PRODUCTION OF SOUND BY RADIANT ENERGY. 195 



then pulled taut at right angles to the plane of the diaphragm. When the inter- 

 mittent beam was focused upon the strip (A), a clear musical tone could he heard 

 by applying the ear to the hearing-tube (C). 



Fig. 5. 



This seemed to indicate a rapid expansion and contraction of the substance 

 under trial. 



But a vibration of the diaphragm (B) would also have resulted if the thin 

 strip (A) had acquired a to-and-fro motion, due either to the direct impact of 

 the beam or to the sudden expansion of the air in contact with the strip. 



2. To test whether this had been the case, an additional strip (D) was attached 

 by its central point only to the strip under trial, and was then submitted to the 

 action of the beam, as shown in Fig. 6. 



It was presumed that, if the vibration of the diaphragm (B) had been due to 

 Si pushing force acting on the strip (A), the addition of the strip (D) would not 

 interfere with the effect ; but, if, on the other hand, it had been due to the lougi- 



Fig. 6. 



tudinal expansion and contraction of the strip (A), the sound would cease, or at 

 least be reduced. The beam of light falling upon the strip (D) was now inter- 

 rupted as before by the rapid rotation of a perforated disk, which was allowed 

 to come gradually to rest. 



l\o sound was heard excepting at a certain speed of rotation, when a feeble 

 musical tone became audible. 



