refraction to glass , &c. by pressure, i6’l 
Proposition IV. 
The tints polarised by plates of glass in a state of compression 
or dilatation, ascend in Newton’s scale of colours as the 
forces are increased; and in the same plate, the tint polarised 
at any particular part is proportional to the compression or 
dilatation to which that part is exposed. 
We have already seen, in illustrating the preceding Proposi- 
tions, that higher tints are developed as the forces are increas- 
ed. If ABCD, Fig. y, (PI. IX.) is a plate of glass, rendered 
concave by bending, and mn the black space which separates 
the dilated portion AB from the compressed portion CD, then 
if efbe the natural distance of the particles of the glass, and 
ab their distance when dilated at the convex edge AB, cd will 
represent the distance of two particles situated at c, and the tint 
at c will be to the tint at a, as cd—ef is to ab^-ef ; but cd — ef : 
ab — ef: :ec:ea; and therefore the tints at any part c will be 
proportional to its distance ce from the limit of compression and 
dilatation. The fringes developed on each side of mn have 
nearly the same breadth, which clearly shows that the tints 
are proportional to the actual compressions and dilatations. 
Proposition V. 
When compressed and dilated plates of glass are combined trans- 
versely and symmetrically, they exhibit all the phenomena which 
are produced by the combination of plates of doubly refracting 
crystals. 
If a plate of compressed glass is combined symmetrically 
with a similar plate, the tint polarised by the combination is 
MDcccxvr. Y 
