300 THE REALITIES OF MODERN SCIENCE 



sidual rays." It is not correct to say "of the natural 

 colors/ ' for some of the "rays" may be of frequencies 

 too low to be visible and others too high. 



The phenomenon of residual rays is one of "reso- 

 nance," of which many illustrations are to be found. 

 Consider a case in sound. If all the keys of a piano 

 are depressed so that the strings are free to vibrate 

 and if then a person sings some note, the string cor- 

 responding to this note is set into "sympathetic 

 vibration." The energy of the sound waves is nob 

 absorbed by the other strings to an appreciable extent, 

 although they are all forced to move somewhat. In 

 the case of the string of the proper natural frequency, 

 each succeeding displacement which the wave causes 

 is in just the same direction as the string would natu- 

 rally move as a result of the energy imparted during 

 the preceding displacement. The energy of the wave 

 train is, therefore, always available for increasing the 

 displacement of the string. In other cases this energy 

 is obviously not always so available. The greater the 

 musical interval between the natural frequency of 

 the string and the impressed frequency of the wave 

 train, the more nearly will it be true that energy is 

 available alternately, and in equal amounts, for reduc- 

 ing the natural motion of the string and for increasing 

 it. Under these conditions the string will have no 

 motion. 



Looking at this matter from a slightly different 

 viewpoint, we see that this means that the string of 

 the wrong frequency does not absorb energy from the 

 wave train. All the energy of the wave train passes 

 by such strings. We, therefore, do not expect a 



