crystallized surfaces upon light. 
1 59 
(p = angle of incident ray with the axis ; and 
C = change in the direction of the polarisation, 
we shall have 
Sin. jC = \/ Sin. <p. 
If we make 
A = complement of the inclination of the reflecting plane 
to the axis ; 
a. — azimuth of the plane of incidence with the principal 
section ; and 
i = angle of incidence reckoned from the perpendicular, 
we shall have 
Cos. « x Tang. A =Tang. z , and 
Cos. ® = P os - A x c <»- 
Y Cos. z. 
In one of the ordinary rhomboidal surfaces where the in- 
clination to the axis is 45 0 23 j # , A = 44 0 36 j # ; and with oil 
of cassia for the incidence of the mean ray, when the polari- 
sation is complete, is about 45 0 17'. I have assumed it at 
45° ( which will be more correct for the mean luminous 
ray than 45 0 17') for the purpose of making the change of 
polarisation commence with zero in o° of azimuth. 
Upon these principles I have computed the following table, 
which shows the change in the direction of the polarisation, 
corresponding to any azimuth and any inclination of the inci- 
dent ray with the axis. 
