The Nineteenth Century , No 182, Apr 1892 [‎81v] (167/244)

652
THE NINETEENTH
April
red. Again, when we combine these, a greenish-blue would induce
a reddish-yellow on the companion shadow, and so on.
The causation of these phenomena 3 is, as may well be imagined,
extremely obscure, arid it has no very direct bearing upon the par
ticular question of colour-vision which we are discussing. The
colour-shadows themselves are, however, of extreme interest to us as
pointing to an association between red and green, and blue and
yellow, respectively, and they supply us with an invaluable means of
testing for colour-defects in colour-vision, inasmuch as it is possible
by the use of Hering's shutter apparatus to obtain two conterminous
colour-shadows illuminated respectively with practically pure red and
green, and also perfectly equalised with respect to illumination.
The conterminous colour-shadows thus obtained naturally appear to
the normal-sighted as quite distinct, by virtue of the fact that the
one is green while the other is red. To the colour-blind, on the
other hand, the conterminous shadows appear as one as soon as all
the differences in illumination have been equalised, and all differences
in blue and yellow colouring have been corrected. It thus becomes
evident that the distinction between red and green is non-existent
for the colour-blind. If, however, we were to fail to correct, or to
introduce, any trace of yellow or blue into one of the colour-shadows,
the contrast blue or yellow would immediately make its appearance
in the companion shadow, and a difference which is appreciable by
the colour-blind person would be established. Similarly, any small
difference in illumination between the shadows would be immediately
noticed. It is thus shown that the colour-blind are properly character
ised as green-red blind, and that their powers of vision are, in other
respects, unimpeachable.
If it were not for this twofold means of discrimination between
objects of similar form which is retained by the colour-blind, no
colour-blind person could escape detection for an instant. As a
matter of fact, we, however, find that the discrimination power in
the colour-blind is so adequate for all ordinary purposes that their
defect in colour-sense is seldom detected unless where it is specially
examined for. How adequate the discrimination power of the colour
blind really is for all ordinary purposes is, however, perhaps best
3 The phenomenon of the colour-shadows can be easily demonstrated by using
any window in which we have a pane of coloured glass. It is only necessary to
darken the whole of the window, with the exception of a narrow strip of the coloured
pane and a similar strip of the contiguous clearer ground-glass pane. A ruler or
lath is then to be held up perpendicularly in such a position that the clear and the
coloured windows may cast contiguous shadows from it upon a piece of white paper.
The clear window is then to be darkened by gradually covering it over, from the top
or from the side, with a piece of cardboard until it admits only as much light as the
coloured pane. The shadow thrown by the coloured window will then come out of
the contrast colour to the coloured window and to its companion-shadow, from which
the colourless light is screened olf.