Colour-blindness and the trichromatic theory of colour vision. Part IV. -Incomplete colour-blindness

Abstract

A Simple Test .—In Parts II and III of this series of papers I have indicated several methods of giving a quantitative value to the amount of green or red sensation which exists in the incomplete green- or red-blind eye as compared with the normal eye. I need not summarise what has already appeared at recent dates in the ‘Proceedings’ of the Royal Society, but I will at once explain a very simple test which gives quantitative measures of the sensations in incomplete colour-blindness. In my last paper of the series a table is given of the luminosity of the arc spectrum and the percentage composition of the tabulated rays of the spectrum, and from the same table the amount of white which exists in these several rays can readily be calculated. Owing to change in hue which white causes in a mixture of a pure red with a green ray I pointed out that the match of the D line by two such rays is open to error, not considerable perhaps, but still appreciable. In my communication “On the Change of Hue of Spectrum Colours by Dilution with White Light, it was shown that the change in hue at the point where the two curves of red and green sensations cut when their areas are made equal is nil . For reasons given later it may also be mentioned that a ray at this point when mixed with a blue where the same red and green sensation curves cut, will, with proper adjustment of the widths of the slits, match the white of the reflected beam of the arc light. The same happens for any other source of light if the curves of equal areas are calculated for such a light. The position of the point of intersection in the yellow part of the spectrum varies with the kind of light forming the spectrum. Looking at the table it will be seen that from S. S. N. 50 to the extreme red there is no blue sensation present in measurable quantities. Suppose that in a beam of the arc spectrum a cell containing a saturated solution of potassium chromate of (say) ¾ inch in thickness is placed; the beam becomes yellow with very little blue present. If a slit is caused to traverse the spectrum in the colour patch apparatus a position will be found for it which exactly matches the hue of the white light which passes through the chromate solution, and will be at S. S. N. 49·6 or λ 5828 to the normal eye. This, like other rays, contains a fixed ratio of green to red sensations, but no blue which is measurable. Making the red sensation unity it will be found that the green sensation present is 0·385. A table of the ratio of red to green sensation (making red unity) for the standard spectrum scale is annexed, as is also a table of wave-lengths with the like ratios:—