(43) 



secutive crests. The aggregate of waves emitted by the 

 sun may be broadly divided into two classes, the one 

 class competent, the other incompetent, to excite vision. 



But the light-producing waves differ markedly among 

 themselves in size, form, and force. The length of the 

 largest of these waves is about twice that of the small- 

 est, but the amplitude of the largest is probably a hun- 

 dred times that of the smallest. Now the force or energy 

 of the wave, which, expressed with reference to sensa- 

 tion, means the intensity of the light, is proportional to 

 the square of the amplitude. Hence the amplitude 

 being one hundred-fold, the energy of the largest light- 

 giving waves would be ten thousand-fold that of the 

 smallest. This is not improbable. I use these figures, 

 not with a view to numerical accuracy, but to give you 

 definite ideas of the differences that probably exist 

 among the light-giving waves. And if we take the 

 whole range of solar radiation into account its non- 

 visual as well as its visual waves I think it probable 

 that the force or energy of the largest wave is a million 

 times that of the smallest. 



Turned into their equivalents of sensation, the differ- 

 ent light waves produce different colors. Red, for ex- 

 ample, is produced by the largest waves, violet by the 

 smallest, while green is produced by a wave of interme- 

 diate length and amplitude. On entering from air into 

 more highly refracting substances, such as glass or water 

 or the sulphide of carbon, all the waves are retarded, 

 but the smallest ones most. This furnishes a means of 

 separating the different classes of waves from each 

 other in other words, of analyzing the light. Sent 

 through a refracting prism, the waves of the sun are 

 turned aside in different degrees from their direct course, 



