IN CRYSTALLIZED MEDIA. 355 



T [v 



COS0 



<— (cos Q cos 0' + sin Q sin 0') - cos ^ cos — sin sin q\ 

 — -^ |— cos Q cos 0' - cos d) cos b\ 



= T| — COS0' — COS0> 



= rl^ -s/l - sin-(^' - cos ^1 



and if ?;i be the velocity of the other wave, its retardation is 



the angle of emergence being supposed the same for both. 

 Hence the difference of retardation is 



t(^ _ M = 7^ Jl _ U nearly. 



4. This hypothesis that the two waves are moving parallel to 

 each other at emergence, is clearly not compatible with the hypothesis 

 that they have the same normal within the crystal. 



If v-> be the velocity of the wave which has a common normal 

 witli that whose velocity is t?,, we have 



retardation of this = TJ— cos^' - cos0> 



= r{jcos,//-x/l-^sin^<^'|. 



Vol.. VI. Part II. Z z 



