514 Prof. A. G. Boll on the Production of 



and more marked as the true pitch of the resonator was 

 neared. When at last the frequency of interruption cor- 

 responded 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 lampblack seemed to me bo very 

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

 periments made in the summer of 1880 with smoked dia- 

 phragms, in which no such reinforcement was noticed. 



Upon examining the records of our past photophonic ex- 

 periments, 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. £., July 18t7i, 1880. 



" Verified the above, but think it somewhat louder than 

 when used without lampblack. — S. T., July 18th, 1880." 



Upon repeating this old experiment we arrived at the same 

 result as that noted. Little if any augmentation of sound re- 

 sulted from smoking the mica. In this experiment the effect 

 was observed by placing the mica diaphragm against the ear, 

 and also by listening through a hearing T tube, one end of 

 which was closed by the diaphragm. The sound was found 

 to be more audible through the free air when the ear was 

 placed as near to the lampblack surface as it could be brought 

 without shading it. 



At the time of my communication to the American Asso- 

 ciation I had been unable to satisfy myself that the substances 

 which had become sonorous under the direct influence of inter- 

 mittent sunlight were capable of reproducing the sounds of 

 articulate speech under the action of an undulatory beam from 

 our photophonic transmitter. The difficulty in ascertaining 

 this will be understood by considering that the sound emitted 

 by thin diaphragms and tubes were so feeble that it was im- 

 practicable to produce audible effects from substances in these 

 conditions at any considerable distance away from the trans- 

 mitter ; but it was equally impossible to judge of the effects 

 produced by our articulate transmitter at a short distance away, 

 because the speaker's voice was directly audible through the 

 air. The extremely loud sounds produced from lampblack 

 have enabled us to demonstrate the feasibility of using this 

 substance in an articulating photophone in place of the elec- 

 trical receiver formerly employed. 



The drawing, fig. 2, illustrates the mode in which the ex- 

 periment was conducted. The diaphragm of the transmitter 

 (A) was only 5 centimetres in diameter; the diameter of the 



