150 ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



lourless. The fracture in the direction of the black fringe is 

 rough, and along the two opposite veins polished ; so that in 

 specimens that have been injured, we can see even ina direc- 

 tion transverse to the axis, as in Fig. 13. the veined struc- 

 ture distinctly marked by a succession of opaque and transpa- 

 rent lines. 



During the preceding experiments, I remarked the follow- 

 ing property of circular polarisation, which enables us to as- 

 certain whether the structure is direct or retrograde, without 

 employing an analysing prism. 



Let AB, Fig. 16. be a plate of amethyst, one half of which 

 APQ. has the direct circular polarising structure, and the other 

 half BPQ. the retrograde structure. Let the plane of primitive 

 polarisation MN coincide with PQ. If we now take a plate of 

 sulphate of lime, which polarises a white of the first order, and 

 place its axis parallel to MN, it will not affect the tints in ei- 

 ther half of the plate AB, which we shall suppose to be a 

 white of the first order. If the axis of the sulphate of lime is 

 shifted to CD, it will raise the white tint of the direct portion 

 to a yellow, growing brighter as it moves round, and becoming 

 orange, red, pink, and blue; the blue becoming fainter, and ter- 

 minating in white, when CD reaches the position BA. If the 

 axis of the sulphate of lime moves from MN to c d, the white 

 tint of the direct portion will become faint blue, then more 

 blue, then pink, orange, and yellow. The effect on the retro- 

 grade structure is quite the reverse of this, the one becoming 

 blue when the other is yellow ; but the resulting tint is always 

 the same, whether a plate of sulphate of lime crosses the direct 

 or the retrograde circular tint. 



The properties of amethyst which have now been described, 

 render a plate of this substance a valuable addition to our ap- 

 paratus 



