CHAPTER XXI 

 OPTICS 



THE adequate stimuli for the eye are certain vibrations of 

 ether, called light because they call forth the sensations of 

 light. In order that an object shall be clearly seen, rays 

 of light must pass out from the object, which by refraction 

 in the eye form an inverted real image of the object on the 

 retina. The cones and rods are the elements of sight; they 

 form a mosaic of nerve elements of which every point upon 

 which light falls can be stimulated. Hence different object 

 points can, by their stimulation of various retinal points, call 

 forth separate sensations of light and can therefore be seen 

 as distinct points. 



1. DIOPTRIC MECHANISM 



Physical observations. 



1. If a ray of light ($,, Fig. 25) passes from the medium J/j into 

 another medium Yl/ , it is refracted at the surface bounding the two 

 media (_/), i e. it takes another direction (*S" a ). The angle a which 

 S t forms with the perpendicular / upon the plane f is called the 

 angle of incidence. The angle ft which S" a forms with / is called 

 the angle of refraction. The sine of the angle of incidence 

 divided by the sine of the angle of refraction is, for any given 

 pair of media, constant, and is called the index of refraction. 

 When the index of refraction of one medium is given, the light 

 passes from the air into that medium. 



2. Homocentric rays, or rays coming from one luminous point, 

 falling upon the spherical surface between two media, are refracted 

 so that after the refraction they either cross each other at a point 

 (the real image point) or, prolonged backward, unite in a virtual 

 image point. This is strictly true only for a part of a bundle of 



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