of 'Crystalline Reflexion described by Prof. Stokes. 263 



In these experiments the light entered and left the crystal 

 by a face parallel to the twin planes. In one specially well- 

 formed and apparently simply twinned crystal I was able to 

 observe a much more oblique reflexion from the internal sur- 

 face or surfaces. The light here entered and left the crystal 

 by cleavage faces making a large angle with the reflecting 

 planes, and thus under conditions widely different from those 

 considered hitherto, and in the latter part of the preceding 

 theoretical discussion. Three reflected images were seen, all 

 completely polarized (the original light being unpolarized),two 

 in one direction and the third in the opposite direction. These 

 images are coloured, and present tolerably discontinuous 

 spectra, giving rise to a suspicion that the twin plane is not 

 really single. These observations were made without special 

 arrangements by merely examining the reflected images of a 

 candle-flame, when the crystal was held close to the eye. 



I have made many experiments on the crystallization of 

 chlorate of potash in the hope of tracing the genesis of the 

 coloured crystals, but without decisive results. Besides the 

 usually small but highly coloured crystals, found by Stokes, I 

 have obtained many larger ones in which the reflexion is 

 feebler and less pure. These appear to be distinct from the 

 exceedingly thin plates which at the early stage of crystalliza- 

 tion swim about in the solution. Mounted in Canada balsam 

 the crystals in question show colours of varying degrees of 

 brightness and purity ; and under these circumstances the 

 effect can hardly be due to the action of the external surfaces 

 (in contact with the balsam). The light disappears twice 

 during the revolution of the plates in azimuth, just as in the 

 case of the more highly coloured specimens. It seems natural 

 to suppose that the reflexion takes place from twin surfaces 

 relatively few in number, and perhaps less regular in dispo- 

 sition. Altogether the existence of these crystals favours the 

 view that fully formed colour is due to a large number of 

 regular alternations. 



Some interesting observations bearing upon our present 

 subject have been recorded by Mr. Madan*. Transparent 

 crystals, free from twinning, were heated on an iron plate to 

 the neighbourhood of the fusion-point. During the heating 

 no change was observable, but "when the temperature had 

 sunk a lew degrees a remarkable change spread quickly and 

 quietly over the crystal-plate causing it to reflect light almost 

 as brilliantly as if a film of silver had been deposited on it." 

 Subsequently examined, the altered crystals are found to 



* K On the Effect of Heat in changing the Structure of Crystals of 

 Potassium Chlorate," < Nature,' May 20, 1S86. 



