438 Sir D. Brewster on the Oxalate of Chrome and Potash. 



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 polarization, the transmitted light is green; 

 but when the axis of the crystal is perpendicular to that plane, 

 the transmitted light is blue. 



When the oxalate of chromium and potash is dissolved in 

 water its double refraction disappears, in consequence of the 

 particles being released from the force of aggregation by 

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

 double refraction is produced. The solution, however, ex- 

 hibits 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 thickness 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 spectrum. This is a property which is not possessed 

 by any solid or fluid body with which I am acquainted, al- 

 though 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 co- 

 lour is scarcely discernible, attacks the yellow rays of the 

 spectrum on the more refrangible side of the line D of Fraun- 

 hofer. 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 side of the lines D, E, are wholly 

 destroyed. In this state the prism gives two distinct images 

 of objects, viz. a red and a greenish blue image, which are 

 considerably separated. As the absorption advances, the green 

 on the blue side of E, and the blue on the violet side of F, 

 gradually disappear, till a pure blue image about F alone re- 

 mains, and this too wholly vanishes by an increased thickness 

 of the solution, leaving the red rays unabsorbed. 



While these changes are going on throughout the spectrum, 

 a specific action is exerted upon a red ray between A and B 

 of Fraunhofer, and in that very part of the spectrum over which 



