982 ON THE EFFECTS OF PRESSURE IN ALTERING THE 
applied at such an angle with the axis, that the compound force 
arising from the combination of the new force with the ordi- 
nary polarising force of the crystal, produced tints far beyond 
the limits of Newron’s scale. In order, therefore, to observe 
the influence of the polarising force generated by pressure, it 
became necessary either to use a crystallised plate. which was 
so thin as to exhibit tints within the limits of Newron’s scale, 
or to apply the force at right angles to the axis, and to transmit 
the polarised light, either along the axis, or at such an angle 
with it as corresponded to a tint below the fifth or sixth order. 
I therefore took a plate of Caleareous spar, (one of the nega- 
tive class of crystals), bounded by planes perpendicular to the 
axis of double refraction, or to the short diagonal of the primitive 
rhomb; and having exposed it to polarised light, I observed 
the beautiful system of circular and highly coloured rings 
which it produced. The force of a screw was now applied to 
the sides of the plate, and the rings instantly began to lose 
their circular shape, to swell and contract in different places, 
and to bend into curves of contrary flexure at the points of 
pressure. By continuing the pressure, the plate was broken to 
pieces.; 
Instead of oriniding ppb the te obtuse solid snipes of 
the rhomboid of calcareous spar, 1 took a complete hoes 
and cemented upon two of its parallel surfaces a prism of flint- 
glass, whose refracting angle was 45°. When a polarised ray 
was incident almost vertically upon one of the faces of the 
prism, so as to be refracted parallel to the diagonal of the 
rhomb, the system of coloured rings was distinctly seen. I now 
pressed. together, by means of screws, the other four parallel 
surfaces of the rhomboid, and observed in a very satisfactory 
manner the change of form induced upon the circular system of 
rings. 
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