146 BULLETIN OF THE 



exposed to an intermittent beam of sunlight. The sound produced 

 was much louder than any heard before. 



Upon smoking a piece of plate-glass, and holding it in the inter- 

 mittent beam with the lamp-black surface towards the sun, the 

 sound produced was loud enough to be heard, with attention, in 

 any part of the room. With the lamp-black surface turned from 

 the sun the sound was much feebler. 



Mr. Tainter repeated these experiments for me immediately upon 

 my return to Washington, so that I might verify his results. 



Upon smoking the interior of the conical cavity shown in Figure 

 I, and then exposing it to the intermittent beam, with the glass lid 

 in position as shown, the effect was perfectly startling. The sound 

 was so loud as to be actually painful to an ear placed closely against 

 the end of the hearing-tube. 



The sounds, however, were sensibly louder when we placed some 

 smoked wire gauze in the receiver, as illustrated in the drawing, 

 Figure I. 



When the beam was thrown into a resonator, the interior of 

 which had been smoked over a lamp, most curious alternations of 

 sound and silence were observed. The interrupting disk was set 

 rotating at a high rate of speed, and was then allowed to come 

 gradually to rest. An extremely feeble musical tone was at first 

 heard, which gradually fell in pitch as the rate of interruption grew 

 less. The loudness of the sound produced varied in the most in- 

 teresting manner. Minor reinforcements were constantly occurring, 

 which became more and more marked as the true pitch of the re- 

 sonator was neared. When at last the frequency of interruption 

 corresponded to the frequency of the fundamental of the resonator, 

 the sound produced was so loud that it might have been heard by 

 an audience of hundreds of people. 



The effects produced by lamp-black seemed to me to be very 

 extraordinary, especially as I had a distinct recollection of experi- 

 ments made in the summer of 1880 with smoked diaphragms, in 

 which no such reinforcement was noticed. 



Upon examining the records of our past photophonic experiments 

 we found in vol. vii, p. 57, the following note : 



" Experiment V. — Mica diaphragm covered with lamp-black on 

 side exposed to light. 



" Result : distinct sound about same as without lamp-black. — 

 A. G. B., July 18th, 1880. 



