ACOUSTICS. 75 



developed will cover the circle with ripples which ap- 

 pear to move around it. 



For the exhibition of the Lissajous curves with such 

 forks as have been described, it is not necessary to use 

 a lens, but the whole light from the porte lumiere may 

 be allowed to enter the room, and the first fork placed 

 with its mirror in the middle of the beam. If, however, 

 it be desirable to admit less light into the room, a dia- 

 phragm may be used that admits a beam only an inch 

 in diameter or less. A lens may be used which will 

 concentrate the light upon a much smaller space, mak- 

 ing a much brighter spot, but will very much reduce the 

 size of the figures. When a lens is used, it must be so 

 placed as to project the mirror upon the second fork. 

 Its focal length should be two feet or more. 



All of these phenomena can be shown by means of 

 a lantern, even an oil lantern will answer. It will be 

 found best to use a beam of parallel rays, which may 

 be used in such a lantern as is represented in Fig. 26 

 by simply removing the front lens of the condenser. 

 With other lanterns it will be necessary to remove the 

 objective, and push forward the light until the beam 

 emerges with parallel rays : then, with a diaphragm cut 

 off all the light except a beam of the size of the mir- 

 ror upon the forks. The conditions are then the same 

 as with sunlight, and a lens may or may not be used. 



SYMPATHETIC VIBRATIONS. 



Let the two forks be brought to unison and at right 

 angles, so as to give, when struck, one of the forms of 

 Fig. 50. If now, but one of the forks be struck, the 

 straight line due to its vibration will slowly swell into 

 an ellipse, which will be due to the absorption by the 

 second fork of the vibrations of the : first. This^may 



