282 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 Newton'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 Newton'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 Calcareous 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 grinding down the opposite obtuse solid angles of 

 the rhomboid of calcareous spar, I took a complete rhomboid, 

 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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