George Gabriel Stokes to Faraday   16 June 1856

Pembroke College Cambridge | June 16, 1856

My dear Sir,

I have looked again for my piece of ruby glass, but without success. But I do not think it of much consequence. I distinctly recollect examining it for fluorescence¹, and finding that it had none, or at least no more than a colourless glass, none which could be in any way connected with the colouring substance. I distinctly recollect noticing that there was more “false dispersion” or scattered light than in colourless glasses, but it did not occur to me that this was connected with the colour by transmitted light. According to the best of an imperfect recollection the dispersing particles were somewhat sparse, not giving an apparently continuous beam like what is seen in the gold mixture from phosphorus. But I am by no means sure of this. It was fluorescence I was seeking after, and finding that this was only a case of “false dispersion” I paid little attention to it. I think it much more probable that the ruby glass I examined was like what you have examined than that my vague supposed recollection of the discontinuity of the dispersed beam was correct.

You have clearly shown that the colouring matter in the phosphorus- gold-mixture is in suspension, but I cannot believe that it is metallic gold. Such a supposition is utterly at variance with my optical experience. I know of no instance in which the same substance exhibits two totally different characters as to absorption, such as do the FeO.SO(3) gold and phosphorus-gold mixtures. Many cases occur in which the tint is quite different according to the thickness looked through; but the prism shows that these are among the instances in which the identity of the character of the absorption is most markedly exhibited. Moreover the transmitted colour of the FeO.SO(3) -gold mixture agrees, but that of the phosphorus gold mixture does not agree, with what might have been predicted from the reflected colour of gold. I can not help believing in the existence of a purple oxide².

Yours very truly | G.G. Stokes