PRODUCTION OF SOUND BY RADIANT ENERGY. 193 



thin diaphragms, the sound heard was due to the vibration of the disk 

 or (as Professor Hughes had suggested) to the expansion and contrac- 

 tion 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 claims 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 : 



^ TVhen an intermittent beam of sunlight is focused 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 telephone connected in the same galvanic 

 circuit with the carbon button (A), Fig. 4. Good effects are also produced when 

 the carbon button (A) forms, with the battery (B), a portion of the primary cir- 

 cuit of an induction-coil, the telephone (C) being placed in the secondary circuit. 



In these cases the wooden box and mouth-piece of the transmitter should be 

 removed, so that no air-cavities may be left on either side of the diaphragm. 



It is evide7itj therefore, that in the case of thin disks a real vibra- 

 tion 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. 



Lord Rayleigh has shown mathematically that a to-and-fro vibra- 

 tion, of sufiicient 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 dis- 

 carding the obvious explanation that the sounds in question are due to 

 the bending of the plates under unequal heating " (" Nature," vol. xxiii, 

 p. 274). Mr. Preece, however, seeks to prove that the sonorous effects 

 can not be explained upon this supposition ; but his experimental proof 

 is inadequate to support his conclusion. Mr. Preece expected that, if 

 Lord Ray leigh's explanation was correct, 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 test the point at issue, for Lord Rayleigh has shown (" Proceedings 

 of the Royal Society," 1877) that an audible sound can be produced 

 by a vibration whose amplitude is less than a ten-millionth of a centi- 

 metre, and certainly such a vibration as that would not have sufliced to 

 operate a " make-and-break contact" like that used by Mr. Preece. 

 The negative results obtained by him can not, therefore, be considered 

 conclusive. 



The following experiments (devised by Mr. Tainter) have given re- 



VOL. XIX. 13 



