as exhibited in its propagation along plates of glass. 49 
are held in the air, the same effect is produced, but in a less 
degree. See Prop. XIV. 
Proposition III. 
When heat is propagated along a plate of glass , its particles as- 
sume such an arrangement that it exhibits distinct neutral and 
depolarising axes , like all doubly refracting crystals , the neutral 
axes being parallel and perpendicular to the direction in which 
the heat is propagated. 
When a ray of light polarised in a plane inclined 45 0 
to the horizon, is transmitted through a glass plate DCEF 
Fig. 1. (Pl. II.) placed upon a piece of hot iron AB, lying hori- 
zontally, it is completely depolarised ; but when the plane of 
primitive polarisation is parallel or perpendicular to the hori- 
zon, no change is produced upon the polarised ray, an inter- 
mediate effect being exhibited in intermediate positions, as in 
regularly crystallized bodies. Hence DE is the neutral axis, 
and DF the depolarising axis of the plate. 
Proposition IV. 
When the depolarising structure is communicated to glass by heat 
in the manner already described , the glass acquires the property 
of arranging polarised light into its complementary colours. 
The apparatus being arranged as in Prop, III, let the light 
transmitted through the glass DCEF be analysed by a prism 
of calcareous spar, or by reflection at the polarising angle from 
a plate of black glass, having a motion of rotation round the 
polarised ray. When the plane of reflection from the black 
glass is perpendicular to the plane of primitive polarisation, 
the whole surface of the glass plate will be covered with 
beautiful and highly coloured fringes parallel to CD* as 
MDCCCXVI. H 
