DR. BREWSTER ON REFLECTED LIGHT. 195 



sented by the superposed images when the film of fluid has its surfaces parallel. 

 If the two prisms A, B give separately the same periods of colours, but at dif- 

 ferent angles of incidence, then the resulting tints are very irregular and indi- 

 stinct ; but if the maxima of the periods produced by one prism coincide with 

 the minima of the periods produced by the other, the colours will be almost 

 wholly obliterated, though it is not easy to ensure the condition on which this 

 compensation depends. When the separate prisms give exactly the same pe- 

 riods at the same angles of incidence, then the minima of the one will corre- 

 spond with the minima of the other, and the maxima with the maxima ; so that 

 the combination jjroduces the same periods of colours that were produced by 

 each prism separately ; but the intensity of the tints is doubled. This duplica- 

 tion of the tints is easily observed by bisecting a prism which produces distinct 

 periods, and separating the two halves by a fluid film. 



Although the preceding experiments are sufficient to establish the existence, 

 and explain the nature of this class of phsenomena, yet, as they will probably 

 lead to very important consequences in the theory of light, I shall make no 

 apology for giving an account of another series, of a very instructive kind, and 

 performed with fluids particularly fitted for the investigation. I continued to 

 use the same prisms of plate glass ; but as the oil and balsam formerly employed 

 dififered considerably in refractive power from the glass, I sought for two oils 

 with nearly the same mean refraction as the prisms ; and those which I select- 

 ed were oil of cummin and distilled wood oil*, which were fortunately capable 

 of being mixed together with great facility. Their refractive powers for the 

 mean yellow rays were nearly as follows : 



Indices of Refraction. 



Oil of cummin 1.512 



Plate glass, prism B 1.510 



Oil of cummin and wood oil mixed 1.5085 



Plate glass, prism A . 1.508 



Wood oil 1.506 



As nothing depends on the numerical accuracy of these indices, I did not 

 measure them with any peculiar attention ; but by immersing a right angle 



• This oil was sent to me from the East Indies by Geoege Swinton, Esq., Secretary to the Go- 

 vernment at Calcutta. 



2 C2 



