1 56 Dr. Brewster on the action of 
56® 16', the change of polarisation is about 40°; and in the 
azimuth of o°, where the polarising angle is 54, 0 18', the 
change of polarisation was a little more than 90° when the 
obtuse angle was farthest from the eye, and about 10 6° when 
the obtuse angle was nearest the eye. In all these positions 
the image reflected from the surface of the oil and the spar, 
is nearly as bright as that from the surface of the prism and 
the oil. 
In order to determine the change of polarisation when the 
plane of reflection was perpendicular to the axis, it was neces- 
sary to have a prismatic crystal of calcareous spar with a 
polished summit ; but I have always found this summit rough 
and unpolished. There was therefore no alternative but to 
polish an artificial face cut in this direction ; and upon the 
application of oil of cassia, I found that in every azimuth the 
change of polarisation was about 75°. The colour of the 
image was a bright yellow, and a little blue light remained at 
the point of evanescence. 
In extending these experiments to other crystals I have 
obtained similar results ; but there are none so well fitted for 
this species of examination as calcareous spar. In apptying 
oil of cassia to a very fine prism of chromate of lead, the 
direction of the polarisation was not in the slightest degree 
altered, as the ordinary action was not sufficiently weakened 
to render visible the influence of the interior force. When 
the plane of reflection passed through the axis of the prism, 
blue light remained in the vanishing image ; but in a plane 
rectangular to this, the light was completely polarised, as in 
the experiment when the reflecting surface was in contact 
with air. 
