;28 



THE POPULAR SCIENCE MONTHLY, 



of the tuning-fork (A), and the passage of the beam is more or less 

 obstructed by the vibration of the opaque screens (C D) carried by 

 the prongs of the fork. 



As the tuning-fork (A) produces a sound 

 by its own vibration, it is placed at a suffi- 

 cient distance away to be inaudible through 

 the air, and a system of lenses is employed 

 for the purpose of bringing the undulating 

 beam of light to the receiving lens (E) 

 with as little loss as possible. The two 

 receivers (F G) are attached to slides (H I) 

 which move upon opposite sides of the 

 axis of the beam, and the receivers are 

 connected by flexible tubes of unequal 

 length (K L) communicating wdth the 

 common hearing-tube (M). 



The length of the tube (K) is such 

 that the sonorous vibrations from the re- 

 ceivers (F G) reach the common hearing- 

 tube (M) in opposite phases. Under these 

 circumstances silence is produced when 

 the vibrations in the receivers (F G) are 

 of equal intensity. When the intensities 

 are unequal, a residual effect is perceived. 

 In operating the instrument the position 

 of the receiver (G) remains constant, and 

 the receiver (F) is moved to or from the 

 focus of the beam until complete silence 

 is produced. The relative positions of the 

 two receivers are then noted. 



3. Another 

 mode is as fol- 



lows : The loud- 

 ness of a musi- 

 cal tone pro- 

 duced by the action of light is compared 

 with the loudness of a tone of similar 

 pitch produced by electrical means. A 

 rheostat introduced into the circuit en- 

 ables us to measure tlie amount of re- 

 sistance required to render the electrical 

 sound equal in intensity to the other. 



4. If the tuning-fork (A) in Fig. 11 is 

 thrown into vibration by an undulatory 



instead of an intermittent current passed through the electro-magnet 

 (B), it is probable that a musical tone, electrically produced in the re- 



