74 SPECIAL SENSES. 



rate of propagation is usually assumed to be about 192,000 

 miles. 



The intensity of light is in proportion to the amplitude 

 of the vibrations. The intensity diminishes as the distance 

 of the luminous body increases, and is in inverse ratio to the 

 square of the distance. 



In the theory of the colors into which pure white light 

 may be decomposed by prisms, it is assumed, as a matter of 

 demonstration, that the waves of the different colors of the 

 solar spectrum are not of the same length. The decomposi- 

 tion of light is produced by differences in the refrangibility 

 of the different colored rays as they pass through a denser 

 medium than the air. The differences in the wave-lengths 

 for different colors is very simply set forth by Tyndall as 

 follows : 



" The color of light is determined solely by its wave- 

 length. The ether-waves gradually diminish in length from 

 the red to the violet. The length of a wave of red light is 

 about 39060 of an inch ; that of the wave of violet light is 

 about g7soo ^ an inch. The waves which produce the other 

 colors of the spectrum lie between these extremes. 



" The velocity of light being 192,000 miles in a second, 

 if we multiply this number by 39,000, we obtain the num- 

 ber of waves of red light in 192,000 miles ; the product is 

 4T4,439,680,000,000. All of these waves enter the eye in a 

 second. In the same interval 699,000,000,000,000 waves of 

 violet light enter the eye. At this prodigious rate is the 

 retina hit by the waves of light. 



" Color, in fact, is to light, what pitch is to sound. The 

 pitch of a note depends solely on the number of aerial waves 

 which strike the ear in a second. The color of light depends 

 on the number of ethereal waves which strike the eye in a 

 second. Thus the sensation of red is produced by imparting 

 to the optic nerve four hundred and seventy-four millions of 

 millions of impulses per second, while the sensation of violet 

 is produced by imparting to the nerve six hundred and ninety- 



