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XIV. Colour Photometry .—Part III. 
By Captain W. de W. Abney, C.B., R.E., D.C.L., F.R.S., and Major-General 
E. Pv. Festing, R.E., F.R.S. 
Received December 14, 1891,—Road January 28, 1892. 
§ XLIY .—Measurement of Luminosity * 
In the paper on Colour Photometry (Bakerian Lecture, 1886) a curve of luminosity 
of the spectrum of the light from the “ crater” of the positive pole of the electric (arc) 
light was given. 
The apparatus used for measuring the luminosity was described in that paper, and 
certain modifications afterwards made were described in the appendix, and in a 
further paper with the same title in ‘ Phil. Trans.,’ 1888. 
Shortly, the arrangement of the apparatus was as follows : a collimator, two prisms, 
and lens were used to form a spectrum ; a second lens, placed a little obliquely, re-com¬ 
bined the rays so as to form a white patch 3 inches square on a screen. A slide, 
having a slit in it, being placed in the spectrum, any ray could be selected and made 
to fall on the patch. 
The beam of white light reflected from the surface of the first prism was, by an 
arrangement of mirror and lens, made to fall on the same patch. By placing an 
upright rod in the path of the two beams, one half of the patch was illuminated by 
the monochromatic ray 7 , and the other by the beam of white light, which, for con¬ 
venience, we call the “ reference ” beam, as it has been used throughout our late 
observations as the standard of reference. The relative luminosity of the two beams 
could then be compared by reducing one or other by the rotating sectors until the 
two halves of the patch appeared of equal brightness, the aperture of the sectors 
being a measure of the proportional brightness of the two beams. 
The patch of light was viewed at a distance of somewhere about three feet, its 
image thus occupied an angular field on the retina of 5°. As all the observations 
referred to in both papers, whether taken by ourselves or by others, were made with 
the same apparatus and under similar circumstances, they were strictly comparable 
* The numbering of the paragraphs and figures in this paper is a continuation of that of Parts I. 
and TL, ‘Phil. Trans,’ 1886 and 1888. 
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