1885.] Phenomenon of Crystalline Reflection. 181 



twin stratum. The stratum usually lies a good deal nearer to one 

 C face than the other. Now when the two faces are turned uppermost 

 alternately, and the distances between the first and second shadows 

 are observed, they are found to be, as nearly as can be estimated, in 

 the same proportion as the distances from the twin stratum to the 

 two faces respectively. 



Again, one of the crystals showed the exposed section of the twin 

 stratum slightly inclined to one of the broad faces, which though smooth 

 to the touch did not give a perfect reflection of objects viewed in it. 

 On holding different parts of the blade of a penknife opposite to 

 different parts of this face, the distance between the first and second 

 shadows was found to vary, as nearly as could be guessed, in pro- 

 portion to the thickness of crystal between the upper face and the 

 twin stratum. 



The conclusion was confirmed by observations made with sunlight ; 

 but the simple method of shadows is quite as good, and even by itself 

 perfectly satisfactory. 



13. Another useful method of observation, not so very simple as 

 the last, is the following. A slit, suppose horizontal, not very narrow, 

 is placed in front of the flame of a lamp at some distance, and an 

 image of the slit is formed by a suitable lens, such as the compound 

 achromatic objective of an opera-glass. The crystal is placed so as to 

 receive in focus the image of the slit, being inclined at a suitable 

 angle, usually in a plane perpendicular to the plane of symmetry. 

 The eye is held in a position to catch the reflected light, and the 

 images formed by the different reflections are viewed through a watch- 

 maker's lens. If the slit be not too broad, the images formed by 

 reflection from the upper surface, from the twin stratum, and from 

 the under surface are seen distinct from each other, so that the light 

 reflected from the twin stratum may be studied apart from that 

 reflected from the upper and under surfaces. 



In this mode of observation it can readily be seen, by turning the 

 crystal in its own plane, and noticing the middle image, which is that 

 reflected from the twin stratum, how very small a rotation out of the 

 position in which the plane of incidence had been the plane of 

 symmetry suffices to re-introduce the coloured light, which had 

 vanished in that critical position, which appears to be a position not 

 merely of absence of colour, but of absence of light altogether ; at 

 least if there be any it is too feeble to be seen in this mode of 

 observation, though from theoretical considerations we should conclude 

 that there must be a very little reflected light, polarised perpen- 

 dicularly to the plane of incidence. 



14. On allowing a strong solution of chlorate of potash in hot 

 water to crystallise rapidly, in which case excessively thin plates are 

 formed in the bosom of the liquid, 1 noticed the play of colours by 



