of Focal Ailjufni:c:!t in the E\e ? 307 



t^iplain, but which was evidently occafioned by the greater dcnfity of the central parts. 

 Suppoling, with Haukfbec and others, the refradion of the aqueous and vitreous huuiourJ 

 equal to that of water, viz. as loooo to 7465, the ratio of refraction of the cryftailine in 

 the eye will be as ioodo to 8806, and it would colledl. parallel rays at the diftance of 1226 

 thoufandths of an inch ; but the diftance of the retina from the cryftailine is 550 thou- 

 fandths ; and that of the anterior furface of the cornea 250 ; hence (by Smith, art 367.) 

 the focal diftance of the cornea and aqueous humour alone muft be 2329. Now fuppofing 

 the cryftailine to aflume a fphciical form, its diameter will be 642 thoufandths, and its focal 

 diftance in the eye 926. Then, difregarding the thicicnefs of the cornea, he deduces (by 

 Smith, art. 370.) that fuch an eye will coUetl thofe rays on the retiiia which diverge from 

 a point at the diftance of 12 inches and S-ioths. This is a greater change than is ne- 

 ceflary for an ox's eye : for, if it be fuppofed capable of diftind vifion at a diftance fome- 

 what lefs than 12 inches, yet it probably is far ftiort of being able to colled parallel rays. 

 The human cryftailine, he fays, is fufceptible of a much greater change of form. ' 



The ciliary zone may admit of as much cxtenfion as this diminution of the diameter of 

 the cryftailine will require ; and its elafticity will affift the cellular texture of the vitreous 

 humour, and perhaps the gelatinous part of the cryftailine, in reftoring the indolent form. 



He queftions whether the retina takes any part in fupplying the lens with nerves; but 

 from the analogy of the olfaftory and auditory nerves, he thinks it more reafonable to 

 fuppofe that the optic nerve ferves no other purpofe than that of conveying fenfation to the 

 braiti. 



Although a ftrong light and clofe examination are required in order to fee the fibres of 

 the cryftailine in its entire ftate, yet their direftion was demonftrated, and their attach- 

 ment fhewn, without much diftjculty. In a dead eye, the tendons are difcernible through 

 the capfule, and fometimes the anterior ones even through the cornea and aqueous humour. 

 When the dry cryftailine falls, it very frequently feparates, as far as the centre, into three 

 portions, each having a tendon in its middle. If it be carefully ftripped of its capfule, and 

 the fmart blaft of a fine blow-pipe be applied clofe to its furface, in different parts, it will 

 be found to crack exactly in the diretlion of the fibres above defcribed, and all thcfe cracks 

 will be ftopped as foon as they reach either of the tendons. The application of a little ink 

 to the cryftailine is of great ufe in fhewing the courfe of the fibres. 



Mr. Young was not at fitft aware that the mufcularity of the cryftailine had ever 

 been fufpecled, either by Defcartes or any other perfon. But the laborious and ac- 

 curate Lewenhoek, to whofe writings he refers, has defcribed the courfe of the fibres of 

 the cryftailine in a variety of animals, and has even gone fo far as to call it a mufcle. He 

 did not, however, attempt to account for the fowl adjuftment of the eye from its muf- 

 cularity. 



The remaining part of Mr. Young's paper is employeil in the fulution of fome optical 

 queries, not immediately relating to the prtfent objed. 



In Plate XIII, Fig. 1. reprefents a vertical fcdion of the ox's eye, of the natural fizc. 

 A, the cornea covered by the tunica conjundiva ; UC15, the fclerotica, covered at Bli by 

 tlie tunica aibuginea and tunica conjundiva ; DI), the choroid, confifting of two laminas ; 

 EE, the circle of adherence of the choroid and fclerotica; FG, FG, the orbiculus ciliari-j} 



P- I i III, UlC, 



