File:Colour Photometry. Part III (1892) (14761963551).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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se in the luminosity of the red and a further decrease in that of the green.What the limit may be where no further change takes place we are not at presentprepared to say. If a star or a distant light be observed in the point where thevisual axis of the eye cuts the retina, and then on the part of the retina slightlyremoved from this point, the different colour of the images will be evident. * Added July 20. ON COLOUR PHOTOMETRY. 537 8 XLVII.—The Limit of Colour Vision, It is well-known that as light of any colour becomes enfeebled the eye fails to seecolour, though it can recognise the presence of light. From a physical as well asfrom a physiological point of view, it appeared to be of interest to ascertain the amountof illumination of a screen at which all appreciation of colour in the different rays of thespectrum disappeared, leaving a sensation of what, for want of a better word, we maycall grey light. In order to ascertain this, an apparatus (fig. 36) was devised as a Fig. 35.
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Curves showing the luminosity of the spectrum when measured (1) with the fovea centralis, (2) with thearea of the yellow spot or macula lutea, (3) with the retina 10° from the fovea centralis and outside themacula lufcea. supplement to that already described, by which a white light of very low intensitycould be compared with the spectrum colours. A.t one end of a box, shown in plan, is an eye-piece E. The other end has at itscentre a patch S9 1^ inches square, whitened with zinc oxide, the rest of the inside ofthe box being blackened. The monochromatic beam a coming from the spectrumthrough the side slit, and the reference beam 6, are reflected by plain glass mirrorsM^M^ to apertures in opposite sides of the box, and from just inside these apertures,by right-angled prisms PXP2 so as to fall on and cover S. Rods R±R^ are inserted inthe box in the paths of the beams so that they illuminate opposite halves of 8. MDCCCXCII.—A. 3 Z 538 CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. FES

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