4 
; 
A. G. Bell—Production of Sound by Radiant Energy 483 
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 pur- 
pose 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 which move upon the graduated sup- 
ports (H I) on opposite sides of the axis of the beam, and the 
receivers are connected by flexible tubes of unequal length (K L) 
communicating 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 receiver (F 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 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 electro-magnet, (B,) it is probable that a musical 
tone, electrically produced in the receiver (F) by the action of 
same current, would be found capable of extinguishing the 
effect produced in the receiver (G) by the action of the undu- 
latory 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. 
Upon the nature of the rays that produce Sonorous effects in 
different substances. 
In my paper read before the American Association last 
August and in the present paper I have used the word “light” 
in its usual rather than its scientific sense, and I have not hith- 
