702 SPECIAL SENSES. 



That the accommodation of the eye to different degrees of illumination is 

 due to the changes in the colors produced by the pigmentary layer of the 

 retina and not to different degrees of dilatation of the pupil, is shown by the 

 fact that a person does not see better in the dark when the pupil has been 

 dilated by atropine (Loring). In a very dim light there is no possibility of 

 exact accommodation for near objects, which, when small, can not be seen 

 distinctly ; and the contraction of the pupil which attends accommodation 

 for near vision does not occur. It is possible that under dim illumination, 

 parts outside of the fovea, which are insensible to vision under a bright light, 

 receive visual impressions. Under these conditions the pupil is dilated 

 and rays impinge on portions of the retina not used in direct vision. A 

 natural extension of this idea would confine distinct vision and the apprecia- 

 tion of minute details to the action of the fovea centralis, in which there is 

 no visual purple, other parts of the retina, under full illumination, not being 

 used. To express this in a few words, the fovea centralis is used by day, and 

 the adjacent parts of the retina, by night. 



MECHANISM OF KEFBACTION IN THE EYE. 



An object that is seen reflects rays from every point of its surface, to the 

 cornea. If the object be near, the rays from each and every point are diver- 

 gent as they strike the eye. Kays from distant objects are practically parallel. 

 It is evident that the refraction for diverging rays must be greater than for 

 parallel rays, as a necessity of distinct vision ; in other words, the eye must 

 be accommodated for vision at different distances. Leaving, however, the 

 mechanism of accommodation for future consideration, it may be stated 

 simply that the important agents in refraction in the eye are the surfaces 

 of the cornea and the crystalline lens. Calculations have shown that the 

 index of refraction of the aqueous humor is sensibly the same as that of 

 the substance of the cornea, so that practically the refraction is the same 

 as if the cornea and the aqueous humor were one and the same substance. 

 The index of refraction of the vitreous humor is practically the same as 

 that of the aqueous humor, both being about equal to the index of refrac- 

 tion of pure water. Refraction by the crystalline lens, however, is more 

 complex in its mechanism ; depending first, upon the curvatures of its two 

 surfaces, and again, upon the differences in the consistence of different por- 

 tions of its substance. In view of these facts, the conditions of refraction 

 in the eye in distinct vision may be simplified by assuming the following 

 arrangement : 



The cornea presents a convex surface upon which the rays of light are 

 received. At a certain distance behind its anterior border, is the crystalline, 

 a double convex lens, corrected sufficiently for all practical purposes, both for 

 spherical and chromatic aberration. This lens is practically suspended in a 

 liquid with an index of refraction equal to that of pure water, as both the 

 aqueous humor in front and the vitreous humor behind have the same refract- 

 ive power. Behind the lens, in its axis and exactly in the plane upon which 

 the rays of light are brought to a focus by the action of the cornea and the 



