Holmgren’s test, although a qualitative one, is most accurate in
allowing a diagnosis to be formed, but it sometimes happens that a
candidate is not satisfied that he has failed in passing the test, and
wishes for another examination. Such a re-examination is best carried
out by the spectrum method, which I will now describe.
The test with the spectrum is a very decisive one, and can be carried
out with the patch apparatus (Fig. 3), page 19. Personally, I like to
have some idea of the kind of colour blindness, if any, which exists by
first using the Holmgren test. Should these tests show that a candidate
is colour blind in any degree, a very excellent beginning is to try and
find his neutral point in the spectrum--if he has one. To arrive at
it we place two patches of light on the screen, one of colour and the
other of white, the rotating sectors being in the last-named beam, and
ask him to say when the two colours appear alike. It must be remembered
that white is coloured from the effect of contrast as long as the
colour alongside differs from it. A good _point de depart_ is with the
slit in the yellow, then to move it into the red, and then gradually
to push it into the green. When here, if colour blind, he will say,
“The two patches are nearly alike, but that the white is rather pink or
green,” as the slit gets further towards the blue. The operator, whilst
changing the colour, alters the sectors so that the luminosities are
about the same. A point will be reached when the colour blind will say,
“Now they are both alike, but one is rather darker than the other.”
The sectors are altered until he says they are both alike, and the
observation is satisfactory when he declares the two patches of light
are both alike in colour and in darkness. It is curious how misleading
the word brightness is to some people who are uneducated. I find it
much safer to ask which is the darker colour, rather than which is
the brighter. A little patience will always enable you to get a good
observation. The place in the spectrum which is the neutral point is
now noted. The neutral point is again found, but this time commencing
in the blue. The same procedure is adopted as before, and we thus get
a second reading for it, and the two will be found to be very close
to one another. In difficult cases, four or five observations may be
made, and the mean taken as a close approximation. So far the spectrum
test has not shown whether the observer is red- or green-blind, except
by comparing the position of the neutral point with that usually
found by the two types. We have, however, an unerring criterion by
the luminosity method. The red is placed beside the white, and he is
asked to say which he considers the darker; he will give an answer of
some kind, and probably protest that the two colours are not alike. A
soothing answer will disarm his objection, and he will quickly see what
you mean. If he be red-blind he will match in brightness a brilliant
red and a feeble white; if he be green-blind he will make a match very
similar to normal vision. In the case of the red-blind the slit is then
moved into the extreme red, when he will say he sees but one patch of
light, whilst the green-blind will see it as a person of normal vision
would do. If time permits, the whole luminosity curve may be taken and
registered. This is not essential, but interesting for reference. Where
complete colour blindness exists, it should be possible to cause him to
match a green with a red. To do this a second instrument, as described
in page 18, may be used, but it is quite sufficient if a piece of
red glass, such as is used for railway signals, or of bottle-green
glass, be placed in the white beam. There is then a red or green patch
alongside the patch of spectrum colour. The red will stimulate the red
sensation of the green-blind, but not being spectrum red it contains
a certain amount of yellow, which stimulates the green sensation if
the observer be red-blind. The green is of such a colour that it will
stimulate both the red and the green sensations. In the path of the
reflected beam between G and the prisms (Fig. 3) a sheet of plain
glass is inserted, which reflects a proportion of white on to the red
patch. The sectors are placed in this beam. If the red glass is being
used, the slit is moved into the green near E, and the colour blind
will say that both are the same colour, but one darker than the other.
By opening or closing the slit in the spectrum, he will possibly say
that both colours are alike and of the same darkness, but he may say
one is paler than the other, in which case the white light must be
increased or diminished by means of the sectors till equality of tone
is established. This applies to the red-blind and the green-blind. The
former will require a very bright red to match a feeble green, whilst
with the latter the red will require a fairly light green. When the
green glass is used the spectrum colour patch should be red, and the
match be made as before. With the violet-blind the neutral point will
be in the yellow, and with monochromatic vision matches can be made
throughout the spectrum. So far it will be seen that no mention of any
colour is required. It may next be advisable to ask him the names of
colours. This is best done by placing the white patch of light over the
spectrum colour patch, and opening and closing, as may be required,
the sectors. If the sectors are closed it is very probable that
correct guesses may be made, for then the colours will be saturated,
and the colour blind, if they are intelligent, will know that a green
to them is white or pale in colour compared with red, though of the
same hue. If white be mixed with the red the wrong name is bound to
be given, for they will be unable to distinguish it from the green,
because it is then a less saturated colour. Passing from green to red
and mixing the colour more or less with white, the most--I was going
to say grotesque--telling mistakes are made. A further excellent test
is to place a cell containing a solution of bichromate in the path of
the reflected beam, and cause the observer to match its colour with
the light coming through two slits, one in the red near C, and the
other in the green near E. Defective colour perception will be well
demonstrated. There are various other artifices which can be employed
in the spectrum test, which would be too long to recount here, and if
there be two sets of apparatus the tests are practically unlimited in
number.
There are cases in which an observer who may have normal vision may
wish to be reported as colour blind. A seaman’s life is not always a
happy one, and a boy on a training-ship, knowing that a failure in
colour vision will free him from a sea life, may be anxious to be told
he has failed in colour vision. By “coaching” in the Holmgren test he
might manage to obtain a “failure,” but a malingerer is sure to be
detected by the spectrum method of testing. He may call diluted red
green, and he may declare he sees a neutral point in the spectrum, but
if he be tested with the diluted colours near his supposed neutral
point, he is sure to fall into a trap. He will make a mistake in
calling a patch green when it ought to be white, or white when it ought
to be green, if he were truly colour deficient--indeed, a malingerer
has no chance of escaping detection with the spectrum tests. It is not
an uninteresting experiment to get an acute observer who has normal
colour vision, and is accustomed to the spectrum test, to feign colour
blindness, and examine him in this manner. He never fails to make such
mistakes as would lead to his detection.
With the partially colour blind the same procedure may be adopted. In
examination by the Holmgren wool test, slight mistakes will be made in
matching the first two test-skeins. With the spectrum test the red will
require a greater dilution with white before it will be matched with a
green, even if it can be matched at all. Measures of the luminosity at
four or five positions in the spectrum, extending from near the extreme
red to the blue, will give an unerring criterion of the kind and extent
of colour blindness from which they are suffering. The existence of
a neutral point in the spectrum is sufficient to indicate that their
blindness is of a nature to be dangerous in certain occupations. To
some it may be a difficulty how a neutral point can be found in such
cases, since all sensations are more or less present. The reason,
however, was explained on page 96.