176 Dr. Brewster on the communication of double 
in the middle are the same as those marked in the figure. 
When this plate was inclined in the plane AC, the following 
results were obtained ; but from the inequalities of the plate 
the measures must be very rude. 
Angle of incidence 
from the perpendicular. 
A red of the first order. 
- 0 ° 
Violet, 
1 5 
Blue, - 
28 
Green, - 
34 
Yellow, - - 
42 
Pink red. 
54 
Blue, - 
61 
Green of the third order. 
Proposition IV. 
71 
The polarising force of distended isinglass exceeds that of beryl, 
and is far greater than that of glass , whether it has received the 
doubly refracting structure from heat or from pressure. 
A soft film of isinglass -j- of an inch thick, developed by 
dilatation a blue of the second order, when it broke. 
Another film ~ of an inch thick was brought nearly to a 
state of induration. When it was dilated, which was done 
with some difficulty, it polarised distinctly the bright red of 
the second order. 
A third film, about — of an inch, and prepared after the 
manner described in Proposition III, polarised a red of the 
fifth order. By comparing this tint, which is higher than 
any of the rest, with a thickness of a plate of glass which 
gives the same tint, we shall find that the constant factor by 
