Sound by Radiant Energy. 515 



receiver (B) was also 5 centimetres; and the distance between 

 the two was 40 metres, or 800 times the diameter of the trans- 

 mitting diaphragm. TVe were nnable to experiment at greater 

 distances without a heliostat, 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 lampblack receiver. 



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

 for producing distant effects without the use of lenses. Two 

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

 rapid rotation, while the other remains stationary. This form 

 of interrupter is also admirably adapted for work with artificial 

 light. The receiver illustrated in the drawing consists of a 

 parabolic reflector, in the focus of which is placed a glass 

 vessel (A) containing lampblack or other sensitive substance, 

 and connected with a hearing-tube. The beam of light is inter- 

 rupted by its passage through the two slotted disks shown at 

 B; and in operating the instrument, musical signals like the 

 dots and dashes of the Morse alphabet are produced from the 

 sensitive receiver (A) by slight motions of the mirror (0) 

 about its axis (D). 



In place of the parabolic reflector shown in the figure, a 

 conical reflector like that recommended by Prof. Silvanus 

 Thompson * can be used, in which case a cylindrical glass 

 vessel would be preferable to the flask (A) shown in the 

 figure. 



In regard to the sensitive materials that can be employed, 

 our experiments indicate that in the case of solids the physical 

 condition and the colour are two conditions that markedly 

 influence the intensity of the sonorous effects. The loudest 

 sounds are produced from substances in a loose, porous, spongy 

 condition, and from those that have the darkest or most absorbent 

 colours. 



The materials from which the best effects have been pro- 

 duced are cotton-wool, worsted, fibrous materials generally, 

 cork, sponge, platinum and other metals in a spongy condition, 

 and lampblack. 



The loud sounds produced from such substances may perhaps 

 be explained in the following manner. Let us consider, for 

 example, the case of lampblack — a substance which becomes 

 heated by exposure to rays of all refrangibilities. I look upon 

 a mass of this substance as a sort of sponge, with its pores 

 filled with air instead of water. When a beam of sunlight 

 falls upon this mass, the particles of lampblack are heated, 



* Phil. Mag. April 1881, vol. xi. p. 28G. 



