244 



SCIENCE. 



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 lamp-black re- 

 ceiver. 



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 w : ith 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 lamp- 

 black or other sensi- 

 tive 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 (C) about its axis (D). 



In place of the parabolic reflector shown in the figure, 

 a conical reflector like that recommended by Prof. Syl- 

 vanus Thompson! can be used, in which case a cylin- 

 drical glass vessel would be preferable to the flask (A) 

 shown in the figure. 



In regard to the sensitive materials that can be em- 

 ployed, our experiments indicate that in the case of 

 solids the physical con- 

 dition and the color are 

 two conditions that 

 markedly influence the 

 intensity of the sonorous 

 effects. The loudest 

 sounds are produced 

 from substances in a 

 loose, -porous, spongy con- 

 dition, 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, fi- 

 brous materials gener- 

 ally, cork, sponge, plat- 

 inum 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 sub- 

 stance 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 ex- 

 pelled, 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 



♦See page 247 for illustrations, 

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



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 be formed among the particles, and the outside 

 air would then be absorbed, as water is by a sponge when 

 the pressure of the hand is removed. 



I imagine that in some such manner as this a wave of 

 condensation is started in the aimosphere each time a 

 beam of sunlight falls upon lamp-black, and a wave of 



rarefaction is origin- 

 ated w^hen the light 

 is cut off. We can 

 tints understand how 

 it is that a sub- 

 stance like lamp- 

 black produces in- 

 tense sonorous vibra- 

 tions in the sur- 

 rounding air, while 

 at the same time it 

 communicates a very 

 feeble vibration to 

 the diaphragm or 

 solid bed upon which 

 Fig. 5. it rests. 



This curious fact was independently observed in Eng- 

 land by Mr. Preece, and it led him to question whether, 

 in our experiments with thin diaphragms, the sound 

 heard was due to the vibration of the disk or (as Prof. 

 Hughes had suggested) to the expansion and contraction 

 of the air in contact with the disk confined in the cavity 

 behind the diaphragm. In his paper read before the 

 Royal Society on the 10th of March, Mr. Preece describes 

 experiments from which he claims to have proved that 

 the effects are wholly due to the vibrations of the con- 

 fined air, and that the disks do not vibrate at all. 



I shall briefly state my 

 reasons for disagreeing 

 with him in this conclu- 

 sion : 



1. When an intermit- 

 tent beam of sunlight is 

 focussed upon a sheet of 

 hard rubber or other 

 material, a musical tone 

 can be heard, not only 

 by placing the ear im- 

 mediately behind the 

 part receiving the beam, 

 but by placing it against 

 any portion ot the sheet, 

 even though this may be 

 a foot or more from the 

 place acted upon by the 

 light. 



2. When the beam is thrown upon the diaphragm of 

 a "Blake Transmitter," a loud musical tone is produced 

 by a telephone connected in the same galvanic circuit 

 with the carbon button (A) Fig. 4.* Good effects are also 

 produced when the carbon button (A) forms, with the 

 battery (B), a portion of the primary circuit of an induc- 

 tion coil, the telephone (C) being placed in the secondary 

 circuit. 



In these cases the wooden box and mouth- piece of the 

 transmitter should be removed, so that no air-cavities 

 may be left on either side of the diaphragm. 



It is evident, therefore, that in the case of thin disks 

 a real vibration of the diaphragm is caused by the ac- 

 tion of the intermittent beam, independently of any ex- 

 pansion and contraction of the air confined in the cav- 

 ity behind the diaphragm . 



Lord Rayleigh has shown mathematically that a to-and- 

 fro vibration, of sufficient amplitude to produce an audible 



* Sec page 248 for illustrations. 



