226 APPENDIX. 



polarized light is transmitted through them, in one posi- 

 tion they suffer it to pass freely, but if they are turned 

 round 90 degrees, they arrest and absorb it entirely. 



When the experiment has been successfully conducted, 

 the crystals will not in this position allow the smallest 

 portion of light to pass. 



If now we consider the extreme thinness of these crys- 

 talline films, it will appear how energetic must be their 

 action upon light ; since, although white and transparent, 

 they are able to produce an absorption equalling that of 

 the best tourmalines, notwithstanding that the effect of 

 the latter is aided by their natural dark colour. 



But if these crystals are analogous to the tourmaline, 

 they must have the power which that substance has of 

 analyzing the light that has been transmitted through 

 other crystals. 



Accordingly, if we place in the path of the polarized 

 ray a plate of sulphate of lime of a proper thickness, the 

 crystal which before absorbed the light and appeared 

 black, becomes splendidly coloured with that colour 

 which the sulphate of lime produces, and which a tourma- 

 line would shew if it were employed as an analyzing plate. 



On reversing the polarization of the ray (or turning 

 round the crystal), the complementary tint appears. The 

 same results occur if the crystal is employed in the first 

 place to polarize the light, and tourmaline or calcareous 

 spar is used to analyze it, so that the analogy or rather 

 identity of effect with the tourmaline is complete. 



