Dr. Brewster on the laws of polarisation, &c. 243 
and the actual progress of the tints became a matter of simple 
* observation. In this way I constructed the following table. 
Angle of 
refraction 
Angular 
dist. from 
the axis. 
Azimuth 
of o° 
Azimuth 
of 22 0 30' 
Azimuth of 
the resultant 
axis 30° 
Azimuth 
of 45 ° 
Azimuth 
of 67° 30' 
Azimuth 
of 90° 
0° 
go 0 
1. 0000 
1.0000 
1. 000 
1. 0000 
1.0000 
1 .0000 
10 
80 
0.978 
0.979 
0.980 
0.984 
0.992 
0.995 
20 
70 
0.9 1 3 
0.919 
0.927 
0.938 
0.962 
o -973 
3 ° 
60 
0.811 
0.824 
0.835 
0.861 
0.913 
0-939 
40 
5 ° 
0.689 
0.710 
0.729 
0.768 
0.853 
0.898 
5 ° 
4 ° 
0.5 5 o 
o.j 7 6 
0.588 
0.662 
0.782 
0.850 
60 
30 
0-435 
0.438 
0.448 
0.538 
0.719 
0.810 
70 
20 
0.340 
0.294 
0.304 
0.410 
0.659 
0.775 
80 
JO 
0.274 
0.125 
0.154 
0.305 
0.614 
0.760 
90 
0 
0.250 
0.105 
0.000 
0.246 
0.597 
0-753 
In the preceding table I have not given the value of the 
tints to more than three decimal places, as even the third 
decimal place is partly the result of estimation. 
To those who may repeat these experiments, it will be 
necessary to state, that in almost all crystals with two axes, 
the tints in the neighbourhood of the resultant axes, when 
the plate has a considerable thickness, lose their resemblance 
to those of Newton’s scale, as will be more minutely described 
in another paper. The rings, however, are perfectly formed ; 
and the numbers in the table are the values of the tints dedu- 
ced from their position, and not from their actual colour. Thus, 
in the third ring or order of colours, reckoned from the re- 
sultant axes. I call the value of the middle point about 17, 
although the tint is not a yellowish green, as in Newton’s 
scale. This mode of proceeding is strictly correct, for the 
cause which prevents the tint from being a yellowish green, 
disappears in general by diminishing the thickness of the 
plate. 
In comparing these observations with the general law, we 
shall suppose that one of the axes has the same situation in 
