104: SPECIAL SENSES. 



phenomena noted in experiments upon other portions of the 

 sympathetic system. 



Accommodation of the Eye to Vision at Different Dis- 

 tances. 



The mechanism by which the eye is adjusted for distinct 

 vision at different distances is one of the most interesting and 

 important points connected with the physiology of the sight. 

 At the present day, this point may be regarded as definitely 

 settled, particularly since the variations in the thickness and 

 the curvatures of the crystalline lens have been so accurately 

 measured by Helmholtz. We shall have little to say with 

 regard to the various theories of accommodation advanced by 

 the older physiologists, except to indicate, in a very general 

 way, the most plausible views that have been adopted from 

 time to time by physiological writers. In the first place, we 

 shall note certain physical laws and their application to the 

 eye, which show the necessity for accommodation. 



Supposing the eye to be adapted to vision at an infinite 

 distance, in which the rays from an object, as they strike the 

 cornea, are practically parallel, it is evident that the foci of 

 the rays, as they form a distinct image upon the retina, are 

 all situated at the proper plane. Under these conditions, in 

 a perfectly normal eye, the image, appreciated by the indi- 

 vidual or seen by means of the ophthalmoscope, is perfectly 

 clear and distinct. If the foci be situated in front of the 

 retina, the rays, instead of coming to a focus upon a point in 

 the retina, will cross, and, from their diffusion, or dispersion, 

 will produce indistinct vision. Under these circumstances, 

 a distinct point is not perceived, but every point in the image 

 is surrounded by an indistinct circle. These are called " cir- 

 cles of diffusion." If, now, the eye, adjusted for vision at an 

 infinite distance, be brought to bear upon a near object, the 

 rays from which are divergent as they strike the cornea, the 

 image will be no longer distinct, but will be obscured by cir- 

 cles of diffusion. It is the adjustment by which these circles 



