The readiest means of investigating the stimulation of the different
sensations necessary to produce colour is evidently by eyes in which
one or two sensations are absent, and this applies not only to the
Young theory, but also to that of Hering.
In former days, not much more than a century ago, the existence of
colour blindness, as it is now named, was a matter of great curiosity,
and in the Philosophical Transactions of the Royal Society of 1777,
the case of a shoemaker named Harris is described by a Mr. Huddart,
who travelled all the way from London to the Midlands on purpose to
see if all the alleged facts regarding the patient were true. Harris
mistook orange for green, brown he called black, and he was unable to
distinguish between red fruits and the surrounding green leaves. At
first, colour blindness was called Daltonism, from the fact that the
great chemist Dalton suffered from it, and investigated the variation
which he found existed in his vision from that of the majority of his
fellow-creatures. It was in 1794 that Dalton described his own case of
colour blindness. He was quite unaware of his defect till 1792, when he
was convinced of its existence from his observations of a pink geranium
by candle-light. “The flower,” he says, “was pink; but it appeared to
me almost an exact sky-blue by day. In candle-light, however, it was
astonishingly changed, not having any blue in it; but being what I
call a red colour which forms a striking contrast to blue.” He goes
on to remark that all his friends except his _brother_ (mark this
relationship), said: there was not any striking difference in the
two colours by the two lights. He then investigated his case by the
solar spectrum, and became convinced that instead of having the normal
sensations, he only had two or at most three. These were yellow, blue,
and perhaps purple. In yellow, he included the red, orange, yellow,
and green of others, and his blue and purple coincided with theirs.
He says, that “part of the image which others call red, appears to me
little more than a shade or defect of light; after that, the orange,
yellow and green seem _one_ colour, which descends pretty uniformly
from an intense and a rare yellow, making what I should call different
shades of yellow. The difference between the green part and the blue
part is very striking to my eye, they seem to be strongly contrasted.
That between the blue and purple much less so. The purple appears to be
blue much darkened and condensed.”
Dalton said a florid complexion looked blackish-blue on a white ground.
Blood looked like bottle green, grass appeared very little different
from red. A laurel leaf was a good match to a stick of sealing-wax.
Colours appeared to him much the same by moonlight as they did by
candle-light. By the electric light and lightning, they appeared as in
day light. Some browns he called red, and others black.
Mr. Babbage, in Scientific London (1874), gives an account of Dalton’s
presentation at Court.
Firstly, he was a Quaker, and would not wear a sword, which is an
indispensable appendage to ordinary Court-dress. Secondly, the robe
of a Doctor of Civil Laws was known to be objectionable on account of
its colour--scarlet, being one forbidden by the Quakers. Luckily, it
was recollected that Dalton was affected with that peculiar colour
blindness which bore his name, and that as cherries and the leaves
of a cherry-tree were to him of the same colour, the scarlet gown
would present no extraordinary appearance. So perfect evidence was the
colour blindness, that the most modest and simple of men, after having
received the Doctor’s gown at Oxford, actually wore it for several days
in happy unconsciousness of the effect he produced in the street. The
rest of the description we need not reproduce. Both the above cases
we shall see shortly come under the category of red-blindness in the
Young theory. Recent investigations tell us that such colour blindness
is by no means rare, nor can it have been then. Statistics, derived
from carefully carried out examinations made in various parts of the
world by an approved method of testing, show that about four out of
every hundred males suffer from some deficiency in colour perception,
but that so far as the more limited statistics regarding them are to be
depended upon, only about four out of every 1000 women suffer in the
same manner.
Colour blindness in a healthy subject is usually hereditary, and is
always congenital. It is curious to trace back in some instances the
colour blindness, where it is to be found, in a family. It often
happens that colour blindness--as the gout is said to do--skips a
generation. This is usually traced to the fact that the generation
skipped is through the mother. Thus, the maternal grandfather may
be colour blind, as may be the grandsons, but the mother will very
frequently have perfectly normal vision for colour. On the other hand,
the paternal grandfather may have defective colour perception, and this
may be inherited both by the grandsons and the father. The remark made
by Dalton regarding his brother’s eyesight points to the fact that his
own colour blindness was probably hereditary. Deaf mutes, Jews and
Quakers, seem to be more liable to colour blindness than other people,
statistics giving them 13·7, 4·9, and 5·9 as the percentages. It may
be well to point out that the deficiency in colour perception to which
we are alluding is totally distinct from that which may arise from
disease. This last form has such marked characteristics of its own that
it can at once be distinguished from the congenital form.
Of the four per cent. of males who suffer from congenital colour
deficiency of vision, a large number are not totally lacking in any
one or more colour sensations. Those in which one sensation, on the
Young theory, is entirely missing are called “completely red-, green-,
or violet-blind,” whilst those in which the sensation is but partially
deadened are called “partially red-, green-, or violet-blind.”
When two sensations are entirely absent, and such cases are very
rare indeed, they are generally said to have monochromatic vision;
that is, every colour to them is the same, as is also white, the
only distinction between any of them being the superior brightness
of some over others. The best illustration of this form of colour
vision is perhaps to say that the retinæ of such people have the same
characteristics in regard to sensitiveness as has a photographic
plate, the resulting prints in black and white representing what it
sees in nature. When we have to adopt the terms used by the followers
of Hering’s theory--the theory which obtains most followers amongst
the physiologists, since it endeavours to explain colour vision in a
physiological way, though it fails to meet all the requirements of the
physicist--we should restrict our terms to red-green and yellow-blue
blindness, still perhaps retaining the term monochromatic vision for
the rare cases specified above. As we must employ some terms to express
our meaning, we shall in these lectures adopt those of the Young theory.
Now taking a red-blind person and examining him with the spectrum, we
find that he sees no light at all at the extreme limit of our red,
and only when he comes to the part where the red lithium line marks
a certain red does a glimmer commence; he then sees what he may call
dark-green, or he may call dark-yellow. When questioned about what to
us are greens he also calls them green or yellow, some being bright,
others saturated hues, and others again paler. When he gets to the
bluish-green he calls it grey, and will say it is indistinguishable
from, and in fact will match with, a white degraded in tone. From this
point he will say he sees blue, near F pale-blue, and in the violet
dark-blue. Too much importance must not be attached to the nomenclature
adopted by the colour blind. They have to take the names of the colours
from the normal eyed. Yellow objects are generally brighter than red,
and having annexed the idea that what to them is bright red is called
yellow, they give it that distinguishing name. His limit of vision at
the violet end will be the same as the majority of mankind, but it will
be considerably shortened at the red end. The point in the spectrum
which he calls grey is an important point, and corresponds to the place
where the violet and green curves cut in Fig. 16. This point can be
very accurately determined by placing alongside the colour patch A
(Fig. 6) the white patch, which is reduced in brightness as required
by rotating sectors. As the slit is moved along the spectrum it will
eventually reach a point where he will say both patches of light are
exactly similar in hue. To the normal eye one will be white and the
other the kind of green indicated above (see frontispiece).
If a similar examination be made of the green-blind, the red end of the
spectrum will be called red or yellow, but the spectrum itself will
be visible between the same limits as it is to the person who has the
normal sense of vision. A grey stripe will be seen in the spectrum, but
in this case it will be a trifle nearer the red end of the spectrum
than the point which the red-blind calls grey; from this point to
the extreme violet, the green-blind will name the spectrum colours
similarly to the red-blind. The part of the spectrum where grey exists
to the green-blind is even more important than that part at which it
exists to red-blind, for it marks the place where the red and violet
curves cut each other in Fig. 16, and is in the majority of cases the
place in the spectrum where to the normal eye the green sensation is
unmixed with any sensation except that of white, as quite recently
explained. This green evidently is the colour which is most usefully
employed in making colour mixtures in order to obtain the three
sensation curves of the Young theory, since white can be added to the
colour matched. To avoid verbiage, we shall call the points where the
red- or green-blind see a grey in the spectrum their neutral points,
and the grey they see at those points their neutral colours. The three
curves we shall call the red, green, or violet curves, and the slits,
when placed in the red, green, or violet of the spectrum, as the red,
green, and violet slits.
We have already mentioned the case of those who possess monochromatic
vision, and shown in what respect they will differ in their description
of the spectrum from those more common cases of defective vision. If
the visual sensation they possess be the violet, they will see no light
at the extreme red of the spectrum, and very little in the orange. They
must match every colour with some shade of grey, for they will only
perceive that sensation, in what to ordinary normal eyes is white.
We need not detail how those who possess monochromatic vision due to
some other sensation would describe the different colours. The diagram
will tell us. Suffice it to say, that one colour will only differ from
another and from white in brightness.
It is a very remarkable fact how many people who are defective in
colour vision pass through a good part of their lives without being
definitely aware of it. It is very doubtful whether, in the majority
of cases, they themselves discover it. They may quite possibly
attribute the descriptions of colour which they hear, and which appear
to them absolutely false or meaningless, as due to mental or moral
defects in their friends. I have had two cases of this recently. One
was a gentleman of seventy-four, who had no conception that he had
anything but normal colour vision; his daughters, however, had a
suspicion that something was not quite right in it, and after a good
deal of persuasion brought him to me to examine. The first mistake
that he made was to state that he was sitting on a black velvet
chair, whereas the seat was a deep crimson plush. He laughed at his
daughter’s description of the mistake he made, and declared he was only
colour ignorant, and that she was the one who was colour blind. The
examination showed that colour ignorant he was, but that the ignorance
was due to complete red-blindness. For the seventy-four years he had
lived he was unaware of his deficiency, suspecting it in others, and
it was only an accidental circumstance which made him acquainted with
the true state of his colour perception. Another elderly gentleman,
in a high position in life, was also accidentally tested, and he
proved to be completely green-blind. He, too, was quite unaware of
his defect, and protested that, yachtsman as he was, he would never
mistake a ship’s lights; but a very brief test showed his friends
who were with him that his declaration had to be received with a
certain amount of reservation. Others there are who certainly do know
that some peculiarity exists in their sense of colour, and, foolish
as it may appear to be--though, after all, it is quite consistent
with a sensitive nature--they have tried to hide their defect from
their fellow-creatures. Such examples, no doubt, some of my audience
have met with, and experience tells me that they have just as much
reluctance to pass an hour in my darkened room as they would have to
occupy a police cell. In those few cases that have come voluntarily
to me for examination, the peculiarity in colour sense was first
brought to notice by the patient--if patient I may call him--failing
to distinguish between cherries and the cherry leaves, or strawberries
and the strawberry leaves. Such mistakes committed publicly are usually
the source of unbounded merriment and curiosity to schoolboys when made
by their schoolfellows, and I am bound to say that even persons of
graver years are not unapt to be amused at what they consider to be a
shortcoming in their fellow-creatures. To the student of colour vision
the discovery of curious cases of colour deficiency is looked upon in a
very different light--a good case of colour blindness, or still better
one of monochromatic vision, is eagerly sought after, with the hope of
submitting it to a rigid examination. When we look at the diagram (Fig.
16) we shall find why it is that the colour blind describe the spectrum
as they do. Literally for those whose vision is dichromic, it is made
up of two sensations alone, and the colours to which these sensations
give rise are mixed throughout a large part of the spectrum, the pure
unmixed sensations being at each end of the spectrum as they are in
normal colour vision. The annexed diagram (Fig. 18) gives the curves
for a red-blind person as made by observations under Clerk Maxwell’s
directions. The standard colours here have been badly selected, for
one of them stimulates the two sensations possessed.
[Illustration: FIG. 18.]
An easy and instructive experiment can be made to give an idea of
the kind of colour that these colour blind imagine as white, whether
they be red-, green-, or violet-blind. (For those who have only
monochromatic vision, as before stated, white is coloured with the
one colour they possess.) Three slits are now in the spectrum, one
near the extreme end of the red, another well in the violet, and the
third in that part of the spectrum in which the green-blind see their
neutral colour (see page 66). With the three colours issuing from
these apertures a match is made with the white patch, and in this case
the match is made as seen from a distant point, so that the resulting
deductions may be true to the audience. If a colour-blind person be in
this theatre, he will agree with me that the match is as correct to him
as it is to myself and the rest of you. So far we could not distinguish
his colour perception from the normal, but if he be red-blind, and
the red slit be covered, he will still say that the match holds good,
for, as a matter of fact, the red with which we helped to build up the
white is non-existent to him. The white that he now sees is to us the
greenish-blue patch which the mixed violet and green make. If he be
a green-blind person he will tell us the colour is a very pale blue,
but when the green slit is covered up and the red uncovered, the match
will once more be correct, though the purple, formed by the mixture of
red and blue, will appear to him to be a little darker than the white.
This is what one would expect, for you must recollect this green in
the spectrum he would call white or grey. If then, from what to him is
also white, though formed by the rays coming through the three slits,
we take away a certain amount of degraded white (green to us), he must
still see white, but darker. We have, however, met with what is an
apparent paradox. The green, coming through the now covered slit, he
calls white, as he also does the purple. To impress this point more
strongly upon you, I will place in front of the green slit a small
prism which has an angle of about one and a-half degrees. This is
just sufficient to throw the green colour on the neighbouring white
surface. Here we have both the colours which the green-blind calls
white side by side. If the brightness of each be the same, he would see
no difference in them. Is it possible that on any theory this can be
correct? To explain this apparent paradox, and without reference to
the mathematical proof that white subtracted from white leaves white,
we have only to look at our diagram (Fig. 16), and it is immediately
apparent how it arises. The red and the blue curves cut at this point;
and if we take away the green sensation entirely, the residue will be
a mixture of the red and blue, which is the identical purple colour
forming the patch.
If we are wishful to ascertain the colour that the violet-blind calls
white, we have only to cover up the violet slit and a yellow is left
behind as the result. I would have you remark that these colours which
are seen as white would only be of the hues shown you, supposing the
colour sensations were identical with those in normal vision. Whether
this is the case we cannot absolutely say, and the only way in which
this can be authoritatively settled is by examining some person who has
normal colour vision in one eye and defective colour sense, _not due
to disease_, in the other. One such person has been examined abroad,
but in what way I am unable to say. It is recorded that he sees the red
end of the spectrum as yellow with the eye that is defective. Another
person I have heard of in England, but so far have not had the good
fortune to get hold of him for examination. When I can lay my hands on
him, he will be able to help to confirm or disprove what should be a
general rather than a particular case.
So far I have only met with what appears to be one genuine case of
violet blindness. It is very remarkable, on account of the eccentricity
of the colour nomenclature. The only two colours which the subject
saw were red and _black_. He named all greens and blues as black, the
distinction between the two being that the former was “bright black”
and the latter “dark black.” Yellow he called white, and a glance at
Fig. 16 will show that at this place in the spectrum the neutral point
of a violet-blind should occur. By shifting the slit gradually into
the green, he called it grey, instead of “bright black,” though it did
not match the white patch when darkened. He called a green light a
“bright black” light. We shall have to refer to this case when we are
describing other investigations.