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. [Illustration: FIG. 34. B. C.’s Luminosity and Extinction Curves.] The curve of luminosity, Fig. 34, is a very remarkable one, being different in character to that of P. and Q., the maximum being well on the D side of E. A great falling off in the luminosity when compared with that measured by the normal eye will be noticed both in the blue and in the red. (For measures see page 225.) The evidence was therefore presumptive that B. C.’s colour sensation was neither red nor blue, but probably a green.