1904.] on Shadows. 429 



way, upon a lantern plate ; and from this Fig. 1 has been repro- 

 duced. 



It is at once evident that at the centre of the shadow, where one 

 would expect the darkness to be most complete, there is a distinct 

 bright spot. This result has always been considered a valuable 

 confirmation of the wave theory of light. 



I now propose to speak of acoustic shadows — shadows thrown by 

 sound. The most suitable source of sound for the following experi- 

 ments is the bird-call,* which emits a note of high pitch — so high, 

 indeed, that it is inaudible to most elderly people. The sound emitted 

 has two characteristics, valuable for our purpose — the wave length 

 is very short; and the sound is thrown forward, without too much 

 tendency to spread, thus differing from sounds produced by most other 

 means. 



Since the sound emitted is nearly inaudible, some objective method 

 of observing it is required. For this purpose we may utilise the 

 discovery of Barrett and Tyndall, that a gas flame issuing under some- 

 what high pressure from a pin-hole burner flares when sound waves 

 impinge on it, but recovers and burns steadily when the sound ceases. 

 The sensitiveness of the flame depends on the pressure of the gas, 

 which should be adjusted so that flaring just does not occur in the 

 absence of sound. If the bird-call is directed towards the sensitive 

 flame, the latter flares so long as the call is sounded and no obstacle 

 intervenes. On interposing the hand about midway between the two, 

 the flame recovers and burns steadily. Thus the sound emitted by 

 the bird-call casts a shadow, and to this extent resembles light. 



It will now be shown that the sensitive flame flares when it is 

 placed at the centre of the acoustic shadow thrown from a circular 

 disc, but recovers in any other position within the shadow ; thus 

 proving that there is sound at the centre of the shadow, although at 

 a small distance from this point there is silence. The part of the 

 flame which is sensitive to sound is that just above the pin-hole orifice, 

 so that it is necessary to arrange the bird-call, the centre of the disc, 

 and the pin-hole orifice in a straight line. For the disc, it is con- 

 venient to use a circular plate of glass about 18 inches in diameter 

 with a piece of black paper pasted over its middle portion, a small 

 hole being cut in the paper exactly at the centre of the disc. The 

 glass disc is hung by two wires, and the positions of the bird-call and 

 sensitive flame can be adjusted by sighting through the hole in the 

 paper. If the disc is caused to oscillate in its own plane, the flame 

 flares every time that the disc passes through its position of equi- 

 librium, and recovers whenever the disc is not in that position. The 

 analogy between this experiment, and that in which a bright spot is 

 formed at the centre of the optical shadow of a small disc, is suffi- 

 ciently obvious. 



The approximate theory of the shadow of the circular disc is 



* See * Proc. Boy. Inst.,' Jan. 17, 1902. 



2 G 2 



