as exhibited in its propagation along plates of glass. 
Proposition XIV. 
When a plate of glass heated uniformly , and having a temperature 
considerably above that of the atmosphere , receives a crystalline 
structure in cooling , as described in Prop. IL the parts which 
produce the four sets of fringes have each a structure opposite to 
that which they had when the plate was crystallized by the in- 
troduction of heat from without. That is, the parts of the glass 
which afford the two exterior sets of fringes, have the same 
structure as the class of doubly refracting crystals , in which the 
extraordinary ray is repelled from the axis, and the parts which 
form the tzvo interior sets of fringes, have the same structure as 
the class in which the extraordinary ray is attracted to the axis. 
I took 12 plates of mirror glass, and brought them to an 
uniform heat by laying them successively on their sides and 
edges upon a bar of hot iron. Having ascertained, by expos- 
ing them to a polarised ray, that they had no action upon 
light, I placed them with their edges upon a cold iron, so as 
to exhibit distinctly the white fringes of the four different sets. 
When the axis of a plate of a sulphate of lime, which pola- 
rised a blue of the second order, was placed at right angles to 
the direction of the fringes, the white of the two exterior sets 
was converted into a brozvnish red, and the white of the two 
interior sets into a light green. The converse of this hap- 
pened, when the axis of the sulphate of lime was coincident 
with the direction of the fringes. When the four white fringes 
are produced by placing the glass upon a hot iron, all these 
phenomena are reversed, the green tint being now produced 
MBCCCXVI. K 
