856 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



equal refracting power. Of the one a double convex lens is made, of the 

 other a double concave, and the two combined. They must be of dif- 

 ferent dispersive power, or the degree of concavity which would correct 

 the chromatic aberration would also destroy the converging power of the 

 convex lens. In crown glass and flint glass we have such media. The 

 flint glass has greater dispersive power, hence the degree of concavity 

 necessary to correct the chromatic aberration will be attained before the 

 degree of concavity will be reached which would destroy the converging 

 power of the convex lens of crown glass. 



In the different refractive media of the eye such combinations are 

 to a certain extent represented, and serve, together with the action of 

 the pupil in shutting off the circumferential rays, to correct chromatic 

 aberration. 



The eye as an optical instrument is analogous to the camera obscura, 

 and forms, in a manner to be described directly, an inverted image in 



FIG. 377. DIAGRAM ILLUSTRATING THE DECOMPOSITION, IN PASSING THROUGH 

 A PRISM, OF WHITE LIGHT INTO THE SEVEN COLORS OF THE SPECTRUM. 

 (Btclard.) 



r, red ; o, orange ; j, yellow ; v, green ; b, blue ; i, indigo ; vi, violet. 



reduced size of objects before it. Instead of a single lens, as in the 

 camera, the eye is composed of a number of different refractive media 

 placed behind each other the cornea, the aqueous humor, and the lens. 

 The field of projection, or the point on which the image is focused, is the 

 retina, and from changes which have recently been discovered to take 

 place in the retina in which the visual purple becomes bleached the 

 analogy to the process of photography is very striking. 



As is well known, by means of a convex lens an image of any object 

 may be formed upon a screen ; thus, if a convex lens be held before a 

 window, and a piece of paper placed behind it as a screen, at a certain 



