164# -Er. Brewster on the communication of double 
Proposition VII. 
If a plate of glass is subject to compressions or dilatations exerted 
in different directions , the same effects are produced as when 
separate plates influenced by the same forces are combined in a 
similar manner. 
I took a plate of glass AB, Fig. 12, (PL IX.) and having 
compressed its extremity A by means of the screw S, a bright 
white of the first order emerged from the points of pressure 
PjO. By a force applied at B, I now bent the glass so as to 
make the lower side concave, and to produce the white tints on 
each side of the interjacent black space m, n. The effects of 
bending were now combined towards m, with the effects of 
compression, so that in the line mo, a black fringe appeared, 
the compressed structure produced by bending having acted in 
opposition to the compressed structure produced by the screw 
S ; while in the line mp, a yellow tint emerged, the dilated 
structure produced by bending, acting in conjunction with the 
compressed structure produced by the screw. These results 
will appear perfectly conformable to Prop. V., when we con- 
sider that the axis of compression produced by the screw is 
PQ, while the axis of compression and dilatation produced by 
bending is parallel to m n , and consequently at right angles 
to PQ. 
