658 Mr. stokes, on the formation of the central spot, etc. 



contact when the incident light is polarized at an azimuth of 45", or thereabouts. The theoretical 

 investigation does not present the slightest difficulty in principle, but would lead to rather long 

 expressions ; and as the experiment would be difficult, and is not likely to be performed, there is no 

 occasion to go into the investigation. 



30. In viewing the spot formed between a prism and a lens, I was struck with the sudden, or 

 nearly sudden disappearance of the spot at a considerable angle of incidence. The cause of the 

 disappearance no doubt was that the lens was of lower refractive power than the prism, and that the 

 critical angle was reached which belongs to refraction out of the prism into the lens. Before disap- 

 pearing, the spot became of a bright sky blue, which shows that the ratio of the refractive index of 

 the prism to that of the lens was greater for the blue rays than for the red. As the disappearance 

 of the spot can be observed with a good deal of precision, it may be possible to determine in this 

 way the refractive index of a substance of which only a very minute quantity can be obtained. The 

 examination of the refractive index of the globule obtained from a small fragment of a fusible 

 mineral might afford the mineralogist a means of discriminating between one mineral and another. 

 For this purpose a plate, which is what a prism becomes when each base angle becomes yo", would 

 probably be more convenient than a prism. Of course the observation is possible only when the 

 refractive index of the substance to be examined is less than that of the prism or plate. 



G. G. STOKES. 



