A. G. Bell—Production of Sound by Radiant Energy. 471 
This curious fact was independently observed in England by 
Mr. Preece, and it led him to question whether, in our experi- 
ments with thin diaphragms, the sound heard was due to the 
vibration of the disk or (as Professor Hughes had suggested) 
to the expansion and contraction of the air in contact with the 
disk confined in the cavity behind the diaphragm. In his 
paper read before the Royal Society on the 10th of March, Mr. 
Preece describes experiments from which he ¢laims to have 
proved that the effects are wholly due to the vibrations of the 
confined air, and that the disks do not vibrate at all. 
I shall briefly state my reasons for disagreeing with him in 
this conclusion : 
1 en an intermittent beam of sunlight is focussed upon 
a sheet of hard rubber or other material, a musical tone can be 
heard, not only by placing the ear immediately behind the 
part receiving the beam, but by placing it against any portion 
of the sheet, even though this may be a foot or more from the 
place acted upon by the light. 
2. When the beam is thrown upon the diaphragm of a 
“Blake Transmitter,’ a loud musical tone is produced by a 
diaphragm. 
1 1s evident, therefore, that in the case of thin disks a real wibra- 
hon of the diaphragm is caused by the action of the intermittent 
beam, independently of any expansion and contraction of the air 
confined in the cavity behind the diaphragm. 
ord Rayleigh has shown mathematically that a to-and-fro 
vibration, of sufficient amplitude to produce an audible sound, 
would result from a periodical communication and abstraction 
of heat, and he says: “ We may conclude, I think, that there 
Is at present no reason for discarding the obvious explanation 
that the sounds in question are due to the bending of the plates 
under unequal heating.” (Nature, xxiii, p. 
r. Preece, however, seeks to prove that the sonorous effects 
cannot be explained upon this supposition; but his experi- 
mental data are not sufficient to support his conclusion. Mr. 
Preece expected that if Lord Rayleigh’s explanation was cor- 
rect, the expansion and contraction of a thin strip under the 
influence of an intermittent beam could be caused to open and 
close a galvanic circuit so as to produce a musical tone from a 
telephone in the circuit. But this was an inadequate way to : 
