as exhibited in its propagation along plates of glass. 63 
represented by the ordinates of some curve m n op q, cutting 
the axis at the neutrai points n p. They reach their maximum 
at m and q, where they have the same character, and also at 
o, where they have an opposite character ; and they vanish 
at. n,p, the points which correspond to the black spaces. 
Pkoposition XIII. 
The upper edge of the plate which polarises the highest tint in the 
second exterior set of fringes, has received no sensible accession 
of heat, and the central parts of the plate, which form the two 
interior sets of fringes, exhibit no variation of temperature con- 
nected with the colours which they polarise. JVhen the number 
and form of the plates of glass, and the temperature of the 
source of heat remain the same, the magnitude of these three sets 
of fringes depends upon the law of the decrease of temperature at 
that part of the glass which produces the first exterior set. 
It will be seen from experiments given under a subsequent 
Proposition, that the depolarising structure is communicated 
to the upper edge of the plate of glass, even when it is 2, 4, 5, 
6 , and 7 inches high. In some of these cases, the edge of 
the glass has the same temperature as the circumambient air, 
although the heat necessary to produce the same fringe at the 
lower edge of the plate, is much greater than that of boiling 
water. 
By spreading over the surface of the plate a thin film of oil of 
mace, which melts with a slight degree of heat, I was enabled 
to ascertain that there was no particular variation of tempe- 
rature connected with the tints which were polarised by the 
three sets of fringes mentioned in the Proposition. 
