PHILOSOPHICAL SOCIETY OF WASHINGTON. 147 



" Verified the above, but think it somewhat louder than when 

 used without lamp-black."— 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 resulted 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-tube, one end of which was closed by the dia- 

 phragm. The sound was found to be more audible through the 

 free air when the ear was placed as near to the lamp-black surface 

 as it could be brought without shading it. 



At the time of my communication to the American Association 

 I had been unable to satisfy myself that the substances which had 

 become sonorous under the direct influence of intermittent sunlight 

 were capable of reproducing 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 sounds emitted by thin diaphragms and tubes were so 

 feeble that it was impracticable to produce audible effects from 

 substances in these conditions at any considerable distance away 

 from the transmitter; 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 lamp-black have 

 enabled us to demonstrate the feasibility of using this substance in 

 an articulating photophone in place of the electrical receiver for- 

 merly employed. 



The drawing (Fig. 2) illustrates the mode in which the experi- 

 ment was conducted. The diaphragm of the transmitter (A) was 

 only 5 centimeters in diameter, the diameter of the receiver (B) 

 was also 5 centimeters, and the distance between the two was 40 

 meters, or 800 times the diameter of the transmitter diaphragm. 

 We were unable to experiment at greater distances without a heli- 

 ostat on account of the difficulty of keeping the light steadily 

 directed on the receiver. Words and sentences spoken into the 

 transmitter in a low tone of voice were audibly reproduced by the 

 lamp-black receiver. 



In Fig. 3 is shown a mode of interrupting a beam of sunlight 

 for producing distant effects without the use of lenses. Two sim- 

 ilarly-perforated disks are employed, one of which is set in rapid 

 rotation, while the other remains stationary. This form of inter- 



