148 BULLETIN OF THE 



rupter is also admirably adapted for work with artificial light. 

 The receiver illustrated in the drawing consists of a parabolic re- 

 flector, in the focus of which is placed a glass vessel (A) containing 

 lamp-black, or other sensitive substance, aud connected with a hear- 

 ing-tube. The beam of light is interrupted 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(C) about its axis (D.) 



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

 reflector like that recommended by Prof. Sylvanus 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 color are two conditions that markedly influence the inten- 

 sity 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 colors. 



The materials from which the best effects have been produced are 

 cotton-wool, worsted, fibrous materials generally, cork, sponge, 

 platinum and other metals in a spongy condition, and lamp-black. 



The loud sounds produced from such substances may perhaps be 

 explained in the following manner : Let us consider, for example, the 

 case of lamp-black — a substance which becomes heated by exposure 

 to rays of all refrangibility. 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 lamp- 

 black are heated, and consequently expand, causing a contraction 

 of the air-spaces or pores among them. 



Under these circumstances a pulse of air should be expelled, just 

 as we would squeeze out water from a sponge. 



The force with which the air is expelled must be greatly increased 

 by the expansion of the air itself, due to contact with the heated 

 particles of lamp-black. When the light is cut off the converse 

 process takes place. The lamp-black particles cool and contract, 

 thus enlarging the air spaces among them, and the enclosed air also 

 becomes cool. Under these circumstances a partial vacuum should 



*Phil. Mag., April, 1881, vol. xi, p. 286. 



