MECHANISM OF REFRACTION IN THE EYE. 793 



Retinal Red. In 1876, Prof. F. Boll published a short account of a discovery which 

 may possibly revolutionize our ideas of the mechanism of the appreciation of images 

 formed upon the retina. He discovered in the outer segments of the rods a peculiar red 

 or purple color, which disappeared after ten or twelve seconds of exposure to light. This 

 was first observed by Boll in the retina of frogs that had been kept for a certain time in 

 the dark. From his preliminary researches, Boll concluded that this coloration of the 

 retina exists only during life and persists but a few moments after death ; that it is con- 

 stantly destroyed during life by the action of light and reappears in the dark ; and finally 

 that it plays an important part in the act of vision. Kiihne and others have since con- 

 firmed and extended the original observations of Boll ; and the " retinal red " has been 

 noted in the mammalia and in man. It has been extracted from the retinas of frogs and 

 dissolved in a five-per-cent. solution of crystallized ox-gall, still presenting in solution its 

 remarkable sensitiveness to light. Finally it has been found possible to fix images of 

 simple objects, such as strips of black paper pasted upon a plate of ground glass, upon 

 the retina, by a process very like that of photography. These observations constitute 

 one of the most remarkable of recent discoveries ; but they are as yet too incomplete for 

 extended discussion in this connection. 1 



Mechanism of Refraction in the Eye. 



A visible object sends rays from every point of its surface to the cornea. If the 

 object be near, the rays from each and every point are divergent as they strike the eye. 

 Rays 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, we shall endeavor 

 to show how the rays of light as they penetrate the eye are refracted and brought to a 

 focus at the retina. The important agents in refraction in the eye are the surfaces of the 

 cornea and the crystalline lens. Careful calculations have shown that the index of 

 refraction of the aqueous humor is sensibly the same as that of the substance of the cor- 

 nea, so- that, practically, the refraction is the same as if the cornea and the aqueous hu- 

 mor were one and the same substance. The index of refraction of the vitreous humor ig 

 practically the same as that of the aqueous humor, both being about equal to the index of 

 refraction 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 portions of its substance. In view o 

 these facts, we may simplify the conditions of refraction in the eye by assuming tl 

 lowing arrangement : 



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

 tion. This lens is practically suspended in a liquid with an index of refraet.on o,,ua o 

 that of pure water, as both the aqueous humor in front and the *^'"T*" 

 have the same refractive power. Behind the lens, in its ,. and esactl; 

 which the ravs of li-ht are brought to a focus by the action of the cornea and i 

 SSS^SS, which is the centre of distinct vision The ana 1 clon,,, s ; 

 the fovea are capable of receiving visual impressions, wh.ch are conveyed to 

 by the optic nerves. All impressions made upon other portion, of t 

 pLatively indistinct; and the point of entrance of the ont.c nerve " 

 Inasmuch as the punctum ctecum is situated in either eye upon the nas 



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July, 1878, p. 190. 



