Researches in colour vision and the trichromatic theory / by Sir William de W. Abney.
- William de Wiveleslie Abney
- Date:
- 1913
Licence: Public Domain Mark
Credit: Researches in colour vision and the trichromatic theory / by Sir William de W. Abney. Source: Wellcome Collection.
62/452 page 42
![It should also be of such a diameter that the ratio of diameter to focal length is not less than the ratio of the diameter of the collimating lens to its focal length. If it be less, the collimating lens will not be filled with light. [It should be noted that the smaller the ratio of focal length to diameter of the collimating lens the brighter will be the spectrum.] If Fig. 11 be examined, it will be seen that any variation in the brightness of the spectrum is accompanied by a corre¬ sponding variation in the light reflected from the first surface of the prism. This is a most valuable property, as the brightness of any colour is most frequently re¬ ferred to in terms of the brightness of the reflected white beam. Apparatus for using two Spectra simultaneously. A later form of colour patch apparatus1 is arranged to enable two spectra formed by the same source of light to be used either separately or together. This arrangement allows a comparison of any differing mixtures of spectrum colours to be made, and it also allows the addition of any desired quantity of white light to the colour patches formed by the aid of either of the two spectra. In this apparatus, as in the last, the intensity of the white light used for comparison with the colours varies with the intensity of the spectrum. The same white light is used as before to form the spectrum and the reflected white light as the comparison light, but, in addition, the main light, after passing through the two prisms, passes through a half-silvered mirror, inclined at about 45° to the axis of the lens. The rays reflected are again reflected so as to pursue a course roughly parallel 1 See Paper No. 6.](https://iiif.wellcomecollection.org/image/b31350574_0062.jp2/full/800%2C/0/default.jpg)
