MECHANISM OF REFRACTION IN THE EYE 679 



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 REFRACTION IN THE EYE 



An object that is seen reflects rays from all points of its surface to 

 the cornea. If the object is near, the rays are divergent as they strike 

 the eye. Rays from distant objects are practically parallel. It is evident 

 that refraction for diverging rays must be greater than for parallel rays, 

 as a necessity for distinct vision ; in other words, the eye must be ac- 

 commodated for vision at different distances. Leaving, however, the 

 mechanism of accommodation for later 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 refraction of pure water. 1 Refraction by the crystalline lens, 

 however, is more complex in its mechanism ; depending first, on the 

 curvatures of its two surfaces, and again, on differences in consistence 

 of different portions of its -substance. In view of these facts, the con- 

 ditions of refraction in the eye in distinct vision may be simplified by 

 assuming the following arrangement : 



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

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

 talline, a double-convex lens, corrected sufficiently for practical purposes; 

 both for spherical and chromatic aberration. This lens is 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 refractive index. 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 lens, is the fovea centralis, in which is the centre 

 for distinct vision. The anatomical elements of the fovea are capable of 

 receiving visual impressions, which are conveyed to the brain by the 

 optic nerves. All impressions made on other portions of the retina are 

 comparatively indistinct ; and the point of entrance of the optic nerve 

 is insensible to light. Inasmuch as the punctum caecum is situated in 



1 The index of refraction is the ratio between the sine of the angle of incidence and the sine 

 of the angle of refraction. This is practically the same for the cornea, the crystalline lens, the 

 aqueous humor and the vitreous humor. For the aqueous and vitreous humors, it is 1.34 to 

 1.36 ; for the crystalline lens, it is 1.40 to 1.45, a little higher. 



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UNIVERSITY 



