exhibited by certain Non-metallic Substances. 395 



new salt of quinine appeared*, I was prepared to connect the 

 metallic green reflected light with an intense absorbing action 

 with respect to green rays. Having prepared some crystals 

 according to his directions, I was readily able to trace the pro- 

 gress of the absorption in the case of light polarized in a plane 

 perpendicular to what is usually the longest dimension of the 

 crystalline plates, and to observe how the light passed from red 

 to black as the thickness increased. Even the thickest of these 

 crystals was so thin as to show hardly any colour by light 

 polarized in the plane of the length. The result of crossing two 

 such plates is of course obvious to any one who is conversant 

 with physical optics. The intense absorption was readily con- 

 nected with the metallic reflexion. An oral account of these 

 observations was given at a meeting of the Cambridge Philo- 

 sophical Society on March 15,1852; but it was not till the obser- 

 vations were a second time described, with some slight additions, 

 at the meeting of the British Association at Belfast, that any 

 account of them was published. A notice of this communication 

 appeared in the Athenaum of September 25, 1852, and from 

 this the report in Cosmos seems to be taken, though the latter is 

 not free from errors. In the report in the Athenteum, the colour 

 of the more rapidly absorbed pencil is briefly described in these 

 words : " But with respect to light polarized in the principal 

 plane of the breadth, the thicker crystals are perfectly black, 

 the thinner ones only transmitting light, which is of a deep red 

 colour." The comparative transparency of the crystals with 

 regard to red light is afterwards expressly connected with the 

 green colour of the light reflected as if by a metal. But in the 

 report in Cosmos, the passage just quoted is replaced by " tandis 

 que pour le cas de la lumiere polarisee dans le plan principal de 

 la largeur ils sont opaques et noirs, quelque minces qu'ils soient 

 d'ailleurs." This error led M. Haidinger to suppose that my 

 observations were opposed to his law ; whereas the fact is, that, 

 without knowing at the time what he had done, I had been led 

 independently to a similar conclusion. 



In mentioning my own observations on safflower-red, Hera- 

 pathite, &c, nothing is further from my wish than to neglect the 

 priority of those to whom priority belongs. M. Haidinger had 

 several years before discovered the phenomenon of the reflexion 

 of differently coloured oppositely polarized pencils, which Sir 

 David Brewster shortly afterwards, and independently, discovered 

 in the case of chrysammate of potash. M. Haidinger had from 

 the first observed a most important character of the phenomenon 

 in the case of many crystals, namely, the orientation of the po- 

 larization of the reflected light, which Sir David Brewster does 

 * Phil. Mag. for March 1852 (vol. iii. p. 161). 

 2D2 



