POLARISING STRUCTURE OF DOUBLY REFRACTING CRYSTALS. 283 
Similar experiments were made with Quartz, a crystal of the 
positive class, and with various other crystals, both with one and 
two axes of double refraction ; and in every case the tints were 
either raised or depressed in the scale of colours. The same 
effects were obtained at various angles with the axis, either by 
reducing the thickness of the plates, or by bringing the tints 
within the limits of Newron’'s nd by the opposite action of 
plates of sulphate of lime. 
~We may therefore consider it as an established fact, that the 
phenomena produced by the polarising forces of all crystals, 
whether they have one or more axes, and whether their action 
is positive or negative, are very considerably affected by sub- 
jecting the crystals to compressing or dilating forces. 
The effect which we have now described may arise from 
two causes, either from an actual modification of the original 
polarising force of the crystal, or from the developement of a 
new force, which merely combines its effects with those of the 
original force. The first of these cases is exemplified, when 
we subject to pressure a plate: of glass along which heat is in 
the act of being transmitted. The pressure which is thus ap- 
plied, alters the state of aggregation into which the glass is 
thrown by the passing heat, and produces a real modification 
of its former polarising force. When, on the other hand, we 
combine a plate of sulphate of lime with a plate of didisaduis 
spar, the resulting tint arises merely from a @ombination of 
the tints which these crystals produce: separately ; ; the po- 
larising foree’ of the calcareous spar remaining mie same as be- 
fore. 
In order to determine whether pressure modifies the origi- 
nal force, or creates a new one in doubly-refracting Feil 
I cut the crystals into different shapes, and found that the ef- 
fect produced by pressure varied with the external shape of 
Nn 2 the 
