510 NOTES TO BOOK IX. 



center, and many other phenomena, were studied with the 

 greater attention, because the general theory was deemed 

 so convincing : and the same cause makes the remaining 

 exceptions objects of intense interest to astronomers and 

 mathematicians. The mathematicians and optical ex- 

 perimenters who accept the undulatory theory, will of 

 course follow out their conviction in the same manner. 

 Accordingly, this has been done and is still doing, as in 

 Mr. Airy's mathematical investigation of the effect of 

 an annular aperture ; Mr. Earnshaw's, of the effect of a 

 triangular aperture; Mr. Talbot's explanation of the 

 effect of interposing a film of mica between a part of 

 the pupil and the pure spectrum, so nearly approaching 

 to the phenomena which have been spoken of as a new 

 Polarity of Light ; besides other labours of eminent ma- 

 thematicians, elsewhere mentioned in these pages. 



The phenomena of the absorption of light have no 

 especial bearing upon the undulatory theory. There is 

 not much difficulty in explaining the possibility of absorp- 

 tion upon the theory. When the light is absorbed, it 

 ceases to belong to the theory. 



For, as I have said, the theory professes only to 

 explain the phenomena of radiant visible light. We 

 know very well that light has other bearings and pro- 

 perties. It produces chemical effects. The optical polar- 

 ity of crystals is connected with the chemical polarity of 

 their constitution. The natural colours of bodies, too, 

 are connected with their chemical constitution. Light is 

 also connected with heat. The undulatory theory does 

 not undertake to explain these properties and their con- 

 nexion. If it did, it would be a Theory of Heat and 

 of Chemical Composition, as well as a Theory of Light. 



