110 FKAGMENTS OF SCIENCE. 



the individual particles of the waves. In water-waves 

 it is the vertical height of the crest above the trough, 

 while the length of the wave is the horizontal distance 

 between two consecutive crests. The aggregate of 

 waves emitted by the sun may be broadly divided into 

 two classes: the one class competent, the other in- 

 competent, 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, 

 expressed with reference to sensation, means the in- 

 tensity 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-thousandfold 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 more than a million times 

 that of the smallest. 



Turned into their equivalents of sensation, the dif- 

 ferent light- waves produce different colours. Red^ for 

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

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

 mediate length and amplitude. On entering from air 

 into a more highly refracting substance, such as glass 

 vr 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 analysing the light. 



