588 



MANUAL OF PHYSIOLOGY. 



tailed one into the other that they form but one layer. They are 

 not known to be connected with the nerve fibres ; but, even sup- 

 posing they be not in any way connected with the nerves, they 

 might still be influenced by the light, and by some kind of motion 

 communicate the effect to the contiguous sensitive nerve termi- 

 nals, which are elaborately adapted to appreciate subtle differen- 

 tiations of stimulus. 



COLOR PERCEPTIONS. 



If a beam of pure white sunlight be allowed to pass through 

 an angular piece of glass or prism, it is decomposed into a num- 

 ber of colors, which may be seen by looking through the prism, 

 or may be thrown on a screen, like that of a camera. These 



FIG. 231. 



Epithelial cells of the retina. a, seen from the outer surface; 6, seen from the side as in 

 a section of the retina; c, shows some rods projecting into the pigmented protoplasm. 



colors, which look like a thin slice of a rainbow, are together 

 called the spectrum. The white solar light is thus shown to be a 

 compound of rays of several colors which possess different de- 

 grees of refrangibility, and hence are separated on their way 

 through the prism. The violet rays are the most bent, and the 

 red the least, so that these form the two extremes of the visible 

 spectrum. The difference of color depends upon the different 

 lengths of the waves, the vibrations of violet (667 billions per 

 sec.) being much more rapid than those of red (456 billions per 

 sec.). Beyond the visible spectrum at the red end there are other 

 rays which, though they look black to the eye, are capable of 

 transmitting heat. This thermic power is best developed in these 

 ultra-red rays, and fades gradually toward the middle of the 



