562 
CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. FESTING 
Addendum. 
(Added July 20, 1892.) 
§ LII .—A Case of Green Monochromatic Vision. 
Since the foregoing paper was read a very phenomenal case of colour-blindness has 
been investigated by us for the Colour Vision Committee of the Royal Society. It is 
the case of a type so rare that we have not hesitated to publish it at the earliest 
opportunity. 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. 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 bright¬ 
ness. 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 illumi¬ 
nation was decreased, he pronounced it to pass first into dirty blue, and then into a 
full blue. 
Maxwell’s 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 “ 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 —f- 45 white. 
360 green = 258 black + 102 white. 
360 blue = 305 black + 55 white. 
