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Charles Piazzi Smyth
... and I was now fully occupied in preparing the photographic plates [Using magnesium-light in the Great Pyramid]
C. Piazzi Smyth Life and Work at the Great Pyramid during the Months of January, February, March, and April, A.D. 1865; with a Discussion of the Facts Ascertained, In three volumes. (Edinburgh: Edmonston and Douglas, 1867), Vol.I, Chap.XVI, Magnesium Photography, p.494-495.
... and I was now fully occupied in preparing the photographic plates. How much magnesium should be burned in order to produce a photograph, was a question only to be settled by actual trial; I suspected much more than what had been employed hitherto in portrait-taking in England; first, because a large surface, say fifteen feet by ten, had to be illuminated; and secondly, because that surface was excessively dark and unreflective. At the first attempt, therefore, we tried sixty grains, burning it in the shape of hanging tapers in Mr. Brother's (of Manchester) very convenient shield-holders. A faint picture appeared. With another pair of plates, aperture l-5th all the time, one hundred grains were consumed; and still the picture was faint. So then one hundred and twenty grains were tried with a third pair of plates; and hardly any impression was obtained! This bad result arose from the vapour of the burning magnesium diffusing itself throughout the room, in spite of all our efforts to condense it on surfaces of glass or porcelain. So by ten o'clock, finding the atmosphere of the King's chamber was so thick we could hardly see the further end, we left the apparatus standing, and under special guardianship of two Arabs, who were to remain at the head of the Grand Gallery all day. About three P.m. I returned, to find the top of the said Grand Gallery smelling fearfully of Arab men; and the room still smoky. A pair of plates was tried, but one of the white-robed Muslims looming through dense haze, was all that was pictured by an expenditure of another one hundred and twenty grains of magnesium. The ordinary caudles, too, burned very palely; and we deeply deplored once again, that the ancient ventilating channels had been surreptitiously stopped up within the last few years.