210 
Dr. Brewster on new properties of heat, 
produced the yellow of the first order ; and when one plate 
only was used, the black spaces, and the bluish white fringes 
tvere distinctly visible. A temperature of about 8o°, that of 
the glass being 6o°, when applied to 20 plates, polarised in 
the central fringe a yellow of the first order, which corres- 
ponds to a tint whose value is 4 in the scale of colours. Hence, 
one plate would have produced a tint corresponding to = 
0.20 of the scale. 
When one of the plates was placed upon a bar of red hot 
iron, just visible in daylight, it polarised in the central fringe 
the commencement of the green of the second order, which 
corresponds to 9.35 in the scale. 
Now the difference of temperature answering to 0.20 was 
8o° — 6o° = 20°. Hence we have 
As 0.20 : 9.35 = 20° : 935 0 
the difference of temperature of the iron and the glass. The 
temperature of the iron is therefore 935°+ 6o°== 995 °. 
If we suppose the tints to be so indefinitely marked that 
the eye can only observe units of the scale of colours, we 
shall, even in this case, have a scale of 187 to measure the 
temperature of 935 0 — 20° = 9i5°, which is a scale having 
each of its divisions equal to nearly 4 0 . 9. The tints, 
however, are much more definite than we have supposed, for 
in the second order of colours, in which the observations 
may always be made, the eight different tints have the follow- 
ing measures. 
