316 DR. BREWSTER ON COLOURS PRODUCED BY GROOVED SURFACES. 



of the metal, we should witness the periodical colours without any grooves at 

 all on the reflecting surface ; so that the phenomena would then become iden- 

 tical with those which are developed at the separating surface of transparent 

 bodies. 



We can scarcely, therefoi-e, avoid the conclusion, that the removal of the 

 substance from the grooves, whether they are made on metal or on transparent 

 bodies, diminishes the refractive power of the intermediate spaces. On the 

 hypothesis of emission, this abstraction of the reflecting matter may be re- 

 garded as equivalent to a diminution of the density of the surface ; while on 

 the imdulatory hypothesis, the effect may be ascribed to the condition of the 

 ether arising from a variation in its density or elasticity towards the extremities 

 of a number of salient points. 







ot ht' 



