92 SIR D. BREWSTER ON THE DOUBLE REFRACTION, ETC. 



lutely impervious to the sun's rays. In this state their colour, seen by reflected light, 

 is nearly black ; but their powder is green in daylight, and of a French grey colour 

 by candlelight. In the smaller crystals, which are generally the best formed, the 

 colour both of reflected and transmitted daylight is blue, but that of candlelight is 

 jmrple. I have not been able to find any distinct traces of cleavage. 



This salt possesses a powerful double refraction, which is no doubt related to two 

 axes. In reference to the axis of the prism the double refraction is negative, like 

 that of calcareous spar. The greatest refractive index is about 1*605, and the least 

 about 1*506, reckoning from a line near the boundary of the blue and green rays. 



One of the most remarkable properties of this salt is the difference of colour in the 

 two images formed by double refraction. At a certain small thickness the least re- 

 fracted image is bright blue, and the most refracted image bright green, in daylight, 

 or bright pink in candlelight. The blue contains an admixture of green when ana- 

 lysed by the prism, and the green an admixture of red, the red predominating over 

 the green in candlelight. At greater thicknesses the blue becomes purer and fainter, 

 and the green passes into red ; and at a certain thickness the least refracted blue 

 image disappears altogether, and the most refracted image is olive green. At still 

 greater thicknesses this image disappears also, and absolute opacity ensues. 



When the crystal is exposed to polarized light, with its axis in the plane of polari- 

 zation, the transmitted light is green ; but when the axis of the crystal is perpendi- 

 cular to that plane, the transmitted light i^ blue. 



When the oxalate of chromium and potash is dissolved in water its double refrac- 

 tion disappears, in consequence of the particles being released from the force of ag- 

 gregation by which they are held together in the solid state, and by which double 

 refraction is produced. The solution, however, exhibits the same general action upon 

 light as the solid. At moderate thicknesses its colour is a dark blueish green by 

 daylight, and a bright blood red by candlelight ; but when we increase the thick- 

 ness of the fluid it becomes of a blueish pink by daylight, and of a deeper blood red 

 by candlelight, the red rays continuing to increase both in day- and candle-light, as 

 we lengthen the path of the ray through the solution. 



The most remarkable property of the oxalate of chromium and potash, and the one 

 on account of which I have submitted this paper to the Royal Society, is its specific 

 action upon a definite red ray lying near the extremity of the red portion of the spec- 

 trum. This is a property which is not possessed by any solid or fluid body with 

 which I am acquainted, although I have submitted some hundreds of coloured bodies 

 to direct experiment. Like all coloured bodies, the oxalate under our consideration 

 exercises a general absorbent action on the spectrum. The smallest thickness of it, 

 in which colour is scarcely discernible, attacks the yellow rays of the spectrum on the 

 more refrangible side of the line D of Fraunhofer. As the thickness of colour of the 

 solution increases, the violet rays are absorbed, and also all the yellow, orange, and 

 less refrangible green, till the whole space D E, and part of the spaces on the other 



