N I 
158 Br. Brewster on the communication of double 
A and 0 the tint is a white of the first order, passing into 
yellow at 0, then advancing up the scale to r, and descending 
by similar gradations to B. The effect produced by turning 
the glass round 45 0 is shown in Fig. 2. 
If the axis of pressure mn, Fig. 3, (PL IX.) is near one side 
of a plate of glass AB, an effect is produced at B exactly like 
the four sets of fringes exhibited by crystallized glass. When 
the axis of pressure mn is in the middle of a plate AB, Fig. 4, 
(PL IX.) about i£ inch long, the same effect is produced 
towards A and B, as if the two pieces Amn, Bmn , had been 
crystallized separately by heat.* 
I experienced considerable difficulty in applying a dilating 
force to glass, till I discovered the method described under 
Proposition III. 
Proposition II. 
When a plate of glass is under the influence of a compressing force 
■ its scructure is the same as that of one class of doubly refracting 
crystals, including calcareous spar, beryl , &c.; but when it is 
under the influence of a dilating force , its structure is the same 
as that of the other class of doubly refracting crystals, including 
sulphate of lime, quartz, &c. 
When a piate of dilated glass was combined with a similar 
plate of compressed glass, so that the direction of the dilating 
force coincided with the direction of the compressing force, 
the difference of their effects was produced, and vice versa . The 
truth in the Proposition was also established by combining 
the glass with standard plates of sulphate of lime. 
* See Phil. Trans. 18:6, p, 90, %. 35. 
