214' Brewster on the Affections of Light 
7. Amber. As this substance possesses no crystalline form, 
and does not split into laminse, I found it impossible to make 
any satisfactory experiments with it. The enormous breadth 
of its coloured rings was conspicuous in every specimen, but 
though I ground and polished more than tw'enty plates of it, 
I could not obtain one which exhibited any thing more than 
broad coloured segments. With a parallelopiped of amber 
0.^66 of an inch long, 0.300 broad, and 0.367 deep, the co- 
loured segments were visible in every direction in which the 
light was transmitted. They appeared most distinct through 
the thickness 0.367 ; and through the thickness 0.566 they 
were still so broad, that no more than one colour of each ring 
could be seen. In a piece of amber of an inch thick, the 
rings were broader than in a plate of topaz of an inch 
thick. 
8. Ice. The difficulty of making experiments upon ice with- 
out melting it, the want of a crystalline form, and the im- 
practicability of shaping it into parallel plates prevented me 
from obtaining any accurate results. The following experi- 
ments, however, will throw some light upon this subject. 
A piece of ice of an inch thick gave rings much broader 
than those exhibited by a plate of topaz of an inch thick. 
The rings w'ere also seen by the reflection of common light 
from the posterior surface of the ice, the light reflected from 
the anterior surface being extinguished by a prism of calca- 
reous spar. 
A piece of ice of an inch thick exhibited rings larger 
than those given by a plate of topaz of an inch thick. The 
breadth of one of the fringes shewn by a plate of ice 
of an inch thick w^as nearly 5° 26', which compared with the 
