light at the surfaces of different media. 217 



briefly notice the phenomena which are presented by the su- 

 perposed images when the film of fluid has its surfaces paral- 

 lel. If the two prisms A, B give separately the same periods 

 of colours, but at different angles of incidence, then the re- 

 sulting tints are very irregular and indistinct ; 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 obUterated, though it is not easy to in- 

 sure the condition on which this compensation depends. 

 When the separate prisms give exactly the same periods at 

 the same angles of incidence, then the minima of the one will 

 correspond with the minima of the other, and the maxima 

 with the maxima ; so that the combination produces the same 

 periods of colours that were produced by each prism sepa- 

 rately ; but the intensity of the tints is doubled. This du- 

 plication 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 esta- 

 blish the existence, and explain the nature of this class of phe- 

 nomena, yet, as they will probably lead to very important con- 

 sequences 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 investiga- 

 tion. I continued to use the same prisms of plate glass ; but 

 as the oil and balsam formerly employed differed 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 selected 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 



• This oil was sent to me from the East Indies by George Swinton, 

 Esq, Secretary to the Government at Calcutta. 



