apparatus which showed two or three lights instead of only one. I
will therefore give my reasons for adopting only one. This point was
one which occupied my attention for a considerable time, especially
in view of the fact--which, as far as I am aware, I was the first to
discover--that simultaneous contrast is increased to the colour-blind.
I was naturally anxious to turn this fact to account. I found,
however, that I gained nothing by increasing the number of lights, and
that in many cases it was a source of error. A second or third light
could have been easily added to my lantern, but besides unnecessarily
complicating the apparatus it would have served no useful purpose. All
the results which are obtained with simultaneous contrast are obtained
even more effectively with successive contrast. It will be noticed
that when lights are seen in ordinary conditions they are conditions
of successive contrast, and not of simultaneous contrast. An observer
rarely keeps his eyes definitely fixed on one light whilst he names
those adjacent to it, but moves his eyes so that the images of the
respective lights fall successively on his foveas. When more than one
light is employed, all the disadvantages of matching as against naming
are introduced. It will be seen that by presenting one light after
another we are fulfilling all the necessary conditions, only that the
light is moved instead of the eyes, sufficient time being allowed to
elapse to enable a normal-sighted person to readily recognise the true
colour of the next light without being confused by the after-image of
the one he has just seen. Many nervous normal-sighted would name a
yellow light seen between two red lights as green, and it does look
green to them from ordinary physiological conditions. They look first
at one red light, then immediately at the yellow light, then at the
second red light, and then again at the yellow light until they feel
sure that the centre light is a green light, and say so. I have never
met with a normal-sighted person who has miscalled the unmodified light
of my lantern, either red or green. Many humble, nervous normal-sighted
persons are under the impression that they are colour-blind, and yet
would make perfectly efficient officers. Many of these men have been
told by their wives or other persons that they are colour-blind, and,
believing this, try to see colours which are not visible to them. I
have examined many persons of this description, and have noted the
ease and accuracy with which they have gone through the tests for
colour-blindness when they have been assured by me that they were
normal-sighted. On the other hand, it is often very difficult to
convince a self-reliant, colour-blind person that he is colour-blind.
He is on the look-out for the small differences which he notices
between colours, and the fact of having another light for comparison
gives him the desired clue, and, though colour-blind, he passes the
test.
The material is the best possible, as it will not fade like all dyed
substances, and therefore all records made with one set of apparatus
will be uniform. Again, a coloured light has none of the accessory
qualities which enable the colour-blind to pass through other tests.
Thus many dichromics will call the yellow glass red or green, who would
not think of putting a yellow with a green or red wool, on account of
the difference in luminosity. He will, in the same way, if told to pick
out colours in the Classification Test to match the colour of the light
shown, have to depend upon his colour-perception. This is a useful
method with nervous and undecided candidates. The objection to it is
that it cannot be carried out in the dark or in a dark room. The Test
is not open to any of the objections which may be urged against the
method of simply naming colours, because the character and intensity of
the colour may be changed at will.
The method is better than that of direct comparison, because the
candidate is forced to use his colour-perception, and has to compare
the colour seen with previous impressions of colour in his mind. By the
use of neutral glasses, etc., I have obviated the fallacy of the method
of naming colours (namely, that these can be distinguished by their
saturation and luminosity), and forced the individual to depend upon
his colour-perception, and not upon some other accessory quality of the
object seen.
No amount of coaching will enable a colour-blind person to pass this
test, whilst almost any other may be passed in this way. I have tried
on many occasions to coach a man so as to pass my lantern, and without
success. The combinations are so numerous that the only result is to
make the colour-blind man nervous and doubtful and more easily detected
than before. This has occurred with men who could pass other tests with
ease.
The test also has a quality possessed by no other--namely, that of
enabling the examiner to reject dangerous persons and dangerous persons
only, the lower degrees of colour-blindness being allowed to pass.
3. _Special directions for conducting the test._--(1) The candidate
should be seated at a distance of twenty feet from the lantern. (2) He
should be asked to name the colour of the light produced by a coloured
glass (1 to 6) alone, or in combination with another coloured glass
or glasses, or with the modifying glasses (7 to 13). (3) A candidate
should be rejected (i) if he call the red, green, or the green,
red, in any circumstances; (ii) if he call the white light, in any
circumstances, red or green, or vice versa; (iii) if he call the red,
green, or white lights, black, in any circumstances. (4) A candidate
who makes mistakes, other than those mentioned above, should be put
through a very searching examination. It is not necessary to have the
room absolutely dark; in fact, I prefer a certain amount of light. The
examiner can, if he wish, make the test at night in the open air.
The examiner should on no account conduct the examination on any
regular plan, because the candidate, anxious to pass, finds out
from persons who have already passed the order and method of the
examination, and so, though colour-blind, might obtain a certificate.
Any one of the glasses may be shown first, and the candidate required
to name the colour of the light. The following will serve as an example
of the method to be employed in testing a candidate. A red being shown,
the candidate is required to name its colour. Then a blue or green may
be substituted. It is best to use the largest aperture at first and
to show all the colours on one disc. This will give confidence to
the normal-sighted candidate, whilst most of the colour-blind will be
detected. In the case of candidates who appear to be normal-sighted
and yet very nervous, there is no harm in telling them after they have
named all the colours on the disc correctly that this is the case.
No comment should, however, be made on individual answers. Then one
of the neutral, ground, or ribbed glasses should be inserted, not
the slightest intimation being given to the candidate of the nature
of the colour. He should be asked to name or describe the light, and
the answer, if incorrect, together with his other replies, carefully
recorded. The other glasses may then be shown, a combination of the
neutral, ground, ribbed, and coloured glasses being used at irregular
intervals.
When the candidate has been examined with the largest aperture,
the examiner can go through the same procedure with one of the
smaller apertures. I have found the third aperture the one which is
most generally useful. On account of the great diminution of total
luminosity caused by the diminished area of the light source, the three
smallest apertures can only be used in a dark room.
If a candidate hesitate about a colour and ultimately name it
correctly, a second and, if necessary, a third glass of the same
colour should be combined with the first. The fact that in one case a
single glass is used, and in another two or three of the same coloured
glass, makes very little difference in the colour of the light to
the normal-sighted. This is not the case with the colour-blind; a
dichromic who has hesitated about a green and then correctly named it
may emphatically call the light red when another green glass is put in
front of the first.
Care must be taken when the candidate is going to be examined with two
glasses at once, such as one of the neutral, ground, or ribbed glasses,
and a coloured glass, that he does not see the light until both are in
position, or else he may see the colour before it is modified in the
necessary way.
If the candidate call the standard red, green; or the standard green,
red, in any circumstances--that is, either alone or in combination with
the modifying glasses--he is to be rejected.
The examiner should ascertain for himself how far the various colours
are visible when modified with the neutral glasses. If the red and
green be not visible with the thickest neutral in the conditions of
luminosity and external lighting which the examiner is employing, he
should use the darkest neutral which allows the colours to be plainly
visible to the normal-sighted. In all cases of doubt the examinee
should be asked to walk towards the lantern and told to say when the
light is visible, and asked to name its colour. The distance at which
the light is visible, and then that at which the colour is visible,
should be noted and compared with the normal.
Particular attention should be paid to the answers given to the
combination of the thickest neutral glass with the standard red and
green respectively.
The examiner should utilise the fact that successive contrast is
increased in the colour-blind, as this is an easy method of detecting
the trichromics. The red having been shown, the light should be quickly
changed to yellow or clear, the examiner’s hand being placed over
the aperture if there be any intervening colours. It is necessary
that the yellow should be shown immediately after the red without
any intervening colours being first seen by the candidate. The
normal-sighted do not see any change in the yellow or clear when they
are shown after the red light, but the trichromic call the yellow
light, green. The examinee should then be shown the green light, and
then the yellow or clear, in the same way as mentioned for the red. The
normal-sighted will easily recognise the yellow, but the trichromic
will call it red. This portion of the examination must never be omitted
in any examination in which the candidate is passed. The two divisions
of the test--that is, showing yellow immediately after red and after
green--may be used at different periods of the examination, and, if
there be any doubt, repeated.
An examiner should, as far as possible, with the exceptions given in
the instructions, avoid all conversation with the candidate, simply
asking, “What colour is this?” and recording the answer without
comment. If an examiner after each answer say, “Quite right,” or some
such expression, the following is likely to occur. The candidate after,
say, six correct answers, makes a mistake; the examiner says, “Are you
sure?” Then the candidate knows at once that he has made a mistake,
and makes a guess, very probably a correct one. When a similar colour
is shown subsequently, he remembers the mistake he made, and gives the
second, and probably the correct answer.
In addition to being an efficient test, it is a very rapid test, as
many men who have been certified as normal after a lengthy examination
with other tests have at once disclosed their defect by calling the
green light of the lantern red. Many are under the impression that in
an examination with the lantern the dichromics simply guess. This is
entirely wrong. A man who did guess would know that he was incompetent.
I find that men have named the coloured lights in strict accordance
with their colour-perception. A man may, however, guess if examined
by an inexperienced and ignorant examiner, who when the examinee has
made a mistake promptly corrects it in a cross tone. A normal-sighted
person will guess when examined in this way. The examiner must receive
the examinee with a smiling face and courteous manner, and appear
pleased and satisfied with the answers, no matter what they may be.
The candidate is then placed at his ease, and answers according to
his colour-perception. It will be noticed that the lantern detects
those who have a slightly diminished colour-perception, as well as
the dangerous varieties of colour-blindness. The former undoubtedly
are not as efficient as those who have a normal colour-perception,
so that a definite standard will have to be fixed, as in the case
of visual acuity. Further details will be found in my book on
Colour-Blindness.[12]
[12] _International Scientific Series._ Kegan Paul & Co., 1909.
_Summary of method of examination._--(1) Show all the colours on one
disc with the largest aperture. (2) Show the reds, greens, and yellow
modified by the neutral glasses. (3) Show all the colours on one disc
with Number 3 aperture. (4) Show red, then immediately afterwards
yellow with largest aperture. Then show green and yellow immediately
afterwards. (5) Test the candidate with the red, green, and yellow with
the smallest aperture. (6) Show the neutrals or ground glass alone. (7)
Show blue made by combining blue or purple with the signal green. (8)
Show a colour, for instance, green, and then combine another glass of
the same colour. (9) Show the red produced by the combination of purple
with red A. (10) Give the combination of red A and signal green.
VI. _Other tests for colour-blindness._--I have three other tests for
colour-blindness: the Classification Test, the Pocket Test, and the
Colour-perception Spectrometer. I have also devised an instrument for
estimating the exact amount of red, at different wave-lengths, which is
necessary to neutralise the complementary in different persons.
1. _The Classification Test._--(_a_) _Description._--This test
consists of 4 test colours and 180 confusion colours; 150 coloured
wools, 10 skeins of silk, 10 small squares of coloured cardboard, and
10 small squares of coloured glass. The whole series of colours is
represented. In addition, there are a large number of colours which
have been chosen by colour-blind persons as matching the test colours.
The test colours are Orange, Violet, Red, and Blue-green, labelled I,
II, III, and IV respectively. The colours are chosen with the view
of presenting as much difficulty as possible to the colour-blind,
and as little as possible to the normal-sighted. The colour-blind
find especial difficulty in matching or naming a colour lying at the
junction of two of their colours. As the normal-sighted often find
difficulty in saying which colour predominates in a blue-green, so do
the tetrachromic with their purple-green, or the trichromic with their
red-green. A colour-blind person may, however, match a colour correctly
which corresponds to the centre of one of his colours. In addition to
choosing those colours for tests which are particularly liable to be
mistaken for other colours by the colour-blind, I have used coloured
materials of different kinds--wools, silks, glass, and cards--so as to
force the colour-blind to judge by colour, and not by saturation or
luminosity. (See Fig. 5.)
[Illustration: Fig. 5.]
(_b_) _Method of examination._--The candidate should be given the four
test colours, and, having named each, he should be told to select all
those which are similar in colour to the test colour. He should be told
to pay no attention to the fact of a colour being lighter or darker; as
long as it is the same colour it should be put with the test skein. The
examiner should not go through the test before the candidate first of
all, neither should one candidate be allowed to watch another making
his selection. A shrewd colour-blind person might pass the test if he
had seen a normal-sighted person go through it previously. In order to
show the candidate the difference between a shade and a colour, the
examiner should take one of the wools which is not a test colour--blue,
for instance--and pick out four or five shades of the colour. The wools
should be arranged without the knowledge of the candidate, so that a
yellow or a grey is placed beside a red and the examinee asked to name
its colour. At another period of the examination the yellow should be
placed adjacent to a green, and the examinee again asked to name it.
The examinee may pick out a certain number of colours correctly, and
then stop, saying that there are no more exactly like the test colour.
This may embarrass the examiner; he should, however, examine any
candidate who has omitted any colours as carefully as if mistakes had
been made. He should ask the candidate to match one of the omitted
colours.
The examiner will soon find out from experience those colours which are
named and matched wrongly by colour-blind persons; he should ask the
examinee to name some of these colours.
Any candidate should be rejected who calls an orange or red, green
or brown; black, red or vice versa; or green, either purple, rose,
red, grey, brown, or violet. Similar mistakes in matching necessitate
objection. A candidate who puts purple, rose, or blue with violet, or
yellow-brown with orange is most probably dangerously colour-blind
and should be very carefully examined. There are cases which pass the
Holmgren test with ease that fail in the most conclusive manner with my
Classification Test. They put green with orange, brown and black with
red, and grey with blue-green. This is due to a different selection
both of test colours and confusion colours. Orange is by far the most
important test colour, and its confusion with green by the dichromics
is very conclusive. The three other test colours, violet, red, and
blue-green, represent both ends of the spectrum and the neutral point
in dichromic cases, and practically these colours are those with which
most mistakes are made. This test can only be regarded as supplementary
to the Lantern Test.
2. _The Pocket Test._--This consists of nineteen cards, on nine of
which are 112 single threads of wool, and 14 pieces of twisted silk,
similar to those in the Classification Test. These are numbered
consecutively, with the exception of the first thread of the first four
cards, and the last thread of the next four cards. The end threads of
the first four cards, I to IV, form the tests; they are Orange, Violet,
Red, and Blue-green. There are also cards on which red, orange, green,
blue, violet and purple, and grey, respectively are to be found. There
are also two special cards marked “Without Red” and two special cards
marked “Without Green.” (See Fig. 6.)
[Illustration: Fig. 6.]
Many normal-sighted persons might object to the inclusion of some of
the colours on the orange card, but this card clearly shows the colours
which may be taken as a match. Fine distinctions are not wanted. The
series of colours I have selected and arranged so as to confuse the
colour-blind and force them to be guided by their colour-perception,
whilst the quantity of colour is amply sufficient for the
normal-sighted to pick out the colours with the greatest ease. The
cards should be arranged irregularly on a white cloth in a good light.
The two most important tests colours are the Orange and Violet, Nos. I
and II. The person examined should be asked to point out the shades of
colour similar to No. I (Orange). A piece of paper rolled to a point
should be used for this purpose. If he do this correctly, he probably
possesses normal colour-perception. If, however, he match the test
with reds or pinks, he is more or less colour-blind, at best belonging
to the pentachromic class. If, in addition, he match the Violet test,
No. II, with blue, he at least belongs to the tetrachromic class. The
trichromic, in addition, may match the Blue-green test, No. III, with
brown and grey. The dichromic will match the Orange test, No. I, with
yellow-green and yellow-brown. Similar mistakes will be made to those
described in connection with the Classification Test. The examinee
should be asked to name all the colours on one of the cards. He should
also be asked to point out on which of the cards the four test colours
are to be found, and which contain none of the test colour.
The examiner should continually change the order of the cards. Most of
the varieties of the colour-blind will be readily detected in this way.
The special advantages of this test are: (1) The colour-blind can be
ranged definitely in their proper classes. (2) Central scotoma can be
detected with its aid. (3) The series of colours are arranged so as
to confuse the colour-blind, whilst the normal-sighted easily match
the test colours. (4) On account of the introduction of different
materials, the relative luminosity and saturation of colours does not
serve as a guide to the colour-blind. (5) Portability. (6) The wools
and silks are kept clean. (7) An important colour is not likely to be
lost.
3. _The Colour-Perception Spectrometer._[13]--(_a_) _Description of
apparatus._--This instrument is a spectrometer so arranged as to
make it possible to expose to view in the eyepiece the portion of a
spectrum between any two desired wave-lengths. In the focal plane of
the telescope are two adjustable shutters with vertical edges; the
shutters can be moved into the field from right and left respectively,
each by its own micrometer screw, and to each screw is attached a drum,
the one being on the right and the other on the left of the telescope.
On each of these drums is cut a helical slot in which runs an index,
and the drum is engraved in such a manner that the reading of the index
gives the position in the spectrum of the corresponding shutter in
wave-lengths direct. (See Fig. 9.) Thus it will be seen that if, for
instance, the reading on the left drum-head is 5320 and that on the
right drum-head is 5920, the region of the spectrum from wave-length
5320 to wave-length 5920 is exposed to view in the eyepiece.
[13] Made by A. Hilger, 75a, Camden Road, London, N.W.
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
[Illustration: Fig. 9.]
(_b_) _Directions for using the instrument._--It should be used as
far as possible with a known quality and intensity of light. A small
oil-lamp is quite suitable for the purpose. The observer should
first ascertain the exact position of the termination of the red
end of the spectrum, the left-hand shutter being moved across until
every trace of red just disappears. The position of the pointer on
the left-hand drum is noted, and the wave-length recorded. The left
drum is then moved so that the shutter is more towards the middle of
the spectrum. The right-hand drum is then moved, until the pointer
indicates the wave-length recorded as the termination of the red end
of the spectrum. The observer then moves the left-hand shutter in
and out until he obtains the largest portion of red, which appears
absolutely monochromatic to him, no notice being taken of variations
in brightness, but only in hue. The position of the index on the
left-hand drum is recorded. The left-hand shutter is then moved towards
the violet end of the spectrum, the right-hand shutter being placed at
the position previously occupied by the left-hand shutter. In this way
the whole of the spectrum is traversed until the termination of the
violet end of the spectrum is finally ascertained with the right-hand
shutter. The variation of the size of the patches and the terminations
of the spectrum with different intensities of light can be noted. The
instrument can also be used for ascertaining the exact position and
size of the neutral patch in dichromics, the position of greatest
luminosity, and the size and extent of pure colours. When it is used
to test colour-blindness, the examinee should first be shown some
portion of the interior of the spectrum, and then asked to name the
various colours which he sees. In this way he will have no clue to the
colours which are being shown him.