312 POPULAR LECTURES AND ADDRESSES. 



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, corresponds to the different 

 sounds I spoke of at first. The wave-length of 

 light is the distance from crest to crest of the 



FIG. 46. Waves of Red Light. 



FIG. 47. Waves of Violet Light. 



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. 1 In this 

 lower diagram (Fig. 47) you have a wave-length of 



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

 violet wave of light (reproduced in Figs. 46 and 47). 



