القرن التاسع عشر ، رقم ١٨٢، أبريل ١٨٩٢ [و‎‎٨‎٤] (٢٤٤/١٧٢)

1892
COLOUR-BLINDNESS
657
Prof. Hering was ascertained to occupy a kind of intermediate
position in respect of his susceptibility to yellow and blue rays.
It thus became evident that there are physiological differences of
sensibility to the blue and yellow rays to be found in persons of per
fectly normal sight, and Professor Hering was able to show, by means
of an ingenious method of experimentation with excised retinas, that
such differences were not referable to any disparity of colour nomen
clature, while they might quite well be accounted for by individual
differences in the depth of pigmentation over the central region, the
so-called macula lutea, or yellow spot of the retina. It was pointed out
by Hering that an increased depth of pigmentation would naturally
entail a greater absorption of the bluish rays during their transmission
to the deeper-lying sensitive layer of the retina, and further that the
result of such an excessive weeding out of the blue rays would be to
give every colour judgment which emanated from a person with a
deeply pigmented retina a distinct yellowish bias. This we have seen
was the condition which was found to obtain in the case of Dr.
Biedermann's colour judgments.
On the other hand, the colour judgments of a person whose retina
contained less than the average of pigment would be characterised by
a bluish bias, for the light which would to the ordinary eye appear as
colourless would to such an eye appear to be bluish, owing to a less
than average elimination of the blue rays.
Having realised these facts, we have now to follow Professor
Hering's application of them to the explanation of the differences
which obtain among the colour-blind. We saw that the colour-blind
had, by the advocates of the Helmholtz hypothesis, been classified
•upon the basis of certain differences into a class of green-blind
persons and into a class of red-blind. We have now, however, to see
/that the differences which are designated by these ' question-begging
epithets' really resolve themselves into merely physiological variations
in retinal pigmentation which come in and modify the always essen
tially red-green blindness in either of two opposite senses. It will
thus, upon this view, only be an accident of excessive or deficient
retinal pigmentation which will determine a particular colour-blind
person to one or other of these categories, while the great majority
/of the colour-blind will follow the and will not fit into
either category. Green-blindness would, upon this basis, be explain-
.able as merely our ordinary green-red colour-blindness occurring in
a person who belonged in the matter of retinal pigmentation to the
Biedermann or deeply pigmented type, while red-blindness would be
the same condition occurring in a person who happened to resemble
Dr. Stilling in having a retina with less than an average amount of
pigment. We must, however, here take into consideration the fact
that reds as well as yellows would be at an advantage in passing
through a deeply pigmented retina, while not only blues but greens