On the change in hue of spectrum colours by dilution with white light

That a change takes place in the hue of the spectrum colours when white is added to them has been noticed, but I am not aware that any measurement of the extent of the change has been made. The need of a quantitative investigation has been forced upon me by the fact that in a research on the colour of fluorescence in various bodies the value of the dominant colours obtained by a photographic method did not correspond exactly with that obtained from visual observations. The photographic results also showed that mixed with the dominant colour of the fluorescence was more or less white light, and it occurred to me that this added white might account for the difference. The colour-measuring apparatus, which has been described in various papers and most recently in the paper “On a Modified Apparatus for the Measurement of Colour,” was employed in the investigation, a slight modification in the method of obtaining the two similar spectra being carried out. In the apparatus described in that paper a portion of the beam of light which had traversed the collimator, the prisms, and the lens which brought the spectrum to a focus was diverted by means of a partially silvered mirror placed in the path of the beam. In that used in the present instance only the lower half of the beam was deflected at right angles to the axis of the beam, and again deflected nearly parallel to its original direction. Both deflections were made by silvered mirrors. The two spectra were of approximately the same intensity. Two colour patches could now be formed side by side on a white surface when slits were inserted in each spectrum. The beam of whie light reflected from the first surface of the first prism could be thrown on either of the patches. In the investigations here given the white was thrown on the right-hand patch which was produced by the spectrum formed the diverted beam.