526 



OBSXRVATIONS ON" VISION. 



fhrougli the lenticular ganglion, formed from 

 branches oHhe third and fifth pair of nerves, 

 by the filaments perforating the sclerotica, to 

 the orbiculus ciUaris, which may be consi- 

 dered as an annular plexus of nerves and 

 vessels ; and thence by the ciliary processes 

 to the muscle of the crystalline, which, by 

 the contraction of its fibres, becomes more 

 convex, and collects the diverging rays to a 

 focus on the retina. The disposition of fibres 

 in each coat is admirably adapted to produce 

 this change ; for, since the least surface that 

 can contain a given bulk is that of a sphere, 

 the contraction of any surface must bring its 

 contents nearer to a spherical form. The li- 

 quid of the ci'ystalline seems to serve as .1 

 synovia in facilitating the motion, and to 

 admit a sufficient change of the muscular 

 part, with a smaller motion of the capsule. 



It remains to be inquired, whether these 

 fibres can produce an alteration in the form 

 of the lens sufficiently great to account for the 

 known efl'ects. 



In the ox's eye, the diameter of the crys- 

 talline is 700 thousandths of an inch, the 

 axis of its anterior segment 223, of its pos- 

 terior 350. In the atmosphere it collects par- 

 allel rays at the distance of 235 thousandths. 

 From these data we find, that its ratio ofre- 

 iVaction is as 10000 to 0574. Hauksbee makes 

 it only as 10000 to G832.7, but we cannot 

 depend on his experiment, since he says, that 

 the image of the candle, which he viewed, 

 was enlarged and distorted : a circumstance 

 that he does not explain, but which was evi- 

 dently occasioited by the greater density of 

 the central parts. Supposing, with Hauksbee 

 and others, the refraction of the aqueous 

 and vitreous humours equal to that of water, 

 that is, as 10000 to 7465, the ratio of refraction 

 ;«/i'tlie crystalline in the eye will be as 10t)OQ 



to 88O6, and it would collect parallel rays at 

 the distance of 1226 thousandths of an inch: 

 but the distance of the retina from the crys- 

 talline is 550 thousandths, and that of the 

 anterior surface of the cornea 250; hence 

 the focal distance of the cornea and aqueous 

 humour alone must be 2329. ~ Now, suppos- 

 ing the crj'stalline to assume a spherical 

 form, its diameter will be 642 thousandths, 

 and its focal distance in the eye 920. Then, 

 disregarding the thickness of the cornea, we 

 find, that such an eye will collect those rays 

 on the retina, which diverge from a point at 

 the distance of 12 inches and 8 tenths. This 

 is a greater change than is necessary for an 

 ox's eye, for if it be supposed capable of 

 distinct vision at a distance somewhat less 

 than 12 inches, yet it probably is far short of 

 being able to collect parallel rays. The hu- 

 man crystalline is susceptible of a much 

 greater change of form. 



Tlie ciliary zone may admit of as much 

 extension as this diminution of the diameter 

 of the crystalline will require ; and its elasti- 

 city will assist the cellular texture of the vi- 

 treous humour, and perhaps the gelatinous 

 part of the crystalline, in restoring the indo- 

 lent form. 



It may be questioned, whether the retina 

 takes any part in supplying the lens with 

 nerves; but, from tlie analogy of the olfac- 

 tory and auditory nerves, it seems more rea- 

 sonable to suppose that the optic nerve serves 

 no other purpose than that of conveying sen- 

 sation to the brain. 



Although a strong light and close exami- 

 nation are required, in order to see the fibres 

 of the crystalline in its inlire state, yet their 

 direction may be demonstrated, and their at- 

 tachment shown, without much difficulty. 

 In a dead eye the radiating lines are discerni- 



