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<p>This paper describes in detail the apparatus and method of operation for reliable determinations of spectral transmission throughout the green, blue and violet by a photographic null method. The arrangement of apparatus is essentially the same as that used by Richtmyer [Abs. 1915A01639], but adapted to the needs of spectrophotometry. The apparatus has been in continual use since April, 1918, and has proved very satisfactory. All errors have been eliminated, as well as the necessity of anytests, calibrations or corrections, in connection with the current- irradiation law or the dark currents of the photoelectric cells, or with electrometer deflection methods. The only variable in the process of making a determination of the transmission at any wave-length is the accurately measurable%2
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