ON COLOUR PHOTOMETRY. 
563 
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. 
The curve of luminosity, fig. 47, 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. The evidence was, therefore, pre¬ 
sumptive that B. C.’s colour sensation was neither red nor blue, but probably a green. 
The next test was made to throw light on this point. He made observations of 
the extinction of the different parts of the spectrum (see § XLVIII.). His observa¬ 
tions were very fair, except on the violet side of F, where they became slightly 
erratic, but by requesting him to use all parts of his retina to obtain the last glimpse 
of light, a very concordant curve resulted, as shown in fig. 47. Some of his observa¬ 
tions at this part were evidently made with the centre of the retina, for they gave 
readings which, when the “ persistency ” curve was calculated, and these observations 
treated as part of the extinction, agreed with the luminosity curve. We may, there¬ 
fore, conclude that B. C. has a region in the retina in which there is an absorbing 
medium corresponding to the yellow spot of the normal eyed. This is diagrammati- 
cally shown in fig. 47 by the difference in height of ordinates in the persistency and 
the luminosity curves. On the red side of the maximum the two curves are practically 
identical, except from Scale number 54. At this point for similar reasons, as given in 
§ XLVIII., it is probable that the white light which illuminated the prism vitiated 
the readings to some degree, as Column VI. of the following table shows. At the 
violet end something similar, doubtless, occurs, but it is masked by the difference in 
extinction by the central part of the retina and that of the whole eye. 
It must, however, be remarked that the amount of reduction of the intensity of a 
ray to produce extinction is very different for B. C. and for the normal eyed, or for 
the red- and green-blind or for P. and Q. B. C. can bear nearly 200 times less 
4 c 2 
