THE WAVE THEORY OF LIGHT 269 



imagine a line of particles in a straight line, and then you 

 must imagine them disturbed into a wave-curve, the shape 

 of the curve corresponding to the disturbance. Having 

 seen what the propagation of the wave is, look at this 

 diagram and then look at that one. This, in light, cor- 

 responds to the different sounds I spoke of at first. The 

 wave-length of light is the distance from crest to crest 

 of the wave, or from hollow to hollow. I speak of crests 

 and hollows, because we have a diagram of ups and downs 

 as the diagram is placed. 



Here, then, you have a wave-length.* In this lower dia- 

 gram (Fie. 119) you have a wave-length of violet light. ' 

 It is but one-half the length of the upper wave of red ) 



FIG. 1 1 8 Waves of Red Light 



FIG. 119 Waves of Violet Light 



light; the period of vibration is but half as long. Now 

 there, on an enormous scale, exaggerated not only as to 

 slope, but immensely magnified as to wave-length, we have 

 an illustration of the waves of violet light. The drawing 

 marked "red" (FiG. 118) corresponds to red light, and this 

 lower diagram corresponds to violet light. The upper 

 curve really corresponds to something a little below the red 

 ray of light in the spectrum, and the lower curve to something 

 beyond the violet light. The variation in wave-length be- 

 tween the most extreme rays is in the proportion of four and 

 a half of red to eight of the violet, instead of four and 

 eight ; the red waves are nearly as one to two of the violet. 



To make a comparison between the number of vibra- 

 tions for each wave of sound and the number of vibrations 



Exhibiting a large drawing, or chart, representing a red and a violet 

 wave of light (reproduced in f IG*. 118 and 119). 



