62 Dr. Brewster on new properties of heat , 
highest tint is polarised at B where the temperature BD is 
greatest. A lower tint in the scale appears at G, depending 
on the temperature GH, and a still lower tint at K where the 
temperature is reduced to KL. As the temperature of the iron 
RS diminishes, and the diffusion of the heat over the glass be- 
comes more uniform, the temperature at B will be changed 
to B d, the temperature at G to G h , and the temperature at 
K to K l. So that the curve will now have the form mlh d. 
When this happens, the fringes grow broader and diminish in 
number* 
When the diffusion of the heat is uniform, the temperatures 
and consequently the ordinates will every where be equal, 
and the curve will change into a straight line, in which case, 
the fringes completely disappear. When the plate CDEF is 
lifted from the iron, it begins to cool at CD. The fringes pass 
off at the edge CD, exhibiting a broad fringe of the same tint. 
The differences of the temperatures now vary less rapidly, 
and the line TLHD, becomes a curve of contrary flexure, such 
as TLHVW or TLHX, when the cooling has made greater 
progress. 
I have not been able to ascertain exactly the relation be- 
tween the thickness corresponding to the polarised tints at 
different distances from the source of heat, and the tempera- 
ture of the glass at the same points ; but by assuming the 
most probable law of the decrease of temperature, and com- 
paring it with the magnitude of the fringes, there is reason to 
believe, that the thicknesses are nearly proportional to the 
temperature. 
The tints polarised at different parts of the glass plate (a sec- 
tion of which is shown in Fig. 6. (PI. II.) by ACB) will be 
