Page:Cyclopaedia, Chambers - Volume 1.djvu/744

 EYE

[ 378

J

EYE

fewer or more Rays, as the Object is nearer or more Tivid • or remoter and more languid. See PurlL.

Now the flatter the Figure of the Cornea is, the lels does ic'coilefl the Rays emitted from any lucid Point ; whence fewer arrive at the Cryftalline, and thofe more diverging, unlefs when they come from a very remote Object: On the contrary, the rounder it is, the more 01 the Ravs from any Point does it colled, and throw on the Cryftalline; and thofe the more : converging: Whence one great Caufc of the Defects in the Eyes both of Old Men

an AEam° F tlic Ray s tranfmitted thro' the Pupil to the Cry- ftalline 'are there refraded a-new, further collected, and render'd converging; fo as that thofe which came from the fame Point of the Object, are now thrown in one Point thro' the Vitrious, upon the Retina ; where they paint or exhibit that prccife Point of the Object whence they flow'd. Accordingly, if the Cryftalline be very denfe, or fpherical, the Focus, or the Point wherein they are united, will be too near ; and if too flat, or thin, the Point will be too remote : The Effect: of both which is Confufion. And hence another Caufe of the Defects of Myopes and 1>resbyt£. See Myoi>es and Presbytje.

'Tis not, however, Myopes and old Men alone, that would labour under thefe Defects, and have their Virion, la moft Cafes confufed, as in Objects very near, or very remote, very fmall, or very great : But thofe would be the common Condition of Virion.

Diftinct, Virion, depending abfolutely on the Union of all the Rays coming from the fame Point of the Object, on the fame precife Point of the Retina ; and Rays from Objects at different Diftances, being united at different Diftanccs behind the Cryftalline ; it were impoflible, e. gr. for the fame Eye to fee diftinctly any two Objects differently removed from it. But Nature has made a Frovifion againft thefe Defcfts ; and that principally, by bringing the Cryfhdlinc nearer to the Cornea, or further from it, occasionally ; which is effected two Ways, viz. either by comprefling the Bulb of the Eye by the four Mufcles all ftrongly contracted at once, which changes the Figure of the aqueous Humour, and renders the Eye oblong ; or by the Ligamentnm Ciliare, augmenting and diminifhing the Convexity of the Cryftalline, and fetting it nearer or further from the Retina. See Seeing, Sight, £Sc.

As complex as the Mechanifm of the Eye may feem, and as manifold as the Parts are which have a Relation hereto : The Juftnefs of Virion feems to require an exact Habitude in them all. — Thus, tho' the Pupil be no fub- ftantial Part of the Eye, but only an Aperture of the TJvea almoft perpetually changing its Bignefs according to the different Degrees of Light the Eye chances to be expofed to ; and therefore ftiould feem, while this Hole remains open, to perform its Office, by giving Entrance to the incident Rays of Light : Yet Mr. Boyle faw a Woman, who, after a Fever, not being able to dilate the 'PupilUe of her Eyes, as before, tho' they were very little narrower than ordinary ; yet had /he thereby almoft loft her Sight. — And on the other Side, tho' a compe- tent Widenefs of the Pupil be requifite to a clear and diftinct Viiiion, yet if its Dilatation exceeds the due Li- mits, there is theicby produced a coniiderable Diftemper of Sight.

It may feem alfo but a flight Circumftance, that the tranfparent Coats of the Eye fhouid be devoid of Colour ; and of as little Moment, that the Cornea ihould be very fmooth, provided it remain tranfparent : Yet when either of thefe Circumftances is wanting, the Sight is greatly vitiated. — Thus we fee, that in the yellow Jaun- dice, the adventitious Colour, wherewith the Eye is tinged, makes the Patient think he fees many Objects yellow, which are of a contrary Colour.

It has of late been an Opinion, that tho' both Eyes be open, and turned towards an Object, yet only one of them at a Time is effectually employed in giving the Reprcfen- tation of it : So that the having of two Eyes ftiould feem in fome Sort a Redundance. — But Mr. 'Boyle furniflies feve- ral Confederations which invalidate this Opinion, and fliew that both the Eyes are of Ufe at the fame Time. — He affures us, he has found by frequent Experiment, that his two Eyes together, beheld an Object in another Situation, than either of them apart would do. — He adds, that he has met with a Perfon, who had a Cataraft in his Eye for two or three Years, without finding any Impediment in his Sight, tho' others had, during that Time, taken Notice ot a white Film that had crofted his Eye ■ til], at length, happening to rub his found Eye, he was fur- prized to find himfelf in the dark : And that a very in- genious Perfon, who by an Accident had one of his Eyes (truck out, told him, that lor fome Months after, he

was apt to miftake the Situation and Diftances of Thins* for having frequent Occafion to pour Liquors out f f n | Vial into another, after this Misfortune he often fpilt them, and let them run quite befides the Necks ot the Vials, he thought he was pouring them directly into.

The like was related to him by another Perfon, Vho had, by a Wound, loft the ufe of one of his Eyes ; viz. that for fome Time after, he often, in pouring our his' Wine, mifs'd the Mouth of the Bottle, or Glafs.

A yet more confiderablc Inftance of this Kind, the fame Author gives us, of a noble Perfon, who, in a Fight, had one of his Eyes ftrangely mangled by a Mulket Ball, which came out at his Mouth ; after which Accident, he could not well pour Drink out of one Veflel into ano- ther; but had broken many Glaffes by letting them fall out of his Hand, when he thought he had given them to another, or fet them down on the Table : He added, that this Aptnefs to misjudge of Diftances and Situation, continued with him, tho' not in the fame Degree, for two Years.

The comparative Structure, and Anatomy of the Eye, is very curious : The Situation, Number, Conformation, &c. of this Organ, in different Animals, being finely and wonderfully adapted to their different Circumftances, Occa- sions and Manners of Living.

In Man, and fome other Creatures, an ingenious Author ob- ferves, the Eye is placed chiefly ro look forwards ; but withal fo ordcr'd, as to take in nearly the Hemifphere before it. — In Birds, and fome other Creatures, the Eyes are fc feated, as to take in near a whole Sphere, that they may the better feek their Food and efcape Danger. — In others they are feated, fo as to fee behind them, or on each Side, whereby to fee the Enemy pur'uing them : Thus, in Hares and Conies, the Eyes are very protuberant, and placed fo much towards the Side ot the Head, that their two Eyes take in nearly a whole Sphere : Whereas in 2Jogs, that purfue 'em, the Eyes are fet more forward in the Head, to look that Way more than backward.

Generally, the Head is contrived to turn this and that Way, chiefly, for the Occafions of the Eyes ; and generally the Eyes themfelves are moveable upwards, downwards, backwards, fidewife, for the more commodious Reception of the vifual Rays. Where Nature deviates from thefe Me- thods, fhe always makes ufe of very artful Expedients to anfwer the fame End.

Thus, in fome Creatures, rhe Eyes are fet out at a Diftance from the Head, to be moved here and there, the one this Way, and the other that ; as in Snails parti- cularly, whofe Eyes are contain'd in their four Horns, like atramentous Spots fitted to the End of their Horns, or lather to the Ends of thofe black Filaments, or Optic Nerves, fheath'd in the Horn. 'Power, Exper. Phil. Obf. 31.

And in other Creatures, whofe Eyes or Head is with- out Motion, as in divers Infects ; that Defect is fometimes made up by their having more Eyes than two ; as in Spiders, which having no Neck, and confequcntly the Head being immoveable, the Defect, is fupplied by the Situation and Multiplicity of their Eyes ; fome having four, fome fix, and others eight, all placed in the fore- front of the Head, which is round, like a Locket of Diamonds. The Reafon Dr. 'Power gives, is, that being to live by catching fo nimble, and fhy a Prey as Flies, they ought to fee her every Way, and fo take her per [ahum, without any Motion of the Head to dis- cover them.

Again, Men, and moft Quadrupeds, are found to have feveral Mu'cles belonging to their Eyes, by Help whereof they can turn them any Way, and fo obvert the Organ of Senfe to the Object. — But Nature not having given that Mobility to the Eyes of Flies, fhe in Recompcnce furnifhes them with a Multitude of little protuberant Parts, finely ranged upon rhe Convex of their large bulging Eyes ; fo that by Means of thefe numerous littfe Studs, numberlefs Rays of Light are reflected from Ob- jects placed on either Hand, above, or beneath the Level of the Eye, and conveniently thrown upon that Organ, to render the Objects they come from, vilible to the Animal ; and by the Help of a good Microfcope, and a clear Light, fome Hundreds of thefe little round Protuberances may be difcover'd, curioufly ranged on the Convexity ot a fingle Eye of an ordinary Flefli Fly.

So Scorpions are found to have above an hundred Eyes ; and Swammendam has obferved no iefs than two thoufand in the little Infect call'd Ephemera.

In other Creatures, the like Deficiency is fupplied by hav- ing their Eyes nearly two protuberant Hemifpheres, each confifting of a prodigious Number of other little Segments of a Sphere.

The Eyes of a Cameleon, Dr. Goddard obferves, refera- ble a Lens, or Convex Glafs, fet in a verfatile globular

Socket,