as exhibited in its propagation along plates of glass. 79 
Proposition XXIII. 
When a plate of glass brought to a red heat is cooled in the open 
air , or is placed with one of its edges upon a bar of cold iron , the 
different sets of fringes described in Section I. are developed 
during its cooling , and they have the same character with those 
which are produced by placing cold glass upon a hot iron. When 
the cooling is completed, the structure which affords the f ringes, 
becomes permanent, and the colours, when thus fixed, possess the 
same brilliancy which they displayed during their formation. 
When the red hot plate is exposed to a polarised ray, it 
exhibits at first no action upon light ; the tints advance slowly 
from the edges, and, after the lapse of 12 or 15 minutes, the 
glass is cooled, and the crystallization complete. 
In this way I have formed various plates of glass which 
possess a permanent structure, and exhibit the phenomena 
described in the Proposition, but not havipg obtained a com- 
plete series of different heights and thicknesses, I have not 
yet taken any exact measurements of the fringes. 
The following results with four different plates of glass, 
will convey some idea of the nature of the tints which are deve- 
loped. All the plates were brought to a red heat so as not to 
lose their shape, and were cooled by placing their lower edges 
upon iron of the same temperature as the surrounding air. 
Thickness of the 
plates. 
Maximum tint at 
the lower edge. 
Tint in the middle. 
Numbers in New- 
ton’s Table corre- 
sponding to the 
maximum tint. 
No. 1. 
2. 
3 - 
4 - 
c.1125 inch 
0.2000 
0.2833 
°-4375 
| Beginningof blue 
( of the 2d. order 
t Green of the 3d 
l order. 
$ Green of the 4th 
{ order. 
f Nearly end of the 
< red of the 5th 
[ order. 
Blue of the 1st. 7 
order, 3 
Beginningofblue 7 
of the 2d. order, j 
Beginningofpur- \ 
pleoftheist. order ) 
Pink of the 2d. 7 
order. j 
8.7 
16.2 
22.7 
35-5 
