

562 Prof. Chuckerbutti on Deformation of '" Rings 



crystal formation, and bow is it in the case of Iceland-spar 

 tbat the initial conditions are re-established after a tbin layer 

 is formed under tbe changed conditions. The difficulty is 

 still greater when we come to consider the case of repeated 

 twinning as in the case of potassium chlorate crystals, where 

 it is found that a very large number of twinning layers may 

 be formed with a most surprisingly regular periodicity and 

 constancy of thickness. Thus there is a good deal in regard 

 to the mechanism of formation of these twinning layers that 

 is as yet but imperfectly understood. 



2. Optical behaviour of a Spath Hemitrope. 



On looking at a source of light through a twin crystal of 

 Iceland-spar, generally it is found that three images of the 

 source are formed, the central one of which is always sta- 

 tionary. If the source of light is an incandescent electric 

 lamp, then the images are beautifully coloured, the nature of 

 the colour changing with the orientation of the plane of the 

 crystal to the incident beam. It is also observed that the two 

 outside images are polarized in perpendicular planes. On 

 rotating the crystal, the two outside images are found to 

 rotate about the central one, and in the course of a revolu- 

 tion there are positions in which both of them disappear, the 

 remaining one becoming the most brilliant for the time. 



The author has also found that when a beam of light is 

 allowed to fall upon a crystal cut perpendicular to the axis 

 and also polished in such a way that the lines, in which the 

 twinning lamina cuts the planes of the main crystal, are on 

 the surface, some interesting diffraction effects are observed. 

 Fringes are noticed in the region of the transmitted light 

 which show a remarkable asymmetry. The edges of the 

 lamina also appear to be luminous. The phenomena 

 appear to belong to the class of laminar diffraction effects, 

 but there are certain features regarding them of which the 

 explanation is not clear. The author is at present engaged 

 in a fuller study of these effects, and the investigation men- 

 tioned above will probably prove of considerable interest in 

 relation to the determination of the optical nature of the 

 twinning layer. 



On examining the crystal with convergent plane polarized 

 light and between crossed nicols, the ordinary " rings and 

 brushes" are found to be distorted, the amount and the 

 nature of the distortion changing with the orientation of 

 the crystal. However, in some cases, the distortion more 

 or less resembles that produced by the superposition of a 



