7o 
Dr. Brewster on new properties of heat. 
Proposition XVII. 
To explain the effect produced upon the fringes by varying the 
shape of the glass plates . 
When the breadth of the plates is very small compared 
with their height, the black spaces have the form shown in 
Fig. 10, 11. (PI. II.) the breadth AB of the former being 
0.8 of an inch, and that of the latter 2.25 inches. In Fig. 10. 
(PI. II.) the upper black space ABCD was indistinct about 
CD, and was scarcely separated from the lower black 
space Eg F. Coloured fringes appear at 1, 2, 3, and a white 
space between AB, CD extending pretty high, and grow- 
ing gradually fainter, the parts 1, 2, have the same tint, 
and the parts 3, 4, the opposite tint With a piece of glass 
whose height AC was 4^ inches, its breadth AB 4 inches, and 
its thickness f— of an inch, I obtained an effect similar to 
what is represented in Fig. 11. (PI. II.) The lowest part of 
the black fringe above 2 was \\ inch above CD, and the 
space at 4, all the way to the edge AB, was a pale bluish white. 
There were numerous fringes between C and D. In Fig, 11. 
(PL II.) the black spaces AB CD, and EF were separated by a 
whitish space 2. The portions 1, 2, 3, had the same tint, and 
the portions 4, 5, the opposite tint. See Prop. XXXVI. 
The form of the black spaces and the fringes varies in 
general with the outline of the plates. When the lower edge 
CD, Fig. 12. (PI. II.) had a waving form, and was placed 
upon a hot iron, the adjacent black space had likewise a wav- 
ing form, and the parts M, N, though the most distant from 
the source of heat, polarised tints higher in the scale than th§ 
parts O, P. 
