i 6 q Dr. Brewster on the communication of double 
that which is due to the sum of their thicknesses ; but if they 
are combined transversely , the effect is that which is due to the 
difference of their thicknesses. The same is true of plates of 
dilated glass. 
If a plate of compressed glass is combined symmetrically 
with a plate of dilated glass, the effect is that which is due to 
the difference of their thicknesses ; while a transverse com- 
bination gives an effect due to the sum of their thicknesses. 
The action of plates of compressed and dilated glass are regu- 
lated by the same laws which M. Biot has investigated for 
the different classes of doubly refracting crystals. 
In order to observe the effects of crossing plates of glass that 
possess both structures, I took a stiff bar of iron AB, Fig. 8., 
( PI. IX.) and placed upon it the glass plate CD, which was 
separated from the iron by the supports E, F ; and by means 
of the screw S, I kept it in such a bent state, that it exhibited 
the fringes shown in Fig. 5. (PI. IX.) When this plate was 
crossed by another similar plate at right angles, the intersec- 
tional figure had the form shown at Fig. g. (PI. IX.) At the 
angles o,y>, where the dilated portions cross the compressed 
portions, the colours rise in the scale, and the maximum tints 
of each plate are exactly doubled at the angular point; but at 
the other angle m , where the dilated portion of the one, 
crosses the dilated portion of the other, or where the com- 
pressed portions cross each other, the tints of the one plate are 
counteracted by those of the other, and therefore a black fringe 
mn, extends across the diagonal of the intersectional figure. 
When a plate of bent glass is crossed by a plate of glass 
crystallized by heat, as shown in Fig. 10, (PI. IX.) it produces 
an intersectional figure which can easily be determined a priori * 
