The patient (B. C.) had been examined by Mr. Nettleship, who kindly secured his attendance at South Kensington for the purpose of being examined by the spectrum and other tests. [Mr. Nettleship states that this case is without doubt a genuine case of congenital colour blindness, without any trace whatever of disease.] B. C. is a youth of 19, who has served as an apprentice at sea. His form vision is perfect, and he is not night blind. He can see well at all times, though he states that on a cloudy day his vision seemed to be slightly more acute than in sunshine. He was first requested to make matches with the Holmgren wools in the usual manner, with the result that he was found to possess monochromatic vision. He matched reds, greens, blues, dark yellows, browns, greys, and purples together; and it was a matter of chance if he selected any proper match for any of the test colours. Finally, when pressed, he admitted that the whole of the heap of wools were “blue” to him, any one only differing from another in brightness. The brighter colours he called “dirty” or “pale” blue, terms which eventually proved to be synonymous. We then examined him with patches of monochromatic spectrum colours by means of the colour patch apparatus. He designated every colour as “blue,” except a bright yellow, which he called white, but when the luminosity of this colour was reduced he pronounced it a good blue. So with white, as the illumination was decreased, he pronounced it to pass first into dirty blue, and then into a full blue.
Colour discs were then brought into requisition, and it was hard at first to know how to make the necessary alterations, owing to the terms he employed to express the difference which existed between the inner disc and the outer grey ring. By noting that a pale “blue” passed into a pure blue when the amount of white in the outer ring was diminished, and that the inner disc was described as “pale” or “dirty” when the outer ring was described as “a very full blue,” we were enabled to make him match accurately a red, a green, and a blue disc separately with mixtures of black and white.
The following are the equations:—
360 red = 315 black + 45 white.
360 green = 258 black + 102 white.
360 blue = 305 black + 55 white.
With these proportions he emphatically stated that all were good blues, and that the inner disc and outer ring were identical in brightness and in colour.
It may be remarked that this is a case of congenital colour blindness, and that there is reason to believe that some of his ancestors were colour blind.
Before using the discs an attempt was made to ascertain the luminosity of the spectrum as it appeared to him. His readings, however, were so erratic that nothing could be made out from these first observations, except to fix the place of maximum luminosity, the terms “pale” and “dirty” puzzling us as to their real meanings. After the experience with the discs we had a clue as to what he wished to express by pale or dirty blue, which only meant that the colour or white was too bright, and on making a second attempt he matched the luminosities of the two shadows as easily as did P. and Q., the other cases of monochromatic vision. The method adopted was to diminish the white light illuminating one shadow to the point at which he pronounced it a good blue, when a slight alteration in the intensity was always sufficient to secure to his eye equality of luminosity between it and the coloured shadow without his perceiving any alteration in the saturation.
Fig. 34.
