with which I have met has fallen naturally into one of the classes
I have given; that is to say, every person is either heptachromic,
hexachromic, pentachromic, tetrachromic, trichromic, dichromic,
or totally colour-blind. When I first gave this classification of
colour-blindness, the facts that I discovered were so at variance with
those generally stated that it was very difficult for those who were
not well acquainted with the subject to compare the two sets. The
general knowledge of the subject has, however, steadily increased, and
the facts which I had so great a difficulty in getting recognised now
form part of our common knowledge. It would be well, therefore, to
describe the two main varieties of colour-blindness which are of chief
practical importance, and to show the relation which they bear to the
writings of other persons. These two main varieties have dichromic and
trichromic vision.
1. _Dichromic vision._--The cases which come under this head form
the class of the ordinary red-green blind. It is under this head
that nearly every one of the recorded cases may be classed. Vision,
as far as colour is concerned, is dichromic, the neutral point being
situated in the green of the normal-sighted at about λ 500. All the
colours on the red side tend to be confused with each other; therefore
red, orange, yellow and half of the green are seen as one colour, the
remainder of the green, blue and violet as the other. The luminosity
curve in uncomplicated cases is similar to the normal. There may be
shortening of the spectrum at either the red or the violet end of
a varying degree. All degrees of shortening of the red end of the
spectrum may be found. Dichromics with normal luminosity curves are
those which were formerly designated green-blind; but this designation
is not in accordance with the facts, because there is no defect of
light-perception in the green, and the so-called diagnostic mistakes,
as, for instance, putting a bright green with a dark red, are not made.
Cases of so-called red-blind are dichromics with shortening of the red
end of the spectrum. I have shown that the defective perception of
the red end of the spectrum will not account for the dichromic vision
which is found in these cases. We may also meet with shortening of the
spectrum with otherwise normal colour-perception. We also meet with
dichromic cases forming a series from almost total colour-blindness
to those bordering on the trichromic. Any theory must account for the
fact that there are varying degrees of colour-blindness in dichromic
vision, and why there is a large neutral band corresponding to the
colours of the centre of the spectrum in some cases, and in others the
neutral band is so small that the dichromic cannot mark it out. The
two colours seen by the dichromic are red and violet, though where no
distinction is seen between yellow and red, and blue and violet, the
brighter colour will often be selected; that is why so many dichromics
say that they see yellow and blue in the spectrum. Those who have had
practical experience of colour-blindness will know, however, that many
dichromics make no mistakes with the red test. The following will
give the normal-sighted the best idea of colour-blindness, and it is
how the dichromic see the spectrum, and explains why they are able to
distinguish between colours. Let him regard the dichromic as a man who
has two colours--red and violet and white. The purest red is at the
red end of the spectrum; this becomes less and less saturated as the
violet is approached until the neutral or white point is reached; then
violet comes into the white, and this increases in saturation to the
termination of the violet. The ordinary dichromic therefore sees green
as a much whiter and less saturated colour than red.
2. _Trichromic vision._--These persons see three distinct colours
in the spectrum--red, green and violet. They describe the region
intermediate between red and green; that is to say, the orange and
the yellow as red-green, and blue as violet-green. It will be seen,
therefore, that their chief difficulty is distinguishing yellows and
blues. A yellow, for instance, which is situated next to a green will
be called red, and the same yellow when adjacent to a red will be
called green. There are various degrees of trichromic vision, varying
from those who are little better than dichromic to those who are
tetrachromic. The trichromic rarely find any difficulty with their
three main colours--red, green and violet.
These cases have been described under the name of anomalous
trichromatics. This name is one which has been given to those persons
who in making a match between a yellow corresponding to the sodium
flame and a mixture of thallium-green and lithium-red make a mixture
which is different to that of the normal.[9] A man who puts too much
green in the mixture is called a green anomaly; whilst a man who puts
too much red in the mixture is called a red anomaly. The red anomaly
is only a trichromic with shortening of the red end of the spectrum,
and this may be as extensive as in any case of dichromic vision. I
have, however, described trichromic cases which had all the symptoms
attributed to the anomalous trichromatics, but they were not anomalous
trichromatics, as they made an absolutely normal match.[10]
[9] _Proceedings of the Royal Society_, vol. B 76, 1905.
[10] _Transactions of the Ophthalmological Society_, 1907, p. 255.
_Proceedings of the Royal Society_, vol. B 82, 1910.
LECTURE II
_Delivered on February 3rd_
THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW
I. _Object of a test for colour-blindness._--Tests for colour-blindness
are of two kinds; namely, those which are used for the purpose of
ascertaining the special phenomena of colour-blindness, and those which
are employed when the inquiry is made for some practical purpose. As
with visual acuity, it is necessary to fix an arbitrary standard. As we
do not wish to exclude a greater number than is absolutely necessary,
the object of the test should be to exclude dangerous persons and
dangerous persons only. These persons may have other duties to perform
which do not require them to possess a perfect colour-sense. I should,
however, like to see those persons who are specially qualified for
a certain position, occupy it, for instance, men who have to keep a
look-out on our most important ships being selected because of their
accurate colour-vision and visual acuity. I do not mean that I would
select only those men and reject the others, but that I should like
to see a second object of a test, namely, to select those who are
specially efficient so that the Captain might know on whom to rely in
conditions of exceptional difficulty.
II. _The requirements of a test for colour-blindness._--A test for
colour-blindness, when it is to be employed for some definite and
specific purpose, as, for instance, excluding dangerous persons from
certain callings, should be such as to show definitely that the
persons rejected are dangerous. It is very useful to demonstrate to
the men and their fellows that a rejected candidate is dangerous. The
colleagues of a rejected candidate would refuse to risk their lives
with a man who before their eyes called a red light, green. I was
expressing these views when a superintendent of a railway company,
who is using my lamp, told me that he had adopted this method with
great satisfaction to himself and to the men. A man, for instance, who
has been working twenty years on the railway has been rejected for
colour-blindness. He has complained bitterly to the superintendent,
at the same time declaring emphatically that he is normal-sighted.
The superintendent has replied, “You know red?”--“Yes.” “You know
green?”--“Yes.” “You will therefore agree that if you call green,
red; or red, green, you ought to be rejected. Bring two or three of
the other men in with you and I will test you.” The man has readily
agreed to this. The superintendent has then tested him by asking him to
name various coloured objects in the room, and knowing by experience
exactly the coloured objects which are miscalled by the colour-blind
readily exposes his defect. It is noteworthy that on some occasions a
colour-blind man has been tested by another person in the same room
without making any of the mistakes which he subsequently made, because
none but coloured objects which he could readily recognise were shown
to him. This is an example of the necessity of a practical knowledge
of colour-blindness in an examiner. On account of the arrangement of
signals by sea and land, it is necessary that persons employed in the
marine and railway services should be able to recognise and distinguish
between the standard red, green, and white lights in all conditions
in which they are likely to be placed. An engine-driver or sailor has
to name a coloured light when he sees it, not to match it. He has to
say to himself, “This is a red light, therefore there is danger”; and
this is practically the same as if he made the observation out loud.
Therefore, from the very commencement we have colour-names introduced,
and it is impossible to exclude them. The engine-driver is told that
red is a “danger” signal, green a “caution” signal, and white an “all
right” signal. Therefore, it is absolutely necessary that he should
know the meaning of these colour-names. A test should be such as to
make it impossible for the examinee to be coached through it. This is
one of the most important requirements of a test for colour-blindness
and one that is rarely fulfilled. Nearly every one of the tests in
general use fail on this account.
A test should be one which can be carried out as rapidly as is possible
with absolute efficiency; of two equally efficient tests the one which
takes the least time must be selected. A test, therefore, should have
no unnecessary details which though of theoretical interest are not
concerned with the object in hand. The test should be made as easy and
as little complicated for the examiner as possible.
III. _Persons to be excluded._--We wish to exclude all those
individuals who are included in the following three classes: (1)
Those who see three or less colours in the spectrum. (2) Those who,
whilst being able to perceive a greater number of colours than three,
have the red end of the spectrum shortened to a degree incompatible
with their recognition of a red light at a distance of two miles. (3)
Those who are unable to distinguish between the red, green, and white
lights at the normal distance through defect or insensitiveness of the
cerebro-retinal apparatus when the image on the retina is diminished in
size.
I will now explain why these three classes of persons should be
excluded. The first class includes the trichromic, the dichromic, and
the totally colour-blind, in accordance with the facts previously
stated. The trichromic never, in ordinary circumstances, mistake green
for red, but confuse yellow with green or red. Colour is a feeble
quality of objects to them, and nervousness or excitement may reduce
them to the condition of the dichromic. The dichromic are liable to
mistake a green light for red, and vice versa. It is very important
that persons belonging to the second class should be excluded, and
yet none of the ordinarily used tests detect them. The rays of red
at the extreme left of the spectrum are the most penetrating, as may
be seen by looking at a light or the sun on a foggy day, or through
several thicknesses of neutral glass. It is chiefly by these rays that
we recognise a red light at a distance; and it is therefore of great
importance that a sailor or engine-driver should be able to perceive
them. The third class contains persons who are able to distinguish
colours easily when they are close to, but fail to distinguish them at
a distance, owing to the nerve-fibres supplying the central portion
of the retina being impaired. As a light at a distance occupies
the central portion of the visual field, it is essential that the
corresponding portion of the retina should be normal. There are cases
of central scotoma for colours with perfect form-vision; these would,
therefore, not be detected by a test for visual acuity. This class also
includes those who without having a scotoma are unable to distinguish
between colours at the normal distance when the image on the retina is
diminished in size.
IV. _The construction of a test for colour-blindness._--In
the construction of a test for colour-blindness, the facts of
colour-blindness must be utilised so that the object and requirements
of the test are fulfilled. The following facts are of practical
importance.
1. _Most colour-blind make mistakes with certain colours, but are
correct with regard to others._ This may be illustrated in the
following way. Let us take an ordinary dichromic, and, having given
him the set of wools belonging to the Classification Test, ask him to
pick out all the reds. On examining the pile of wools selected as reds,
it will be found that the majority are red, but in addition there will
be some browns and yellow-greens. If he be then told to pick out the
whole of the greens the greater number of those selected will be green,
but there will be also greys, browns, and reds. In each case, it will
be seen that the majority of wools are of the desired colour.
If another dichromic be examined in the same way it will be found
that, though he may not make exactly the same mistakes, he will in all
probability pick out the same greens to put with the reds, and the same
reds to put with the greens. The same result will be obtained if the
colour-blind persons be asked to name a large number of colours. They
will in most cases name the colour correctly. It will be noticed that
the greens which were put with the reds when classifying the colours,
will be called red in naming them. It is evident that the same idea has
guided the colour-blind in each case. This shows that, though a person
may be red-green blind, he is not absolutely red-green blind in the
sense of being totally unable to distinguish between the two colours.
The fact that they are actually judging by colour may be demonstrated
by giving them coloured materials of different kinds, or by asking them
to name a large number of coloured objects.
It will be seen that if we take a dichromic and ask him to name a
number of red and green wools, in the majority of instances he will
name them correctly. But as, almost invariably, the same wools are
chosen, for all practical purposes the same result would be obtained
by asking a person to name a few of these wools. What more decided and
brighter greens could we have than Nos. 76 and 94 of my Pocket Test?
yet these are two of the greens which are called reds by the dichromic.
We should have accomplished as much by asking a colour-blind person to
name Nos. 76 and 94 as if we had asked him to name a large number of
greens. The colours in a test should, therefore, be those which the
colour-blind are particularly liable to miscall. At the same time,
their nature should be unmistakable to the normal-sighted.
2. _The colour-blind name colours in accordance with their
colour-perception, and thus show definitely to which class they
belong._ I have not come across a man who has guessed correctly when
examined with my test. A man who did guess would know that he was
incompetent. As the colour-blind are often not aware of their defect
they answer as they see, only guessing when they feel uncertain as to
the nature of the colour shown. There is probably more misapprehension
on this point than on any other in the practical testing of
colour-blindness.
3. _Colours may be changed to the colour-blind, whilst leaving them
unaltered to the normal-sighted._
4. _The phenomena of simultaneous and successive contrast are much more
marked for the colour-blind than for the normal-sighted._ Two colours,
which have not changed in the slightest degree to the normal-sighted
on being contrasted, have apparently altered very considerably to the
colour-blind. As an example of this, let us take a pure deep yellow,
a bright red, and a bright green. To the normal-sighted the yellow
will be altered very little by comparison with the red or the green,
but a trichromic would say that the colour was green when contrasted
with the red, red when contrasted with the green. This principle of
exaggerated contrast must be borne in mind when examining a candidate.
Thus if a trichromic be doubtful about a yellow, but seems inclined to
call it green, he should be given a pure green to compare with it. In
the same way, in showing the coloured lights, the same colour produced
in a different way should often be shown. Thus an orange-red may be
shown immediately after a pure red. This will not alter the colour
to the normal-sighted, but greatly facilitate the examination of the
colour-blind.
5. _Many colour-blind match correctly, but name the principal colours
wrongly._ Therefore the test must be a naming test, the examinee being
rejected if he confuse the colours which it is essential he should
distinguish between in his occupation.
6. _Many colour-blind recognise colours easily when they are close to
them, or the surface is large, but fail to distinguish between them
when they are at a distance or the image on the retina is small._ The
test must be constructed in conformity with these facts.
7. _The colour-blind are more dependent upon luminosity than the
normal-sighted, and are liable to mistake a change in luminosity for a
change of colour._ The test should have a means of rapidly changing the
luminosity of a colour.
8. _The colour-blind find special difficulty with faint and dim
colours._ The test should have colours of this kind.
9. _The colour-blind who have shortening of the red end of the spectrum
cannot see reds reflecting or transmitting only rays corresponding to
the shortened portion._ It is essential that reds of this kind should
form part of the test.
10. _The colour-blind find more difficulty in comparing colours when
different materials are used, than when the coloured objects are all of
the same nature._
11. _Most colour-blind find more difficulty with transmitted than with
reflected light._
12. _The colour-blind have a defective memory for colours._
13. _Colours may be changed to the normal-sighted whilst leaving
them unchanged to the colour-blind._ When three colours of the
normal-sighted are included in one of the colour-blind, it is obvious
that a change from one colour to another of the three will make no
difference to the colour-blind. Also when the spectrum is shortened,
the addition or rays corresponding to the shortened portion to another
colour will not alter its appearance to the person with the shortened
spectrum. For instance, to a person with shortening of the red end of
the spectrum, a blue will still remain blue, when so many red rays from
the shortened portion have been added to it as to make it appear rose
to the normal-sighted.
14. _The colour-blind may have a sense of luminosity similar to that of
the normal-sighted._
15. _The dichromic distinguish between the colours of the
normal-sighted, which are included in one of theirs by their relative
luminosity and the difference of saturation which is apparent to
them._ A test should therefore have the means of presenting colours of
different saturation in succession.
16. _Colour-blindness is frequently associated with very high
intelligence and exceptional ability._
V. _The Lantern Test._[11]--1. _Description of apparatus._ The lantern
contains four discs: three carrying seven coloured glasses, and one
with seven modifying glasses. Each disc has a clear aperture. The
other mechanical details are: an electric or oil lamp with projecting
accessories, a diaphragm for diminishing the size of the light
projected, handles for moving the discs and the indicator showing the
colour or modifier in use. The diaphragm is graduated in respect to
three apertures to represent a 5-1/2-inch railway signal bullseye at
600, 800, and 1000 yards respectively when the test is made at 20 ft.
The glasses are as follows:---
_Coloured glasses._
1. Red (A and B).
2. Yellow.
3. Green.
4. Signal Green.
5. Blue.
6. Purple.
_Modifying glasses._
7. Ground glass.
8. Ribbed glass.
9. Neutral (No. I).
10. " ( " II).
11. " ( " III).
12. " ( " IV).
13. " ( " V).
[11] Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, 1a,
Old Bond Street, W.
[Illustration: Fig. 3.]
It will be noticed that three of the discs are similar in every
respect. In some of my lanterns the two reds are put at the end of the
series of colours and numbered Red 1 and Red 2. This makes no important
difference, but the arrangement given here is more convenient. It
should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If
the electric lamp should get broken the projecting apparatus can be
removed and an ordinary kerosene lamp placed behind the aperture.
[Illustration: Fig. 4.]
2. _Reasons for special construction._--The lantern has been
constructed conformably with the requirements and facts of
colour-blindness. All the facts I have given have been considered in
constructing the lantern.
The examiner, on possessing a lantern for the first time, should
carefully test himself with it and ascertain how the different lights
appear to him with different conditions of general illumination.
It is probable that certain improvements may suggest themselves to
him, therefore, I think it will be advisable to deal with certain of
these points, as it will help the examiner in the use of the lantern.
The colours have never been altered, and I certainly should have
altered them if I could have improved the lantern by doing so. I
have never met a single colour-blind person who has not been readily
and easily detected with my lantern, though I have examined many who
have passed other lanterns and in some cases a number of other tests
for colour-blindness. In most cases one turn of the wheel will be
sufficient to make a colour-blind person disclose his defect.
The examiner may be dissatisfied with the colour of the blue; let us,
therefore, compare an examination of a normal-sighted person with that
of an ordinary dichromic. The normal-sighted person will name every
one of the colours with ease and certainty, with perhaps the exception
of the blue, with which he is in some doubt. Here is the result of an
examination of an ordinary dichromic: he called the yellow, green; the
green, red; the signal green, no colour; the blue, blue; the purple,
green; red A, no colour or light; red B, green. It will be noticed that
the only colour that he has correctly named is the blue. We can try
him again and again, and though he will mistake all the other colours
he will always name the blue correctly. The examiner will have learnt
from this several important facts. He will see that the colour-blind
are really guided by their sensations of colour, and that it is not
simply a matter of guessing. The more an examiner has practical
experience of colour-blindness, the more will he recognise the fact
that the colour-blind are guided by their sensations of colour. He
will notice that the dichromic has readily recognised the blue which
was scarcely apparent to him (the examiner), and therefore cannot have
overlooked as a matter of carelessness colours which are much more
apparent to the normal-sighted. The blue is a valuable colour for other
reasons, for though it is not a colour on which I reject candidates,
anyone miscalling it must be very carefully examined before he is
passed. The trichromic generally call this blue, green. If we wish to
obtain a purer blue, we can do so by combining the blue or the purple
with the signal green.