refraction to glass , &c. by pressure . 163 
and which is exactly one half of the intersectional figure that 
would be produced by crossing AB with another plate of 
crystallized glass, having the same tints as CD in its four sets 
of fringes. 
Proposition VI. 
If a plate of glass resting on two supports, is bent by any force 
applied between the pomts of support , the tints are a maximum 
at the part where the pressure is applied , and ascend gradually 
in the scale of colours towards the points of support. 
I took a plate of glass ABCD, Fig. 11, (PI. IX.) six inches 
long, \\ broad, and 0.28 thick, and having placed its extre- 
mities upon the points of support C, D, I bent it by a screw 
applied to the surface at M. Seven orders of colours were 
now distinctly seen on each side of the black fringe in the sec- 
tion Mm. In the sections 1.1, the first order of colours only 
was developed; between the sections 1 1, and 2 2, the second 
order of colours appeared, and so on with the succeeding 
orders, till the seventh was seen near Mm. 
Scholium. 
It follows from the preceding experiments, that the mecha- 
nical contractions and dilatations at the points, 1, 2, 3, 4, g, 6, 
are as the numbers, 7 13—, 22, 2 38, 45 -J, the values of 
the corresponding tints in Newton’s scale. * 
• See Newton’s Optics, Book II. Part II. p. 206. 
