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XII. The Bakerian Lecture. —Colour Photometry. 
By Captain Abney, R.E., F.R.S., and Major-General Festing, R.E. 
Received February 18,—Read March 4, 1886. 
[Plates 24, 25.] 
We think it may possibly be of some interest to the Boyal Society if we lay before 
them the account of the method which we have used, and the results of some experi¬ 
ments which we have made, in measuring the relative illuminating intensities of 
different parts of the spectrum, as seen by ourselves and by others, and those of 
different parts of spectra produced under varying circumstances, more particularly 
as these results have considerable bearing on practical photometry. 
§ I. Formation of a Patch of Monochromatic Light. 
In the Phil. Mag. (ser. 5, vol. 20 (1885), p. 172), one of us described a method 
which he had previously demonstrated to the Physical Society of forming patches of 
monochromatic light upon a white screen, and alluded to the possibility of adapting it 
to researches on colour. The method, briefly described, is to pass light through a 
spectroscope, using a photographic camera in place of the observing telescope, then to 
isolate different portions of the spectrum and make them fall in a patch on a white 
screen by means of a lens as shown in fig. 1. Any variations in the apparatus are 
described further on. 
This forms the basis of the method on which we carried on our experiments. 
Having paid much attention to the determination of the electrical energy which is 
required in carbon filaments, and platinum, and other wires, to give a certain total 
radiation per unit of area, we have endeavoured to extend these researches further by 
ascertaining not only the energy evolved in the whole visible portion of the spectrum, 
but also the intensity of radiation in various parts of it. 
§ II. Photometric Methods Used. 
To pursue this investigation it was necessary to elaborate some photometric method 
which should enable us to compare the visual intensity of one ray with that of another 
of different colour. Our first idea was to place white paper with black lines closely 
ruled on it in the colour patches of different parts of the spectrum, and then to ascertain 
the distance at which the lines would just not be separated by the naked eye. Crova 
