القرن التاسع عشر ، رقم ١٨٢، أبريل ١٨٩٢ [و‎‎٨‎١] (٢٤٤/١٦٦)

1892
COL 0UR-BLINDNESS
651
occurrence of absolute colour-blindness). On the other hand, the
form of colour-blindness which is so commonly met with is in every
case a green-red colour-blindness. 2
It is thus evident that the facts are in substantial harmony with
the requirements of the theory, and we can also see that we may, for
all practical purposes, confine ourselves to the consideration of the
particular variety of partial colour-blindness which we shall call red-
green colour-blindness.
This colour-blindness would, in Hering's view, not be a condition
in which the eye is insensitive to either green or red, but rather a
condition in which the eye is blind to that distinction between red
and green rays which comes as a matter of course to the normal eye.
This - view of the nature of colour-blindness is based upon observa
tions which were made by Hering in connection with the optical phe
nomenon known either simply as ' colour-shadows,' or more commonly
as ' Hering's colour-shadows,' in consequence of the important im
provements in the methods of demonstrating them which were
introduced by that physiologist. The phenomenon itself has, how
ever, been long known, and Goethe, for instance, experimented upon
the subject with a lead-pencil, and the shadows which were cast from
it by his candle and the full moon at his window.
The principle involved in the production of colour-shadows is,
that if we have two sources of light placed so that the rays emanating
from them shall be incident in the same plane upon any opaque
object which is placed to intercept their rays, two contiguous shadows
will be cast on a background of, let us say, a sheet of white paper.
If, now, the light from the one source is a coloured light (we may
assume for the sake of fixing our ideas that it is a blue light we are
dealing with), while the light from the other source is a white
light of somewhat comparable— i .e. of duly diminished—intensity, the
portion of the paper which is screened off from the white light will,
under these circumstances, naturally come out of a relatively saturated
blue. So much might have been expected On the other
hand, it is at first somewhat startling to find that the companion
shadow from which the blue light is screened off by the intercepting
body comes out a yellow which is quite comparable in intensity to
the brilliancy of the blue shadow. In the same way, a red light
would have induced the contrast green on the companion shadow,
and, vice versa, a yellow would have induced a blue, and a green a
2 The faculty of distinguishing between red and green is for many reasons
nssnmed to be less primitive than the faculty of distinguishing between blue and
yellow. This view is based on the fact that the power of making the distinction
between red and green is limited to the central area of the retina, while yellow can
be distinguished from blue in any part of the field. Further, it is based on the fact
that the faculty is not unfrequently congenitally absent. Lastly, it is based on the
fact that it may be lost either by accidental injury to the eyes, or, as we have seen,
by the abuse of drugs.