136 FRAGMENTS OF SCIENCE. 



edge or of memory from causing any rent in our picture, I 

 here propose to run rapidly over a bit of ground which is 

 probably familiar to most of you, but which I am anxious to 

 make familiar to you all. The waves generated in the ether 

 by the swinging atoms of luminous bodies are of different 

 lengths and amplitudes. The amplitude is the width of 

 swing of the individual particles of the wave. In water- 

 waves it is the height of the crest above the trough, while 

 the length of the wave is the distance between two con- 

 secutive crests. The aggregate of waves emitted by the sun 

 may be broadly divided into two classes : the one class com- 

 petent, 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 smallest, but the amplitude 

 of the largest is probably a hundred times that of the 

 smallest. Now the force or energy of the wave, which, ex- 

 pressed with reference to sensation, 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 different 

 light-waves produce different colors. Red, for example, is 

 produced by the largest waves, violet by the smallest, while 

 green is produced by a wave of intermediate length and 

 amplitude. On entering from air into more highly refract- 

 ing substances, such as glass or water, or the sulphide of 



