The detection of colour-blindness from a practical point of view / by F. W. Edridge-Green.

  • Edridge-Green, F. W. (Frederick William), 1863-1953.
Date:
1889
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Licence: Public Domain Mark

Credit: The detection of colour-blindness from a practical point of view / by F. W. Edridge-Green. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    with certainty, because colours which appear similar have not necessarily the same composition. Indeed, some of the features of this test are due to the use of impure colours. The red test skein (II. b), in both the Holmgren sets which I have obtained from Berlin, reflects a large quantity of violet light. The skein appears to differ very little from a pure red to the normal-sighted, but seen through a blue-green glass it appears violet, whilst a pm'e red appears black. This makes very little difference in testing an ordinary case of red-green blindness, because the preponderance of red rays causes the colour to fall into the red psycho-physical miit. But the presence of the violet rays causes the colour to be perceived as red, mixed with a large amomit of grey, because a mixtm-e of Adolet and red appears grey to the two-unit. The green, there- fore, that he matches with this colour will be a blue-green containing similar proportions from each unit. The red (III.) in my Pocket and Classification Tests is a much purer red, and the two-unit, therefore, match it with bright yellowish greens (78 and 94), instead of the dull greens represented by 10 and 12 in Holmgren's plate. Then, if the red end of the spectrmn be much shortened, the red test (II. h) will not be classed with reds, but with blues, as happened to me in one case. The reason of this is, that as a considerable number of the red rays are not perceived, they have to be subtracted from the composi- tion of the colour. The violet rays predominating causes the colour to fall into the violet psycho-physical unit. There can be no division into red and green bhnd. If any two unit colour-blind be set to classify a large number of colours, he will make the mistakes of both classes. It is difficult to say who should be excluded by Holmgren's test; if we exclude only the completely red- green blind, we shall allow persons who are dangerous to officiate as signalmen. If we exclude all the partially colour-blind, we shall exclude many who are competent to act. Then, the fading of the wools has to be taken into account. Another objection is that the greens which it is necessary to pick out will become more soiled than the others, and so afford a mark of distinction to the colour- blind, who, not being confused by the striking differences of colour, are able to notice minutia? of this kind. Holmgren himself relates a case of a man who could dis-