1889. | NEW YORK ACADEMY OF SCIENCES. 131 
physicist therefore can ever be expected for the ‘‘ substantial ” 
theorists. 
Huygens’ principle, that a disturbed particle in an elastic ma- 
terial medium must necessarily become acentre from which wave 
motion is propagated spherically in all directions, was illustrated ; 
and a discussion was given of its application to the phenomena 
of diffraction, which constitute a crucial test of the wave theory, 
whether this be applied to sound, heat, or light. For exhibiting 
diffraction of light, special apparatus is needed ; but, on account 
of the extreme shortness of light-waves, measurements of great 
accuracy can be made upon them. By the use of a diffraction 
grating, the lecturer projected a luminous spectrum on the 
screen, and gave the familiar computation for the length of a 
wave of red light. Sound-waves are so long that their diffrac- 
tive encroachment on sound-shadows almost masks the existence 
of such shadows. ‘To define these, and to make even coarse 
measurements upon the diffraction of sound, tones of very short 
wave-length are needed, and consequently of such high pitch and 
slight intensity that audition becomes difficult. 
An account was given of observations made on sound-shadows 
in water by Colladon in 1826, and Le Conte in 1874; also of 
sound-shadows in air due to dynamite explosions. Lord Ray- 
leigh has recently made some remarkable experiments on dif- 
fractive effects upon aérial sound-shadows, using a high-pitched 
whistle as a source of sound, and a sensitive flame as an in- 
dicator. 
The discovery of sensitive flames by Le Conte in 1857 was 
then recounted, and also the subsequent development of this in- 
dicator by Barrett, Tyndall, and others. The sensitive flame 
was exhibited, and all of the experiments thus far published by 
Lord Rayleigh were repeated. Quite sharply defined sound- 
shadows were produced, wave-lengths in open air were measured, 
the theory of the diffraction-grating for sound was explained, 
and one of these gratings was employed to converge sound by 
diffraction to a well-defined focus, and to exhibit around this 
alternate rings of noise and silence. 
The lecturer concluded by describing experiments which he 
had lately made on the sensitive flame as a means of research, 
