32 6

OBSERVATIONS ON VISION.

through the lenticular ganglion, formed frombranches of the third and fifth pair of nerves,by the filaments perforating the sclerotica, tothe orbiculus ciliaris, which may be consi-dered as an annular plexus of nerves andvessels ; and thence by the ciliary processesto the muscle of the crystalline, which, bythe contraction of its fibres, becomes moreconvex, and collects the diverging rays to afocus on the retina. The disposition of fibresin each coat is admirably adapted to producethis change; for, since the least surface thatcan contain a given bulk is that of a sphere,the contraction of any surface must bring itscontents nearer to a spherical form. The li-quid of the crystalline seems to serve as asynovia in facilitating the motion, and toadmit a sutfieient change of the muscularpart, with a smaller motion of the capsule.

It remains to be inquired, whether thesefibres can produce an alteration in the formof the lens sufficiently great to account for theknown effects.

In the ox’s eye, the diameter of die crys-talline is 700 thousandths of an inch, theaxis of its anterior segment 225, of its pos-terior 350. In the atmosphere it collects par-allel rays at the distance of 235 thousandths.Prom these data we find, that its ratio of re-fraction is as 10000 to 0574. Hnuksbee makesit only as 10000 to 0832.7, but we cannotdepend on his experiment, since lie says, thatthe image of the candle, which he viewed,was enlarged and distorted : a circumstancethat he does not explain, but which was evi-dently occasioned by the greater density ofthe central parts. Supposing, with JIauksbeeand others, the refraction of the aqueousand vitreous humours equal to that of water,that is, as 10000 to 7405, the ratio of refractionof the crystalline in the eye will be as 10000

to 8800, and it would collect parallel rays atthe distance of 1226 thousandths of an inch:but the distance of the retina from the crys-talline is 550 thousandths, and that of theanterior surface of the cornea 250; hencethe focal distance of the cornea and aqueoushumour alone must be 2329. Now, suppos-ing the crystalline to assume a sphericalform, its diameter will be 642 thousandths,and its focal distance in the eye 926. Then,disregarding the thickness of the cornea, wefind, that such an eye will collect those rayson the retina, which diverge from a point atthe distance of 12 inches and 8 tenths. Thisis a greater change than is necessary for anox’s eye, for if it be supposed capable ofdistinct vision at a distance somewhat lessthan 12 inches, yet it probably is far short ofbeing able to collect parallel rays. The hu-man crystalline is susceptible of a muchgreater change of form.

The ciliary zone may admit of as muchextension as this diminution of the diameterof the crystalline will require ; and its elasti-city will assist the cellular texture of the vi-treous humour, and perhaps the gelatinouspart of the crystalline, in restoring the indo-lent form.

It may be questioned, whether the retinatakes any part in supplying the lens withnerves; hut, from the analogy of the olfac-tory and auditory nerves, it seems more rea-sonable to suppose that the optic nerve servesno other purpose than that of conveying sen-sation to the brain.

Although a strong light and close exami-nation are required, iu order to see the fibresof the crystalline in its inlire state, yet theirdirection may be demonstrated, and their at-tachment shown, without much difficulty.Iu a dead eye the radiating lines are discerni-