ON LIGHT. 



245 



the " refractive index " of that medium be ascer- 

 tained. 



\ 



Fig. a. 



(29.) When refraction takes place out of any 0;/ trans- 

 parent medium into any other in close and perfect con- 

 tact with it such contact as exists, for instance, be- 

 tween a fluid and a solid that it wets, or between two 

 fluids of different specific gravities, which do not mix, 

 resting the one on the other experiment shows that, so 

 far as the mere direction of the refracted ray is concerned, 

 it is the same as if the two media were separated by an 

 exceedingly thin film of air. In that case, the same per- 

 pendicular being common to both surfaces at the point 

 of contact, the angle of refraction out of the first medium 

 is the same with that of incidence on the second. And 

 from this it results that the proportion of the sine of in- 

 ternal incidence on the surface of the first to that of 



