356 Mr KELLAND, ON THE TRANSMISSION OF LIGHT 



and the difference of retardation is 



= Tvcosd)'\ A 



= Tv I -\ i{ (p' be very small; 



hence the hypothesis that the angle of incidence is small, reduces this 

 case to the same form as the former, and we may in such circum- 

 stances consider the difference of the retardation as proportional to the 

 difference between tlie two refractive indices. 



5. In the applications of this formula, we must introduce the 

 relations which are given by tlie constitution of the crystal determined 

 by the passage of light through it. Such relations must, I conceive, 

 depend on the refractive energies of the crystal in different directions. 



Now the refractive energy has undoubtedly no connexion whatever 

 with the velocities of transmission of the rays, since these velocities 

 are merely nominal ones ; that is, they are not estimated in the direc- 

 tion in which the effect is transmitted. Indeed, I do not suppose we 

 have auy notion of these velocities independent of theory, whilst the 

 velocity of the wave is a physical motion, apart from the idea wliich 

 is suggested by the expression. 



I have been imder the necessity of giving the term radial to 

 M. Fresnel's axes, since they are not at all the same thing as the 

 optic axes. M. Fresnel himself remarks, that "although the difference 

 between them is very slight in almost all crystals, there are sOme 

 where it becomes more sensible, and in which we must not confound 

 the two." 



6. We are concerned only with waves which have a common 

 direction in air, and must consequently assume that the difference of 

 the velocities of tlie two corresponding refracted waves, is very nearly 

 the same as the difference of the velocities of two waves which travel 



