242 Dr. Brewster on the laws of polarisation, &c. 
In determining the tints by new experiments, I adopted 
two different methods of investigation. By one of these, which 
was analogous to that of M. Biot, I observed the tints for all 
angles of refraction up to 6o°, and by another mode I observed 
the remaining tints up to 90°, and I endeavoured to avoid as 
much as possible the secondary effects produced by obliquity. 
The first of these methods is shown in PL xvi.fig. 11, where 
ABCD is a plate of sulphate of lime, whose natural surfaces, 
AB, CD, are parallel to the plane passing through the resultant 
axes. Upon these surfaces I cemented two prisms of crown glass 
M, N, by a thin and equal film of Canada balsam, and having 
placed them upon the goniometer, I was able to observe the 
tints with great correctness at angles of refraction considerably 
beyond those which could be obtained in air. At angles of re- 
fraction between 6o° and go°, I resorted to the second method, 
which consisted in transmitting the polarised light through 
the parallel faces AD, AC. This method is attended with 
peculiar difficulties; and much perplexing labour must be 
submitted to before good plates can be obtained, as the laminae 
are constantly separating from each other ; and by admitting 
the materials employed in grinding and smoothing the surfaces, 
the transparency of the plate is destroyed. I found it neces- 
sary, indeed, to bind the laminae together by wax, sometimes 
by plates of wood, and at other times by placing them in a 
small hand vice. By these means my experiments at last 
succeeded, and all the mysterious actions of sulphate of lime, 
with the origin and classification of which M. Biot had been 
so much perplexed, were immediately unravelled* The two 
resultant axes and their attendant rings were observed at an 
angular distance of 6o°; the relative intensities of the two 
axes, from which ail the irregularities arose, were thus given. 
