524 Prof. A. G. Boll on the Production of 



duced from D and E are of equal intensity. The relative 

 positions of the receivers are then noted. 



(2) Another method of investigation is based upon the pro- 

 duction of an interference of sound ; and the apparatus em- 

 ployed is shown in fig. 11. The interrupter consists of a 

 tuning-fork (A), which is kept in continuous vibration by 

 means of an electromagnet (B). 



A powerful beam of light is brought to a focus between the 

 prongs of the tuning-fork (A) ; and the passage of the beam 

 is more or less obstructed by the vibration of the opaque 

 screens (0, D) carried by the prongs of the fork. 



As the tuning-fork (A) produces a sound by its own vibra- 

 tion, it is placed at a sufficient 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 re- 

 ceiving lens (E) with as little loss as possible. The two re- 

 ceivers (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) commu- 

 nicating with the common hearing-tube (M). 



The length of the tube (K) is such that the sonorous vibra- 

 tions from the receivers (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 re- 

 ceiver (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 follows: — The loudness of a musical 

 tone produced 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 enables us to 

 measure the amount of resistance required to render the elec- 

 trical sound equal in intensity to the other. 



(4) If the tuning-fork (A) in fig. 11 is thrown into vibra- 

 tion by an undulatory instead of an intermittent current passed 

 through the electromagnet (B), it is probable that a musical 

 tone, electrically produced in the receiver (F) by the action 

 of the same current, would be found capable of extinguishing 

 the effect produced in the receiver (G) by the action of the 

 undulatory beam of light, in which case it should be possible 

 to establish an acoustic balance between the effects produced 

 by light and electricity, by introducing sufficient resistance 

 into the electric circuit. 



