) 
refraction to glass, &c. by pressure. '- 165 
/ - y . , •, . , - . . , . , j 
\ • 
Proposition VIJ I. 
If two plates of bent glass are placed together at their con- 
cave or compressed edges , the compound plate has exactly the 
same properties as a plate of glass transiently or permanently 
crystallized by heat, which gives the usual series of fringes. But 
if the two plates are placed together at their convex or dilated 
edges, tbe compound plate has the same properties as plates of 
glass transiently crystallized by heal, which produce the unusual 
series of fringes. 
The plates described in the Proposition exhibit the same 
intersectional figures as the plates of crystallized glass, and 
have in every respect the same action upon polarised light. 
Proposition IX. < 
Jf the compressing and dilating forces are applied to the centre of 
a plate of glass, the principal axes of the particles will be di- 
rected to the point of compression or dilatation, and the glass 
will exhibit the black cross , and the other phenomena which are 
seen in doubly refracting crystals. 
Having procured a strong lens of considerable convexity, I 
pressed it by means of a screw upon the centre of a plate 
of glass. When exposed to polarised light, it exhibited the 
appearance shown in Fig. 13, (PI. IX.) where ABCD is part of 
the lens, and m, n, o,p, four rectangular sectors, separated by 
a black cross. When the pressure is increased, different tints 
and fringes are developed, as in crystallized bodies. 
