as exhibited in its propagation along plates of glass. 53 
As the heat of the iron becomes more uniformly diffused 
•s 
over the plate of glass, the fringes between MN and CD di- 
minish rapidly in number, and pass off at CD, those which 
remain always increasing in magnitude. The same effect takes 
place at EF, but more slowly, so that there is a particular time 
when the fringes between EF and OP are equally numerous 
as those between CD and MN. The two interior sets diminish 
and disappear in a similar manner, the part AB re-exhibiting 
all its former colours in an inverse order. Nothing is now 
seen but the white and black fringes, which gradually die 
away, and at last disappear when the temperature of the glass 
becomes uniform. 
Proposition VII. 
The colours of the fringes in all the six sets ascend in Newton’s 
scale as they recede from the black spaces MN, OP, the fringes 
adjacent to these spaces being composed of the colours of the 
first order. 
The truth contained in this proposition might have been 
safely deduced from a comparison of the tints with those in 
Newton’s scale, or with the table of colours which I have found 
in the rings exhibited by topaz when exposed to a polarised ray.* 
In order, however, to obtain a more convincing proof, I took a 
plate of sulphate of lime, which polarised a bright blue of the 
second order, and combined it with the plate of glass CDEF. 
When the axis of the sulphate of lime was parallel to the 
axis CD, the blue of the second fringe below MN was con- 
verted into black, a tint due to the difference of their ac- 
tions ; but when its axis was at right angles to CD, the same 
* See Phil. Trans, 1814, p. 204. 
