IX. An account of a method of measuring the comparative intensities of the light emitted by luminous bodies. By Lieutenant-General Sir Benjamin Thompson, Count of Rumford, F. R. S. In two letters to Sir Joseph Banks, Bart. P. R. S

Abstract

Dear Sir, About two years ago, being employed in making a number of experiments to determine, if possible, the most œconomical method of lighting up a very large workhouse, or public manufactory, which has been erected in the suburbs of this city under my directions, where the poor, old and young, and all industrious people who are in want of work, are employed in a great variety of different manufactures, a method occurred to me for measuring the relative quantities of light emitted by lamps of different constructions, candles, &c. which is very simple, and which I have reason to think perfectly accurate. I sent you a verbal account of this little invention about a year and a half ago, by Dr. Baader, who wrote to me that you seemed to be pleased with it; but, as I think it very probable, that he might not have been able to give you a complete idea of the matter from memory, I have determined to send you a short account of it in writing, which, if you should think it worthy of that honour, you will be pleased to lay before the Royal Society. The method is shortly this:— Let the two burning candles, lamps, or other lights to be compared, A and B, be placed at equal heights upon two light tables, or moveable stands, in a darkened room; let a sheet of clean white paper be equally spread out, and fastened upon the wainscot or side of the room, at the same height from the floor with the lights, and let the lights be placed over against this sheet of paper, at the distance of 6 or 8 feet from it, and 6 or 8 feet from each other, in such a manner, that a line drawn from the centre of the paper, perpendicular to its surface, shall bisect the angle formed by lines drawn from the lights to that centre; in which case, considering the sheet of paper as a plane speculum, the one light will be precisely in the line of reflection of the other.