268 Dr. Brewster on the laws of polarisation, &c. 
tion ; and the necessity that it should have its resultant axes 
considerably inclined to each other, in order to obtain a 
measurable separation of the images at several points, be- 
tween these resultant axes, rendered all my experiments for 
a long time completely unsuccessful. The discovery, how- 
ever, of crystals which possessed, in some degree, the most 
important of these requisites, has enabled me to resume and 
to complete the investigation. 
After ascertaining the position of the diameters of no 
polarisation in a crystal with two axes, I formed a prism 
with a great refracting angle, so as to have a flat surface as 
perpendicular as possible to one of these diameters. I then 
placed this prism upon the goniometer, and having marked 
the position when the point of no polarisation, or the 
centre of the rings (namely P, Plate xv. fig. 4.) was 
coincident with the polarised image of a candle, I substi- 
tuted the direct image of a candle, and observed that this 
image was single, and therefore that the force of double 
refraction, as well as the polarising force, had completely 
vanished. Upon turning the goniometer to both sides of 
these positions, the deviation of the extraordinary ray became 
perceptible and gradually increased ; and in continuing to 
observe this deviation in the plane COD passing through the 
resultant axes, I found that it increased to the middle point O 
between these axes, where it became stationary ; and that it 
again gradually diminished towards the other resultant axis P' 
where the image again became single. I now measured the 
deviation of the extraordinary ray at the points O, A, C, 
and at various other points, both in and out of these 
rectangular directions, and I found that the force of double. 
