1 66 Dr. Brewster on the communication of double 
Proposition X. 
If a plate of glass in a state of compression or dilatation is inclined 
to the polarised ray in a plane parallel to the axis of dilatation 
and compression, the tints will descend in the scale ; but if they 
are inclined in a plane at right angles to these axes , the tints will 
ascend. 
This result was obtained by the inclination of plates com- 
pressed by screws, and of plates compressed and dilated by 
bending. 
Proposition XL 
If a plate of glass that has already received the doubly refracting 
structure from heat, is exposed to compression, the tints of the 
interior fringes rise in the scale, and those of the exterior fringes 
descend, when the axis of pressure is perpendicular to the direc- 
tion of the fringes ; the opposite effect being produced by a 
dilating force. The same results are in this case obtained as if 
an uncrystallized plate similarly compressed or dilated, had been 
similarly combined with the crystallized plate. 
I took a plate of crystallized glass, which displayed in the 
middle fringes a blue of the second order, and having com- 
pressed it by a screw in a direction perpendicular to the 
fringes, the tint of the interior set rose to the red of the second 
order, while that of the exterior set descended in the scale. 
When the plate was pressed in a direction perpendicular to the 
fringes, the tint of the interior set descended to a faint yellow, 
and that of the exterior set rose in a similar proportion. 
When a piece of crystallized glass is bent by a screw, as in 
Fig. 8, (PI. IX.) the exterior fringes on the upper or concave side 
