File:Colour Photometry. Part III (1892) (14761965071).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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hese observations/^ and thedotted curve B is that derived from curve A, supposing that each ray had an originalluminosity of one amyl light at the distance of 1 foot. It will be seen that the colour of the central portion of the spectrum is discerniblewith much greater reduction of light than is that of the extremities. This accountsfor the fact that objects illuminated by moonlight appear of a greenish hue. Thelight from the full moon, as is well known, is somewhere about half a million timesless bright than that of the sun, or about t^q of an amyl lamp at 1 foot. The figure * The extreme left end being plotted to a different scale so as to bring it within the paper. 3 z 2 540 CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. FESTING shows that at this point most of the spectrum has lost its colour. The faint spectrumwhen re-combined will be pale green, mixed, of course, with a fair proportion ofgrey, due to those portions of the spectrum from which the colour has disappeared. Fiff. 37,
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Diagram of limit of colour vision. § XLVIII.—Extinction of the Light of Different Parts of the Spectrum. A preliminary note has already been published by one of us (Captain Abney) onthis subject. It was expected that the results then given might be liable to correction,as in work of this kind it is only after repeated experiment that sources of errorcan be discovered and eliminated. The errors which have since been discovered bycomparing the results of some hundreds of observations are not serious, though theircorrection alters the extinction curve to some small extent. In the first experiments the readings of the scale had to be made by the observer,and it has been found that the light used to illuminate the scale, small though it was,fatigued the eye sufficiently to vitiate, to a small but sensible extent, the observations.This source of error has, in the latter experiments, been eliminated. ON COLOUR PHOTOMETRY. 541 The necessity of resting the eye for some time in darkness in o

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