28 John LeConte—Sound-Shadows in Water. 
3. The contrast, in this respect, between sound and light is 
well expressed by Lord Rayleigh: ‘ en waves of sound im- 
pinge upon an obstacle, a portion of the motion is thrown back 
as an echo, and under cover of the obstacle there is formed a 
sort of sound-shadow. In order, however, to produce shadows 
in anything like optical perfection, the dimensions of the inter- 
vening body must be considerable. The standard of compari- 
son proper to the subject is the wave-length of the vibration; 
it requires almost as extreme conditions to produce rays in the 
case of sound, as it requires in optics to avoid producing them.’* 
In other words, the difference between sound and light results 
from the well-known fact, that an ordinary obstacle bears an 
immense ratio to the length of a wave of light; but does not 
bear a very great ratio to the length of a sound-wave. Hence 
minous source, the d bance vanishes, while at any point 
outside of the geometrical projection, the disturbance is the 
same as if the primary 1 the screen unimpeded 
waves, in consequence of their considerable length; it is rigor- 
ously true only when, as in optics, the diameter of the obstacle 
is large in comparison with the wave-length. 
4. e are, however, other causes depending upon the 
differences between the sense of hearing and of sight, which 
. “Tn 
* “Theory of Sound,” vol. ii, p. 106, art. 283. London, 1878. 
+ Phil. Mag., 5th series, vol. iii, p. 458, 1877. 
