The speculation above referred to is of such intrinsic importance, in view of the developments of the last decade, that it compels further notice. Faraday himself further expanded it in a letter to Richard Phillips, which was printed in the _Philosophical Magazine_ for May, 1846, under the title “Thoughts on Ray-vibrations.” In this avowedly speculative paper Faraday touched the highest point in his scientific writings, and threw out, though in a tentative and fragmentary way, brilliant hints of that which his imagination had perceived, as in a vision;--the doctrine now known as the electromagnetic theory of light. At the dates when the earlier biographies of Faraday appeared, neither that doctrine nor this paper had received the recognition due to its importance. Tyndall dismisses it as “one of the most singular speculations that ever emanated from a scientific man.” Bence Jones just mentions it in half a line. Dr. Gladstone does not allude to it. It therefore seems expedient to give here some extracts from the letter itself:-- THOUGHTS ON RAY-VIBRATIONS. _To Richard Phillips, Esq._ DEAR SIR,--At your request, I will endeavour to convey to you a notion of that which I ventured to say at the close of the last Friday evening meeting ...; but, from first to last, understand that I merely threw out, as matter for speculation, the vague impressions of my mind, for I gave nothing as the result of sufficient consideration, or as the settled conviction, or even probable conclusion at which I had arrived. The point intended to be set forth for the consideration of the hearers was whether it was not possible that the vibrations--which in a certain theory are assumed to account for radiation and radiant phenomena--may not occur in the lines of force which connect particles, and consequently masses, of matter together--a notion which, as far as it is admitted, will dispense with the æther, which, in another view, is supposed to be the medium in which these vibrations take place. * * * * *