182 



SCIENTIFIC RECREATIONS. 



vibrations in a second, they will produce equal sound waves, and the sound 

 produced by both together will be uninterrupted, and twice as loud as one of 

 them. But if one string vibrate, say one hundred times, and the other one 

 hundred and one times in a second, they will not be in unison, and one will 

 gain upon the other string, till after it has got to fifty vibrations it will be 

 half a vibration ahead. At that moment they will neutralize each other, and 

 silence will ensue for an appreciable time. 



In the case of light suppose a red ray strikes the eye, and another 

 red ray to come upon it from somewhere else. If the difference between 



Fig. 185. The Megaphone. 



its distance and the other point from the spot in the retina on which 

 the first ray fell, is the -ffo part of an inch, or exactly twice, thrice, 

 four times as much, etc., that distance, the light will be seen twice as strong. 

 But if the difference in the distances between the points whence the light 

 comes be only one-half the -}-- part of an inch, or i^-, 2-J, 31, or 4-^ 

 times that distance, one light will extinguish the other, and darkness will 

 be the result. Now this is precisely what happens in the case of the 

 Topophone. To return to our simile of water waves. If two stones be cast 

 into a pond, and two equal and similar waves produced, and if they reach a 

 certain place at the same moment, they will make one large wave. But if 



