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-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 different 
light-waves produce different colors. Eed, 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 
