V. 



Biological Theories. 



VIII.— THE CRYSTALLINE LENS. 



TWO theories as to the action of the lens in the vertebrate eye are 

 extant, and I have been familiar with both for so long a time 

 that I do not know where I first learnt either of them. Of these, one 

 is to be found in almost any good text-book of physiology, and in 

 some text-books of physics. The other is well-known to physicists, 

 and is the obvious and necessary outcome of the treatment of optical 

 questions by the method of considering wave-surfaces instead 

 of "rays." 



The first and almost universally adopted doctrine treats the lens 

 of the eye like the lens of a photographer's camera, and involves the 

 calculation of the focal length of the lens from its " mean refractive 

 index " and the curvature of its surfaces. The second is based 

 rather on a knowledge of the internal constitution of the lens than on 

 its external conformation. Before entering into it let me describe an 

 exceedingly simple experiment which ought to be seen by every 

 student of physiology. 



Remove the lens from the eye of a rabbit and press it very 

 slightly between two slips of glass so as to reduce its anterior and 

 posterior surfaces to two parallel planes. Now look at a printed 

 page through it and note that, though its surfaces are parallel and 

 plane, it magnifies strongly. Hold it up, still between the glass slips, 

 at a suitable distance in front of a white screen, and note that this 

 body, now no longer a lens, still produces a clear image. 



From the results of this experiment, which I hope every reader 

 will try for himself, it follows that the first of the two doctrines is 

 erroneous, and that neither curvature of the surfaces nor the " mean 

 refractive index " of the lens has any important influence in the 

 formation of the image, and that the function of the lens depends 

 almost entirely upon the arrangement within it of media of different 

 optical densities. 



The second doctrine, the true one as it seems, is that when a light- 

 wave falls upon the lens and traverses it, that portion of the wave 

 which traverses the central portion of the lens, having to traverse 

 media in which its velocity of propagation is less than in the peripheral 

 portions, is retarded in its passage to a greater extent than are other 

 portions of the wave, and thus a wave-front which was before plane 



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