DIVERGENCE OF LIGHT. 257 
be regarded further as a striking mark of respect towards 
the great man whose name, so to speak, has been identi- 
fied with the theory which they think ought to be re- 
jected. As to the theory of waves, the Newtonians have 
not done it the honour to discuss it with the same detail ; 
it has seemed to them that a single objection was suffi- 
cient to annihilate it; and this objection they have drawn 
from the manner in which sound is propagated in air. If 
light, they say, is a vibration like the vibrations of sound, 
it will be transmitted in all directions; just as we hear 
the sound of a distant bell when we are separated from it 
by a screen which conceals it from our eyes, in the same 
way we ought to perceive the light of the sun behind 
every kind of opaque body. Such are the terms to 
which we must reduce the difficulty, for analogy does not 
permit us to say that light ought to extend itself behind 
screens without losing some of its intensity ; since sound 
itself, as every one knows, does not penetrate obstacles 
without being enfeebled in a sensible degree. Thus, in 
speaking of the extension of light into the geometrical 
shadow of a body as an insurmountable difficulty, New- 
ton and his adherents certainly did not suspect the answer 
which it would bring with it; yet this answer is direct 
and simple. You maintain that the luminous vibrations 
ought to extend into the shadow,—they do so. You say 
that in the system of waves, the shadow of an opaque 
body can never be completely dark,—it never ts so. It 
includes a number of rays which give rise to a multitude 
of curious phenomena, of which you may have some 
knowledge, since Grimaldi perceived them in part so 
long ago as before 1633.* Fresnel,—and here is incon- 
* Among the earliest difficulties which seemed to attend the con- 
ception of the wave theory, was the consideration, which appeared so 
