258 W. L. Steoens — Sensitive Flame as a means of Research. 



easy to make a sound shadow manifest. The difficulty in 

 sound is not to produce diffraction but rather to limit it 

 by using' the shortest wave-lengths possible. The pitch em- 

 ployed by Lord Rayleigh was more than 20,000 vibrations per 

 second, corresponding to a wave-length of less than two-thirds 

 of an inch. To measure this the waves are reflected from a 

 surface arranged vertically across the direction of propagation, 

 thus- producing interference with the direct waves. The posi- 

 tion of the nodes and ventral segments is determined by mov- 

 ing the reflector toward or from a sensitive flame interposed 

 between it and the source of sound. The flame flares in a ven- 

 tral segment and burns quietly at a node. The distance be- 

 tween two points of quiescence is a half wave-length, from 

 which the pitch is readily computed. Knowing the wave- 

 length, if this be small in comparison with the diameter of an 

 obstacle such as a disk, it is possible to calculate the deflection 

 necessary for the meeting of secondary waves behind it, from its 

 opposite edges, in order to produce a maximum or minimum 

 of intensity. In this way, as much as eight or nine years ago, 

 Lord Rayleigh repeated acoustically the. celebrated experiment 

 suggested by Poisson to Fresnel, and first performed by Arago, 

 by which a bright point was found at the middle of the shadow 

 of a small disk. Applying the formula for Huygens's zones, an 

 acoustic diffraction grating was made by which sound was con- 

 verged to a focus, as if by a lens, the flaring of the flame at 

 this focus being very violent. Around it, according to theory, 

 there should be seveial successive rings of motion and qui- 

 escence, or, in other words, of noise and silence. The first 

 ring of noise, and the rings of silence that precede and follow 

 it, are detected without difficulty by means of the sensitive 

 flame. 



All of these experiments by Lord Rayleigh have been re- 

 peated by me. The source of sound used is Galton's adjust- 

 able whistle, through which a blast is sent from a cylinder of 

 compressed air or oxygen. The sensitive flame is fed from a 

 similar cylinder of compressed coal gas, the pressure of the 

 supply being carefully regulated in each case by means of a 

 water manometer gauge. The whistle is capable of giving a 

 pitch as high as 18,000 or 20,000, but as this limit is approached 

 the intensity becomes too much diminished, and practically the 

 best pitch it yields is about 13,000 vibrations per second. Lord 

 Rayleigh' s whistle is slightly different in construction, and 

 probably better than the Galton whistle. But there is no 

 difficulty in attaining good results with this pitch. The great- 

 est practical difficulty is that of keeping the sensitiveness of 

 the flame exactly right, the slightest variation of pressure 

 making it inconstant, and causing it to give misleading indica- 



