CHAPTER XXI 



VISION 



IF we wish to investigate an object by means of the tactile sense, we must 

 be able to feel the different parts of it. In doing this different nerve fibers 

 are stimulated; each nerve fiber produces a special sensation, which, owing to 

 its "local sign," differs from those mediated by other nerve fibers; and the 

 sum total of all these different sensations gives us our idea of the object. 



It is the same with the eye. The retina constitutes a mosaic of nerve 

 endings sensitive to the light ; each of these nerve endings produces a sensation 

 endowed with its own peculiar " local sign " ; and just as with the skin, the 

 total result of all these sensations constitutes our idea of the object as obtained 

 by vision. 



From this it is evident that a clear idea of an object perceptible to the 

 eye can only be obtained, if each point of the object acts upon its own particu- 

 lar point of the retina. 



Since light emanating from or reflected from an object radiates in all 

 directions, becoming more and more divergent the farther it proceeds, in order 

 to form a sharp picture of the object on the retina the light must be collected 

 by refraction of its rays, in such a manner that they will be focused on the 

 retina. This is the purpose of the refracting media of the eye. 



The physiology of the visual organ must begin therefore with a considera- 

 tion of the eye as an optical instrument. After that we shall study the visual 

 sensations, and the movements of the eye. 



FIRST SECTION 



THE EYE AS AN OPTICAL INSTRUMENT 



1. THE OPTICAL CONSTANTS OF THE EYE 



The eye contains a number of refracting media separated from one another 

 by approximately spherical surfaces. These media named from anterior to 

 posterior are: (1) The layer of tears; (2) the cornea; (3) the aqueous humor; 



(4) the crystalline lens composed of many layers of different refracting power; 



(5) the vitreous body. 



In order to follow the course of light rays in the eye we must determine 

 (1) the refractive indices * of the various media; (2) the radii of the refract- 



1 The ratio between the velocity of light in a vacuum and its velocity in a given me- 

 dium, as glass, is the refractive index of that medium. Since however, the velocity in air 

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