I have so far spoken only of normal, or physiological, colour
blindness; a peculiarity, or defect, present at birth, and, as far
as is at present known, irremediable, but not associated with any
defect of the visual functions, or with any disease or any optical
peculiarities. What the nature and seat of this defect may be--whether
in the eye or in the sensorium--is at present unknown, although some of
the characteristics of the deficiency in colour sensation, I believe,
seem to indicate the existence of a special part of the brain endowed
with the functions for perceiving colour.
But cases are well known to medical men in which colour vision,
normal to start with, fails in greater or less degree in connection
with disease. This part of the subject is large and very complex, and
requires for its full elucidation an acquaintance with the diseases
and disorders of the eye. Many of the phenomena accompanying acquired
colour blindness, however, are of great interest to the physicist
in his study of colour vision, more particularly in regard to the
test of the truth of any particular theory. Through the kindness of
several medical men, and Mr. Nettleship in particular, I have had the
opportunity of examining by the colour apparatus several types of
colour blindness due to disease. One feature, common, I understand,
to all, or nearly all, cases, is the presence of some disease of the
optic nerve. Defective sight--from loss of transparency of the cornea,
the crystalline lens, or other transparent parts of the eye--does not
interfere with the perception of colour; nor is true colour blindness,
as I am informed, well marked, if present at all, in disease limited
to the choroid and retina (see Fig. 1). Even in cases of the disease
of the optic nerve, medical authorities tell us that great differences
exist in the amount of colour defect, and that _although the colour
defect always goes along with some other serious visual loss, either
of form, light, or field_, the relation between these several factors
of the visual defect is by no means always the same, so far as can be
judged by the tests commonly used by ophthalmic surgeons. They tell us
that in some cases of disease of the optic nerve, colour vision when
tested by the wool test, which will be described shortly, may be almost
perfect, whilst the capacity for reading test letters of the alphabet
may be extremely bad, and _vice versâ_. It seems that in some cases
these discrepancies cannot be accounted for; but in others the facts
can be explained by the limitation of the disease to certain fibres of
the optic nerve. Thus, if those fibres which supply the yellow spot
region of the retina are alone involved, direct, or central, vision
will be much damaged both for form and colour, whilst a little further
from the centre of the field, the visual functions in such a case are
often quite normal. From what has been said in the opening chapters,
this will be understood to be that the colour vision is perfect, but
the definition of form more or less imperfect. We are told that cases
of this type have long been known and are comparatively common, and
often favourable as regards recovery; that the mischief may affect one
optic nerve, or both; that when both are diseased the malady is usually
due to the action of some toxic substance, and that of all substances
known to have this particular effect on the optic nerves tobacco is
the most important. I dwell a little on this variety--damage to form
and colour sense at the centre of the visual field of each eye from
limited, and usually curable, disease of the optic nerve--on account
of its interest to myself in the investigations I have made, and also
on account of the degree of practical importance which it assumes in
connection with the proper reading of signals and coloured lights.
These cases of “tobacco amblyopia,” as it is pathologically called,
are, of course, always found in men; and it may occasionally happen
that such a man, if an engine driver, signalman, or a look-out man on
board ship, may still see form sufficiently well to see his signals,
but may mistake their true colours. From evidence given before the
Committee of the Royal Society on Colour Vision, it appears that the
disease causing this type of colour blindness is usually produced by
the over-use of tobacco, aided by mental depression and a low state
of health. As we have no sumptuary laws, cases of tobacco blindness
must frequently occur, and it should be the care of all who have the
management of railways or shipping to take measures for preventing
persons suffering from this disease from occupying posts which require
perfect colour vision in order to prevent the possibility of loss of
life.
Congenital colour blindness can at once be discovered, and its
possessor be excluded from any post in which normal colour perception
is necessary, but with this type a single examination is no safeguard,
as it may be developed at any period of a man’s career. The disease
is, I believe, a progressive one, and at first is most generally
unrecognised, the deficiencies of vision being usually slight at its
commencement. It is very often brought to the notice of the sufferer
by finding he is unable to read. The words at first seem only slightly
indistinct, but later become undecipherable, and as time goes on he is
unable to even see the letters. He or his friends then usually think
it time to consult the specialist. In tobacco amblyopia the area of
insensibility is central, and it may subtend a very small angle or one
which covers a considerable portion of the field. I am not aware that
it ever extends over it all, but it very generally covers the yellow
spot. Now as the eye naturally receives the image on the centre of the
retina, it follows that, as the ability to distinguish some colours is
absent in that particular region, the patient is practically colour
blind, though he can distinguish them on most parts of the retina which
are not affected. As regards form vision, it was mentioned in the first
chapter that in a healthy eye it is much more acute at the centre
than towards the periphery, and instances were given of the angular
distances apart that black dots on a white ground were required to be
placed to allow their being seen as separate objects when the images
were received on the centre of the retina, and at the periphery. Sharp
definition may be said to be almost confined to 3° of angular distance
at the centre, and most probably this is a happy state of affairs, for
if we could see equally distinctly with the whole field of vision, the
mind would be distracted from the object which it wished primarily to
contemplate.
Bearing in mind the want of definition beyond 3°, and the
indistinctness caused by a diseased central area, it will not be
surprising to find that form vision in these cases is imperfect
throughout, though the colour perception outside such area may be
unimpaired. But, practically, men suffering from this disease are
colour blind to coloured objects, such as a signal light on a railway
or a ship’s light at sea. They may see that there is light at the
distant signal or on the bow of a vessel, but will be unable to
interpret correctly the colour. The colours which fail to make visual
impressions are the reds and greens. Some will distinguish yellow, and
very nearly all will distinguish blue with the centre of the eye. If
a bright spectrum be thrown on the screen, and a tobacco-blind person
be requested to name the colours of the different parts pointed out
to him, it is often the case that as his eyes follow the pointer he
will tell you that in the extreme red he sees no light, but in the
bright red he sees dull white. The bright yellow he will tell you is
a pale yellow or white, according as his case is a moderate or bad
one; the green he will call white, and the blue and violet he will
designate correctly. At the same time that his eye is turned away to
another colour, he will see the true colour of the part of the spectrum
which he has just incorrectly named, but it will disappear again as he
turns his eyes back again. This tells us that his sense of colour is
apparently unaffected outside the diseased area.
[Illustration: FIG. 35.
_Left Eye._ _Right Eye._]
At page 10, a description has been given of the manner in which the
field for colour and light has been determined, and if this same method
be pursued with persons suffering from this form of colour blindness
we get some remarkable results. Fig. 35 is the chart of the eye for
red and for white, which was made by a case of tobacco blindness. The
yellow spot is entirely affected, and, as is very common, it extends
to the blind spot in the eye. At no place within that area can red be
seen, though blue is immediately recognised. The extent of the field
for white is that found under normal conditions, and except for the
diseased area the same is true for the red. The fields for both eyes
are given: that for the left eye in the left-hand chart, and that for
the right eye in the right-hand chart. The small dark spots within the
5° area are places where the colour sensation is most defective. The
part in the central dark area shaded with lines in this direction ////
shows the portion of the field which is insensitive to red, though not
to _light_, whilst the remainder of the shaded central area indicates
the extent of the field which is sensitive to red. The field for light
generally is also shown by the (approximately) rectangular unshaded
area. Although the area occupied by the insensitive part of the retina
is small compared with the whole, yet it is in that part which is used
for distinct vision.
For testing for colour the apparatus, Fig. 3, arranged so that the
patch of colour has the white patch alongside, is the most useful, but
it is as well then to use a surface of patch about ½ inch square only,
and thus to confine the image as nearly as may be to the spot on the
retina which is defective. These cases of central scotoma are by no
means very easy to test; for it frequently happens that before they are
able to distinguish that there are two patches side by side, they have
to approach very close to the screen. If this be the case, however,
it will usually be found that the patches of ½ inch side are still
efficient, as the near approach of the eyes to the screen indicates
a wide area as being affected, so that the image still lies within
the diseased retinal area. In some instances the colours named will
vary very considerably; sometimes, for instance, a red will be named
as grey, and then immediately after as pale red. This is generally
due to the diseased area being small, and a very slight change in the
direction of the axis of the eye causes it to be seen in nearly its
true colour, part being viewed with the diseased and part with the
healthy portion of the retina. With the wool test, which we shall
describe later, it is the commonest thing possible for colour-blind
persons who have a central scotoma to match accurately the different
test-skeins, for the reason that the images of the skeins of wool are
so large that they are received on the parts of the retina which are
not diseased. These same colours, however, if presented to them in
small patches, will inevitably show the defect in vision.
With this end in view, I have had a set of brick-clay pellets some
3/16-inch in diameter, painted with water-colours mixed with soluble
glass solution of the same colours as the wools. These are placed in
a shallow tray, and presented to patients affected with this central
colour blindness to pick out all the pellets which match reds and
greens. They will tell you that they see neither one nor the other,
though they will pick out the blue pellets unerringly. A red pellet
they will match with a red, green, grey, or a brown one, and a green
one with the same. If, however, you instruct them to direct their eyes
a few degrees away from the tray, they will tell you they see all the
colours, and as they endeavour to pick them out, they, with a natural
instinct, direct their eyes again to the collection, when once more
the colours vanish. It is almost piteous sometimes to see the distress
which this simple test occasions. The sight of the colours for an
instant and their immediate disappearance in the cases that I have
tried, seem indicative of something terrible, for they usually have no
idea of the cause of this (to them almost miraculous) phenomenon. I
have seen these colour blind tested with a pair of ordinary bull’s-eye
lanterns, placed side by side, with diaphragms of moderate size with
coloured glasses, which can be changed at will, in front. At twelve
feet distance they will often see both lights as one, but as they
approach they will make out two lights and call them both white, or
sometimes they will make a guess and call a green red, or _vice versâ_.
It goes without saying that such eyesight is useless for reading
signals, and indeed for any purpose whatever. Sometimes, but I believe
this is rare, no colour whatever can be distinguished.