as exhibited in its propagation along plates of glass. 85 
/ ■ -ns im atfsoqqo adt ol STi4oinJ?'. 
Proposition XXVI. 
When a rectangular plate of glass is brought to a red heat, and 
cooled as already described, it will acquire such a permanent 
structure as to exhibit the coloured fringes when polarised light 
is transmitted through any of the parallel faces by which it is 
bounded; every rectangular plate being considered as a solid 
contained by six parallel planes. The depolarising axes are 
distinctly developed in all these directions, and form angles of 
45 0 with the common sections of the planes. 
The fringes described in the Proposition are extremely 
minute, in plates of glass of an ordinary thickness. They 
consist of the same number of sets, having the same cha- 
racter and properties as those seen through the broad sur- 
faces of the plates, and their maximum tint is generally lower, 
though sometimes higher, than the maximum tint of the large 
fringes produced by the broad surfaces. They are in general 
perfectly regular, even when there is a great degree of irregiu 
larity in the form of the large fringes. In a plate of glass which 
had various breadths, and which polarised a faint yellow of 
the first order in its central fringes, and a bright blue of the 
second order in its exterior fringes, the central tints seen 
through its edges varied with the breadth of the plate, from a 
faint yellow of the first order, to a deep blue of the second 
order. 
In order to examine with more accuracy the fringes formed 
by transmitting polarised light through the different faces of 
a plate of glass, I crystallized a rough parallelopiped of crown 
glass, which was about three inches long, and half an inch 
4 
