424 FRAGMENTS OF SCIENCE. 



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 dis- 

 tance 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 incompetent, to 

 excite vision. But the light-producing waves differ mark- 

 edly 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 sen- 

 sation, 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-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 more than 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 a more highly 

 refracting substance, 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 analyz- 

 ing the light. Sent through a refracting prism, the waves 

 of the sun are turned aside in different degrees from their 

 direct course, the red least, the violet most. They are 

 virtually pulled asunder, and they paint upon a white 

 screen placed to receive them " the solar spectrum." 

 Strictly speaking, the spectrum embraces an infinity of 

 colors; but the limits of language, and of our powers of 

 distinction, cause it to be divided into seven segments: red, 

 orange, yellow, green, blue, indigo, violet. These are the 

 seven primary or prismatic colors. 



Separately, or mixed in various proportions, the solar 



