58 Dr. Brewster on new properties of heat , 
Proposition IX. 
When the temperature of the source of heat remains the same , the 
thicknesses of the glass , whether one or more plates are used , 
which polarise any particular colour , under a perpendicular 
incidence, are proportional to the thicknesses of thin uncry- 
stallized plates, which would reflect the same colour in the 
phenomenon of coloured rings. 
M. Biot has shown with much ingenuity, that the thicknesses 
of sulphate of lime, rock crystal, and calcareous spar, which 
polarise any particular colour, are proportional to the thick- 
nesses of the uncrystallized plates which reflect that colour :* 
and there was reason to believe that the same law would regu- 
late the phenomena exhibited by heated glass. 
I took several plates of glass of various thicknesses, from 
the thinnest German crown glass, about ~th of an inch thick, 
to plate glass J of an inch thick, and, having placed them all 
upon a piece of red hot iron, I found that the number of orders 
of colours which were developed, was nearly related to the 
thickness of the glass. As these plates, however, had not 
the same chemical composition, I employed several pieces of 
thick mirror glass cut out of the same plate. I placed one of 
these by itself on the hot iron, and marked the particular tint 
which it polarised in the first order of Newton’s scale. 
All the rest of the plates having been placed on the hot 
iron at the same time with the first, I took each of them in 
succession, and joined it to the first plate ; the tints which 
were thus produced, ascended in the order of colours as the 
* See Biot’s Recherches sur la polarisation de la lumiere, p. 53. 
