COLOUR VISION
[Illustration: TYPES OF COLOUR VISION.
W. DE W. A., DEL. W. GRIGGS, LITH.
]
COLOUR VISION
BEING
THE TYNDALL LECTURES
DELIVERED IN 1894
AT
_THE ROYAL INSTITUTION_
BY
CAPT. W. DE W. ABNEY, C.B., D.C.L., F.R.S.,
LATE ROYAL ENGINEERS
WITH COLOURED PLATE AND NUMEROUS DIAGRAMS
NEW YORK
WILLIAM WOOD AND COMPANY
CONTENTS.
PAGE
PREFACE vii
CHAPTER I.
The Eye 1
CHAPTER II.
Simple Colours and their Mixture 15
CHAPTER III.
Three Colour Sensations Possible 32
CHAPTER IV.
The Young and Hering Theories of Colour Vision 41
CHAPTER V.
General Aspect of Colour Blindness 58
CHAPTER VI.
Colour Blindness exhibited by Colour Discs and exhibited by
Luminosity Curves of the Spectrum 74
CHAPTER VII.
Luminosity of Colours to Different Parts of the Retina 88
CHAPTER VIII.
Luminosity of a Feeble Spectrum and the Limit of the Perception
of Colour 98
CHAPTER IX.
The Extinction of Light from the Spectrum 108
CHAPTER X.
The Extinction of the Perception of Light by the Colour Blind 122
CHAPTER XI.
Tobacco Blindness 137
CHAPTER XII.
Examples of Colour Blindness due to Disease 148
CHAPTER XIII.
The Holmgren Test for Colour Blindness 167
CHAPTER XIV.
The Spectrum Test for Colour Blindness 180
CHAPTER XV.
The Young and Hering Theories of Colour Vision Compared 187
APPENDIX 201
INDEX 229
PREFACE.
The writer had for some years past, in conjunction with General
Festing, and recently as Secretary and Member of the Colour Vision
Committee of the Royal Society, carried out a series of investigations
on colour vision, and selected that subject when he was invited, in
1894, to deliver the Tyndall Lectures at the Royal Institution.
The brief time allotted for these lectures--an hour on three successive
Saturday afternoons--restricted the discussion of some aspects of the
question, and confined its treatment in the main to those features most
readily explicable by the physicist, and to bringing into notice the
latest results which had been obtained from physical experiments. How
far the writer has succeeded in the task which he then outlined it is
for the reader to determine.
There was no intention in the first instance to publish these lectures.
After their delivery, many persons expressed a desire that the
information they contained should be rendered accessible to such as
were interested in the theory of colour vision, and in deference to
that desire the lecture-notes have been re-cast in book form. For the
reader’s convenience the matter is now divided into chapters instead
of into lectures, and a few additions have been made in the text to
explain some of the experimental work to those who have not facilities
for its repetition.
The writer has to acknowledge several debts of gratitude. First, to Mr.
E. Nettleship, for his kindness in looking over the proofs, and making
valuable suggestions whilst the work was passing through the press; and
also, as will be seen throughout its pages, for many of the interesting
cases of defective colour perception which have been examined by the
somewhat novel methods described. Next, the writer’s gratitude is
due to Professor M. Foster for the permission he has given to use his
admirable description of the Hering theory; and, lastly, to the Royal
Society for the permission it accorded to use various diagrams which
have served as illustrations to papers which have appeared in its
“Philosophical Transactions” and “Proceedings.”
COLOUR VISION.
CHAPTER I.
I must commence this course by saying that I feel the honour that has
been done me in asking me to undertake it, connected as it is with the
name of Tyndall, whose recent removal from our midst has been deplored
by all lovers of science, and by none more than by those who have had
the privilege of listening to him at this Institution. It is my duty
to speak on some subject of physics, and the subject I have chosen
is Colour Vision. I hope it will not be considered inappropriate,
since it was Thomas Young, the physicist, whose connection with this
Institution is well known, who first propounded a really philosophical
theory of the subject. Interesting as it may be to trace how old
theories have failed and new ones have started, I feel that for those
who, like myself, have but little time at command in which to address
you, the historical side of this question must of necessity be treated
incompletely.
Colour vision is a subject which enters into the domains both of
physics and physiology, and it is thus difficult for any one individual
to treat of it exhaustively unless he be a Helmholtz, who was as
distinguished in the one branch of science as he was in the other.
I am not a physiologist, and at the most, can only pretend to an
elementary knowledge of the physiology of the eye, but I trust it is
sufficient to prevent myself from falling into any grievous error. I
shall try and show you, however, that the subject is capable of being
made subordinate to physical methods of examination. I must necessarily
commence by a very brief description of those parts of the eye in which
it is supposed the seat of vision lies, but in terms which are not
too technical. As to the mere optical properties of the eye I shall
say but little, for they are not necessary for my purpose, although
more particularly adapted to mathematical treatment than the other
properties I have to describe.
The eye may be diagrammatically represented as in the figure which is
supposed to be a horizontal section of it, the figure being reproduced
from Professor Michael Foster’s Physiology.
[Illustration: FIG. 1.
_Scl_ is the sclerotic coat. _Ch_ the choroid coat, with _CP_ the
ciliary process. _I_ is the body of the Iris. _R_ is the retina
or inner wall. _PE_ the pigment epithelium or outer wall. _L_ the
lens held by the suspensory ligament _sp.l._ _VH_ is the vitreous
humour. _ON_ the optic nerve, _ox_ is the optic axis, in this case
made to pass through the fovea centralis, _f.c._
]
As far as the perception of colour is concerned, the principal part
of the eye which is not distinctly optical--_i.e._ for the production
of images--is the retina, and this it will be seen is in reality an
outcrop of the brain, the connection between the two being the optic
nerve. Owing to this connection, it is not easy to determine where the
seat of colour perception is located; but for the purpose of physical
investigation this is not of first-rate importance, nor does it affect
the discussion of rival theories except in a minor degree. There are
other subsidiary adjuncts in the eye to which, however, I must call
attention, as they have a distinct bearing on the apparent intensity
of some colours and of the hue that mixtures of others are perceived.
The first is what is called the “macula lutea,” or yellow spot, a spot
which it may be assumed exists in every eye. It is horizontally oval
in form, and is situated in the very centre of the retina, embracing
some 6° to 8° in angular measure. It has a brownish or yellowish tint,
and the retina at this part is slightly depressed, being bounded by a
slightly raised rim. In the centre of this area the retina becomes very
thin, having a depression about 1/100 of an inch or ·3 millimetres in
diameter, which is named the “fovea centralis,” where it is said that
vision is the most acute. This statement can be well credited when we
come to consider where the seat of the stimulation of sensation lies.
The colour which tints the yellow spot is strongest at the crater-like
rim, and fades away centrally and peripherally, and is said to be
wholly absent in the fovea centralis.
As the colour of this spot is yellow or brown in the living eye (and
that it is probably brown the absorption indicates), it follows
that white light passing through it must be deprived of some of its
components, though in differing degrees. If the seat of sensation is
at the outer layer of the retina, as we shall shortly see must be the
case, it will further be seen that when light of any colour which the
brown pigment will absorb more or less completely falls on different
parts of the oval area, the absorption must vary at each part, and the
intensity of the perceived light will be least at the rim and increase
centrally and peripherally. As the centre of the yellow spot or fovea
is coincident approximately with the point where the axis of the eye
cuts the retina, the image of an evenly illuminated object, when looked
at directly, must fall on the yellow spot. If, therefore, a patch of
such light, the image of which more than covers the spot, be observed,
it ought to exhibit a varying brightness of colour corresponding to
the strength of the colouring matter which exists at the different
parts. This it but rarely does, for habit and constant interpretation
of what should be seen prevents the mind from distinguishing these
variations; but if the colour brightness, as perceived by the different
parts, be submitted to measurement by proper means, the variations in
brightness of the image can be readily recognised. A very common method
of exhibiting the presence of the pigment is to look at a bright white
cloud through a layer of chrome alum. Chrome alum transmits red and
blue-green rays. Now as the spectrum-blue rays are those which the
pigment will absorb, it follows that the colour of the solution should
appear ruddy to the central part of the eye, but on the rest of the
retina it should appear of its ordinary purplish colour. At a first
glance, and before the eye has become fatigued, this is the case, but
the phenomenon soon disappears. Another way of forming an idea as to
what the yellow spot absorbs is to throw a feeble spectrum on a white
surface and cause the eye to travel along it. If the spectrum be viewed
so that it does not occupy more than about 40° of the retina, the
movement of the eye will show a dark band travelling along the green,
blue, and violet regions as the image of these parts of the spectrum
fall on the yellow spot, and their apparent brightness will increase as
they fall outside the absorbing area. This proves that an absorption
takes place in this area.
[Illustration: FIG. 2.]
The retina consists essentially of an inner and outer wall, enclosing
matter which is similar to the grey matter of the brain. On the inner
wall are the vessels which are connected with the optic nerve. The
outer wall is epithelium coloured with a pigment, and it is here that
the visual impulses begin, although the rays of light giving rise
to them have to pass through the thickness of the retina before so
doing. It has already been stated that the light has to pass through
the thickness of the yellow spot before a visual sensation is felt in
the centre of the field, and the experiments just given offer a fair
proof of the truth of the assertion, but there is still another which
is perhaps more conclusive. Suppose we have a hollow reflecting ball,
as shown in Fig. 2, and through an orifice A we project a beam of light
to B, which meets an obstruction, S, in its path, then A B would be
reflected from B to C on a screen C F, and the obstruction S would be
marked at C. If another beam from D was directed so as to meet the same
obstruction, its presence would be marked at F. Knowing the distance
of the centre O of the hollow sphere from F C and its diameter, and
measuring the distance between F and C and their respective distances
from the axis of the sphere, the distances S B and S E can be
calculated. This method is applied in the formation of what are known
as Purkinje’s figures. The simplest case is where a beam of light is
directed through the sclerotic and transmitted through the lens. Images
of the retinal vessels are distinguished as at S, and it is found that
they cast shadows, which are seen as dark lines in the glare of the
field of vision. The sensation of light must therefore come from behind
these vessels, and calculation shows that the seat of the sensation is
close to the pigmented inner wall of the retina.
Lying here is a layer of what are known as rods and cones, which have
a connection, either actual or functional, with the optic fibres which
largely compose the inner wall of the retina, and are connected with
the optic nerve. In the yellow spot the cones are much more numerous
than the rods, but in the peripheral part the reverse is the case. In
the fovea the rods appear to be altogether absent. The total number
of cones in the eye has been calculated to be about 3,000,000, of
which about 7,000 are in the small fovea. The number of cones will
give an idea of their dimensions. This detail has been entered into
as it has been supposed that these rods and cones are all-important
in translating light-waves into visual impulses. The inner wall of
the retina of most human eyes, as has been mentioned, is stained with
a black pigment, fuscin, though in albinos it is absent. What its
particular use may be is still unknown, for its change by light is so
slow that it can scarcely be the cause of vision. In the outer parts of
the rods is, however, diffused a substance highly sensitive to light,
called the “visual purple,” from its colour, and a theory founded on
chemical action, produced by a change in this substance, has been
promulgated. Fascinating, however, as such a theory must be, it lacks
confirmation. The fact that the cones do not contain it, and that in
the fovea are cones alone, renders it difficult to reconcile the theory
with the fact that this part of the retina possesses, we are told, the
greatest acuteness of sensation as regards light and colour.
The eyes of most vertebrate animals, it may be remarked, have this
visual purple, but in those of the bat, owl, hen, and some others the
colouring matter seems to be absent. Visual purple is an interesting
substance, however, and as it is found in the eye it probably exercises
some useful function, though what that function may be is at present
unknown. That images of objects can be formed on the retina, owing to
the bleaching of this substance, has been proved by experiment. The
purple is first changed to a yellow colour, and then passes into white.
These “optograms,” as they are called, can be fixed in an excised eye
if the retina be detached, and then be treated with a weak solution of
alum.
[Illustration: FIG. 3.]
Many persons are not aware of the extent of the field of view which the
eye embraces. Vertically it takes in about 100°, whilst horizontally
it will take in some 145°, more or less. The field is smaller on the
nasal than on the temporal side. When both eyes are used, the combined
field of view is larger horizontally, being about 180°. The field of
view which is common to both eyes is roughly a circle of about 90°.
There is, however, a marked difference in the distinctness with which
objects are perceived in the different parts of field of view. On the
fovea centralis two dots placed so as to subtend an angle of 60″ will
be perceived as double. That is to say, if a piece of paper, on which
are two dots 1/30 of an inch apart, be placed 10 feet away from the
observer, these dots will be seen as separated, whilst dots (in this
case they should be black and of good dimensions) placed half-an-inch
apart would still appear as one if viewed at the same distance near
the periphery of the retina. In the yellow spot the distance apart of
the cones is such that they subtend about the same angle as the dots
when they are seen separate, viz., about 60″; that is, they are about
16/100000 of an inch apart, and hence may have something to say to
the limit of separation. The field for the perception of colour is
different to that for light.
The diagrams (Fig. 3) will show the fields in a satisfactory manner.
The concentric circles are supposed to be circles lying on the retina
corresponding to parallels of latitude on a globe, and are not,
therefore, equi-distant when seen in projection. To make these circles
it must be imagined that we have a bowl, in the middle of which is a
thin rod standing upright and passing through the centre, and another
rod attached to it at the centre of the sphere of exactly the length
of the radius. If this last arm be opened to make an angle of 5° with
the fixed rod, and be twisted round like the leg of a compass against
the bowl, it will make a circle, the projection of which will give the
innermost circle of the diagram; if opened to 10° it will give the next
circle, and so on for every subsequent 10°. The lines passing through
the centre are 30° from one another, the line stretching from 360° to
180° being a line supposed to be vertical. By means of an instrument
called the perimeter, the field of vision for each eye can be measured.
With its aid any small object can be made to fall on any part of the
retina by directing the axis of the eye to a fixed point and moving
the object along one of the diameters. Suppose we wish to ascertain
the field for a white object, a small white disc is moved, say, along
the horizontal line, and the angles at which the retina just no longer
sees it are noted. This gives two points in the field, and they are
plotted on the chart--in Fig. 3 one touches the outside circle, and the
other is at an angle of about 65°. The field of vision is next tested
along another line, say 300° to 120°, and other points noted and marked
on the chart. When the whole circle has been examined, the various
points are joined together, and we have the boundary of vision for a
white object. The boundaries of the _colour_ perception for (say) small
red and green discs are found in the same way. The former is depicted
in the left-hand chart and gives the field for the right eye, and the
latter with that for white in the right-hand chart for the same eye.
It will be noticed that two boundaries are given, one taken at mid-day
and the other at 6 p.m. The brighter the colour, the larger is the
boundary in both cases, showing that the field of colour vision varies
according to the illumination. Now it is difficult from this method of
experimenting to determine whether the fields for different colours are
the same or differ in extent, as we have no information as to whether
the colours themselves which were used were physiologically equal. The
only way by which this can be satisfactorily determined is by using
spectrum colours each of known brightness and area. (Some preliminary
experiments made by myself regarding the colour fields will be found in
the appendix, and will be referred to later.) It must not be thought
that the various colour boundaries mark the limit at which _light_ is
perceived, but only the limit at which colour is seen; outside the
boundaries the objects appear of a nondescript colour, to which we
shall by-and-by call attention. The yellow spot lies within the circle
of 5°, and the blind spot on which no sensation of light is stimulated
is shown by the black dot about 15° away from the centre.
I have only attempted to sketch, in unphysiological language, the
primary apparatus with which our experiments in colour have perforce to
be made.