which he expoſing the Eye to the Aſſaults of the open Air, brings on that Ophthalmia which we were endeavouring to prevent: In this Caſe he himſelf is blamable, and not the Surgeon, for all the ill Conſequences that may attend it, one of which I have found often to be the intire Loſs of Sight.

Another Misfortune, which too frequently attends the unhappy Patient, is, when the Cataract is accompanied with a Gutta Serena, perhaps of as long ſtanding as the Cataract itſelf; this we find when upon depreſſing the Cataract, (and there's no Poſſibility of judging of it before) and the Pupilla remaining clear, the Patient notwithſtanding continues in total Darkneſs: This Caſe is indeed deplorable. I ſhould be unwilling to think that there was any Part of Mankind ſuch Strangers to common Senſe, as to imagine this to be a Fault of the Surgeon; but, I myſelf have more than once been calumniated on this very Account: I can't help ſaying, I have been unfairly dealt with by ſome of the Gentlemen of my own Profeſſion, who have miſrepreſented the Caſes, and deluded the common People, as well as injured me; this was the Caſe of Three Subjects that I lately Couch'd in Yarmouth.

N. B. I have met with the ſame Treatment on Account of a Subject which I Cut for the Stone, where I will undertake to demonſtrate to any Man in England, that it was as impoſſible to meet with Succeſs in that Operation as it was circumſtanc'd, as it is to ſucceed in the Operation of Couching where the Cataract is attended with a Gutta Serena; for unleſs a Man could directly look into the Body in the one Caſe, or down to the Bottom of the Eye in the other, he can be no more able to make a poſitive Prognoſtick, than he would be to alter the Circumſtances.

I ſhall make here another Obſervation, That where there is a Cataract in one Eye and the other Eye ſound and good, the Patient don't ought to be encourag'd to ſuffer the diſeas'd Eye to be Couch'd; becauſe the Symptoms which may poſſibly attend it, may be the Occaſion of the Loſs of both.

I ſhall take the Liberty of mentioning a Caſe of my own, wherein I have met with ſome of the afore-mention'd Accidents. About Five Months ſince, a Patient came to me with a Confirm'd Cataract in one Eye, and a Suffuſion in the other. I found the Cataract good, and (after the common Preparations) depreſs'd it with no apparent ill Accident. The Eye continu'd clear, and the Patient ſaw for ſome Days: At which Time an Inflammation came on, which equally affected both Eyes; and, notwithſtanding all I could do to abate it, the Coats and Humors were ſo thicken'd by it, that the Eye which had the Suffuſion in it became totally blind: And ſome Time after, that which I had couch'd was in like Manner intirely depriv'd of Sight; tho' the Cataract remain depreſs'd, and the Pupilla perfectly clear.

Obſervation. It ſeems pretty plain, That the Suffuſion above-mention'd was Nothing more than the Beginning of a Cataract, cauſed by the Cryſtalline Humor growing thick and muddy: And that this Inflammation, by increaſing the Viſcidity of the Matter ſeparated for the Nouriſhment of the ſaid Humor, increas'd the Thickneſs ſo, as to render the Humor intirely opake; or in other Terms, ripen'd the Cataract. The Reaſon why I did not venture to Couch it afterwards was, an uncommon Dilatation of the Pupilla, which had been from the Beginning of the Suffuſion: The Diſeaſe of the other was a direct Gutta Serena, caus'd by the Inflammation in the Manner before deſerib'd.

The Subject I have been treating of being altogether new, I ſhall beg Leave to add the following Queries, to be conſider'd by the Gentlemen of my Profeſſion, not troubling myſelf whether they make for or againſt my Hypotheſis.

Query I. Whether the Reaſon of the Dilaation of the Pupilla in one Eye and not the her may not be this: That the Cryſtalline Humor being render'd ſomewhat leſs tranſparent than formerly, the Patient finds that th • Rays of Light do not ſtrike ſo clearly and for cibly as they us'd to do; or in other Words the Eye is darker than in its natural State To amend which, he naturally and one may almoſt ſay involuntarily dilates the Pupilla in order to receive more Rays of Light, and recompenſe the Defect of the diſeas'd Humor?

The ſame is done whenever we want to ſee Objects in the Dark; but in this caſe the Cauſe remaining conſtant, the Dilatation is conſtant too, till the Pupilla by the frequent Uſe of this Motion becomes actually wider than it ought naturally to be. In ſuch a Dilatation as I here mention, we can't promiſe any great Succeſs from the Uſe of the Needle: For was the Humor itſelf perfectly clear, ſuch a dilated Pupilla would admit ſuch a Glare and Flow of Rays to the Retina, as would render Viſion confus'd and indiſtinct; much leſs can we hope to have a good Eye, when that Humor ſo neceſſary to Viſion is intirely removed.

Query II. Whether the Cataract ſometimes appearing at firſt in a white Point, and afterwards ſpreading itſelf like a Line drawn from a Center to a Circumference, is not a Proof that 'tis a tranſparent Body already form'd, growing thick and cloudy, rather than the Beginning of a new Membrane?

N. B. The Compoſition of the Cryſtalline Humor, in the Anatomy of the Eye.

Query III. Whether Cataracts very old and diſcolour'd, declining in the ſame Manner, and becoming angulous and uneven, is not a Proof of the ſame Thing?

Query IV. Whether it would not be worth while to obſerve after the Couching a young Subject, if the ſame Subject is ever troubled with a ſecond Cataract, properly diſtinguiſhing whether it be the ſame Cataract riſen again or not? And this I think is beſt to be done, by obſerving the Interval of Time which paſſes between his being Couch'd and the Appearance of this ſecond Cataract. Whether if this Obſervation be juſt, and a ſecond Cataract be form'd in the ſame Eye, and in the ſame Subject, this would not deſtroy my Hypotheſis?

Query V. Whether our not being able to remove a Cataract at its firſt Appearance, is not a Proof that 'tis not the Cryſtalline Humor? Becauſe for all what we know certainly, the Cryſtalline Humor is equally removable at all Times.

Query VI. Whether or no in that Stage of the Diſeaſe which we call a Milky Cataract, there may not be ſuch a Solution of Continuity of the Parts of the Cryſtalline Humor, as may ſuffer the Needle to paſs backwards and forwards in it, without being able to remove it out of its Place?

Query VII. And whether in a farther Stage of this Diſeaſe, the Parts may not again cloſe, and ſtick together in ſuch a Manner, as that the whole Body of the Humor may be at once remov'd, by only preſſing upon a particular Part of it?

Query VIII. Whether there may not be a Gradation of Colouring ſettled from Sir Iſaac Newton's Theory, beginning at a light Blue and ending at a greeniſh Yellow; whereby we might be able to determine the exact Age of a Cataract, as ſoon as we ſee it? And whether ſuch a particular Colour will or will not ſucceſsfully bear the Needle?

Query IX. Whether 'tis not poſſible to invent a Speculum Oculi, which may keep the Globe of the Eye without Motion, and without giving that Pain to the Patient which too frequently contributes to the bringing on of an Inflammation? I mention this, becauſe I have ſeen ſo many Inconveniencies attend the Uſe of the Common Speculum Oculi; that, rather than uſe it myſelf, I have choſe to have Nothing to aſſiſt me, except the Preſſure of the Thumb and Finger?

Query X. Whether a Trembling, which I have obſerv'd in the Aqueous Humor ſome Time after Couching, does not proceed from a Loſs of Part of that Humor in Couching? And whether this Trembling is one Reaſon why the Rays of Light paſs confus'd into the Eye?

Query XI. Whether there is Room for the Needle to paſs between the Cryſtalline Humor and the Uvea, without removing the former ſo as to ſpoil the Eye, or cutting or lacerating the latter?

Query XII. Whether, if this laſt is practicable, there's not a Poſſibility of removing a Film that may be in the Forepart of the Pupilla, notwithſtanding the Needle enters in at the Backpart of it?

Query XIII. Whether there might not be a nice Experiment made, by placing an Eye like a Lens in the Hole of a dark Chamber? I think (if I remember rightly) I have met with the following Account in an Author of Experimental Philoſophy. Stripping the Fund of the Eye of its Integuments, he plac'd it as above-mention'd; and holding a Peice of white Paper within the Room, at a ſmall Diſtance from the Eye, he diſtinctly perceived the Images of all Objects from without painted upon the Paper; as they would have been upon the Retina, if the Eye had been in its natural Poſition.

Had a nice Hand been there who would have gon on the Outſide and with a Needle as in Couching depreſs'd the Cryſtalline Humor, and if notwithſtanding this Depreſſion the Perſon within could ſtill perceive the Images as before, it would be a certain Proof that the Eye might ſee, notwithſtanding the Loſs of the Cryſtalline Humor.

The Eye ſhould be taken down, and diſſected; and by obſerving in what Part the Cryſtalline Humor after its Removal was lodg'd, we ſhould receive a great Light into the Theory of Couching, and much Satisfaction, as to the Truth of this Hypotheſis.

I only mention this as it ſeems to me poſſible, without being able to determine whether 'tis practicable or not.

CONCLUSION.

Before I conclude, I thought it my Duty to mention, (and in the Words of a Great Man) That as a candid and ſincere Behaviour ought to be preferr'd to all other Conſiderations, I would not let my Heart reproach me with a Conſciouſneſs of having gain'd even the Poſſibility of a Praiſe, which is not in every Particular my Right: I therefore thought proper to acknowledge, that I am oblig'd to a very ingenious Gentleman (whom I have the Honour to be acquainted with) for Two or Three Hints that he has given me in the latter Part of this ſhort Diſſertation, which otherwiſe probably might have been omitted.

N.B. I believe it will be neceſſary to obſerve to ſuch Perſons as are not Judges of the Subject I have been treating of, That I have therein demonſtrated the Reaſon of my not meeting with Succeſs in ſome Patients that I have Couch'd, particularly in Yarmouth.

FINIS.