1885.] Phenomenon of Crystalline Reflection. 



179 



width, I viewed through the crystal a sheet of white paper illumi- 

 nated by a soda flame. A dark ring was seen on the paper, which 

 was circular, or nearly so, and was interrupted in two places at 

 opposite extremities of a diameter, namely, the places where the ring 

 was cut by the plane of symmetry. The light of the refrangibility of 

 D was so nearly excluded from the greater part of the ring that it 

 appeared nearly black, though slightly bluish, as it was illuminated 

 by the feeble radiation from the flame belonging to refrangibilities 

 other than those of the immediate neighbourhood of D. The ends 

 of the two halves of the ring became feeble as they approached the 

 plane of symmetry. A subordinate comparatively faint ring lay in 

 this crystal immediately outside the main one. 



10. Suspecting that the production of colour- was in some way con- 

 nected with twinning, I examined the cleft edge of some of the 

 crystals which happened to have been broken across,, and found that 

 the bright reflection given by the exposed surface was interrupted by 

 a line, much finer than a hair, running parallel to the C faces, which 

 could be easily seen with a watchmaker's lens,, if not with the naked 

 eye. This line was dark on the illuminated bright surface exposed 

 by cleavage, a surface which I suppose illuminated by a source of 

 light not too large, such as a lamp, or a window at some distance. 

 The plane of incidence being supposed normal to the intersection of 

 the cleavage plane by the C faces, on turning the crystal in a proper 

 direction round a normal to the plane of incidence, the light ceased to 

 be reflected from the cleavage surface, and after turning through a 

 certain angle, the narrow line, which previously had been dark, was 

 seen to glisten, indicating the existence of a reflecting surface, though 

 it was much too narrow to get a reflected image from oif it. The 

 direction of rotation required to make the fine line glisten was what 

 it ought to be on the supposition that the fine line was the cleavage 

 face of an extremely narrow twin stratum. 



11. On examining the fine line under the microscope, it was found 

 to be of different thicknesses in different crystals, though in those 

 crystals which showed colour it did not vary very greatly. On 

 putting a little lycopodium on the cleavage face interrupted by the 

 fine line, it was seen that in those crystals which showed colour the 

 breadth of the twin stratum varied from a little greater to a little less 

 than the breadth of a spore. The thickness accordingly ranged some- 

 where about the thousandth of an inch, such being the diameter of the 

 spores. The stratum was visibly thicker in those crystals which 

 showed their bright band in the red than in those which showed it in 

 the blue. 



12. That the thin twin stratum was in fact the seat of the colour, 

 admitted of being proved by a very simple experiment. It was suffi- 

 cient to hold a needle, or the blade of a penknife (I will suppose the 



