File:Colour Photometry. Part III (1892) (14742137626).jpg

Identifier: philtrans04238067 (find matches)
Title: Colour Photometry. Part III
Year: 1892 (1890s)
Authors: Abney, W. Festing, E.
Subjects: Proceedings of the Royal Society of London Philosophical Transactions of the Royal Society
Publisher: Royal Society of London

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nnerthe relative luminosity of the rays varied when the intensity of the light whichformed the spectrum was altered in a definite ratio. Evidently the most satisfactorymethod of ascertaining this was to throw a patch of white light on the screen andthen to diminish its luminosity to known amounts, and having selected some ray ofthe spectrum, to equalize their luminosities. The box already described (fig. 36) wasbrought into requisition, and a beam of white light was caused to illuminate one halfof the white patch on the screen at the end of the box, and the other half wasilluminated by the ray whose luminosity was to be tried. Rotating sectors wereplaced in each beam ; the apertures of those in the white were fixed at differentangles, whilst those of the sectors in the coloured beam, were opened or closed tillthe luminosities appeared the same to the eye, a series of readings being taken foreach ray. The results thus obtained were plotted, and some typical ones are shown Fig. 46. ^ 40
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Decrees of ir er o0 90 100white beam 160 170 180 Relative luminosities of rajs with different intensities of the spectrum. in fig. 46. The ordinates are the apertures of the sectors in the monochromatic rays,and the abscissa the apertures of the sectors in the white beam. The tangent ofthe inclination to the vertical of the curve at any point, therefore, represents theratio of the luminosity of the coloured to that of the white beam for a certainintensity of light. If this ratio were the same for all intensities the curve wouldbecome a straight line starting from the origin. This is the case, it will be seen,with one ray only, that at scale number 46*3, or about X 5618. This ray and 560 CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. EESTING white light would therefore be extinguished together. It may be more than acoincidence that this ray does not differ much in wave length from that ray which, asstated by one of us in a paper on the Transmission of Sunlight through the EarthsAtmosph

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