go Dr. Brewster on new properties of heat, 
was green, or that which was due to the sum of their actions. 
The same result was obtained when I combined with the above 
plate the central part of another crystallized plate which had 
the direction of its fringes inclined 45 0 to the plane of primitive 
polarisation. 
When the axis of the plate of sulphate of lime was turned 
round go 0 , or when the blue tint was taken from the lateral 
fringes of a plate of crystallized glass, having the direction of 
its fringes inclined 45 0 to the plane of primitive polarisation, 
an opposite effect was produced, that is, the resulting tint of 
the portions S, S', was black, and that of the portions N, N', 
green. 
In two crystallized plates of a square form which afforded 
the lateral sets of fringes C, D, and the terminal sets A, B, 
but no central sets, as shown in Fig. 32. (PI. IV.) the por- 
tions N, N', S, S', had the structure described in the Propo- 
sition. When the plate was held with the line A, B, parallel 
or perpendicular to the plane of primitive polarisation, it exhi» 
bited the phenomenon shown in Fig. 33. (PL IV. ) 
When any plate of crystallized glass, as AB, Fig. 31 . (PI. IV.) 
is cut through at CD, either by a diamond or upon a lapidary's 
slitting wheel, new fringes, n, n' , s, s' similar to N, N' S, S' 
start up at the new extremities of the plate. 
The fringes described in this Proposition may be called the 
diagonal fringes . 
