On the extinction of colour by reduction of luminosity

In “Colour Photometry,” Part III, XLVII, ‘Phil. Trans.,’ 1892, the results of some measures for the extinction of colour from the spectrum colours are recorded and the method of making the observations is explained. What was attempted in these observations was to reduce all the colours to a hue of the steel grey which a pure white becomes when it is of feeble intensity. During the observations then made it was found that some rays near the yellow lost their colour earlier than shown, but that the white they then matched was not completely the steel grey alluded to above. In the present results the hue of the white has been disregarded and no judgment of the hue of the white was required, making it easy for anyone to go through a series of readings. The instruments used were almost the same in principle as those employed on the previous occasion, but in some details they differed. Instead of sectors being employed to dim both colour and white, the annulus described in my paper on “The Sensitiveness of the Retina to Light and Colour” was employed, and the figure from that paper is inserted here to show the apparatus, as is also its brief description.