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 comjiletely dark, — it nei^er is 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- 



* Amoncf the earliest difficulties which seemed to attend the con- 

 ception of the wave theory, was the consideration, which appeared so 



