IV. On the nature of the light emitted by heated tourmaline

Some months ago I had the honour of submitting to the Royal Society a paper on the light radiated by heated bodies, in which it was endeavoured to explain the facts recorded by an extension of the theory of exchanges. Having mentioned the difficulty which I had in maintaining the various transparent substances at a nearly steady red heat for a sufficient length of time in experiments demanding a dark background, Professor Stokes suggested an apparatus by means of which this difficulty might be overcome; and it is owing to his kindness in doing so that I have been enabled to lay these results before the Society.