118 FRAGMENTS OF SCIEyCE 



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 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 consecu- 

 tive 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 ligbt-producing waves differ markedly among them- 

 selves 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 of energy of the wave, 

 which, expressed with reference to sensation, means the 

 intensity of the light, is proportional to the square of the 

 amplitude. Hence, the amplitude being one-hundredfold, 

 the energy of the largest light- gi\dng waves would be 

 ten-thousandfold that of the smallest. This is not im- 

 probable. I use these figures not with a view to numer- 

 ical accuracy, but to give you definite ideas of the differ- 

 ences 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 larg- 

 est wave is more than a million times that of the smallest. 

 Turned into their equivalents of sensation, the differ- 

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

 ample, 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 or water, 



