316 DU. 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, therefore, 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 undulatory 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. 
