92 Dr. Brewster on new properties of heat , 
Proposition XXXI. 
If a crystallized plate a b, Fig. 34. (P/. IV. ) is placed symmetri- 
cally above A B, Fig. 31 . (P/. IV . ) either with the two anterior 
or the two posterior faces coincident , or with the anterior face 
of the one coincident with the posterior face of the other , or with 
the end a above A or b above B, or with b above A or a above 
B, in all these positions the tints polarised by the portions 
N, N' S, S' will ascend in the scale of colours , and be that 
which is due to the sum of the thicknesses of the plates. If the 
extremity a or b is placed above B or above A 6 so that the lines 
AB, a , b,form a continuous straight line , the tint polarised by 
the combination , will descend in the scale , and be that which is 
due to the difference of the thicknesses of the plates. 
The truth contained in this Proposition, has been established 
by direct experiment, although it might have been deduced 
from the Propositions which precede it. 
Proposition XXXII. 
When the neutral axes of a plate of crystallized glass are parallel 
or perpendicular to the plane of primitive polarisation, both the 
exterior and interior sets of fringes vanish , if the polarised ray 
is incident perpendicularly upon the plate ; but , if the plate is 
inclined to the incident ray, four sets of fringes are developed. 
They are separated from each other by three black spaces, and 
the fringes on each side of the central black line have the same 
character. 
When the lateral and the central fringes have vanished, the 
four diagonal fringes A, B, C, D, Fig. 3 6. (PL IV.) alone 
