254 



" Now the measure of the axis of the eye we have seen to be only 

 •833 inch, according to Treviranus, and -855 according to Krause ; 

 consequently, rays of mean refrangibility (to which Brewster's mea- 

 sures refer) converge to a point no less than -227 inch behind the 

 retina, when the rays fall parallel on the cornea, and '302 when the 

 object viewed is at 10 inches' distance. The axis of the eye, as even 

 measured by Dr Young, though somewhat greater than we have reck- 

 oned it above, (Dr Young makes it "Ol), does not come up to the 

 requisite dimensions ; and Dr Young, with his usual acuteness, 

 ascribes the difference to the gradually varying density of the strata 

 or coats of the lens,* the dense small nucleus evidently acting as a 

 lens of comparatively short focus ; and this explanation is probably 

 the correct one, to which we may add, that the conBguration of the 

 coats of equal density, which, near the surface of the lens, are very 

 elliptical, become, near its centre, gradually nearly spherical. On 

 this account, it is all but impossible to predict the exact course of 

 the rays through a structure of so much complication. 



" Dr Young had considered the case with his usual attention and 

 penetration. He investigates the focus of a spherical lens, or lens 

 with surfaces which are segments of spheres, and whose density is 

 variable, and the result may be recalled here as one which, perhaps, 

 has not been sufficiently remarked. " On the whole," he says, " it 

 is probable that the refractive power of the human crystalline in its 

 living state is to that of water nearly as 18 to 17 [gives index 

 refr. = 1-415] ; that the water imbibed after death from the humour 

 of the capsule reduces it to the ratio of 21 to 20 [1-403], but that, 

 on account of the unequable density of the lens, its effect on the eye 

 is equivalent to a refraction of 14 to 13 [1 -439] for its whole size."| 



" On the whole, these calculations, as well as the considerations 

 into which I entered in a former paper, read to the Society in 1844,+ 

 on the mechanism of the focal adjustment, have left on my mind the 

 conviction that the optical and mechanical structure of the organ of 

 sight is even less understood than it is commonly believed to be. 

 Simple as are its general arrangements, and comparable, in some 

 respects, to those of artificial combinations, we perceive surfaces 

 figured in a complex manner, and structures of varying refractive 



* Nat. Phil., vol. ii., p. 580. t >'at. Phil., vol. ii., p. 82. 



J Transactions Royal Society of Edinburgh, vol. xvi., p. 1. 



