refraction to glass , &c. by pressure, 159 
Proposition III. 
If a long plate or slip of glass is bent by the force of the hand , 
it exhibits at the same time , the two opposite structures described 
in the preceding Proposition. The convex , or dilated side of the 
plate affords one set of coloured fringes, similar to those pro- 
duced by one class of doubly refracting crystals ; and the con- 
cave or compressed side , exhibits another set of fringes similar ' 
to those produced by the other class. These two sets of fringes 
are separated by a deep black line where there is neither com- 
pression nor dilatation. 
This curious result may be obtained by plates of glass of 
any size, provided they are a few inches in length, but the 
experiment is more easily made with a long and narrow slip. 
When a very small degree of force is employed in bending it, 
a faint bluish white fringe appears at both edges. As the force 
increases, these fringes encroach upon the interjacent black 
space, and gradually become white, yellow, orange, purple, in- 
digo, blue , green , yellow , &c. till three or four orders of colours 
are distinctly developed on each side of the black space. These 
phenomena are represented in Figs. 5 and 6 . (PI. IX.) Fig. 5, 
shows the effect produced by a very small force, and Fig. 6 , the 
effect produced by a considerable force. In one of these expe- 
riments, when the plate of glass was if inch broad, 0.28 
thick, and 6 inches between the points of support, I developed 
by the force of a screw no fewer than 7 orders of colours. 
The black fringe was scarcely perceptible, and the white tint 
arising from the mixture of all the colours, was on the eve of 
being produced when the plate broke in pieces. 
