166 MINERALOGY 



Oil of cedar . . . . 1.520 



Oil of cloves 1.540 



Canada balsam 1.548 



Carbon bisulphide 1.627 



Methylene iodide 1.742 



Fluorite 1.423 



Potash alum 1.456 



Crown glass 1.515 



Rock salt 1.544 



Garnet 1.807 



Spinel (Chrome) 2.096 



Diamond 2.467 



Proustite 3.08 



The index of refraction of any substance is different for light of 

 different wave lengths, and also varies slightly with the temperature; 

 as regards the wave length, it is inversely as X 2 , or for light of long 

 wave length, as red, n the index of refraction is less and the angle 

 of refraction r would be greater than in case of violet light, with a 

 short wave, or the violet ray would be bent more in entering the 

 water, Fig. 310, than the red ray. The violet ray would lie nearer 

 the normal than the red ray. This division of white light into 

 colors is known as dispersion. 



The relative velocity of light in any substance is the reciprocal of 



y 



the index of refraction, as, n = 7 ; where v, the velocity in air, is 



Violet light, which has a larger index of refraction in water, will 

 travel more slowly than red. 



Critical angle. In the equation, n = - , sin i may have any 



value between zero and one ; when all values are considered, there 

 are two special cases, those of the limiting values, o and i . In the 

 case when sin i = i, or the angle of incidence is 90, Fig. 311, then 



n = - or sin r = - ; substituting the value of n in case of water, 

 sin r n 



sin r = - , or the angle r is 48 36'. 

 1-333 



When light is traveling along the surface SS', that entering at any 

 point o will take the direction oai, in which the angle aioni is 48 36'. 



