8(> Dr. Brewster on new properties of heat , 
thick, and when it was properly cut, and polished on a lapi- 
dary's wheel, it had the dimensions shown in Figs. 22, 23, and 
24, (PI. III.) The fringes seen through its two broadest 
surfaces are represented in Fig. 22. (PI. Ill,) The maximum 
tint of the central fringes is the commencement of the green 
of the second order, and that of the exterior fringes a green 
of the third order. In the fringes seen through the edges of 
the plate, which are shown in Fig. 23. (PI. III.) the maxi- 
mum tint of the interior set is a yellow of the second order, 
and that of the exterior set is a green of the third order. 
The fringes seen through the ends of the glass plate are very 
curious, and are represented in Fig. 24. (PI. III.) where A 
shows their form when the line AB is inclined 45 0 to the plane 
of primitive polarisation, and B their form when the line AB 
is parallel, or perpendicular to that plane. I have another 
parallelopiped of flint glass, about 4.3 inches, by 1 broad, and 
1 inch deep, which was crystallized when in the form of a 
cylinder, and afterwards ground into the shape of a paralle- 
lopiped. It exhibited the same phenomena as the preceding, 
and equalled it in the fine display of numerous orders of 
colours. The beautiful figures produced by crossing these 
two pieces, surpass in splendour every optical phenomenon 
that I have seen. 
In these and several other specimens of very thick crystal- 
lized glass, the maximum tint was always diminished by the 
operations of grinding and polishing. 
The following descriptions of four specimens of crystal- 
lized glass will point out the effects which are produced by 
changing the form of the plate. 
