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AN INTRODUCTION TO THE STUDY
OF COLOUR VISION

CAMBRIDGE UNIVERSITY PRESS

C. F. CLAY, Manager

Hotliloti: FETTER LANE, E.G.

CBlJmtiutgf) : loo, PRINCES STREET

ILontfon: H. K. LEWIS, 136 GOWER STREET, W.C.

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STokoo: THE MARUZEN-KABUSHIKI-KAISHA

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AN INTRODUCTION

TO THE STUDY OF

COLOUR VISION

J. HERBERT PARSONS, D.Sc, F.R.C.S.,

OPHTHALMIC SURGEON, UNIVERSITY COLLEGE HOSPITAL;
SURGEON, ROYAL LONDON (mOORFIELDS) OPHTHALMIC HOSPITAL

Cambridge :
at the University Press

New York :
G. P. Putnam's Sons
1915

Cambritgr :

PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS

.'> \

PREFACE

rpHE vast literature on colour vision consists almost entirely of
-^ papers written in support of some particular theory. It is
peculiarly difficult to obtain a general and unbiassed view of the
subject. I have here endeavoured to separate the best established
facts of colour vision from the theories, and have then discussed the
chief theories in the light of these facts.

I wish to express my great indebtedness to Sir WilUam Abney,
K.C.B., F.R.S., Professor W. Watson, F.R.S., and Dr Myers for their
invaluable assistance in a very difficult task, which demands a not
inconsiderable knowledge of such diverse subjects as physics, physiology,
and psychology. The chief references to the literature have been given
in footnotes. The frequent references to " Abney " are to his Researches
in Colour Vision (London, 1913), and to " v. Helmholtz" to his Hand-
buch der physiologischen Optik. In the latter case each of the three
editions contains material which is absent from the others, and the
exact reference is given in the footnote.

Note. Fig. 12, p. 47. The discrepancy in these curves has been
found to be due to a technical defect in the apparatus. More recent
observations show the identity of the curves obtained by the two
methods, thus confirming Ives's results (Fig. 38).

J. H. P.

November, 1914

/ G 0 zt

CONTENTS

Section
Chap.
Chap.
Chap.

Section
Chap.
Chap.
Chap.
Chap.

Section
Chap.
Chap.

Section
Chap.

Chap.
Chap.

Section
Chap.
Chap.

Section
Chap.
Chap.
Chap.
Chap.

PART 1

THE CHIEF FACTS OF NORMAL COLOUR VISION

I. The Bases of Colour Vision. page

I The Physical Basis 1

II. The Anatomical Basis 7

The Psychological Basis . . . . . . . 17

The Spectrum as seen by the Light-adapted (Photopio) Eye.

The Spectrum: Hue, Luminosity, Saturation ... 27

The Discrimination of Hue in the Spectrum ... 30

The Mixture of Pure-colour Stimuli 33

Tlie Luminosity of the Spectrum 42

The Spectrum as seen by the Dark-adapted (Scotopic) Eye.

III.

IL
I.
II.
Ill,
IV.

III.
I.
IL

IV.
I.
II.

V.
I.

II.
Ill

VI.
I.
II.

VII.
I.
II.
Ill
IV.

Adaptation or Temporal Induction

Scotopia or TwiUght Vision

Regional Effects.

The Field of Vision for Colours

The Macula lutea and Fovea centralis ....

Temporal Effects.

Recurrent Vision; the Talbot-Plateau Law; the Flicker
Phenomenon ........

Successive Induction or After-images ....
, The Effects of "Fatigue"

Areal Effects.
The Local Quantitative Effect ......

Simultaneous Contrast or Spatial Induction

The Evolution of Colour Vision.

Introduction .........

The Comparative Psychology of Colour Vision .

The Colour Vision of Primitive Races . . . .

The Development of Colour Vision in the Child

49
52

85
101
112

117
125

130
131
145
152

PART II

THE CHIEF FACTS OF COLOUR BLINDNESS

Chap. I. Introduction: Colour Names 158

Chap. II. Dichromatic Vision 162

Chap. III. Anomalous Trichromatic Vision 182

Chap. IV. Monochromatic Vision 186

Vlll

CONTENTS

PART III
THE CHIEF THEORIES OF COLOUR VISION

Section I.

General Review.

Chap. I.

Introduction .....

193

Chap. II.

Historical Review of Modern Theories of Colour Vision

196

Section II.

The Duplicity Theory

203

Section III.

The Three-Components Theory (Young-Helmholtz).

Chap. I.

Statement of the Theory

213

Chap. II.

Researches based upon the Theory

Normal Colour Vision

220

II.

Dichromatic Vision

• •

232

III.

Anomalous Trichromatic Vision :

Approximate

Dichromatism

> •

235

IV.

Anomalous Trichromatic Vision : Shift of a Sensatior

Ctu-ve

243

Section IV.

The Opponent Colours Theory (Hering).

Chap. I.

Statement of the Theory

, ,

251

Chap. IT.

Researches based upon the Theory

263

Section V.

Other Theories.

Donders' Theory

270

II.

Ladd-Franklin's Theory

271

III.

McDougall's Theory

274

IV.

Schenck's Theory .

285

Wundt's Photochemical Theory

289

VI.

G. E. Muller's Theory

290

VII.

Edridge-Green's Theory

291

Index of Subjects .

301

Index of Authors .

305

The Prismatic Spectrum of Sunlight, showing the positions of the chief

Fraunhofer lines and wave-lengths . . .... Frontispiece

The author wishes to make grateful acknowledgment for permission to repro-
duce the following iDustrations :

to the Royal Society for Figs. 68 — 74; to the Royal Society and Messrs
Longmans, Green & Co. for Figs. 11, 12, 14, 19—23, 28—33, 40, 41, 49, 61—65;
to the Kaiserliche Akademie der Wissenschaft of Vienna for Figs. 5, 60, 66 and 67 ;
to the proprietors of the Journal of Physiology for Figs. 2, 3 and 17, of the British
Journal of Psychology for Figs. 34 — 37, 42 and 43, of the Philosophical Magazine
for Figs. 38 and 39, of Mind for Fig. 75, and of the Zeitschrift fur Psychologic und
Physiologie der Sinnesorgane for Figs. 15, 16, 25 — 27, 45, 46 and 50 — 55; and to
Messrs Vieweg und Sohn, Braunschweig, for Figs. 1, 6, 7, 13 and 57.

PART I

THE CHIEF FACTS OF NORMAL
COLOUR VISION

SECTION I

THE BASES OF COLOUR VISION
CHAPTER I

THE PHYSICAL BASIS

What is generally understood by the term " light " is a composite
congeries of allied manifestations of energy, comprising such apparently
various phenomena as heat, Hght in the narrower sense of the word,
and chemical action. Various as these phenomena are, they are
physically identical in character, all consisting of radiant energy in the
form of waves of identical character, differing only in the length and
rapidity of the vibrations. Broadly speaking, the longest waves cause
the sensation of heat, the shortest give rise to chemical action, whilst
those of intermediate length cause the sensation of light.

If we take ordinary sunlight as the basis of our investigations, it is
possible to split it up by appropriate means into its component "rays,"
differing from each other in wave-length. Of these certain are visible,
and constitute light in the narrower sense of the word, but instead of
giving rise to the sensation of white light, they, to the majority of
people, show certain pure colours, viz. red, orange, yellow, green, blue,
and violet, in order, the red having the longest and the violet the shortest
wave-length. The visible spectrum extends from about 723 ixfi at the
red end to 397ju,/li at the violet end. v. Helmholtz under the most
favourable conditions was able to see as far as about 8^5 [x/jl. The
limitation of the spectrum at the violet end is less precise, because the
rays in this neighbourhood are changed into rays of greater wave-length
p. c. V. 1

2 COLOUR VISION

by the media of the eye, particularly the lens and retina. This " fluor-
escence " causes them to produce a lavender-hued sensation, which does
not denote true visibility of the short wave-length rays. Beyond the
red end are waves of greater length (extending to 60,000 /^/j), which when
absorbed cause a rise in temperature ; beyond the violet end are waves
of smaller wave-length, which are capable of causing chemical action.
So striking is the physiological phenomenon of the visibility of the
intermediate series that the heat rays are commonly spoken of as
" infra-red," and the actinic or chemical rays as " ultra-violet." This
custom is unfortunate, since it tends to obscure the importance
of the physical uniformity of the series. For example, not every
normal individual is able to see all the rays from 723 fifx to 397 /x/x ; for
most people the range is less extensive, roughly from 700 /x/x to 400 /x/x.
Again, though the ultra-violet rays are particularly potent in inducing
chemical action, the visible rays are also, but in less degree, actinic,
and the same is true, in still less degree, of the infra-red rays. Further,
all rays when absorbed cause a rise in temperature. The most convenient
and striking method of demonstrating actinicity is by the photographic
film, so that we have come to regard a photograph of the spectrum as a
complete analysis of the light under observation, too often forgetting
that the photographic effect varies with the specific sensitiveness of the
film to particular groups of waves. Thus it is only by specially sensitized
films, invented by Sir William Abney, that it is possible to demonstrate
infra-red rays photographically.

It is further essential that the methods employed for analysis of the
light be suitable for their purpose. For example, an ordinary spectro-
scope, with glass prisms and lenses, suffices to demonstrate the visible
spectrum, but is almost useless for showing the ultra-violet rays, since
these are absorbed by the glass. In order to demonstrate the full
extent of the spectrum it is necessary to use a train of lenses and prisms
made of quartz or Iceland spar, which allows a maximum of rays to pass
unimpeded.

Probably more error has crept into the subject of colour vision from
inexact description of experimental conditions and the nature of the
stimuli employed than from any other cause. Two green lights may
appear identical in colour to the eye, yet their physical characters may
differ widely. Again, mixing a yellow and a blue pigment will produce
a green pigment, yet the more general statement that green results
from mixing yellow and blue is not accurate.

The complete range of simple colours can be obtained in a pure state

THE PHYSICAL BASIS

by only two methods, dispersion and diffraction. When white light
is passed through a glass prism, as in Sir Isaac Newton's original experi-
ment, a spectrum is obtained. Only under certain, now well-defined
conditions is such a spectrum pure, i.e. the colours do not overlap. It
is commonly said that the white light is " split up " into its component
parts, which are coloured. Lord Rayleigh has given sound reasons for
the view that white light is not thus analysed into component parts,
but that the periodicities characteristic of the several rays are in reality
imposed by the prism and are not antecedently present in the white
light. Be this as it may, dispersion of white light by prisms enables
us to obtain coloured light in a pure state. By passing white light
through a diffraction grating a pure spectrum can also be obtained.
This method has the advantage that the deviation of the component
rays varies within narrow limits directly with the wave-length, i.e. equal
differences of wave-length are separated by equal distances in the
spectrum. It suffers, however, from the disadvantage that the spectrum
is less bright and less extended than the prismatic spectrum, and from
the still greater objection that the interference spectrum is never free from
scattered Kght. In the prismatic spectrum the dispersion increases as
the wave-length diminishes, so that the violet end is much more extended
than the red end and its intensity is diminished. Moreover the amount
of dispersion depends upon the character of the prism or prisms employed.
Hence it is necessary for accurate observations that each prismatic
spectrum shall be calibrated. The Fraunhofer lines, being absolutely
constant in situation, afford a series of fixed points from which the
calibration curve of the given spectrum can be constructed^. The

A. U.

A = 7606 in extreme red.
B = 6869 in deep red.

Lithium . .

= 6707

bright red.

Hydrogen

= 6564

bright red.

Sodium . .

.. A

= 5897

orange.

Sodium . .

= 5891

orange.

Thallium . .

= 5351

yellow green

= 5271

green.

Magnesium

.. b,

= 5184

green.

Magnesium

b-2

= 5174

green.

Hydrogen

= 4862

blue green.

Strontium

= 4609

blue.

= 4308

violet.

Calcium . .

= 3969

extreme violet

Calcium . .

= 3934

extreme violet

^ Burch, Practical Exarcincs in Physiological Optics, p. 102, Oxford, 1912.

1—2

4 COLOUR VISION

o ^ o

table gives the principal lines m Angstrom units (1 A.U.— one ten
millionth part of a millimetre = 0"1 ixfi).

The most convenient method of calibration, however, is by the
mercury lines as given by the " mercury arc^."

In spite of the necessity for calibration the prismatic spectrum is
more generally suited than the diffraction spectrum for physiological
experiments on account of its greater brightness and relative freedom
from scattered light.

Whatever spectrum be employed the source of hght must be constant.
Lights which we commonly regard as giving " white light," such as
sunlight, the arc light, incandescent light, and so on, vary much in
character and consequently in the constitution of their spectra.
Sunlight varies so much that it is generally unsuitable for the purposes
in view, the variations being not only in intensity but also in com-
position, owing to the unequal absorption of different rays by the
atmosphere, and this absorption again varies greatly according to the
amount and nature of the matter suspended in the air. The arc light
is the most satisfactory, and after this probably the Nernst lamp, though
the latter has not yet been sufficiently investigated^. Less satisfactory
are gas light, petroleum and so on, but as many of the experiments of
earlier observers have been made with such sources they have to be
considered if these researches are to receive due appreciation. Some
sources of illumination, especially used for investigation of the ultra-
violet rays, such as the Schott uviol mercury vapour lamps, are wholly
unsuited, since they do not give continuous spectra. For experiments
on colour vision many such details which cannot be discussed here must
be attended to^.

Suffice it to say that by taking proper precautions it is possible to
obtain a spectrum which is practically constant during the time necessary
to take a series of observations and which can be reproduced from time to
time with a minimum of variation.

If such a spectrum is viewed through the eyepiece of an ordinary
spectroscope a direct spectrum is seen. This method has usually been
adopted, as for example by Aubert, von Helmholtz, Clerk-Maxwell and
others. By a slight change in the optical arrangements the spectrum

1 Watson, Practical Physics, p. 309, 1906.

2 Abney, Researches in Colour Vision, 1913, Chap. v. ; Golant, Ztsch. f. Sinnesphysiol.
XLiii. 70, 1908.

^ E.g. Tigerstedt, Handb. d. physiol. Methodik, Bd. in. Abt. 2, Sinnesphysiologie n.
Lpipzig, 1909.

THE PHYSICAL BASIS 5

can be accurately focussed upon a screen. Such a projected spectrum
can then be viewed by several observers at the same time, a very con-
siderable advantage in testing colour vision. The use of a projected
spectrum necessitates further care in detail, for the character of the
spectrum will depend upon the optical properties of the screen^. A
matt white surface must be used and that obtained with magnesium
oxide is probably best.

In order to obtain the most accurate information from the experi-
ments the observations must be as far as possible quantitative and not
merely qualitative. In many physiological experiments this counsel of
perfection cannot be complied with and we are reduced to the informa-
tion which can be obtained from merely qualitative observations. When,
however, it is possible to obtain quantitative results it is generally
necessary to have a constant light for purposes of comparison. Now,
photometry is admittedly one of the most faulty of physical measure-
ments, chiefly, to use a paradox, because it is, in most cases, not really
physical but physiological. One of the most important and unique
features of Sir William Abney's apparatus is that the intensity of the
comparison light bears a constant physical relationship to that of
the spectrum used, since it is obtained by the reflection of a portion
of the original beam of Hght from the surface of the first prism. Hence
any variation in the original beam will cause similar and simultaneous
variations in both the spectrum and the comparison light^. Measurable
changes in the intensity of the light are best obtained by the use of rotating
sectors, sometimes called the episcotister (Aubert), or by the annulus,
a gelatine wedge impregnated with ivory black^. Reduction of intensity
by means of Nicol prisms as in v. Helmholtz' spectrophotometer, may
not be free from error, since quite an appreciable amount of polarisa-
tion of the light is produced by the prisms used to form the spectrum.
Much of the German work has been done by this method, and care has
by no means always been taken to calculate the corrections necessary
owing to this cause.

Pure spectral colours rarely occur in nature, and much of the litera-
ture on colour vision is devoted to observations with pigments, coloured
glasses and so on. It is necessary, therefore, to say a few words about
these complex colours, chiefly with the object of putting the reader
upon his guard. When white light passes through a red glass or trans-
parent red fluid certain rays are absorbed. The red rays are transmitted

1 Abney, p. 46. - Ibid. Chap. iv. » Ibid. Chap. vi.

6 COLOUR VISION

in greatest quantity, so that the dominant colour of the light reaching
the eye is red ; but it is not pure red. Most blue substances, such as
copper salts, allow the blue rays to pass, but also some of the green
and violet, though few of the red and yellow. Yellow substances allow
much red and green to pass as well as the yellow, but little blue and
violet. The true composition of the transmitted light can only be
determined with the spectroscope.

The case of pigments is similar. Each speck of powder is a small
transparent body which absorbs certain rays of light. When light falls
upon such a powder a small portion is reflected from the upper surface ;
this is white. The remainder passes deeper and is reflected from deeper
layers. The deeper it passes the greater is the absorption and the more
intense the colour. Hence a coarse powder appears more intensely
coloured than one which is finely divided. Reflection varies with the
number of surfaces, not with the thickness of the particles. The larger
the latter the deeper the light must penetrate for the same number of
surfaces to be met as when the particles are smaller. The absorption
is therefore greater in a coarse than in a fine powder. The reflection
at the surfaces is diminished when the intervals between the particles
are filled with a fluid of refractive index nearer their own than that of
air. Hence powders are generally whiter when dry than when mixed
with water or oil.

The amount of absorption of light by a transparent body can be
measured and expressed in the form of a coefficient. If a spectrum
is viewed through an orange glass very little red, orange and yellow,
but much green and all the blue are absorbed, as shown by dark bands
in the regions of absorption. In this case the coefficient of absorption
increases as the blue is approached. By knowing the coefficients of
absorption of different media the effects of combining them in various
ways can be calculated. By empirical experiments colour screens or
filters can be made which transmit certain portions only of the spectrum,
and in some cases approximately monochromatic light can be obtained
in this manner. These filters are much used for photographic purposes.
The characters of the absorption by Jena glass filters and by various
fluid media are described in Tigerstedt's Handbuch der pkysiologischen
Metliodih^, which also gives an excellent resume of the methods which
have been employed for the investigation of colour vision.

No pigments accurately represent spectral colours, for the reflected

1 Bd. III. Abt. 2, Sinnespkijsiologie ii pp. 47 and 52.

THE PHYSICAL BASIS 7

light is always more or less impure. The nearest approach is given by
the following list :

Red — vermilion (not scarlet vermilion) mixed with a small quantity of permanent
violet.

Orange — orange cadmium.

Yellow — chrome yellow.

Green — Prussian blue mixed with aurelin.

Blue-green — viridian mixed with a small amount of cobalt blue.

Blue — ultramarine.

Violet — permanent violet mixed with a small amount of blue, (Abney.)

CHAPTER II

THE ANATOMICAL BASIS

I do not propose to discuss fully the anatomy and physiology of the
eye and visual paths, but it is necessary to draw attention to certain
features of special importance in colour vision. This course will doubt-
less emphasise the great complexity of the subject, which is too often
wilfully ignored.

The eye resembles a photographic camera, in which the cornea and
crystalline lens represent the lens-system, the iris the diaphragm, and
the retina the sensitive plate. The size of the pupillary aperture is not
under voluntary control, but varies with the intensity of light entering
the eye and other causes. This fact has to be taken into consideration
in some experiments. (The reader is recommended to read the earlier
chapters in the author's Manual of Diseases of the Eye, 2nd edition,
J. and A. Churchill, London, 1912.) The optical system of the normal
eye at rest is focussed for distant objects, i.e. parallel rays are brought
to a focus upon the retina. Focussing for near objects is brought about
by automatically altering the " strength " of the crystalline lens (accom-
modation), not by altering the length of the eye as in the photographic
camera. Some eyes are naturally too short or their optical system at
rest is too weak, so that accommodation is required even for distant
objects (hypermetropia). Others are too long or their optical system
at rest is too strong so that it is impossible to focus distant objects with-
out the aid of concave spectacles and near objects may be seen clearly
without the aid of accommodation (myopia). Many eyes show slight
differences in the radius of curvature of the cornea in meridians at right
angles to each other and this is often associated with slight tilting of the
lens (regular astigmatism). The lens always shows slight irregularities

8 COLOUR VISION

causing irregular astigmatism. For these and other reasons the retinal
image of a luminous point is never accurately punctate^.

Further, the optical system of the eye is not achromatic. This fact
might be regarded as of extreme importance in the consideration of
colour vision, but in general the effect is negligible. One would also
expect diffraction at the edge of the pupil, and it can be demonstrated
to occur, especially when the pupil is small ; but this effect is also
negligible under most conditions. There are, however, occasions when
both these factors must be taken into account.

The crystalline lens normally possesses a slightly amber-yellow hue,
which is inappreciable in youth, but increases as age advances. In
elderly people the colour of the lens causes an appreciable absorption
of the more refrangible rays (green, blue and violet). This fact must
be borne in mind in estimating the visual sensations of such people.

The retina is a transparent membrane lining the back of the vitreous
chamber. It is composed of several layers, the outer of which is a
mosaic of rods and cones. The rods and cones are a neuro-epithelium,
connected with bipolar cells which form the outer nuclear layer. A
second set of cells forms the inner nuclear layer. The arborisations of
these two sets of cells form an intermediate outer reticular layer. Arbori-
sations from the inner nuclear layer and processes from the ganglion
cell layer form the inner reticular layer. The ganglion cell layer is com-
posed of larger cells which give rise to the axons of the optic nerve.
These axons form a layer upon the inner surface of the ganglion cell
layer, the nerve fibre layer.

The optic disc or head of the optic nerve is situated about 3 mm.
to the inner or nasal side of the posterior pole of the eye. Light falling
on this area causes no sensory impression (Mariotte's blind spot).

Outside the retina proper is the layer of retinal pigment epithelium.
It consists of cells which are hexagonal in section and have processes
passing forwards between the rods and cones. The cells contain minute
needle-shaped crystals of pigment. This layer bears some resemblance
to the " backing " of a photographic plate.

External to the retina is the choroid, a highly vascular membrane
having the function of nourishing the outer layers of the retina. The
inner layers of the retina are nourished by the retinal blood vessels,
which spring from the disc and radiate over the surface of the nerve
fibre layer. The walls of the retinal blood vessels are transparent, but

^ Parsons, " The Perce^jtion of a Luminous Point," Roy. Lond. Ophth. Hosj). Reports,
xvm. 239, 1012 ; xix. 104, 264, 274, 1913-4.

THE ANATOMICAL BASIS 9

since the colouring matter of the blood is contained in highly refractile
corpuscles the blood column is opaque.

The part of the retina to which most attention must be directed
for the present purpose is that comprising the rods and cones and the
pigment epithelium. It is shown conclusively by Purkinje's experi-
ment, which depends upon the position of shadows thrown by the retinal
vessels upon the percipient layer of the retina, that the primary seat
of the visual impulses is in the layer of rods and cones. Here the most
sharply defined image is formed by the optical system. As already
stated it is not perfectly defined physically, but as will be seen later
physiological and psychological compensation tends to counteract the
physical defects.

The rods and cones, as their names imply, are minute cylindrical
and conical structures. They project vertically — or more accurately
radially — from the surface of the outer limiting membrane of the retina.
Almost exactly at the posterior pole of the eye is situated a small area
in which vision is most distinct. This area is impregnated with a yellow
pigment and hence is called the macula lutea or yellow spot. In the
centre of the yellow spot there is a conical pit, the fovea centralis, caused
by thinning out of the retina. In this minute area the structures are
reduced to little more than neuroepithelium and ganglion cells.
Moreover the neuroepithelium in this region consists entirely of cones,
though the cones are slender and elongated and are more rod-like here
than elsewhere^.

This change in structure of the cones may be evidence of some
physiological combination of the functions of both rods and cones in
this situation.

Passing peripherally in every direction from the central fovea it is
found that rods gradually make their appearance between the cones,
and soon the number of rods in a given area becomes greater than that
of the cones, so that at the extreme periphery of the retina only a few
scattered cones are to be found.

The following are some measurements of the diameter of the foveal
cones : Max Schultze, 2-8 /m ; H. Miiller, not more than 3 ^ ; Merkel, 3/x ;
Welcker, S'l— 3-5;Lt; Wadsworth, 2-5 /x; Kuhnt. 2—2-5^; Kolliker,
4"5 — 5'4ju,; Koster, 4-4: /x; Greeff, 2-5 /x; Dimmer, 3 — 3'5/x.

The foveal region is an elliptical area with the long axis horizontal.
The long axis measures about 0'3 mm., the vertical 0*2 mm., and the
total area is 0"5 — 0*6 sq. mm. Taking the diameter of the inner hmbs

^ Greeff, in Graefe-Saemisch Handb. d. ges. Augenheilkunde, Theil i. Bd. i. Cap. v. 1900.

10 COLOUR VISION

of the foveal cones at 0*3 yCt, there are about 300 in the long axis, and
60 in the short, 1300 — 1400 in 0"1 sq. mm. The diameter of the outer
limbs is 0'6 — 0'75 ijl. The cones are arranged in curved lines (Max
Schultze) or spirals (Fritsch), and are not quite regular. There are
small spaces between them, measuring from 0*05 to 0'27 of the trans-
verse section of the inner limb. Greeif says that the cones are very
closely packed in the fovea, and in a specimen of Heine's were
hexagonal in transverse section.

Koster^ examined three normal children's eyes and found that the
part completely free from rods occupied a circular area 0*44 — 0'55 mm.
in diameter, the part relatively free from rods 0*88 mm. In the eye of
a youth aged 20, the rod-free area was 0-901 mm. He concludes that
in the adult the rod-free area measures about 0*8 mm. in diameter,
subtending a visual angle of 3° 3'. This is probably a maximum, and
there is physiological evidence to show that the rod-free area varies in
size in different individuals.

Three areas must be carefully distinguished :

Fovea Centralis, measuring 0*24 — 0'3 mm. in diameter, subtending
55'— 70' ;

Rod-free area, measuring 0'8 mm., subtending 3° 3' ;

Macula, measuring 1 — 3 mm., subtending 4° — 12°.

Dimmer- describes a fovea centralis, 1*5 mm. in diameter (the macula
of Koster), containing in its centre a foveola (the fovea centralis of
Koster).

Gullstrand^ regards the yellow colouration of the macula lutea as
a post-mortem change, a view which is scarcely consistent with its
absorptive capacity for coloured lights during life.

Fritsch^ describes the site of clearest vision as the area centralis,
possessing a central depression, the fovea centralis, which may or may
not contain a foveola.

Comparative Anatomy. The distribution of rods and cones in the
retinae of lower animals is of great theoretical importance. Many erro-
neous statements have gained currency and have been used as arguments
in favour of certain theories. The great variety in the forms of the
neuroepithelial cells prevents any generalised classification.

GreefE^ says that there are rods and cones in the retinae of most

1 Arch.f. Ophth. XLi. 4, 1, 1895 ; Arch. d'OpM. xv 428, 1895

2 Arch. f. Ophth. Lxv. 486, 1907.

■•» Arch.f. Ophth. lxii. 1, .378, 1905; lxvi. 141, 1907.

* Ueber Bau u. Bedeututig d. Area centralis des Menschen, Berlin, 1908.

^ Graefe-Saemisch Handb. d ges. Augenheilkunde, Theil i. Bd. i. Cap. v. 1900.

THE ANATOMICAL BASIS 11

mammals, amphibia and fishes, the number of rods much exceeding
that of cones. In birds, on the other hand, cones are much in excess of
rods. In most reptiles (lizards, snakes, tortoises), only cones are found.
There are vertebrates possessing only rods, e.g. amongst fishes, rays
and dog-fish ; amongst mammals, hedgehog, bat, mole and night-ape
{Nyctipithecus felinus). There are also animals of nocturnal habits
possessing only rods. Owls, mice, and rell-mice have only a few
rudimentary cones ; rats also possess a few cones.

Hess^ found rods in fowls and pigeons, though they are few in the
posterior and superior parts of the retina, which are most used in pecking.
As regards nocturnal birds Schultze (1866), Krause (1894), CTreeft' (1900)
and Piper (1905) give contradictory statements. Hess found a "not
inconsiderable number " of yellow or brown oil globules in the retinae
of the owl and hawk, and these are present only in cones. They are
fairly uniformly distributed.

Coloured oil globules are found in the cones of birds and reptiles ;
similar colourless bodies are found in fishes and amphibia. The globules
are more deeply coloured, yellow or brown, in night birds and in tortoises
than in day birds. They are absent in the crocodile.

Research has shown that at any rate in lower animals stimulation
of the retina by light is accompanied by structural, chemical, and
electrical changes^.

Structural Changes. The chief structural changes are the photo-
tropic reaction of the pigment epithelium and the contraction of the
cones. To these may be added changes in the Nissl granules of the
ganglion cells^.

When the frog's eye is exposed to light the pigment granules wander
into the cell processes between the rods and cones. This light effect
is complete after 5 to 10 minutes' exposure. The retreat of the granules
to the complete dark position takes one to two hours. The light effect
is limited to the area stimulated, so that an " epithelial optogram "
can be produced (Klihne). The violet end of the spectrum is more
strongly " retinomotor " than the red end (Angelucci^, Engelmann^),
and red light causes little reaction. Light on one eye causes wandering

1 Vergleichende Physiol, d. Gesichtsinnes, Jena, 1912.

2 Garten, in Graefe-Saemisch Handb. d. ges. Augenheilkuiide, Theil i. Brl. iii. Cap. xii.
Anhang, 1907-8.

3 Birch-Hirschfeld, Arch. f. Ophlh. L. 166, 1900; LXiii. 1, 8.5, 1906.
« Arch. f. Anat. u. Physiol. 353, 1878.

'^ Arch J. d. ges. Physiol, xxxv. 498, 1885.

12 COLOUR VISION

of the pigment in both (Engelmann). Light on the skin produces the
effect in frogs ; so too electrical stimulation of the optic nerve, probably
through the centrifugal nerve fibres described by Ramon y CajaF.
The phototropic reaction of the pigment epithelium has not yet been
proved to occur in mammals^.

In the frog's eye in the state of rest (darkness) the cones are extended ;
on exposure to light they become contracted. This reaction occurs in
all animals that have been examined, including man (van Genderen
Stort^) ; but Garten (in Graefe-Saemisch, loc. cit.) found the reaction
doubtful in monkeys. The reaction is slow, taking two or more minutes
even with intense light. The violet end of the spectrum acts most
strongly (Engelmann), but the reaction to the red end is greater than
that of the pigment. Light on one eye causes reaction in both, as also
light on the skin, so long as the brain is intact (Engelmann, Nahmacher'*).

Chemical Changes. Light on the retina causes it to become acid
(Angelucci, Lodato^, Dittler^), and its staining reactions are said to alter.

The most important chemical change, however, has to do with the
visual purple or rhodopsin. This remarkable substance was discovered
in the rods of the frog's retina by H. Muller in 1851. Boll in 1876
discovered that it was bleached by exposure to light. Kiihne in 1878
first studied it exhaustively. It occurs in all animals which possess
rods, and is present in the rods only (Kiihne). Hence it is absent from
the fovea. This statement is denied by Edridge-Green and Devereux
Marshall' for monkeys, but their observations have not been confirmed.
Hering, however, points out that visual purple may possibly not be
wholly absent from the cones, and the peculiar rod-like character of
the human foveal cones makes it not improbable that they contain
some of the substance. The question merits further investigation.
Kiihne investigated the chemical characteristics of rhodopsin, the most
noteworthy facts being its solubility in bile acids and their salts, and its
resistance to strong oxidising and reducing agents. It is not the cause
of the fluorescence of the retina. It can only be seen ophthalmoscopically
in fishes which possess a white tapetum (AbelsdorfE^). Tait^ and Boll

^ Die Retina der Wirbeltiere, Wiesbaden, 1894.

2 Hess, Vergleichende Physiol, d. Gemchtsinnes, Jena, 1912.

3 Arch. f. Ophth. xxxni. 3, 229, 1887. « Arch. f. d. ges. Physiol, liii. 375, 1893.
5 Arch, di Oft. ix. 267, 1902.

* Arch. f. d. ges. Physiol, cxx. 44, 1907.

' Trans. Ophth. Sac. xxn. 300, 1902.

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xiv. 77, 1897 ; Sitz. d. Berliner Akad. 1895.

9 Proc. R. S. ofEdin. vii. 605, 1869,

THE ANATOMICAL BASIS 13

state that it can be seen entoptically on waking in the morning as a
rose-red ring round the fixation point, projected against the white
ceiling. Haab attributes this phenomenon, not to the visual purple,
but to the pigment of the yellow spot. Edridge-Green^ confirms Tait's
and Boll's observations, and states that the purple can be seen to flow
in waves from the periphery of the yellow spot towards the point of
fixation.

The colour of the visual purple varies in different animals and under
different circumstances^, and gives different spectroscopic absorption
bands (Kiihne ; Kottgen and Abelsdorff^). The maximum absorption
in fishes is in the yellow (540 ft/u,), in mammals in the blue-green (500 M/i) ;
hence it is reddish violet in fishes and purple in mammals. The occur-
rence of " visual yellow " as an intermediate stage in the bleaching
of visual purple (Kiihne) seems to have been conclusively disproved by
Kottgen and Abelsdorff. They found that as the visual purple was
bleached the relative absorption remained unchanged. The substance
therefore becomes gradually less concentrated, without passing through
a yellow stage.

The bleaching of visual purple is limited to the area exposed to
light, so that an optogram or image of the luminous object, such as a
window, can be obtained. Such an optogram can be partially preserved
by alum solution, somewhat as a photographic negative is fixed, though
the processes are entirely dissimilar. Two to seven minutes' exposure
to light suffice to obtain a good optogram in the frog's retina. Light
on one eye does not cause any bleaching of the visual purple of the
other.

The bleaching of visual purple by monochromatic light has proved
to be of great theoretical interest. Observations have been made by
Konig^ Kottgen and Abelsdorff, and Trendelenburg^. Trendelenburg
took two specimens of frog's visual purple and exposed one to Hght of
the sodium line (D, 589 /x/x) and the other to another Hght from the
same dispersion spectrum, the diminution of absorption being measured
by the spectrophotometer. The human achromatic scotopic luminosity

1 J. of Physiol. XLi. 263, 1910 ; xlii 428, 1!)11 : xlv. 70, 1913.

'^ Garten, in Graefe-Saemisch Handb. d. ges. Augenheilkuride, Teil i. Bd. ni. Kap.
xii, Plate VII, 1908.

' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xn. 161, 1896.

* Konig, p. 388.

'" Centralhl. f. Physiol, xvii. 1904 ; Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvn.
1, 1904.

COLOUR VISION

3-0

2-0

1 — ■ —

-^^V

^^-^^

1 ^^^ -

^ 'n^^-

t ^^-

J^ \^

t \

55^

_S^

- SS.^

- ^^N

_ ^s"!^^

^^^N

1 __ 1 — 1 — L_--_J — , — 1 — ^1 — 1 — ^1 — L

CJi

Fig. 1. • • — the bleaching values of spectral lights for frog's visual purple,

and X — X — X the human achromatic scotopic luminosity curve. Abscissae, wave-
lengths of the prismatic spectrum of the Nernst light ; ordinates, an arbitrary
scale. (Trendelenburg.)

curve will be referred to later (pp. 53, 55). Actual readings are as
follows :

Wave-length

589

542

530

519

509

491

474

459

Bleaching value

3-40

3-62

3-45

3-09

1-69

0-975

0-299

Scotopic luminosity

3-62

3-91

3-19

2-67

1-42

0-621

0-346

Visual purple is regenerated after bleaching in the living and " surviv-
ing " retina, but only on contact with the pigment epithelium. Regene-
ration commences in the frog after 29 minutes and is complete in one
to two hours. The process is more rapid in the rabbit, commencing in
seven minutes and being complete in 33 to 38 minutes. (It may be
noted here for convenience that increase of sensibility in dark adaptation
in man after good light adaptation commences in 7 to 8 minutes and
reaches nearly the maximum in 40 to 45 minutes (Piper^).)

It has already been stated that the visual purple occurs in nearly
if not all animals possessing rods from petromyzon to man. It is present
in albinotic animals, e.g. rabbit, and has been found in the 9- and 7-
months' human foetus (Klihne, Fuchs and Welponer'^). Kiihne said that
it was absent in the bat {Rliinoloj^lius hijjposiderus), but this statement
has been disproved by Trendelenburg. Kiihne found no visual purple

^ Ztsch. f. Psychol, m. Physiol, d. Siimesorg. xxxi. 161, 1903>
2 Wiener med. Woch. 221, 1877.

THE ANATOMICAL BASIS

in fowls and pigeons, but Boll, Angelucci, and van Genderen Stort
found it in pigeons. Hess dissolved out the oil globules with benzol,
which does not affect the purple in frog's retina, and found traces of
rhodopsin in both fowls and pigeons, but in far less amount than in man,
ox, frog, etc. There was more present in the hawk, buzzard, goose and
duck. Amongst reptiles the crocodile possesses many rods and is rich
in visual purple.

Electrical Changes. The retina, connected through non-polarisable
electrodes with a galvanometer or capillary electrometer, shows a
" current of rest " (du Bois-Reymond). On exposure to light there is
usually a short negative variation followed by a longer positive variation,

ooos

— ^

imn^

"^"^-

• 40I

Utiu /

otr /

000?

0001

luea.

]

r 2' »■ ♦• r ■ 6' 7'. «• »■ 1

0^'

± r-

0 I" *■ 3' •« y 0' r ? r Kf

Fig. 2. Electrical changes in the frog's eye caused by light. Plotted curves from the
analyses of three capillary electrometer records. The illumination in the case of the
upper record was white light, in that of the middle red light, and in that of the lowest
violet light. Abscissae, time after the commencement of the illumination in seconds ;
ordinates, the electromotive force in ten thousandths of a volt. (Gotch.)

VoU
•0007

0 01 02 &3 &4 &5 06 07 08 09 10 II l«

Fig. 3. Curves constructed from typical electrometer records of eyeball responses to the
light from the red, green, and violet regions of the interference spectrum of the arc light.
(Gotch.)

16 COLOUR VISION

and cessation of the light stimulus causes a further positive variation
(Holmgren), a fact of considerable theoretical interest. The electrical
changes have been studied by Dewar and McKendrick^, Klihne and
Steiner^, Fuchs^, Waller^ Himstedt and NageP, and others, but Gotch's
experiments with the capillary electrometer are the most conclusive^.
He found that spectral red light gave a latent period of nearly
-fn second and a difference of potential of 0*0004 volt ; green, -fy" and
0-0005 volt; violet, ^" and 0-00024 volt. Himstedt and Nagel
obtained a slight reaction with ultra-violet and Rontgen rays. They
also found that in the dark-adapted eye the maximum effect was
obtained at 544jLt/x, the site of maximum luminosity in dark adaptation
in man (vide infra) ; in the light-adapted eye at about the D line (589 /x/x),
the site of maximum luminosity in light adaptation ; but though the
light sensibility of the dark-adapted eye is more than a thousandfold
that of the light-adapted for some colours, CI. E. Mliller^ obtained no
appreciable difference in the electrical reaction. Engelmann^ found
that stimulation of one eye caused a reaction in the other also, but
the positive variation on removal of the stimulus was absent. The
relation of intensity of stimulus to strength of response does not
accurately follow the Weber-Fechner law {vide infra, p. 20), but
Talbot's law (v. p. 92) is followed more accurately (de Haas^).

When an object is looked at directly a sharp image is formed on the
fovea and the immediately surrounding area. An object therefore
which subtends less than 3° at the nodal point of the eye will form its
image entirely upon the rod-free area of the retina. Larger objects
subtending 4° — 12° will form their images on the macular region, in which
only a few rods are present in the peripheral parts. Objects surrounding
that fixated form images on the peripheral regions of the retina which
are richly supplied with rods. The acuteness of form vision falls off
rapidly in passing from the fovea to the periphery, but movements of
objects having their images in the periphery are very readily observed.

Not only is form vision different according as the image is at the

1 Trans. R. S. Edin. xxvii. 141, 1873.

2 Heidelb. Unters. in. 327, 1880. ^ ^^j-ch. f. d. ges. Physiol. LVi. 408, 1894.

* Phil. Trans. Roy. Soc. Lond. cxciii. 123, 1900.

* A7m. d. Physik, iv. 1901 ; Ber. d. Naturf. Ges. Freiburg, 1901.
« J. of Physiol. XXIX. 388, 1903 ; xxx. 10 ; xxxi. 1, 1904.

' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xiv. 329, 1897.

8 Helmholtz' Festschrift, 197, 1891.

" Inaug. Dissert., Leiden, 1903; ref. in Nagel's Jahresbericht f. Ophth., 73. 1903.

THE ANATOMICAL BASIS 17

fovea or peripheral, colour vision is also affected. Foveal or central
vision must therefore be clearly distinguished from peripheral or eccentric
vision.

Vision is profoundly affected by the condition of the retina at the
moment of stimulation. The condition of the retina at any given part
is determined by two factors, temporal and spatial. The temporal
factor is the nature of the stimulation to which the retina has been
previously submitted (temporal induction). If the eye has been exposed
to bright light it is said to be light-adapted. I shall speak of vision
under these circumstances as photopia, and the light-adapted eye as a
photopic eye. If the eye has been kept completely free from light for
a considerable period it is said to be dark-adapted. I shall speak of
vision under these circumstances as scotopia (as an equivalent for the
German Ddmmerungssehen), and the dark-adapted eye as a scotopic
eye.

The spatial factor in retinal sensibility is the reciprocal action of
different areas upon each other. The excitability of a given area is
affected by the condition of sensibility and stimulation of the surround-
ing areas (spatial induction).

CHAPTER III

THE PSYCHOLOGICAL BASIS

The physiology of the senses may be regarded as the intermediate
link between the outer world and the consciousness of the individual.
The physical stimuli come into contact with the sense organs and set
up sensory neural physiological changes, which may be transformed
into, or at all events accompanied by, mental processes leading to
sensation, perception, and comparison. In this long and complicated
path there are two points of contact, the physico-physiological and the
physiologico-psychological. Of the former we know a Httle, and the
elements of our knowledge have been discussed in the last chapter.
Of the latter we know nothing, but of the final perfected perceptions we
have some knowledge, the elements of which we must now discuss.

The awakened perceptions, their qualities and attributes, and their
inter-relations belong to the domain of psychology. They depend in
some occult manner upon the sensations aroused by the physiological
processes which are set in activity by the physical stimuli. We have
good reason to think that there are many gradients and junctions upon
p. o. \ . 2

18 COLOUR VISION

the physiological path, so that the resultant nervous activities set up in
the higher centres of the brain are very different from those which
started in the peripheral mechanism. Try as we may to dissociate the
physiological from the attendant psychological phenomena we are
unable to do so, for the act of analysis itself is a psychological process.
We are practically limited to the consideration of the relationships
which can be made out with greater or less certainty to exist between
the stimuli and the resulting sensations.

With regard to these relationships there are certain generalisations
which have been thought to hold good for all the so-called " senses."
One of these generalisations, enunciated in 1785 by Bonnet and
independently by Johannes Midler in 1826^, is called Miiller's Law of
the Specific Energies of the Senses. It is to be noted that the word
energy is not used here in its ordinary physical sense. Miiller stated
that we can experience no kinds of sensation through the inter-
mediation of external causes which we cannot also experience through
the sensation derived from the condition existing in the nervous
organs without external agency. These internal causes call up
different sensations in the different senses according to the nature of
each sense. Further, the same external stimuli arouse in the different
sense mechanisms different sensations according to the particular sense ;
and different external or internal stimuli, acting upon the same sense
mechanism, ahvays arouse the same sensation. The essential features
of Miiller's law are contained in the sentences in italics. He
amplified the law by saying that a sensation is not the transference of
a quality or condition of external bodies to the consciousness, but the
transference of a quality or condition of a sensory mechanism to the
consciousness, though that quality or condition is occasioned by external
stimuli. These qualities or " energies " of the senses are specific to
each sensory mechanism. Miiller left it open as fo the site of the
specific energies, whether in the brain or elsewhere in the nervous paths,
but he considered it certain that the central endings of the sensory
nerve-paths in the brain were capable of arousing the specific sensations
independently of the conducting paths. It is now generally held that
the seat of the specific sensations is in the brain. If this assumption
be true Miiller's law relieves us of the necessity of predicating the con-
duction of different kinds of impulses by the peripheral nerve-paths.
It is customary to divide the stimuli which can arouse a specific

^ Zur vergleichendcn Physiologlt dcs Gesichtssinnes, Leipzig, 1826 ; Ha/idb. d. Physiol,
d. Menschen, Coblenz, 1840.

THE PSYCHOLOGICAL BASIS 19

sensation into adequate and inadequate. The former are those which
arouse the sensation under ordinary circumstances, the latter are all
other effectual stimuli. Thus light is an adequate stimulus for the eye,
pressure on the eyeball an inadequate.

The next great step in the advance of our knowledge of the relation-
ships subsisting between physical stimuli and sensations was made by
E. H. Weber, the founder of modern psycho-physical methods. In
general terms it may be stated that a stimulus must attain a certain
intensity in order to excite a sensation and that stimuli of greater
intensity excite stronger sensations. There is therefore a quantitative
relationship between the stimulus and the sensation. The minimum
effectual intensity of stimulus is called the general threshold or the general
liminal value. A higher value may arouse a sensation differing in
quality from the other ; this value is called the specific threshold or the
specific liminal value. Thus a coloured light of low intensity may excite
a colourless sensation ; when of a higher intensity it may excite a
sensation of colour.

The intensity of the stimulus has to be increased by a definite amount
before a difference in the amount of the sensation becomes appreciable.
This amount may be called the differential threshold or the liminal
discrimination value. Equal increases of physical intensity do not give
equal increases of sensation. There are three chief psycho-physical
methods whereby estimates can be obtained. (1) The method of least
perceptible differences or the limiting method : one of two identical
stimuli is regularly increased by small increments until a just perceptible
difference between the constant and the variable stimulus is experienced.
(2) The method of right and wrong answers or the constant method :
the observer is asked which is the greater in the case of such pairs of
stimuli (constant and variable), the variables here being fewer in number
and presented with 'the constant in irregular order, and the average of a
large number of such estimates being taken. (3) The method of mean
error or the method of production : the observer picks out what he
regards as just appreciably different stimuli ; the sum of the differences
from the standard divided by the number of trials gives a lower limit
of the threshold value. These results are accurate only when submitted
to strict statistical processes^.

Weber stated his conclusions in the form of a law : The just appreci-
able increase of stimulus hears a constant ratio to the original stimulus ;
i.e. two stimuli in order to be discriminated must be in a constant ratio,
^ See Myers, Text-hook oj Exp Psychology, Chaps, x. and xv. London, 1911.

2—2

20 COLOUR VISION

which is independent of the absolute magnitudes of the stimuh. For
white hght Fechner could distinguish a difference of jljy, v. Helmholtz
y^y, of the light intensity.

Fechner went further and attempted to express sensations in terms
of quantitative units. His most important assumption was that all
just noticeable differences of sensation contain an equal number of
sensation-units.

Fechner's law states that the sensation varies as the logarithm of the
stimulus ; i.e. the sensation changes in arithmetical proportion as the
stimulus increases in geometrical proportion.

Stated algebraically, if E is the measure of a sensation and hE the
just appreciable difference, S the measure of the stimulus and hS a small
increment, then

8^=0^ (Weber's Law)

where C is a constant : therefore, on the questionable assumption that
it is permissible to integrate BiwaW finite quantities {hE, etc.)

= C log S + C (Fechner's Law)

where C" is another constant.

Weber's law does not hold good for very low or very high intensities
of stimuli, and is only approximate at the best. An immense super-
structure has been built up upon these psycho-physical foundations^.
The bases are insecure on mathematical as well as on physiological
grounds. So far as the latter are concerned we have no unit of sensation
(cf. p. 61), and the variations, though quantitative, are only relative.
The chief difficulty, however, is to be found in the ever-changing
condition of the sensory apparatus. The deductions are not without
value, for some quantitative relationship certainly exists, even if it be
not so simple as Fechner's law implies.

The problem is still more complicated when we come to comparison
of specific qualities in sensations. Thus we recognise brightness as a
quality of coloured lights and we may say with some degree of certainty
that a given red light has the same brightness or luminosity as a given
blue light. We can thus estimate qualitatively equivalent stimuli and it
is also true that we can attempt to estimate qualitatively equivalent

^ Lipps, Grundriss d. Psychophysik, Leipzig, 1903 ; G. E. Miiller, Ergeb. d. Physiol.
II. 2, 267, 1903 ; v. Kries„ in Nagel's Ha7idb. d. Physiol d. Menschen, m. 16, 1904.

THE PSYCHOLOGICAL BASIS 21

discrimination values. The question therefore arises whether sensations
can be divided up into elementary qualitative parts. The simplest
theory of such psychological analysis is that elaborated by Mach and
others that each psychological element has a physiological counterpart,
which is itself the expression of a physical counterpart. Each is indeed
an accurate image of the other, or, to use Fechner's simile, the psychical
and the physical are the concave and convex sides of the same curve.
V. Kries, McDougall, and many others are of the opinion that this
view is untenable. If two perceptions differ, yet possess a certain similar
quality, that quality may be regarded either as made up of the sum of
similar preformed parts in the various constituents of the two percep-
tions, or as a totally new psychological rearrangement of the under-
lying factors. Of these alternatives the latter is the more likely, and
we must at present remain content to regard certain psychological
similarities as not capable of analysis.

The psychological analysis of our sensations brings out other funda-
mental facts of importance. We not only see a light or a colour, but
we see it at a definite time and in a definite place. Leaving aside the
temporal element for the moment, we find that our orientation in space
is largely dependent on vision. Hence it arises that our visual impres-
sions are projected outwards to definite positions in the outer world.
In this respect visual " sensations " differ from such sensations as pain,
heat, cold, etc.^ This unconscious projection of impressions is responsible
for the fact that we associate our sensations with certain properties of
external objects. We speak of objects as being round, bright, red, and
so on, — an inaccuracy which is responsible for much confusion. A
luminous object sets the ether in vibration ; when these vibrations
stimulate the retina they give rise to sensations, which we describe as
bright, red, and so on. These qualities are therefore subjective and must
be strictly dissociated from the physical stimuli which give rise to them.

This aspect of the subject has been lucidly treated by Hering^. As
he says, " our visual world {Sehivelt) consists essentially of dift'erently
presented colours, and objects, as seen, that is visual objects {Sehdinge),
are nothing but colours of different nature and form^." The whole of

1 Cf. Hering, Grundziige der Lehre vom Lichtsinn, in Graefe-Saemisch Handb. Th. i
Bd. m. Kap. xii. 1905.

- Loc. cit.

^ " The eye sees no form, inasmuch as light, shade, and colour together constitute that
which to our vision distinguishes object from object, and the parts of an object from each
other." Goethe (1810). " AU vision is colour vision, for it is only by observing difference^!
of colour that we distinguish the forms of objects." Clerk-Maxwell (1871).

22 COLOUR VISION

the nervous apparatus of vision constitutes an " inner eye," which
builds up a new visual world under the compulsion of the stimuli derived
from the real objects of the outer world.

Things seen, visual objects, or colour forms must therefore be clearly
distinguished from the real objects. The untutored regard the green
of a leaf as an attribute of the leaf. The physicist, however, knows that
colour depends upon the light reflected from the leaf and calls the reflected
light green. The physiologist knows that the leaf, which appears green
when looked at directly may appear yellow or grey when its image falls
upon the peripheral part of the retina. He is therefore inclined to regard
the colour as an attribute of the eye itself. Finally, to the psychologist
the green is neither an attribute of the leaf, nor of the light, nor of the
eye, but a psychical phenomenon, a definite qualitative entity in con-
sciousness.

Colours therefore are visual qualities, and we are only justified in
speaking of red or green objects, red or green rays, and so on in the broad
sense that the objects or rays appear red or green respectively under the
ordinary conditions of vision. Brightness and darkness, again, are not
attributes of the objects or light rays, but of the colours as visual
qualities. White, grey, and black must be included amongst the colours
as visual qualities, but may be distinguished from the variegated or
toned colours as untoned colours.

The common attribution of colours to the objects themselves, thus
implying that the colours are properties of the objects, is largely a matter
of memory. We say that snow is white, soot black, blood red, because
under the ordinary conditions of life these objects appear to be of those
hues. In this sense the colours may be well termed " memory colours "
{Geddchtnisfarheyi, Hering^). The appearance of a given object at a given
moment is by no means determined solely by the nature and intensity
of the rays falling upon the eye and the condition of the nervous appara-
tus of vision at the time. These are but the primary and fundamental
exciting factors. They awaken unconscious reproductions or memories
of bygone experiences, which act as secondary but potent factors in the
subconscious sphere, modifying and in many cases determining the
ultimate conception. Thus it is that, though we are firmly convinced
that snow is white and blood is red, the pink glow of a snow-clad mountain
and the pallid hue of a face seen by the light of the mercury arc are
regarded as accidental colours in no wise modifying our impressions of
the actual colours of the objects. Every object with which we are

> Cf. Katz, Centralhl. /. Physiol., xx. 1906.

THE PSYCHOLOGICAL BASIS 23

familiar awakens a memory picture in our minds : "we see it through
memory-coloured spectacles." Thus we often see it quite different from
what it is, and our capacity to dissociate accidental colours from the
so-called real colours of objects is very highly developed. Thus, the
shadows on the surface of a body, which largely influence our perception
of its shape, relief and distance, we instinctively regard as an epiphe-
nomenon, and we think that we see the actual or real colours through
the darkness of the shadows. A shadow on white paper appears quite
different to us from a grey spot on the paper, even though both reflect
exactly the same amount of light. Similarly a patch of cigarette ash
on a black coat conveys a different impression from a patch of bright
sunlight. Indeed, the difference manifests itself in words ; the one we
usually call white or grey or black, the other bright or dark.

Hering has devised some simple, but very instructive experiments
to illustrate these facts. One example will suffice. In a room lighted
by a window on one side, the opposite wall being white, standing with
the back to the window and holding up a grey sheet of paper, the paper
looks grey and the wall white. The wall however reflects only a portion
of the light into the eye and " is " therefore grey. By looking through
a tube it is possible to select a grey paper which exactly matches the
greyness of the wall ; yet directly the tube is removed the wall at once
appears white, whilst the paper still remains grey. If, however, the
edge of the paper is fixed with one eye only, the wall appears to be on
the same plane and of the same tint as the paper. In this case different
localisation produces diff'erent colour impressions, and the ultimate
perceptions depend, not upon dift'erences of physical light intensity,
but upon other impressions which simultaneously enter into conscious-
ness and modify judgment.

One does not generally pay special attention to the colours of objects,
but uses them merely as indicators, specially associated with the objects;
hence when the object is seen again the colour impression is immediately
revived. Some dresses look blue by daylight, bluish-green by electric
light, and the wearers often think it strange when in artificial light people
say that they are bluish-green. Such people may even correct themselves
and say that they are certainly blue, when they are told that they are
blue.

Hering points out that these facts have nothing to do with simul-
taneous contrast {v. Section VI, Chap, ii) as has sometimes been thought.
They are indeed examples of the association of ideas or sympsychosis.
They show, however, the necessity for eliminating as far as possible all

24 COLOUR VISION

subsidiary impressions, such as localisation, shape, dimensions, etc., when
comparing colours. Further, it is easy to show that the apparent
brightness and colour of objects can be altered within a wide range
without disabusing our minds of the opinion that the colours are
inherent properties of the objects. Thus, the paper of a book appears
white and the print black, whether we read it in the morning or at mid-
day or in the evening. Yet Hering has shown by accurate measurements
that the print may actually reflect more light at mid-day than the paper
did in the morning. Similarly the paper remains " white " and the
print " black " whether the book be read by daylight or gas light or
electric light or in the shadow of green trees. " The approximate
constancy of the colours of visual objects in spite of gross quantitative
and qualitative variations of the general illumination of the visual field
is one of the most remarkable and weightiest facts in the domain of
physiological optics " (Hering). We shall see that these gross variations
are compensated for by processes of physiological adaptation of the
visual nervous structures as a whole and that stimulation of retinal
areas by light arouses reciprocal activities in neighbouring areas.

These complex processes should deter us from drawing too dogmatic
conclusions from the psychological analysis of colour sensations.
As McDougall^ says, mental activity consists in the process of
establishing in the mind relations between one thing and another.
This process in its best-defined form is apperception, " the process
by which a mental system appropriates a new element or otherwise
receives a fresh determination^." Each mental system is gradually
built up by a series of apperceptive processes, each such process con-
sisting in the presentation of some one aspect or feature of the whole
object through some sense-organ, and the bringing of this feature into
mental relation with various other aspects and features previously
apperceived and incorporated into the mental system. " In almost
every moment of waking life an apperceptive process is taking place ;
whenever an object is attended to the presentation of it is apperceived^."
Mental activity then consists essentially in the perpetual succession of
apperceptive processes, and the essence of apperception is the appro-
priation of the relatively novel presentation by the mental system built
up by previous apperceptions. At each apperception of any given
presentation of an object the appropriation of it by the mental system
is more ready and more complete, while the consciousness excited by

1 Brain, xxiv. 605, 1901. ^ stout. Analytic Psychology, 1896.

* Stout, loc. cil. II. p. 113.

THE PSYCHOLOGICAL BASIS 25

this aspect of the object becomes less and less vivid, until finally, when
the appropriation of it by the mental system becomes complete, it is
implicitly apprehended, or, in terms of conation, the stimulus applied
by this aspect of the object is responded to automatically, while some
other aspect occupies the focus of consciousness.

If, however, bearing in mind the underlying complex factors, we
attempt to make a psychological analysis of visual qualities it will be
generally agreed that they can be divided into two groups of colour
sensations, the untoned and the toned. The untoned or colourless form
a continuous series from the blackest black through all gradations of
grey to the whitest white. The toned or coloured include four, red,
yellow, green and blue, together with all the gradations between them.
So far as the insecure foundations of psychological analysis go red,
yellow, green, and blue are simple or pure visual qualities. All other
hues are psychologically mixtures of these qualities. Thus, orange
obviously partakes of the nature of both red and yellow, purple of both
red and blue, and so on. Yet that psychological analysis is necessary
is shown by the acceptance of green as a simple colour sensation ;
for without analysis most people would say that green is a mixture of
yellow and blue. That such is the case is doubtless due to familiarity
with the behaviour of pigments, yet it may have a deeper significance,
since the distinction between green and blue is vague amongst many
primitive races and frequently amongst the uneducated classes.

Moreover, there are difficulties associated with black and white and
two toned colours, brown and olive-green. " White " is particularly
variable, chiefly owing to complexities arising from adaptation. Hering
regards difiuse sunlight as distinctly yellow, and the " whites " of sun-
light, arc light, incandescent gas light and so on, even when reflected
from a surface of compressed magnesium oxide, show gross variations
which are not submissible to psychological analysis. Opinion differs
as to whether " black " is the negation of all sensation, as generally
accepted by physicists, or an active sensation, as accepted by many
psychologists. It is certain that there is a blacker blackness than that
experienced when the eyes are carefully shaded from the light. Simi-
larly the sensation of brown cannot be elicited by merely reducing the
intensity of a yellowish-red or any other spectral light or mixture of
spectral lights, and the same applies to olive-green. Under these
circumstances the spectral colours approximate more and more nearly
to black. In order that a brown sensation may be experienced the
stimulus effect of the yellowish-red light must be " blackened " by

26 COLOUR VISION

simultaneous or successive contrast, or the appropriate pigment must
be mixed with black.

We shall have to discuss these anomalies more fully in the sequel,
but enough has been said to show that psychological analysis can afford
no infallible criterion.

SECTION TI

THE SPECTRUM AS SEEN BY THE LIGHT-ADAPTED

(PHOTOPIC) EYE

CHAPTER I

THE SPECTRUM: HUE, LUMINOSITY, SATURATION

If a pure spectrum, e.g. that of the arc light, of moderate intensity
is observed a band of colours is seen. Of these, four are clearly defined
as separate and distinct from each other, viz. red, yellow, green and
blue, the red region consisting of the least refracted rays, the blue of the
most refracted. Between the red and yellow we distinguish a region
which is called orange. The gradation from red to yellow is gradual
and it will be generally admitted that orange partakes of the natures
of both red and yellow psychologically, the red element diminishing as
we pass from red to yellow and the yellow element correspondingly
increasing. Between yellow and green a somewhat similar gradation
occurs, the yellow gradually becoming more and more tinged with green
until we fail to recognise any yellow at all and the colour gives the
impression of pure green. Passing further towards the blue an inter-
mediate green-blue region is met with, showing the same gradual
transition until the blue no longer gives any impression of green. Pass-
ing beyond the blue we gradually come to a region in which the pre-
dominant sensation is still of the order " blue," but it is not pure blue.
It is called violet. Now violet is a colour which occurs rarely in nature.
There is, however, a colour in nature which is often called violet, but
which is really purple. True purple does not occur in the spectrum,
but it can be obtained by mixing pure red light with pure blue light,
and we can pass from blue to red through violet and the mixtures of
blue and red which are called purple and carmine.

We have thus travelled in a circle and returned to the original

28 COLOUR VISION

starting place, red. This is a very important fact, for it can be proved
that with the help of the colours thus obtained, either pure or mixed
with each other or with black in various proportions, all known colours
and tints can be reproduced.

We can map out the spectrum into its separate colours, using the
Fraunhofer lines as convenient fixed points, but as the colours pass
gradually into each other the limits are more or less arbitrary,
v. Helmholtz gives the following names to the different regions of the
spectrum :

ine

Wave length in ixn

Colour

760-40

Extreme red.

686-853

Red.

656-314

Junction of red and orange.

f589-625
\ 589 -024

Golden yellow.

526-990

Green.

486-164

Cyan blue.

430-825

Jiniction of indigo blue and violet

396-879

Limit of violet.

Speaking generally then, change of wave-length causes a change in
colour, or in the hue or tone of a colour. The tone changes most rapidly
on both sides of the yellow, most slowly near the ends of the spectrum.
For a certain distance at each end change of wave-length is no longer
accompanied by change of tone, at the red end beyond 655 /x/u, at the
violet end beyond 430 ixfi.

Apart from the change in colour the most striking feature of the
spectrum is the difference in brightness or luminosity of different parts.
The brightest part is in the yellow at about the D line, the luminosity
diminishing continuously on both sides to the extreme ends. The
brightness varies with the intensity of the light, but if the intensity
is increased beyond a certain point the colours also change in tone.
The colours on each side of about 500 fx/n behave differently ; the red,
orange, yellow and green approximate to yellow, the blue-green, blue
and violet approximate to blue.

Though the brightness of the colours increases with the intensity
of the light it does not follow the curve of energy of the spectrum.
Whereas the brightest part of the spectrum is in the yellow the curve of
energy rises continuously from the violet to the red end^.

^ Nichols, Phys.Rev. xxi. 147, 1905; Knirmp, Physisch-ophthalmologische Grenzprobleme.
Leipzig, 1906, p. 4.

THE SPECTRUM: HUE, LUMINOSITY, SATURATION 29

If now the pure spectral colours be successively mixed with gradually
increasing quantities of white light they become paler until eventually
no colour can be distinguished. They are said to become less saturated.

A given colour may therefore be defined by its hue, its luminosity,
and its degree of saturation. In regard to the hue or tone the matter
is relatively simple, so long as we adhere to the term hue or tone in
this definite sense. The terms tint, nuance, shade and so on should
be avoided.

With regard to luminosity and saturation the matter is by no means
so simple. An unsaturated colour is also an impure colour in the
physical sense of the word, for it no longer consists solely of rays from
a single small region of the spectrum. But we are confronted with
another fact, less easy of explanation, viz. that great increase of in-
tensity of the light not only alters the hue, but also alters the saturation,
so that eventually it produces only the sensation of white light. It
would seem therefore that luminosity is in some recondite sense an
inherent " whiteness " in the colour itself, differing in degree in different
spectral colours and varying with the intensity of those colours. Clearly
we are here, at the outset, face to face with a physiological fact of immense
importance, and much of the difficulty of colour vision is concerned
with this fact.

The terms hue or tone {Farbenton), brightness or luminosity {Hellig-
keit), and saturation or purity {Sdttigung) are now generally used in the
well-defined senses given above. One has to be careful, however, in
reading the older and some modern works. Thus Aubert^ uses Farben-
ton for Clerk-Maxwell's " hue " (" one may be more blue or more red
than the other, that is, they may differ in hue ") ; Farbenniiance for
Helmholtz's " Sdttigungsgrad " and Grassmann's Intensitdt des bei-
gemischten Weiss and Clerk-Maxwell's " tint " (" one may be more or less
decided in its colour ; it may vary from purity on the one hand to
neutrality on the other. This is sometimes expressed by saying that
they may dift'er in tint") ; Farbenmtensitdt for Helmholtz's LicktstdrJce
and Clerk-Maxwell's " shade " (" one may be lighter or darker than the
other ; that is, the tints may differ in shade "). Edridge-Green uses
the terms hue, luminosity and purity. Hue is employed in the usual
sense. Of luminosity, however, he says : " No coloured object can
have the luminosity of a white object reflecting practically the whole
of the light impinging upon it. Therefore if we take absolute reflection
as 100, a fraction of 100 will give the relative luminosity of any body."

1 Physiologic der Netzhaut, Breslau, 1865.

30 COLOUR VISION

Purity is " the freedom of the colour from admixture with white light,"
but he says " when I speak of a colour being mixed with white light,
I have a different meaning from that which is signified by most writers
on colour." His explanation should be read carefully^, but he appears
to use the term " white," which denotes a physiological sensation, in the
sense of a physical property of the light.

CHAPTER II

THE DISCRIMINATION OF HUE IN THE SPECTRUM

Observations on the discrimination of hue in the spectrum have
been made by Mandelstamm^, Dobrowolski^, Peirce*, Konig and
Dieterici^ UhthofE^, Brodhun', F. Exner^, Steindler^ Edridge-Green"
and others. Of these, the most accurate and complete are those of
Steindler.

The maximum discrimination sensibility for hue, i.e. the smallest
difference in wave-length (8x) which gives rise to appreciable difference
in colour-tone, occurs in the yellow and the blue-green. It has already
been remarked (p. 28) that there are regions at each end of the spectrum
in which differences of hue are no longer appreciated. Two neighbouring
wave-lengths in these regions may appear to differ slightly in hue, but
only owing to differences of intensity. When the intensities are suitably
modified the differences disappear.

Konig and Dieterici found three maxima, I at 440 /it/', II at 490 /x//,
and III at 570 /^/x (Dieterici) or 590 fxu (Konig). Of these the greatest
discrimination sensibility was II. The minima were at 450 /u./z between
I and II, and at 540 /xju between II and III. These results were obtained
by the method of mean error (i'. p. 19). (Fig. 4.)

1 Colour-blindness and Colour PcrccjjfAon, 2nd ed. London, 1909, p. 60.

' Arch. f. Ophth. XIII. 2, 399, 1867. » Arch. f. Ophth. xviii. 1, 66, 1872.

* Amer. J. of Sc. xxvi. 299, 1883.

5 A7171. d. Physik, XXII. 579, 1884; Arch. f. Opihth. xxx. 2, 158, 1884; Konig,
Ztsch. f. Psychol, u. Physiol, d. Sinncsorg. viii 375, 1895 : in Konig. pp. 23, 105, 367.
« Arch.f Ophth. xxxiv. 4, 1, 1888.
' Ztsch. f. Psychol, u. Physiol, d. Sinncsorg. in. 89, 1892.

* Sitz. d. Wiener Akad. cxi. ii a, 857, 1902

* Sitz. d. Wiener Akad. cxv. ii a, 115, 1906. i" Vide infra. Part iii

THE DISCRIMINATION OF HUE IN THE SPECTRUM 31

Fig. 4. Curves ot discrimination sensibility for hues of the spectrum of gas light. A, A, A,
for normal trichromat ; B, B, B, for a deuteranope. Abscissae, wave-lengths.
(Konig.)

Uhthoff used the method of least perceptible differences {v. p. 19),
and obtained the following results.

(550

640

630

620

610

600

590

580

570

550

530

510

4-70

2-97

1-68

1-24

1-08

102

0-91

0-88

1-10

166

1-88

1-29

490

480

470

460

450

0-72

0-95

1-57

1-95

2-15

His maxima are therefore II at 490 /x/x, and III at 580 /xju,.

V. Kries^ points out that Konig's and UhthofE's methods are not
strictly comparable, since the former measures the general discrimina-
tion sensibility rather than the true specific or hue discrimination
sensibility, which is measured by the latter.

Steindler used the interference spectrum of the arc-light, and
examined twelve persons with normal colour vision. She obtained four
maxima for her own right eye (Fig. 5) : I at 435 /x/x ; II at 497 /x/x ;
III at 585 /x/x ; IV at 636 /x/x. The corresponding minima were between
I and II at 454 /x/x ; II and III, 535 /x/x ; III and IV, 624 /x/x.

Comparing different individuals, II or III may show the greater
discrimination sensibility. Thus her own III was greater than II
whereas we have seen that Konig's and Dieterici's II was greater than
III. The exact positions vary — for I, 15 — 20 /x/x; for II, 30 /x/x; for
III, 15/i/x; for IV, 15 — 20 /x/x. The average positions were — II, 492 /x/x;

1 Nagel's Huudh. d. Physiol, d. Mcnschen, ni. p. 252.

COLOUR VISION

III, bSlfjifx; IV, 635-5 /xju. : and of the corresponding minima — between
I and II, 458 /jifx ; II and III, 533 /x/x ; III and IV, 627 |U./x.

1 )

^<?

5tf

5tf<?

5(7

600

700

Fig. 5. Curve of discrimination sensibility for hues of the interference spectrum of the arc
light. Abscissae, wave-lengths of the lights ; ordmates, difEerences of wave-length
(5X) capable of being discriminated. (Steindler.)

These results are of very considerable theoretical importance, and
will be referred to again later.

The number of discriminable hues in the spectrum can be calculated
from these data^. If A and X + h\ are the wave-lengths of two
just discriminable monochromatic lights, then hX varies with A and
may be regarded as a function of A. The reciprocal value of Sa.
therefore gives the number of hues which can be discriminated in
an interval of the spectrum in which A varies according to the same
chosen unit. The total number of distinguishable spectral hues is there-
fore equal to the integral \j-.d\ over the whole visible spectrum.

Uhthoff's values for h\ are given above. Since from 655 /x/x to the red
end and from 430 /x/x to the violet end there are no changes in hue,

drawing the curve and measuring the area enclosed between it and the
axis of abscissae. From these calculations Konig concluded that
Uhthoff could discriminate 165 hues in the spectrum.
1 Konig, Gesammdte Abhandlungen, p. 367.

is here = 0, and the integration can be performed graphically by

THE MIXTURE OF PURE-COLOUR STIMULI 33

CHAPTER III

THE MIXTURE OF PURE-COLOUR STIMULI

Sir Isaac Newton (1704) first scientifically investigated the pheno-
mena of colour mixtures. The fundamental laws were first enunciated
by Grassmann (1853). They may be stated in the following form :

(1) Unlike lights mixed with like lights produce unlike mixtures :
or, if in a mixture one component is continuously altered the appearance
of the mixture will also continuously alter.

(2) Like lights mixed with like lights produce like mixtures : or,
if two lights that look the same are each mixed with a third light the
resultant mixtures will look alike.

Special Case : Proportional increase of intensity of each component
does not destroy the match. This corollary will be found on further
investigation to require some reservation.

(3) Every mixture of lights can be matched by a definite spectral
light or a definite purple mixture which is mixed with a definite amount
of white light : or, if we take any fixed homogeneous or composite
light and mix it with the whole series of pure spectral lights, completed
by purple, varying the proportions in the mixture from zero of one to
zero of the other, we obtain every known variety of stimulus.

The importance of these laws is that matches of optical mixtures
resemble algebraical equations and can be treated as such, the match
holding good if any addition or subtraction is made to both optical
mixtures.

Owing to the facts already stated on p. 27 it is clear that we can
pass continuously from red to violet by two paths, one via the spectrum,
the other via purple. This is represented graphically by a closed curve,
and Newton naturally chose the circle as the simplest. It is not,
however, the most comprehensive, as will be seen by the results of
observing various colour mixtures. It is at once obvious, for instance,
that the progress from red to violet via purple must be represented
by a straight line, for experiment shows that purple can only be obtained
by mixing red and violet, and therefore a given purple must be
represented as a point on the chord joining the points which represent
these colours.

The same applies to all hues of wave-length greater than 540 ///x
(in the yellow-green). For example, any mixture of red light of 670 /x/x

p. c. V. 3

34 COLOUR VISION

and of yellow of 580 fxjx causes a colour sensation which can be accurately
matched with that derived from some spectral colour between those
wave-lengths. The exact position, i.e. the wave-length, of this colour
depends upon the relative amounts of the two colours in the mixture.
If there is an excess of red the resultant mixture will match a colour
nearer the red than the yellow- green. Moreover, the position of the
colour will be accurately represented by the mass centre of the weights
of the two components, i.e. by the centre of gravity, as Newton showed.-
V. Kries^ gives the following matches of mixtures of 670"8 /x/x and
552 fi/ji. for the spectrum of gas light, the measurements being obtained
with the Helmholtz-Konig spectrophotometer :

Amounts of red (670"8 /xim.) and
green-yellow (552 fxfj.) in
the mixtures

Spectral match

670 8 At/a

552 /xfi

100

(J70-8

2-84

014

628

311

0-38

615

3 05

0-69

603

2-27

0-94

591

1-39

107

581

0-82

1-28

571

0-24

113

561

1-00

552

All the colour sensations in this region are therefore functions of a
single variable and can be represented on a straight line, AB, where A
represents about 800 /zyu and B 540 /u/u,. Each point on the line represents

A C D B

I \ 1 1

Fig. 6. Representation of mixtures of lights of long wave-length on o. straight line.

a colour sensation. If an amount, Wj, of light L^, a.t A is mixed with
an amount, m-., of light, L^., at B, then the resultant sensation is a light,
Xa, at C such that CA : CB as m. : Wj. Similarly, if ?n,j of L,.j at C be
mixed with m^ of L, at B, the resultant sensation is a light, L^, at D,
such that DC : DB as m^ : m-s, and so on. All mixtures therefore
which give a sensation corresponding to a given point give rise to the
same sensation or match accurately. Hence the range from 800 /x^ to
540 /u/x must, like the purple range, be represented in the colour diagram
as a straight line.

' Ztsch. f, Psychol, u. Physiol, d. iSinncsorg. xni. 281, 1896.

THE MIXTURE OF PURE-COLOUR STIMULI 35

Complications arise when we pass beyond these limits. If a yellow
is mixed with a blue-green the resultant mixture, though resembling
a pure intermediate colour, does not match it perfectly. The match
is made perfect by adding a certain proportion of white light to the pure
spectral intermediate. In other words the mixture is paler, or less
saturated, than the spectral match. As the distance between the mixing
colours is increased the saturation becomes continuously less, until
finally at one distance two colours are obtained which, when mixed,
yield a sensation of white, free from all trace of colour sensation. Such
colours are called complementary colours.

The following are v. Helmholtz's estimates of the wave-lengths for
certain complementary colours :

Colour

Complementary Colour

Ratio of Wave-lengths

Red ..

. .

656-2 /J.H

Green -blue

492-1 fxfi

1-334

Orange

. .

607-7

Blue

480-7

1-340

Yellow

585-3

J J

485-4

1-240

573 -'J

482-1

1-190

. .

567 1

Indigo- blue

464-5

1-221

J5 • •

. .

5$4-4

461-8

1-222

Green-yellow

563-6

Violet

433 and beyond

1301

Observations have also been published by v. Frey and v. Kries^,
Konig and Dieterici^, Angier and Trendelenburg^ and others^. If these
results are plotted as curves with wave-lengths from 400-500 /x^u, as
abscissae and wave-lengths from 560-680 [ifx as ordinates the curves
nearly resemble hyperbolae, but differ slightly from each other. The
differences have been attributed in part to macular pigmentation.
Krarup"^ has re-investigated the subject and finds that the comple-
mentary colours change somewhat as the intensity of the illumination
is altered. There is no change due to this cause from 460 to 480 i^fM, but
a gradual increase from that point up to 512 /x/x. With a suitable,
relatively low, illumination the curve is a rectangular hyperbola.

The ratio of the quantities of Aj to X.,, where Xj and A.^ are the wave-
lengths of two complementary colours, is approximately constant and
independent of the intensities of illumination. Glan^ came to the con-
clusion that the energies of \ and X. at the percipient retinal structures

^ Arch. f. Anat. u. Physiol. 336, 1881.

^ Wied. Ann. xxxiii. 1887; in Konig, p. 2()1. i

^ Ztsch. f. Psychol, u. Physiol, d. Sinncsorg. xxxix. 284, 1905. i

1 Hclmholtz, 3rd ed., ii. p. 107.

^ Physisch-ophlhalmologische Grenzprvbleme, p. 100, Leipzig, 1906. i

" Arch. J. d. ges. Physiol, xxix. 53, 1886; Wied. Ann. xlviii. 1893. !

3—2

36 COLOUR VISION

must be equal in order that white may be perceived. This law is not
true, but Krarup points out that the ratio of the energies of \i to \o at
the retina is independent of the intensities of illumination. The ratio
is not constant, still less equal to unity, but if the energy ratios calculated
from Angier and Trendelenburg's quantities (slit-widths) be plotted as
ordinates, against wave-lengths as abscissae a symmetrical curve re-
sembling a parabola and having its apex at about 608 //./x results.

Ebbinghaus^ states that the brightness of the resultant white is
equal to the sum of the brightnesses of the constituent complementary
colours. Most observers have found that the white is brighter.

Complementary spectral colours have seldom the same luminosity,
i.e. they do not look equally bright. The nearest approach to equal
luminosity is orange, 607'7 ^/x, and blue, 489"7 fifji. Colour mixing shows
that the order of saturation of spectral colours diminishes from violet
successively through indigo-blue, red and cyan-blue, orange and green,
yellow (v. Helmholtz).

It will at once be observed that there is a range from about 560 to
492 ju/i, i.e. green, which possesses no spectral complementary. White
can only be obtained from green by mixing it with both red and violet,
i.e. purple.

There are hues with which we are familiar in pigments, etc., that
do not at first sight fall into any of the categories mentioned. Of these
the most striking is brown. Brown, olive green, and greys possessing
some coloured hue are obtained by mixing black with a spectral colour
or mixture of colours. Further evidence will accumulate in the course
of our discussions in favour of the view that black is an actual and
effective stimulus.

We can now return to the unfinished colour table or diagram, of
which only two rectilinear portions have as yet been mapped out. It
has been pointed out that the graph must be a closed figure in one plane
and that the various points upon it follow the law of the centre of
inertia of masses.

If three colours, neither of which can be obtained from a mixture
of the other two, are represented by three points on a plane, then
assigning to them values in terms of any unit, the situations and
quantitative values of their mixtures can be ascertained. The problem
is well stated by Greenwood^. " In order to establish the correctness
of this method it is necessary to prove that, given the experimental

^ Ztsch.f. Psychol, u. Physiol, d. Sinnesorg v. 176. 1893.
^ Physiology of the Special Senses, London, 1910, p. 131.

THE MIXTURE OF PURE-COLOUR STIMULI

laws of colour mixing, this construction is valid in all possible cases,
i.e. that the situation of the mixed colour coincides with that of the
mass centre of two equivalent masses (1) when the two constituents
can be mixed from the three chosen colours ; (2) when one can and the
other cannot so be mixed ; (3) when neither can be so mixed." v. Helm-
holtz^ has supplied the mathematical proof.

The diagram will vary in form according to the source of light and
according to the choice of units and fixed points. It is best represented
in such a form as in Fig. 7.

Green

Yellow

Red

Orange

Pig.

Purple
Diagram of a colour table.

Violet

The position of the pure white sensation is obtained by dividing the
line joining any two complementary colours according to the relative
amounts of those colours required to produce white when mixed to-
gether. The deviation of the curve from a straight line beyond 540 /x/z.
indicates the unsaturated nature of the mixtures.

Since, for example, mixtures of green and violet are less saturated
than spectral cyan-blue, the curve must deviate further from the white
point than the straight line joining green and violet. From experi-
mental results it is found that the curvature is sharpest in the green.

These facts are quite independent of any theory of colour vision
whatever, and their importance is absolutely fundamental. " The
colour table merely expresses in systematic manner directly observed
physiological equivalents of objectively different lights " (v. Kries).
The facts show that the totality of physiological activities comprises
far fewer elements than the objective stimuli. The varieties of stimuli
of all possible mixtures derived from a source of constant intensity can
be reproduced on a co-ordinate system as points in a plane.

If we introduce variations of intensity the law can be stated in general
terms: " The entire physiological valency of every conceivable light and

1 Helmholtz, 3rd ed., n. p. 112.

38 COLOUR VISION

light mixture can he comprehensively represented as the function of three
variables " (v. Kries). Hence a colour diagram representing varying
intensities of colours and colour mixtures must be in three dimensions,
as in Lambert's colour pyramid^ or Runge's sphere^.

Within a certain range, which includes all ordinary conditions of
colour vision except those mentioned on p. 36, this law shows that
every conceivable light or light mixture gives rise to a sensation which
can be accurately matched by the sensation produced by a suitable
mixture of only three lights. In other words, from the point of view
of stimuli normal colour vision is trichromatic.

It is to be noted carefully that the colour table does not express
the change in physiological valency which corresponds to variation
in absolute intensity. The unit of intensity is fixed for the given table,
as is also the choice of the three variables. Theoretically the choice
of unit intensity of the three variables is arbitrary. The choice of
variables merely involves a change in the co-ordinate axes. If the
variables are selected too close together the table involves negative
stimuli and the stimulus values cannot be reproduced experimentally.
This is, however, of no theoretical importance.

We have here the basis of colour equations. For example, if spectral
green-blue {Bg) is mixed with red {R) in certain proportions it matches
a mixture of green {Gr) and violet (F), which may be expressed thus :

aBg h^R = jGr + eV.

Hence we can obtain a value for Bg

aBg = jGr + eV-^R,

which is strictly accurate though incapable of objective interpretation.
As Greenwood well puts it^ — in colour equations " addition is uniform,
the same result being always obtained when the same quantities are
summed ; it is commutative, the order of operations does not affect
the result ; it is associative and homogeneous. . . .If we define subtraction,
in terms of arithmetical quantity as uniform, non-commutative and non-
associative, similar analogies can be observed ; but this is of little
importance, since a justification of the use of the symbol of addition
will suffice for our purposes."

1 Beschreibung einer mit dem Calnu'scJien Wachse ausgemahlten Farhenpyr amide, Berlin,
1772.

- Die. Farhenhujcl. Hamburs;, 1810. Cf. Clicvroul, Expose d^nn moyen de definir ef
de nommer Ics coidcurs, Paris, ISfil ; liofler, Ziscli. f. Psychol, n. Physiol, d. Sinnesorg.
LViii. 356, 1911.

3 Loc. cit. p. 133.

THE MIXTURE OF PURE-COLOUR STIMULI 39

From the experimental point of view a constant spectrum is chosen
and three colours are selected, e.g. a red, a green and a blue. Each
part of the spectrum is then matched by mixing different quantities
of the three together. This process is called " gauging the spectrum "
[Aiclinng cles Spektrums, v. Kries). There are many technical difficulties
and the results are not wholly free from objection. The best published
results are those of Konig and Dieterici^ and Abney and Watson. The
colour tables deduced from these results are seen in Figs. 8 and 9.

Green

Red G,n Blue

Fig. 8. Colour triangle. A, B, C, etc., Fraunhofer lines. (Konig.)

In Konig's curve the sudden bend at the extreme end of the violet
is probably due to fluorescence.

As already mentioned colour mixtures often produce unsaturated
colour sensations. Consequently the match has then to be made
by a suitable addition of white light to the comparison spectral
colour.

No three spectral colours can be chosen which when mixed will
accurately match in hue and saturation all spectral colours. There is
good physiological evidence of colour-sensations of much greater
saturation than the spectral colours {vide infra, Section V, Chap. ii).
Such colours, being less mixed with white, must lie outside the colour
table, in some such positions as shown at the angles of the circum-
scribed triangles in Figs. 8 and 9.

1 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. iv. 241, 1893.

COLOUR VISION
B

49 50 51 52 53 54 55 56 57 58
Fig. 9. Colour triangle. W, white ; B, red ; G, green ; B, blue. The numbers are those
of an arbitrary scale of the spectrum of the arc light. (Abney and Watson.)

TJie Influence of Macular Pigmentation. The macula liitea, as its
name implies, is permeated with a yellow pigment, and therefore absorbs
certain spectral rays more than others. The variations in colour
matches and in the estimation of complementary colours by various
normal-sighted individuals were attributed by MaxwelP to this cause.
Glan^ and Sachs^ examined the absorption of the yellow pigment
from the macula of human eyes. Sachs found that absorption is
inappreciable in the red and orange, commences in the yellow-green
and gradually increases towards the violet end. Moreover, the amount
of pigment varies considerably in different individuals. Sachs examined
nine specimens : the mean coefficients of transmission of monochromatic
lights of these cases are as follows (Krarup)*:

1 Bhil. Trans. Boy. Soc. Lond. CL. 57, 1860.

2 Arch. f. d. ges. Physiol, xxxix. 53, 1886.
^ Ibid. L 574, 1891.

* Loc. cit. p. 18.

THE MIXTURE OF PURE-COLOUR STIMULI 41

X = 670— 590 580 575 570 560 555 550 540 535 530
1 0-991 0-986 0-981 0-971 0-966 0-962 0-951 0-946 0-941

X= 520 510 505 500 490 480 474 470 464 454—420
0-905 0-800 0-770 0-740 0-700 0-680 0-677 0-675 0-672 0-670

Owing to this absorption one would expect variations in colour mixtures,
and it is indeed found that, for example, in matching a homogeneous
yellow with a mixture of red and green, some people require slightly
more green than others, so that a given match appears too green to some,
too red to others. If such people make the match eccentrically, so that
the images fall just outside the macula, where there is no yellow pigment,
their matches agree^. v. Frey and v. Kries^ exhaustively investigated
the matches of numerous normal-sighted individuals (students) and
found that the deviations were such as would be expected from pig-
mentary absorption. This variation in individuals is therefore purely
physical, and must not be confounded with allied variations which
are due to physiological abnormalities. (See Part II, Chap, iii.) Since
the physiological abnormalities very rarely affect the blue end of the
spectrum, a ready method of distinguishing the effects of macular
pigmentation from them is by measuring their match between spectral
blue-green and a mixture of blue and green, v. Kries^ found that
reduction of blue by minimum and maximum macular pigmentation
was in the ratio 1 : 0"31, the corresponding ratio for green (517 /x/x)
being 1 : 0-5. (See also Section IV, Chap, ii.)

The Influetice of Lenticular Pigmentation. With advancing years
the crystalline lens becomes tinged with a yellow pigment. The effect
is similar to that of macular pigmentation, but does not manifest itself
until late adult life. Konig* estimated the coefficients of transmission
of monochromatic lights by the lens of a man, aet. 55.

1 Cf. Hering, Arch. /. d. ges. Physiol, uv. 277, 1893; Breuer, Ztsch. f. Psychol, u.
Physiol, d. Sinnesorg. xin. 464, 1897.
~ Arch. f. Aimt. u. Physiol. 336, 1881.

3 Ztsch. /. Psychol, u. Physiol, d. Sinnesorg. xm. 284, 1896.
" Konig, p. 346.

42 COLOUR VISION

CHAPTER IV

THE LUMINOSITY OF THE SPECTRUM

" It is to be noted that the most luminous of the prismatic colours
are the yellow and orange. These affect the senses more strongly than
all the rest together ; and next to these in strength are the red and green.
The blue compared to these is a faint and dark colour, and the indigo
and violet are much darker and fainter, so that these compared with the
stronger colours are little to be regarded^."

Fraunhofer^ first published in 1817 measurements of the brightness
of the various parts of the spectrum. Vierordt^ published a very similar
curve of the " strengths of coloured light " of the sun's spectrum. He
measured the amounts of the white light which must be added to
various parts of the spectrum in order to produce a just appreciable
diminution in saturation. Fraunhofer's and Vierordt's curves agree
well except towards the violet end, a fact which is of considerable
theoretical importance^. Draper^ however found all parts of the dif-
fraction spectrum equal by this method, and the red brightest in the
prismatic spectrum. Accurate investigations of more importance for
our purpose date from those of Abney and Festing^ and Konig and
Ritter'^, but before entering upon these and other researches we must
attack the difficult problem of realising what exactly is being measured.
It may be stated at once that we shall be unable to give a completely
satisfactory solution to the problem at this stage, but as we proceed in
our discussion the profound importance and significance of luminosity
in colour vision will become more apparent.

Two white lights of different intensity impress the senses with
stimuli which are interpreted as differing in brightness or luminosity.
The measurement of differences of luminosity is the function of photo-
metry.

Weber's and Fechner's laws (p. 20) suffice to show that even with

white light the relationship of the sensations to the stimuli is by no means

simple. When lights of different colours are compared the difficulties

1 Sir Isaac Newton, Optics, Bk. i. Part I, Prop, vii, Theor. vi. 1704.

^ Gesammelte Schriften, p. 1, Miinchen, 1888.

3 Pogcj. Ann. cxxxvii. 200, 1869.

* Cf. Helmlioltz, Zfsch. f. Psychol, u. Physiol, d. Skmcsorg. ii. 1, 1S91.

5 Loncl., Edin. nnd Dublin Philos. Mag. viii. 75, 1879.

6 Phil Trails. Roy. Sac. Land. 1886, 1888, 1892, 1899, etc.
~' 1891, in Konig, p. 144.

THE LUMINOSITY OF THE SPECTRUM 43

are greatly increased. The comparison of brightness or luminosity
of coloured lights is known as " heterochromic photometry " (Briicke).

In conformity with Newton's remarks, quoted at the beginning of
this chapter, and with the general consensus of opinion, brightness
must be regarded as a quantitative conception. When one colour is
added to another on the same field we are conscious that the brightness
of the field is greater than with either colour separately. An exact
match between two colour mixtures means that they have the same
hue and the same luminosity, and v. Helmholtz draws the conclusion
that Grassmann's second law (p. 33) can be extended to include
brightness^. He does not admit, however, that " equally bright lights
added give equally bright mixtures," and there is no doubt that
this statement is incorrect under certain circumstances (Purkinje's
phenomenon, v. p. 57). Apparently this is not the only exception
V. Helmholtz would admit, for as he says " equally bright red or blue
doubled give the red brighter than the blue." Briickner^ also holds
that the brightness of a mixture is not always equal to the sum of the
luminosities of its components, blue and yellow especially giving a
brighter mixture.

V. Helmholtz indeed expresses quite candidly his doubts as to the
accuracy of direct comparison of the luminosities of different coloured
areas. " I scarcely trust my judgement iipon the equivalence of the
heterochromic brightnesses, at any rate upon greater and smaller in
extreme cases. I admit, however, that one can gradually so darken one
of two coloured fields that no doubt remains as to the other being now
the brighter.". .." As far as my own senses are concerned I have the
impression that in heterochromic luminosity ecjuations it is not a question
of the comparison of one magnitude, but of the combination of two,
brightness and colour-glow {Farbengluth), for which I do not know how
to form any simple sum, and which too I cannot further define in
scientific terms."

V. Kries^ says that " it must not be considered as an obvious fact
that e.g. a given blue and a given red sensation are necessarily in the
strictest sense equally bright. For we could only state this definitely
if we were certain that the equality of the luminosity impression
corresponded with a particular physiological condition so that we
could substitute a well defined physiological entity for the subjective
conception of brightness ; but this is by no means the case."

» Helmholtz, 2nd cd., p. 440. 2 ^^c/i. /. d. gi\^. Physiol, xcviii. 90. 1904.

* Nagel's Handb. d. Physiol, d. Menschen, m. 259.

COLOUR VISION

Whether or not we agree with these words, that a high degree of
accuracy in such estimations can be attained is shown by Abney's
results, notably the following^ :

" To test the illuminating value of colour mixtures, three slits were
placed in the spectrum, in the red, green and violet. The luminosities
of the rays coming through each were measured — (1) separately ;
(2) in pairs ; (3) the whole combined.

20.3

(R+G) ..

242

38-5

(G+V) ..

8-5

(R+V) ..

214

(R+G+V)

250

Combining these together we get :

R+ G+ F=250
(R+G)+ F =250-5
(R+ V)+ G =252-5
{G+ V)+ 72=248
{R+ G+ F)=250

=250-25 by least squares.

Within the limits of error of observation the luminosity of the
combined spectrum measured as white is equal to the luminosity of
spectrum colours measured separately."

The very fairly coordinated results of different observers and
different methods show that in these luminosity measurements there is
something measured which appeals to their senses as of the same order.
Most will agree that it is a complex " something " which is expressed
by V. Helmholtz as a combination of " brightness and colour glow,"
and which we shall see later is an integral part of Hering's theory and
is expressed as the " white valency " of the colours. Further analysis
of this " something " must be left till a later stage. In the meantime
we will designate it " luminosity."

Direct comparison or "equality of brightness 2" is not the only method
of measuring the luminosity of spectral colours and mixtures. So
early as 1735 Celsius employed the visual acuity to determine the relative
brightness of various regions of the spectrum, and this method was also
employed by Buffon and Sir Wm. HerscheP. The same principle has
been applied to white light in the discrimination photometers of Houston

1 Abney, p. 105. ^ This is the term usually api^lied to this method by physicists.

3 Parsons, Roy. Lond. Ophth. Hosp. Rep. xix. 277, 1914.

THE LUMINOSITY OF THE SPECTRUM 45

and Kennelly, Fleming and others^. This method, however, is too
inaccurate for the purpose in view, though Crova's curves^ are in-
teresting^.

Charpentier^ and Haycraft^ used the method of finding the dis-
tance at which small areas of different colours become invisible. Another
method, based on the fact that the size of the pupil depends, inter alia,
upon the intensity of the light, has been advocated by Gorham^, and
has also given good results with coloured papers. Rivers', Martins^,
and Briickner^ have suggested other methods.

Heterochromic photometry has become of importance technically
since about 1880 on the introduction of the arc light for commercial
purposes. Hence a considerable amount of attention has been devoted
to it^°. Apart from the equality of brightness method adopted by Abney
and Festing, and Konig and Ritter, the method which provides most
accurate data for physiological purposes is flicker photometry, suggested
by Talbot (1834), but first used by Schafhautl (1855), and subsequently
by Rood^^, Polimanti^^ and others. When before two lights of different
luminosities an episcotister (a metal disc containing alternate closed
and open sectors) is rapidly rotated, an unpleasant flickering of each
light is seen at certain rates of rotation. Now it is found that the
flickering ceases at the same speed of rotation of the episcotister for the
two lights when their luminosities are the same. The causes of flicker
are complex and will require further attention later (Section V, Chap. i).
The results obtained are not free from criticism, but if due precautions
are taken they agree well with those yielded by the equality of brightness
method.

Konig's luminosity curves for the gas spectrum with different light
intensities are given in Fig. 10. They were obtained by the equality
of brightness method.

We are concerned here only with the higher intensities {F, G, H).
The curves have the same ordinate at 535 /x/z, because that ordinate
was taken as unity. It will be noted, however, that they all cross
and reverse their relative positions. This is due to the Purkinje

1 Trotter, Illumination, London, 1911.

■^ Ann. de Chim. et de Phys. vi. 528, 1885 : La Lumiere elec. xvm. 549, 1885.

* Trotter's Illumination, p. 171. * La Lumiere et les Couleurs, Paris, 1888.

* J. of Physiol. XXI. 126, 1897. « Proc. Roy. Soc. Lond. xxxvn 425, 1884.

' J. of Physiol, xxn. 137, 1897. ^ Beitrdge zur Psychol, u. Philos. i. 95, 1896.

* Arch. f. d. ges. Physiol, xcviii. 90, 1904. lo Trotter, he. cit. 101.
" Amer. J. of 8c. xlvi. 173, 189:5 ; Science, vu. 757 ; viii. 11, 1898.

12 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xix. 263, 1899.

COLOUR VISION

3 8-

3-6

3-4-

Light. int.p.nRitip!iH

3-2

3-0

/ /
/ (
/ (
/ /

/ /

2-G

/ /
/ '

. D

;_ , c

2-4-

' ,

_ „ B

2-2

1-8

1 1
1 1

1 1

1 1 1

1-4

I'l
III

1-2

1
i

-■■

--'-

--.^

..\

" 1

/'/

r- ".'■'■■

\S^^^

0 8

'/•

■■''/.■■'

^ ^
x."^

V -v.

00-
0 4
0-2-

1 ■

/
/

...

.■

870 650 625 605 590 575 555 535 520 505 490 470
BCD E F

450

430
G

Fig.

10. Normal trichromat's luminosity curves lor different intensities of light {A, B, C,
cto. ; A being the lowest, and // the highest intensity). Abscissae, wave-lengths of
the prismatic spectrum of gas light ; ordinates, an arbitrary scale. (Konig.)

r— —

'AN

too

\j/

^//

' 1

-A

fin

•in

1
1 \

' /

^i>

' /

i \»

7ft

{±1

===

7 £

1 1

^»~

6 8 10 12 H 16 18 Id ZZ Zt, U Z8 50 32 34 36 18 4o « 44 4€ 48 50 5Z 54 56 5B 60 62 64 66
m 3839« 506 S8-ISS'7 6l-3

Fig. 11. Normal trichromat's photopic luminosity curves for the fovea, macular region,
and 10° from the fovea. Abscissae, arbitrary scale of the prismatic spectrum of the
arc light, the Fraunhofer lines and some important wave-lengths bemg indicated ;
ordinates, arbitrary scale, the maximum luminosity being 100. (Abney.)

THE LUMINOSITY OF THE SPECTRUM

phenomenon (p. 57). Maximum luminosity is at about (JlO/x/x with
these intensities. Krarup^ has calculated the curves from Konig's
results in terms of the energy values of the spectrum, making allowance
for absorption by the macular pigment, so that the results are inde-
pendent of the particular source of light.

Abney's luminosity curves for the spectrum of the arc light with
inclined carbons are shown in Fig. 11. They were also obtained by
the equality of brightness method.

The following are the macular luminosities for the chief Fraunhofer
lines, the source of light being the crater of the positive pole of the
electric arc, with sloping carbons :

Line

Lumincsity

OSfi-G /u/x

Li (red)

670-5

656-2

589-2

09-5

526-0

h (Mff)

518-3

486-0

Li (blue)

460-3

430-7

0-()

The difference between the luminosity curve by the equality of
brightness method and the flicker luminosity curve is shown in Fig. 12.

/oo

90
80
70

60
50
-fO
30
20
/O

SS/^ 26 30 55 'fO -fS SO S5 60 65

Fig. 12. Normal trichromat's photopic luminosity curves with the equality of brightness
and the flicker methods. Abscissae, arbitrarj^ scale of the prismatic spectrum of
the arc light ; ordmates, arbitrary scale, the maximum luminosity by the equality
of brightness method being 100. (Abney.) (Cf. Fig. 38.)

1 —

M LI

mine

5ity (

urve

N Fl

icker

curve

(max)

925)

E
1

I-'

^ Loc. cit. p. 46.

48 COLOUR VISION

The flicker curve was taken with Prof. W. Watson's flicker apparatus
used in conjunction with Sir Wm. Abney's apparatus. The conditions
were — arc light with positive carbon horizontal, current 18 amps.,
D line ^ 1 metre candle, patch 1 sq. inch observed at a distance of 3 ft.
It will be noticed that the flicker maximum is less than the equality
of brightness maximum, and in the proportion 92 '5 : 100, but Ives's
results {v. p. 96, Figs. 38, 39) show almost complete coincidence.

The most striking confirmation of the general accuracy of these
methods is that the luminosity values thus found agree with their
periphery values. We shall see later (p. 67) that at the periphery of
the retina colours of moderate intensity cease to convey the impression
of colour and appear white, so that the periphery values are not com-
plicated by the intrusion of the colour factor. Polimanti^ confirmed
this fact for the flicker method.

1 Loc. cit.

SECTION III

THE SPECTRUM AS SEEN BY THE DARK-ADAPTED

(SCOTOPIC) EYE

CHAPTER I

ADAPTATION OR TEMPORAL INDUCTION

When we pass suddenly from bright sunlight into a dimly lighted
room we can see nothing for a time until we become " accustomed to
the darkness." When we pass from the dark into bright light vision
is also difficult and may be painful. We therefore infer that the sensi-
bility of the retina becomes increased at low illuminations. This
automatic process of levelling the sensibility of the retina to the require-
ments of the moment is called dark or light " adaptation " (Aubert)
or " temporal induction."

Dark adaptation is a relatively slow process. It is characterised
by a rise in the sensitiveness of the retina to light, which is slow during
the first ten minutes of exclusion of light from the eyes, rapid during
the following twenty or thirty minutes, and again slow or almost
negligible after that period. The general character of the curve of
retinal sensibility is the same in all cases, but there are marked individual
variations in the rapidity and amount of the rise, thus explaining the
fact that some people see very much better in a dull light than others,
though variations in the size of the pupils and other factors {vide infra)
are not without importance in this respect. In night-blind people
there may be only a very slow rise, the ultimate sensibility after an
hour being near the normal limit. In severe cases there is very little
rise even after several hours.

Such adaptation is normal in the colour-blind, even the totally colour-
blind^. Strychnin and brucin cause increase in the amount and rapidity

1 Tschermak, Ergebnisse d. Physiol, i. 1, 700, 1902.
p. c. V. 4

50 COLOUR VISION

of the rise of sensibility : santonin has no effect. Mydriatics have an
indirect effect ; the first slow rise is prolonged from ten to twenty
minutes but is followed by the normal rapid rise to the normal height.

Very short exposure to bright light, e.g. striking a match, causes
a very temporary fall without materially altering the course of the curve.
The increase in sensibility after very prolonged dark adaptation is more
transient than the increase during the first hour, i.e. it is more quickly
and completely abolished by exposure to light. Dark adaptation of
one eye has no effect upon the other^.

Besides this temporal variation in the sensitiveness of the retina
there is a well-marked regional variation. In the condition of light
adaptation the fovea is the most sensitive part of the retina, though
little attention has been paid to the degree of adaptation in the researches
published on this subject. (The light sensitiveness of the various parts
of the retina must be carefully distinguished from their visual acuity
for form.) The regional sensibility for colours of the retina of the light-
adapted eye has been worked out by Vaughan and Boltunow^, v. Kries^,
and Guillery*. Vaughan and Boltunow found the sensitiveness at 10°
from the fovea to be \, at 20° to be y^, and at 35° to be ^f'^^ of that of the
fovea itself. In dark adaptation the fovea is the least sensitive part
of the retina^. In other words the fovea is a region of physiological
night-blindness (v. Kries).

The relative central scotoma in dark adaptation was long ago recog-
nised by astronomers, who noticed that stars of small magnitude were
seen better if viewed somewhat eccentrically. " Pour apercevoir un
objet tres pen lumineux, il faut ne pas le regarder " (Arago). It is
strikingly illustrated in viewing the Pleiades : by direct fixation four
or at most five stars are seen ; by indirect fixation a number of weaker
stars become visible. Different observers use different parafoveal^ spots
for clearest vision in dark adaptation' and the spots vary with the
degree of dark adaptation. The nearer the intensity of the stimulus
is to the threshold of the dark-adapted fovea the nearer is the spot to the
fovea : the feebler the light the more eccentric is fixation. With a
given sub-minimal foveal stimulus Simon found that he fixed 2° from

^ Cliarpentier, La Lumiere et les Couleurs, p. 175, Paris, 1888.

2 Ztsch. f. Sinnesphysiol. xlii. 1, 1907.

^ Zl-sch. f. Psychol, ii. Physiol, d. Sinnesorrj. ix. 81, 189fi.

* Ibid. xii. 261, 189() , xiii. 189, 1897. ^ Donders, Brii. Med. J. 1880.

* I.e. in the region near the fovea.

' Christine Ladd-Franklin, in Konig. p. 353 ; Simon, Ztsch. f. Psychol, u. Physiol, d.
Sinmsorg. xxxvi. ]8fi, 1904.

ADAPTATION OR TEMPORAL INDUCTION

the fovea after ten minutes dark adaptation, 1|° after twenty minutes,
and 1° after an hour. The direction is constant for the same eye and
varies with different eyes ; it depends upon muscular balance and
refraction rather than upon the specific sensibility of the parts of the
parafoveal region (Simon).

Although the fovea is night-blind relatively to the periphery it is
capable of a slight degree of dark adaptation^, but the small rise in
sensitiveness of the fovea is only appreciable after previous very strong
light adaptation, such as looking at the open sky.

Breuer and Pertz^ showed that the peripheral rise in retinal sensi-
bility in dark adaptation is rapid from 1° to 4° around the fovea, then
slower to a maximum between 10° and 20° beyond which it falls. This
is seen graphically in Fig. 13, where the sensibility of the fovea and the

Fig. 13. Sensibility of the fovea and parafoveal region for mixed bluish-white light.
Abscissae, to the left of zero degrees to the temporal, to the right, degrees to the nasal
side of the fovea (0) ; abscissae, arbitrary scale. (Breuer and Pertz.)

parafoveal region to bluish-white light are shown. The abscissae to
the left represent the teniporal side, to the right the nasal. Pertz's
experiments show that the scotopic fovea is more sensitive to red light
than the periphery, though the difference is slight. Blue light gives a
curve resembling that in Fig. 13. Yellow shows 'a slight rise in the
paracentral area.

The alterations in sensibility differ according to the size of the area

1 Charpcntier, Arch. iTOpht. iv. 291, 1884 ; xvi. 87, 1 896 ; Fick, Arch.f. d. yes. Physiol
xmi. 481, 1888; Treitel, Arch.f. Ophth. xxxv. 1, 50, 1889; v. Kries, Arch.f. Ophth. xlii.
3, 95, 189G; Tschermak, Arch. f.d. fjea. Physiol, lxx. 320, 1898; Bloom and Garten,
ibid. Lxxii. 1898.

^ V. Kries, Ztsch.f. Psychol, u. Physiol, d. Sinncsorfj. xv. 327, 1897.

4—2

52 COLOUR VISION

stimulated, and the relations between sensibility and the area stimulated
are different in the light- and dark-adapted eye, and also in the foveal
region and the periphery. For foveal vision the sensibility is pro-
portional to the area stimulated (Ricco, Loeser^). In the dark-adapted
periphery the sensibility is proportional to the square root of the area
stimulated'^, but only for composite- white light and objects subtending
a visual angle of 1° to 10°. Above 10° the sensibility rises more slowly.
The rise is still less even for smaller angles with red light. The rise of
the curve of sensibility in dark adaptation therefore varies with the
size of the area of retina stimulated and with the nature of the light.
In the light-adapted eye there is no definite relationship between the
rise of sensibility and the size of the visual angle.

In dark-adapted eyes binocular summation of stimuli occurs, so
that the sensibility is about twice as great with both eyes as with
only one (Piper), though individual variations occur. In light-adapted
eyes no such binocular summation occurs (Fechner^), but care must be
taken that there is good light adaptation, and one eye must be covered
for only a very short period, otherwise partial dark adaptation occurs.
In this respect there is a noteworthy analogy to the effects of the size
of the area of the retina stimulated : with complete light adaptation the
stimuli to different parts of the retina are not summated, given that
the visual angle exceeds a certain (small) size, nor are the stimuli to the
two eyes summated, whereas in the condition of dark adaptation both
summations occur.

CHAPTER II

SCOTOPIA OR TWILIGHT VISION

It will be readily appreciated that complete dark adaptation rarely
occurs under normal conditions of life. Scotopia is the condition of
vision in which there is a relatively high degree of dark adaptation.
It will be best to consider the conditions of vision after prolonged stay
in a feebly lighted room. If now coloured objects are viewed under
feeble illumination the colours cannot be distinguished, but all appear
to be of various shades of grey. The eye is totally colour-blind.

^ Parsons, Roy. Loncl. Ophth. Hosp. Rep. xix. 114, 1913.

2 Piper, Ztsch. /. Psychol, u. Physiol, d. Sinnesorg. xxxn. 98, 1904.

» V. Helmholtz, 3rd ed. ii. 287.

SCOTOPIA OR TWILIGHT VISION 53

A spectrum of low intensity appears as a colourless bright streak,
varying, however, in brightness in different parts. Consequently
accurate matches can be made between any two parts of the spectrum
by merely modifying the intensity of one light.

If the intensity of the spectrum is slightly raised the colours become
evident in a definite order and the relative brightness of the different
parts becomes altered. As the intensity is still further raised the eye
becomes light adapted and the spectrum shows all its hues with the
relative brightnesses described in Section II. Scotopic vision at very
low intensities is therefore achromatic ; with slightly raised intensities
of light it becomes chromatic. We may distinguish the two conditions
as achromatic and chromatic scotopia respectively.

The achromatic scotopic values of different parts of the spectrum
were first investigated by Hering and Hillebrand^ for the dispersion
spectrum of daylight. Abney and Festing's curve^ is shown in Fig. 14.

The striking feature is that the brightest part, instead of being in
the neighbourhood of the D line (yellow), is moved further towards the
violet end, and is at about 530 /Z/u- instead of at about 580 /^t/u,. The
luminosity curve falls slowly towards the violet end, and sharply
towards the red ; the red end is shortened. It may be stated at once
that all types of colour blind give almost identical achromatic scotopic
curves^. Hence we can consider here results obtained both by normal
observers and by colour-blind observers like Nagel* (a deuteranope).
Very accurate observations were made by Schaternikoff^ in v. Kries's
laboratory. Fig. 15 shows the similarity between Nagel's (deuter-
anopic) and Schaternikolf's (normal) curves.

Fig. 16 shows the sunlight, Nernst light, and gas light curves^.

The summits of the gas light curve (537"2/x/x) and the sunlight
curve (529'3;u./x) differ slightly, and light from cloudless sky gives
a rather higher value in the green-blue and blue than direct sunlight.
An Auer lamp, which is rich in green rays, will give a slightly different
curve from a carbon filament electric lamp, which is rich in red rays.
The curve will depend upon the energy distribution in the given
spectrum, and of course the diffraction or interference spectrum curve

1 Siizungsher. d. Wiener AkacL, math.-naturw. Kl. xcviii. 70, 1889.

2 Phil. Trans. Roy. Soc. Lond. clxxxiii. A, 531, 1892.

^ Raehlmann, Zlsch. f. Augcnlilk. ii. 31.5, 403, 1899, for a possible exception ; also
Tschermak, Ergeh. d. Physiol, i. 2, 703, 747, 1902.

* V. Kries and Nagel, Ztsch.f. Psychol, u. Physiol, d. tiinnesorg. xii. i, 1896.
5 Ibid. XXIX. 2fi5, 1902.

* Trendelenburg, Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xxxvn. 1, 1904

COLOUR VISION

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Fig. 14. Normal trichromat's photopic and scotopic luminosity curves. (One amy]
acetate or Hefner unit is equal to 0-9 international candle power. ) Abscissae, arbitrary
scale of the prismatic spectrum of the arc light ; ordinates, arbitrary scale, the maxi-
mum of luminosity being in each case 100. (Abney and Festing.)

300
280

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240
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Fig. 15. Scotopic luminosity curves.

trichromat). — Nagel's curve (deuteranope)

Schaternikoff's curve (normal
X X X X X and o o o o o.

other observations by Nagel. Abscissae, wave-lengths of the prismatic spectrum
of gas light ; ordinates, arbitrary scale. (Schaternikoff.)

SCOTOPIA OR TWILIGHT VISION

will differ from the prismatic curve. Krarup^, using the energy values
for the Hefner light calculated by Angstrom, and allowing for absorption
by the macular pigment on the basis of Sachs's researches, has calculated
the achromatic scotopic luminosity curve from Konig's results. This
curve is independent of the source of light employed and shows a
remarkable agreement with (1) the curve of absorption of visual
purple ; (2) the luminosity curve of the totally colour-blind {vide
infra) ; and (3) the similarly corrected achromatic scotopic luminosity
curve of a protanope investigated by Konig.

3000

2500

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\

1500

1000

1500

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Sunlight

Nernstlight
Gaslight

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Iff If III I

Fig. 10. Scotopic luminosity curves for direct sunlight (Schaternikofi'), gas light (Schater-
nikoff), and Nernst light (Trendelenburg), reflected from a magnesium oxide-coated
surface. Abscissae, wave-lengths of the prismatic spectrum ; ordinates, arbitrary
scale, the maximum of luminosity being in each case 3000. (Trendelenburg.)

The flicker method

gives

concordant results. Fig. 17 shows

Haycraft's^ results for colour-discs with three intensities. The ordinates
show the number of revolutions of a semi-disc per second which were
found necessary to abolish flicker : the abscissae are wave-lengths.

The extraordinary similarity between the scotopic luminosity curve
and the curve of values of different monochromatic lights for bleaching
frog's visual purple (Trendelenburg) (Fig. 1), seems to point to some
underlying physiological fact of great significance. We shall return to
this point later.

^ Loc. cit. p. 21.

2 J. of Physiol, XXI. 12(». 1897.

COLOUR VISION

As may be easily imagined there are many practical difficulties
about the observations. Before examining these and other such
experiments more in detail it will be well to consider briefly some of the
outstanding peculiarities of scotopia or twilight vision as compared with
photopia or daylight vision.

400 460 620 680 640 700 760

Fig. 17. Liiminosity curves determined by the flicker method (critical frequency readings).
The upper curve i-epreseuts a briglit spectrum, the lower a dark spectrum, and the
middle curve one of intermediate intensity. (Haycraft.)

" At nightfall in the summer the order of disappearance of colour
may often be seen ; orange flowers may be plainly visible, yet a red
geranium may appear black as night ; the green grass will be grey when
the colour of the yellow flowers may yet be just visible. An early
morning start in the autumn before daybreak will give an ample oppor-
tunity of satisfying oneself as to the order in which colours gradually
reappear as daybreak approaches. Red flowers will be at the outset
black, whilst other colours will be visible as grey. As more light comes
from the sky the pale yellow and blue flowers will next be distinguished,
though the grass may still be a nondescript grey. Then, as the light
still increases, every colour will burst out, if not in their full brilliance,
yet into their own undoubted hue^." It is instructive with good dark
adaptation in a dull light, to arrange different coloured cards, about

^ Abney.. Colour Vision (Tyndall Lectures), p. 107, 1895.

SCOTOPIA OR TWILIGHT VISION 57

the size of a postcard, according to their apparent brightness. Red
and green will occupy the dark and light ends of the series and orange
will match a dark blue. Similar experiments can be made to give
quantitative estimates with the colour top.

The explanation of these facts is found in the scotopic luminosity
curves. In the bright prismatic spectrum of gas light the red (670 /x/x)
appears about ten times as bright as the blue (480 ixfju). On the other
hand the achromatic scotopic value of the red is less than one-sixteenth
that of the blue. Hence with failing light the brightness of different
coloured objects alters, the colours towards the red end of the spectrum
becoming relatively darker, those towards the violet end brighter, so
that finally the reds appear almost black and the blues bright. This
fact was first investigated by Purkinje (1825) and is known as Purkinje's
phenomenon. Hering^ drew attention to the fact that the brightness
of the blues increases much more rapidly than that of the reds diminishes.
This depends upon the condition of dark adaptation of the retina.
K5nig'2 {v. p. 45) paid more attention to variations in the intensity of
the light. The dift'erences of these two authors on the subject of
Purkinje's phenomenon depend upon the fact that two factors have to
be considered — dark adaptation and stimulus-intensity {vide infra).

It is to be noted that in every-day experience we are not dealing with
complete dark adaptation. The scotopic luminosity curves given above
apply only to very thorough dark adaptation. In such experiments
as Konig's (Fig. 10) the adaptation was certainly changing, though
there must have been a fair degree of dark adaptation, and the main
alteration in the form of the curves is rightly attributed to intensity
of stimulus. It will be seen from these and other such curves that the
shift in sensation intensity with diminished stimulus intensity is gradual.
I wish to avoid as far as possible introducing theoretical considerations
into this part of my book, but it will simplify comprehension of the
subject if it is pointed out at once that the easiest explanation of the
gradual shift of the curves is the simultaneous action of two processes,
one especially related to vision with relatively high stimulus values and
light adaptation, the other to vision with low stimulus values and dark
adaptation. Upon this explanation scotopic vision is a relative con-
dition, its nature depending upon low^ stimulus values and a com-
paratively high degree of dark adaptation. Whether the processes are
subserved by independent physiological mechanisms is a question to be

^ Arch.f. d. ges. Physiol, i.x. 51G, 1895. 2 Konig, p. 114.

58 COLOUR VISION

deferred, though what has been said in the last sentence is hardly in-
telligible on any other assumption.

Complete dark adaptation is only reached after prolonged exclusion
of light. Hence a moderate degree of scotopia is generally present in
every-day life, sufficient indeed to elicit the Purkinje phenomenon
merely by sudden diminution of the intensity of the stimvilus. If after
remaining for a considerable time in a moderately lighted room the
illumination is suddenly diminished reds at once appear much darker
and blues much brighter^.

The relative roles played by dark adaptation and by intensity of
stimulus may be stated somewhat as follows. Dark adaptation deter-
mines the degree of scotopia. This is shown by the fact that the colour
threshold or threshold value of photopic vision {Schivellenivert des
Tagessehens, v. Kries) remains constant whilst the general threshold
becomes lowered. Hence with good dark adaptation scotopic luminosity
values are high. As light adaptation increases the photopic values
increase and ultimately preponderate: hence Purkinje's phenomenon.
With still stronger light adaptation intensities of stimuli below the
photopic values are sub-liminal.

As regards intensity of stiimdus, as soon as it reaches the chromatic
threshold the luminosity values increase throughout the spectrum, but
the increase in the red is very slight as compared with that in the
blue.

The older authors (Dove, Grailich, Helmholtz) directed their attention
almost entirely to the intensity, so that Konig lays stress upon the
alteration in colour matches as " deviations from Newton's law of
colour mixtures^." Hering^ and his pupil Tschermak^, on the other
hand, regard adaptation as the sole cause. Hering^ used two rooms
separated by a partition which had round holes in it filled with coloured
glass. Alteration in the illumination in one room varied the adaptation
and in that of the other the intensity of the light observed. He found
that with continued light adaptation, diminution of intensity of red and
blue caused equal change in the luminosity of the two colours. Red,
however, retained its colour slightly longer and appeared slightly
brighter. With dark adaptation the blue became whiter and brighter

^ Bering, loc. cit.

2 Konig, pp. 108, 4] 6 ; cf . Tonn, Ztach. f. Psychol, u. Physiol, d. Sinnesorg. vii. 279,
1894.

3 Lotos, VI. 1885 ; vii. 1886. « Arch. f. d. ge.'i. Physiol, lxx. 297, 1898.
* Arch.f d. ges. Physiol, lx. 519, 1895.

SCOTOPIA OR TWILIGHT VISION

than the red without bringing about any change in intensity. Even
momentary adaptation on lowering the illumination elicits the Pur-
kinje phenomenon. These experiments cannot be regarded as con-
clusive, partly on account of the different amounts of white light
transmitted by the coloured glasses, but undoubtedly adaptation plays
the preponderant role^.

As might be expected from our previous experiences of luminosity
phenomena the scotopic relative increase in brightness is associated
with a diminution in saturation. Those red rays which have little
(620 — 670jUjLt) or no (beyond QTO/jl/jl) scotopic stimulus value, show no
change.

The eft'ect of Purkinje's phenomenon is seen graphically in Fig. 18,
where the abscissae represent intensities of red lights (670 /x^) and
the ordinates those of blue light (450 fifx) ; points on the curve represent
equivalent luminosities^. The deviation from a straight line at low
intensities shows the relative increase in brightness of the blue light.

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Fig. 18. Curve of equivalent luminosities of red (670 /xfj.) and blue (450 fxfx) lights at difiFerent
intensities (slit-widths). (Konig.)

Although different colours alter their appearance on diminution of
their intensity it might be thought that accurate photopic matches
would remain valid for the scotopic eye. This, however, is not the case.
Albert^ noticed that spectral yellow when matched with a suitable
mixture of red and green ceased to match on diminishing the intensity,
the mixture becoming brighter and less saturated than the homogeneous

^ Cf. Feilchenfeld, Ztsch. f. Sinnesphysiol. XLiv. 51, 1909.
^ Konig and Brodhun, in Konig, p. 144.
^ Wiedemann's Ann. xvi. 129, 1882.

60 COLOUR VISION

colour. Ebbinghaus^ and Christine Ladd-Franklin^ almost simul-
taneously drew attention to the significance of the facts. They showed
that three whites made by mixture of red and blue-green, yellow and
blue, and green-yellow and violet darken unequally with proportionally
diminishing intensity, the first least, the second more, and the third
most. Konig's experiments^ with complementary colours are confirma-
tory. Still more so are observations on the colour-blind.

On the other hand as soon as dark adaptation is sufficiently great
to abolish the sensations of colour any alteration of the intensity of
the light which does not exceed the colour threshold fails to alter the
various matches. It is true that Stegmann^ found slight differences,
but they were too slight to be of much importance, and such as
they were were in a direction opposite to that taken in the Purkinje
phenomenon.

As Lummer^ pointed out, the peculiarities of scotopic vision explain
an observation of Williams^, H. F. Weber'^, and Aubert*^ that a body
heated to redness in the dark first shows a grey glow. This occurs
at 400° C. (H. F. Weber ; 379° C, Gray^) ; as the temperature rises the
yellow-green rays increase and cause a yellowish-grey glow. At about
525° C. (Draper) the red glow commences, but the temperature varies
with the conditions of the experiment. If the observer is light-adapted
these preliminary stages are invisible. Abney^° had previously recog-
nised the explanation of these facts.

From what has already been said we see that there are two thresholds
of vision — a general threshold, the minimal stimulus producing the
sensation of light ; and a specific or colour threshold, the minimal stimulus
producing the sensation of colour. The interval between them is
known as the colourless or photochromatic interval. It depends upon the
scotopic visibility of .the given light below the threshold of photopic
vision, and varies with the condition of adaptation and the nature
of the light stimulus^^ The colourless interval increases with in-
creasing dark adaptation, and this is due to lowering of the general

^ Ztsch. f. Psycliol. u. Physiol, d. Sinnesorg. v. 145, 1893.

- Nature, XLvm. 517, 1893. » Kcinig, p. 373.

* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxv. 226, 1901.

* Wiedemann's Ann. LXii. 14, 1897. '^ Pogg. Ann. xxxvi. 494, 1835.

"> Wied. Ann. xxxii. 25G, 1887. ** Physiologic der Netzhnuf, p. 41, 1865.

9 Proc. Phys. Sac. xiii. 122, 1894.
^° Colour Vision, Tyndall Lectures, p. 35, 1895

1^ Nagel and Schiifer, Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxiv. 271, 1904;
Loeser, Ibid, xxxvi. 1, 1904.

SCOTOPIA OR TWILIGHT VISION 61

threshold, the specific threshold remaining almost or quite constant.
As regards the nature of the light stimulus the colourless interval is
greatest for light of short wave-length and least for light of long wave-
length. In the orange it is small even with good dark adaptation.
In the red beyond 670 /x/x it is almost completely abolished. In fact
even with very good dark adaptation such a red light excites the
sensation of red^, and the only evidence of a colourless interval is the
alteration in the character of the sensation as dark adaptation becomes
more complete, the red disappearing or becoming paler and brighter
according to whether its wave-length is greater or less than about
670 jxfji. A minimal colourless interval can, however, be elicited under
suitable conditions^degree of dark adaptation, size of field, paracentral
or peripheral stimulation (Charpentier)^ {v. p. 81).

The general light-threshold has been held to meet the requirements
of a physiological unit of luminosity, for it has been accepted as an
axiom that at their achromatic scotopic thresholds all lights are of equal
brightness. " The light which can just be perceived has always the
same brightness, no matter what be the light which acts as the stimulus"
(Konig^).

Abney and Festing* early recognised the importance of investi-
gating what are here called the chromatic and general thresholds for
different parts of the spectrum. Light from a monochromatic beam
was matched with that from a white beam, both being altered in intensity
by the use of rotating sectors (episcotister), or in Abney's later experi-
ments by the annulus (p. 5). Accurate photometric observations of
the comparison light were made and the experimental variations were
calculated from the aperture of the sectors or the scale of the annulus.
The " extinction of colour " was measured thus. The light from the
D line was taken as a standard and was arranged so that the illumina-
tion on the screen was 1 foot-candle. The intensity of the monochromatic
light was then reduced by the annulus or episcotister until it appeared
colourless and matched the comparison " white." The amounts of
reduction necessary for various monochromatic lights throughout the
spectrum were thus obtained. From these results the amounts of

^ Pai'inaud, Compt. rend, dc VAcad. Jr., 286, 1881 ; Konig, p. 144 ; v. Krics, Ztscli. f.
Psychol, u. Physiol, d. Sinnesorg. ix. 86, 1896.

2 Arch, de Physiol. 1877 ; Arch. d'Opht. xvi. 337, 1896 ; Hering, Arch. f. d. ges. Physiol.
LX. 535, 1895; Koster, .4rc^./. Ophth. xli. 4, 13, 1895; Tschermak, Arch. f.d. ges. Physiol.
Lxx. 320, 1898 ; v. Krics and Nagcl, Ztsch. f. Psychol, u. Physiol, d. Sinntsorg. xii.
15, 1896.

' Konig, p. 190. « Phil. Trans. Roy. Soc. Lond. CLXXxm. 537, 1892.

COLOUR VISION

reduction necessary when the illumination on the screen by each
monochromatic light was 1 foot-candle were calculated. These results
give the actual illumination at the point of extinction of colour in
terms of foot-candles. Thus, when the illumination with SSN'^ 50
{558[x[x) was 0"0016 foot-candle at the screen the colour just dis-
appeared, and so on. Fig. 19 shows one of the curves obtained.

S (0 15 20 25 30 3S 40 'iS 50 SS 60

Fig- 19. " Extinction of colour " curve. Abscissae, wave-lengths of the prismatic
spectrum ; ordinates, intensity of the illumination in candle-feet on the screen when
the colour just vanishes. (Abney.)

In the " extinction of light " a curve was plotted showing what was
the fraction of the beam from each part of the spectrum which was just
invisible. The absolute luminosity of each part of the spectrum was
determined, and from these values a second curve was plotted with
ordinates representing the absolute luminosities at the points of extinc-
tion. The second curve shows the illumination in fractions of a foot-
candle at the screen by each monochromatic light which would be just
invisible. The branching of the curves (Fig. 20) beyond the green
towards the violet end is due to macular pigmentation, the " whole eye "
curve showing the intensities when the eye was allowed to wander.
If a curve is plotted from the reciprocals of the extinctions the " per-
sistency curve " (Abney and Festing) is obtained.. This is obviously
the luminosity curve of the spectrum at the points of extinction. It is

1 SSN is the scale-number on Abney's arbitrary scale of the spectrum.

SCOTOPIA OR TWILIGHT VISION

identical with the luminosity curve of a feeble spectrum when the
maxima are made equal. Fig. 21 shows such a curve.

Such are the conditions of the chromatic and general thresholds
in the dark-adapted eye. What are the conditions of extinction of
colour and light in the photopic eye ? This difficult problem has been

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Fig. 20. " Extinction of light "curve. Abscissae, wave-lengths of the prismatic spectrum ;
ordinates, the fractions of the beams which are just invisible, the unreduced illumina-
tion of tlie D light at the screen being one Hefner-foot (0-9 candle-foot). Tiie dotted
lines show the proportion on the assumption that each beam has the same intensity
as that of the D light.

attacked for the general threshold by iVbuey^ in the following manner.
A disc of matt white paper, 4 inches in diameter, was illuminated by
white light. Through a hole in the centre, .| inch in diameter, a coloured
light was viewed. The fovea was thus stimulated with monochromatic,

^ Proc. Roy. Soc Lond. A, lxxxvii. 1912.

COLOUR VISION

and the parafoveal zone with white light, of known intensities. A small
spot of shadow was thrown upon the white disc from which the para-
foveal zone was illuminated. The blackness of this shadow is the same,
irrespective of the intensity of the white light, though it appears to be
different according to that intensity. The monochromatic light was
then reduced until the spot illuminated by it matched the shadow spot
in blackness.

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Fig. 21. " Persistency curve " of a deuteranope. The values are the reciprocals of the
" extmction of light " values, which are also shown. The " normal luminosity curve "
is the curve for higher intensities of light (the photopic luminosity curve). The
persistency curve is the achromatic scotopic luminosity curve (compare Fig. 14).
(Abney and Festing.)

The illumination of the white area was modified by the annulus so
that intensities from 0*2 to 0'00078 foot-candle were obtained. Fig. 22
shows the logarithms of the intensities of the light for different wave-
lengths for extinction with different strengths of illumination of the
parafoveal zone with white light.

Fig. 23 shows the corresponding persistency curves. These, as
before pointed out, give the luminosity curves of the spectrum at the
points of extinction, and we see that they correspond with Konig's

SCOTOPIA OR TWILIGHT VISION

and other luminosity curves. With 0'2 foot-candle of parafoveal
illumination the curve is the luminosity curve of the photopic eye.

Fig. 22. " Extinction of light " curves for different degrees of light adaptation. Abscis-
sae, wave-lengths ; ordinates, logarithms of the intensities of the light. (Abney.)

100

Fig. 23. " Persistency of light " curves corresponding to the " extinction of light "
curves for different degrees of light adaptation. Abscissae, wave-lengths ; ordinates,
reciprocals of extinction values. (Abney.)

As the parafoveal illumination diminishes the curve shifts towards the
violet end, so that the scotopic curve for complete dark adaptation is
finally reached.

p. 0. V. 5

66 COLOUR VISION

It has been found in experiments on the colour threshokl that if
the colour is just extinguished very slight illumination of the retina
with white light will cause it to reappear without making any alteration
in the intensity of the coloured light. Boswell^, working under Nagel's
direction, showed that the same phenomenon occurred on the addition
of any light of considerable scotopic value, such as green. Since the
eye is dark adapted the summation of the colourless scotopic values of
the coloured lights throws the value of the stronger above the chromatic
threshold. It is not a question of contrast, as is shown by the action
of white light, nor of complementary colours. The same fact is borne
out by Abney's experiments, where the extinction intensity for retinal
illumination of 0*2 foot-candle is 2 '6 times that of illumination of
0-00078 foot-candle.

^ ZLsch.f Sinnesphysiol xli 364, 1907.

SECTION IV

REGIONAL EFFECTS
CHAPTER I

THE FIELD OF VISION FOR COLOURS

When we look at or fixate a particular object we are conscious of
the presence and movement of other objects situated around the object
fixated. The image of the object fixated is formed at the fovea, those
of surrounding objects at various points in the peripheral parts of the
retina. The area over which such outlying objects can be perceived
is called the field of vision, which is therefore the projection outwards
of all the points upon the retina which can initiate visual sensations.
Its limits are usually plotted upon a chart by means of a perimeter^.

The normal photopic field of vision for a well-illuminated white
object, 10 mm. square at a distance of 45 cm., is a horizontally oval
area extending upwards about 50°, outwards 90°, downwards 70°, and
inwards 60°. It varies with the illumination, the size of the test object,
the contrast of the test object with the background, and the state of
adaptation of the eye^.

The fields for colours are smaller (Purkinje) : by the ordinary
clinical methods those for blue and yellow pigmented objects are about
10° smaller than that for white ; those for red and green 20° smaller,
the red field being usually smaller than the green, and violet smallest
of all.

^ See Parsons, Diseases of the Eye, p. 1(50, 2iid ed. 1912.

- See Landolt, in GraeJeSuemisch Hand. d. (jes. Augenheilkunde, iv. 1, 0-48, 1904 (witli
Bibliography).

5—2

68 COLOUR VISION

Charpentier^ obtained the following results with coloured paper of
20 sq. mm.

Violet Blue- Yellow- Red Orange Yellow Blue

green

Inwards

Downwards

Outwards

Upwards

Hess's field for invariable red and green {vide infra, p. 70) with an
object subtending 3° was 21° inwards, 43° outwards, 17° upwards, and
14° downwards.

The fields for colours are approximately concentric with that for
white. The colours change in appearance in passing from the point
of fixation towards the periphery. Those at the red end of the spectrum
pass through yellow to grey ; those at the violet end through blue to
grey. Blue-green becomes green, then yellow-green, then yellowish
white (Hering).

Baird'^, who has paid particular attention to this point, finds that
the changes in colour in passing from the periphery towards the centre
are as follow. Red first appeared yellowish, then passed through
yellow, orange and orange-red before it finally appeared red. Orange
first appeared yellow, becoming more and more orange. Yellow
appeared yellowish and gradually increased in saturation. Green
appeared yellowish at first, gradually increased in saturation, assuming
a greenish and finally a green tint. Blue became more and more
saturated. Violet appeared bluish, then blue, and finally violet.
Purple gave the longest and richest series of transitions : beginning
with yellowish, its tone gradually moved down the spectrum, passing
through orange-yellow, orange, red, and purplish red, before the pure
tone finally appeared.

Aubert^ found the limits of the coloured fields determined by the
intensity of the light, and this result was confirmed by Landolt, Char-
pentier and others. The periphery is therefore dyschromatopic rather
than achromatopic (Charpentier). The diminution in intensity, as
measured by the distance of visibility of coloured spots, is given in
Fig. 24. The sudden curve in the blue at the macula is due to macular
pigmentation and also probably to dark adaptation.

1 La Lumiere d les Couleurs, p. 193.

2 The Color Sensitivity of the Peripheral Retina, AVasliington, 1905.
^ Physiologie der Netzhaut, p. 116, 1865.

THE FIELD OF VISION FOR COLOURS

The limits of the colour fields vary not only with the intensity of
the light, but also with the saturation of the colour, and, above all,
the size of the object. If these are sufficiently great colours may be
recognised, almost, if not quite, at the extreme periphery (Bonders,
Landolt).

As already mentioned only the foveal region gives the unadulterated
photopic reactions, unless the eye is very completely adapted to light,
so that all traces of scotopia are eliminated from the peripheral field.

Fig. 24. Perception of colours in different parts of the field of vision. Abscissae, de-
grees to the nasal side of the fovea {O) ; ordinates, maximum distance at which the colour
is perceptible in a coloured paper 2 mm. square. (Charpentier.)

Ordinary observations with the perimeter do not afford accurate details
for comparison. If light adaptation is rendered as complete as possible
by exposure to bright sunlight many points of interest are elicited.
Under these circumstances it appears — within the limits of experimental
error — that colour matches, spectral or composite, which hold good for
the fovea remain good when viewed eccentrically, but though the
matches remain matches the values alter, the colours changing in the
raid-peripheral region, and becoming colourless in the extreme periphery.
It may therefore be concluded that in the phofopic eye peripheral vision
differs from central vision only in the direction of a diminution in sensi-
bility, and not in the direction of a change in character of sensation.
Further, all colour mixtures which appear colourless by central
vision remain colourless by peripheral vision. Allowance must, however

70 COLOUR VISION ^

be made for macular pigmentation {vide infra, p. 124). The deduction
is therefore more accurate if paracentral and peripheral regions are
compared^. Care must be taken that light adaptation is complete and
has been induced by colourless light, and that long exposure to coloured
lights is avoided.

Ole Bull^ was the first to employ stimuli the saturations and lumi-
nosities of which were equalised and to discover colours which do not
I change in hue in peripheral vision. His " physiologically pure "
j'?^ colours were a purplish red, a bluish green, a yellow, and a blue.
He found that the limits of the purple-red zone coincide with those
of the blue-green zone, and that those of the yellow and blue approxi-
mately coincide. The yellow-blue zone is considerably wider than
the red-green zone. He employed colour papers.

Hess^, also using coloured papers, similarly showed that with light
adaptation colour pairs of fixed size, intensity, and saturation can be
selected which lose their colour simultaneously and form a grey match
when viewed peripherally, viz. green (495 fjifju) and red mixed with a small
amount of blue; and yellow (574*5 /a/a) and blue (471 /u,/x). These
colours are complementary. All colours of greater wave-length than
549 fjLfjL approximate yellow, all of less wave-length blue, in passing
from central to peripheral vision. Thus orange and green-yellow
become yellow ; blue-green, violet and purple become blue. Hence
there are only four colours which gradually become paler without
altering their colour tone, ultimately becoming colourless, as one passes
from central to peripheral vision. Hess calls these colours " invariable "
red, yellow, green and blue. The limits of the zones at which the
colours become colourless vary with conditions already mentioned,
but the limits for invariable red and green are the same, as also for
invariable yellow and blue.

Hegg^, Baird, and Dreher^ confirmed Hess's results. Baird found
the " stable " or invariable colours to be a purplish-red, a yellow
(about 570ju./i), a bluish-green (about 490 fxf^), and a blue (about 460 fiix).
Dreher's values for the three last were 568 /z/a, 483 /^/x, and 461 /x/x.
Baird found that the coincidence of the zones of each pair of stable
colours holds good for the dark-adapted as well as for the light-adapted

1 Hess, Arch. f. Ophth. xxxv. 4, 1, 1889 : v Kries, Ztsch. f. Psychol, u. Physiol, d.
Sinnesonj. xv. 26.3, 1897.

2 Arch.f. Ophth. xxvn. 1, .54, 1881 ; xxix. 3, 71, 1883.

3 Ihirl. xxxv. 4, ]. 1889.

* Ibid, xxxvm. 3, 14.5, 1892; Ann. d'ocul. rix. 321, 1893; cxi. 122, 1894.

* Ztsch. f. Sinnesphysiol. xlvi. 1. 19]].

THE FIELD OF VISION FOR COLOURS

eye. He used gelatine filters which transmitted approximately mono-
chromatic lights.

The yellow-blue zone is " dichromatic." Beyond this zone there
is an extreme peripheral zone which is " monochromatic " or totally
colour-blind. It is best demonstrated in the nasal and upper and
lower portions of the field, but only with very small test objects in the
peripheral portion. If the luminosity curve for different colours is
worked out for this zone it is found to be quite different from the achro-
matic scotopic luminosity curve, v. Kries^ determined the peripheral
luminosity values and compared them with Nagel's corresponding
values for the achromatic scotopic luminosity curve :

Wavelengths 680 651 629 608 589 573 558

Periphery values 0-6 37-5 77-5 101 100 796 52-2

Scotopic values ? 3-4 14 0 355 100 256 351

The curves are shown in Fig. 25.

530 512
28-5 14-6
321 198

3bU

— J

32 5

275
250

22 5

20 0

17 5

IbO
125

-r

—

100
75
50

*-^

—

■ —

■^

—

..-

■■''

—

680 651 629 608 589 573 558 530

513

Fig. 2a.

Photopic luminosity curve for the totally colour-blind peripheral

zone of the retina Scotopic luminosity curve. Abscissae, wave-lengths of

the prismatic spectrum of gas light; ordinates, arbitrary scale, (v. Kries.)

The peripheral luminosity curve is highest at about 608 /m/x instead
of 544 ju.^, thus nearly approximating the luminosity curve for the fovea.
We have here a further proof that peripheral vision is to be regarded
as central vision diminished in sensibility, whereas scotopia is a difTerent
form of vision ; the former is a quantitative variation, the latter
qualitative.

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesortj. xv. 247, 1897.

COLOUR VISION

The dissimilarity of the peripheral and achromatic scotopic luminosity
curves shows that Hess's experiments do not prove all that he attributed
to them, for he chose colours of equivalent " white value " and stated
that they remained good matches both in peripheral photopic and in
scotopic vision. The matches are valid only for peripheral photopic
or scotopic vision.

On comparison of the peripheral photopic luminosity curve with
Konig's and Abney's luminosity curves, making allowance for the fact
that the two former are with gas light, the latter with arc light, we see
that they agree, so that the statement on p. 71 is correct. Polimanti^
has confirmed the statement on the same individuals using the same
apparatus and source of light with the flicker method (Fig. 26),

1
1

^^^

■ii

/ I

^''

iil

**i^

t
1

687
1 —

664

642

624
1

606
1

589

565
' 1 — 1

543
' 1 1

534

509

A'd-2-5 /Va-2A'4-T5/l'a-1 A'A-0-5 Na.

Na*^

Na-il

/Vd+3

Na.*',

Fig. 26

Photopic periphery luminosity curve (flicker metliod).

Pliotopic central luminosity curve (flicker metliod). Photopic

periphery luminosity curve (equality of brightness method). Abscissae, arbitrary
scale of wave-lengths of prismatic spectrum of gas light, measured from the
D line {No), with certain absolute wave-lengths indicated ; ordinates, arbitrary scale.
(Polimanti.)

Siebeck^, using minimal fields, has adduced further confirmation.
He found that with spectral lights of moderate intensity it was impossible
with accurate foveal fixation to abolish the sensation of colour without
that of light by reduction of the field, but that this was readily effected
1°'5 from the centre. He compared the luminosities of the lights under

1 Ztsch.f. Psychol, n. Pkyftiol. d. Siiinesorcj. xix. 272, 1890.

Ibid. XLi. 89. 1907.

THE FIELD OF VISION FOR COLOUES

these circumstances and found that these " minimal field luminosities "
agree with both the ordinary and the peripheral photopic luminosity
curves, and therefore disagree with the achromatic scotopic luminosity
curve. He found the maximum luminosity by this method at 601*3 fifj. :

Wave-lengths

642-4

620-8

607-8

601-3

595-3

589

Minimal-field

luminosities

66-()

91-5

112-8

128

110-9

100

Wave-lengths

5791

574-1

564-8

551-4

Minimal-field

luminosities

81-6

79-5

68-6

The minimal -field and luminosity curves are shown in Fig. 27.

IJU

.,\

100

}

•^

■

1
it

7 f
/

I'l/

!jl

•^

I*.

"♦O

'•!

/■/

^•

-x^

•>,

^ ^

■-*,

"-.

•■^

^■■4

^;--

■—I — I — I — I — I — I — I — I — I — I — I — I — I — I — I I 1 — I — I — I — I — 1 — I — I — I — I — I — I .

~2 1,8 1.6 1,1 U 1 0,8 Q6 0.1 0.2 0 0,2 0.<. 0,6 0,8 t 1.2 XH 1,6 1.8 2 Z'J. 2.t 2,6 Zs 3 3.2 i.t i.6 i& ^

o
co"

iT)

Fig. 27. "Minimal-field luminosity curve." Photopic

periphery luminosity curve. — Photopic central luminosity curve

(flicker method) " Il'-curve " of a deuteranope (see Fig. 45).

(Siebeck.)

74 COLOUE VISION

Zalin^ has investigated the " minimal-time kirainosities.'' In these
experiments the coloured light is exposed momentarily, when it appears
colourless, and is compared in brightness with a surrounding white
surface. The minimal-time luminosities completely confirm the results
obtained by minimal fields and periphery values, as is shown by the
following values :

Wave-lengths 659 621 601 589 564 542 523 506

Minimal-time
luminosities 22 5 79-0 105-0 100 76-6 59-4 380 15-2

Minimal -field
luminosities 24-4 702 1048 100 74-4 58-3 37-5 20-2

Periphery
values 21-6 73-9 99-6 100 79*9 54-7 36-7 15-2

Hess^, Hering^, and Tschermak* have examined the relative lumin-
osity of different lights in different regions of the retina. Hess found
that red and green pigments on a grey background, when viewed by
dull daylight with moderate dark adaptation, became darker and brighter
respectively in peripheral as compared with central vision. Hering
confirmed this result with spectral lights. Tschermak used Hering's
" double room " {v. p. 58) with spectral lights from Auer-gas or arc light
against a daylit background. He found a relative increase in luminosity
from 516 to 466 /x/x, a relative decrease from 693 to 525 ju/x, and no
change from 525 to 516 /x/x, on indirect fixation. In the dark-adapted
eye there is similarly a change in relative luminosity, and it occurs in
the same sense as the Purkinje phenomenon. Hering^ has shown that
for the dark-adapted eye, even on momentary dark adaptation, lights
of equal brightness by direct fixation appear very unlike in luminosity
on indirect fixation. He describes these alterations in luminosity as
Purkinje's phenomenon produced by change of position in the visual
field without change of light intensity.

The limits of the colour fields of the partially dark-adapted eye for
spectral colours have been worked out very thoroughly by Abney*'.

1 Ztsch. f. Sinnefsphysiol. XLVT. 287, 1911. ^ Arch.f. OpJdJi. xxv. 4, 1, 1889.

3 Arch.f. d. gcs. Physiol, xlvii. 417, 1890.

4 Ibid. Lxxxii. 559, 1900. "• Ihid. Lx. .'".19. 1895
« Abney, p. 190; Phil. Trans. Roy. Soc. cxr. 155, 1897.

THE FIELD OF VISION FOR COLOURS

Fig. 28 shows the fields with

red . . . . 670-5 /t/x 0-27 foot-candle

yellow .. 589-2 „ 3-95

green .. 508-5 „ 1-89

blue .. .. 460-3 „ 0-36

RIGHT.
0

30^VvvV

,WV

u^'^^^^t''^^^^'^^

6s-
;{"7""/'m|Nifm,in.i<

,<^ 120
,<?^ 135

165

,T|r

180

165

Fig. 28. The fields of vision for spectral colours with moderate illumination
(D lisht:=3-95 foot-candles at the screen). (Al)ncy.)

COLOUR VISION

Fig. 29 shows the fields with

red

yellow
green
blue ,

670-5 fi/x

589-2 „ 0-21 foot-candle

530 „

460-3 „

-^vA

180

Fig. 29. The fields of vision for spectral colours witli low illumination (D light =0-21 foot-
candle at the screen). (Abney.)

THE FIELD OF VISION FOR COLOURS

Fig. 30 shows the fields with complementary colours

red . . . . 650-0 /x/x

green.. .. 500-2 ,,

yellow-green.. 56L4 ,,

blue .. .. 460-3 .,

90—

105

Fig. 30. The fields of vision for complementary spectral colours. The relative luminosities
of the red and green were 225 and 270 ; those of the yellow-green and blue 96-5 and
21-5. (Abney.)

COLOUR VISION

If a red and a green light are mixed to match the D light (589*2 jm/x)
in hue and luminosity the D field is considerably larger than that of
the mixture, as are also the fields of the red and green separately.

Colours of pigments do not give the same fields as the spectrum
colours with which they approximately match, since they are impure
colours (Abney^).

Fig. 31 gives the data for the fields for the whole range of the
spectrum with three difl^erent intensities, the luminosities of the D line
being 3 "95, 0'99, and 0"45 foot-candle respectively. (In the first two
the aperture was 0*525 inch at 1 ft., in the last 0'086 inch at 1 ft.)
The abscissae are scale numbers (wave-lengths), the ordinates degrees
of field.

Fig. 31. The temporal and nasal limits of the fields of vision for spectral colours at three
different intensities of illumination. (Abney.)

It will be seen that when the temporal field reads 40° the nasal
reads 30°, and that as the field increases 7|° on the temporal side it
increases nearly 6° on the nasal, irrespective of the particular colour.

The curves for variations in intensity of the light are particularly
interesting. They were taken in the horizontal directions only
(Fig. 32).

The rays used were blue 430*3 /x/it, yellow 589*2 /x^, red 670*5 jjl/jl, green
SSN 41*7 (about 530 fxfM). Unit intensity was yellow = 3*95 foot-candles,
red = 0*45 ft.-c, green = 2*8 ft.-c, blue = 0*27 ft.-c. The abscissae
are intensities of light, the ordinates degrees of field.

1 Abney. p. 203.

THE FIELD OF VISION FOR COLOURS

Since the curves are straight lines it follows that as the intensiti/
diminishes in geometrical progression the angle of field diminishes in
arithnietical progression.

The effect of the area of the retina stimulated upon the size of the
field may be most conveniently considered here. Charpentieri found
that for white the size was immaterial so long as it exceeded a diameter
of about 0"17 mm. If it were smaller the field remained the same
when the brightness was increased in inverse proportion to the size.
60r

^*"»*^,^^^
► *-*»^,

Na.

>al.

1^****.,,^^

^^"•H^^^

§^

Blue

Yellow
Red.

Green .

Blue.

Ynlow.
Red.

Gi-een.

Intensity

Fig. 32. The temporal and nasal limits of the fields of vision for four spectral colours at
different intensities of illumination. (Abney.)

For colours Abney^ found that between apertures subtending 4° 28'
and 10' the fields decrease in extent in such a manner that for each
diminution in aperture to half the diameter the diminution in field is
5° on the temporal and 4° on the nasal side.

1 Arch. d'Opht. vu. 13, 18S7. '^ Abney, p. 208.

COLOUR VISION

This is the same ratio, 5 : 4, as for diminution of intensity of light
by quarters (p. 79). The diminution by each quarter of the area is thus
equivalent to a quarter of the intensity of light. Any a'perture subtending
more than 5° will give the same field.

Guilleryi confirmed Hess's results by using the size of the area
stimulated as a guide to the size of the colour fields. For each colour
a gradual increase is necessary in passing towards the periphery. The
increase for yellow and blue and for red and green agree respectively,
but of course the red-green increase differs from the yellow-blue increase.

50r^

0 I 2 Z '}■ 5 6

Aperture in powers of - Z

Fig. 33. The temporal ( ) and nasal (- ) limits of the fields

of vision for four spectral colours for different areas of stimulation. (Abne}'.)

The general threshold of light at the periphery is difficult to measure.
Abney used a small spot of luminous paint for fixation. At 10° from
the fovea he found that the extinction of all light from red light takes
place when the light is about one-third brighter than is required for
the fovea. With white paraffin light it is somewhat less. With green
light at about the E line and with blue at the Lithium line the necessary
reduction of the light is greater than for the centre of the eye. The
photochromatic interval is greater for the peripheral than for the central
part of the retina.

^ Ztfich. f. Psychol, u. Physiol, d. Sinnesorg. xii. 267, 189fi

THE MACULA LUTEA AND FOVEA CENTRALIS 81

CHAPTER II

THE MACULA LUTEA AND FOVEA CENTRALIS

The Macula Lutea. The chief feature of macular vision is the
unequal absorption of different rays by the pigment of the yellow spot.
It has been necessary to refer to this disturbing feature at an earlier
stage (pp. 40, 70).

The Fovea Centralis. The photopic luminosity curve for the fovea
is shown in Fig, 11, and calculations show that it is about one-sixth
more sensitive to the D light than the macula (Abney), To the green
and blue it appears to be less sensitive, and Konig^ even says it
is blue-blind. This observation is certainly untrue and is probably
due to the low scotopic luminosity value of the fovea combined with
the absorption by the pigment which probably extends to this region.

We have already referred incidentally to many features in which
foveal vision differs from paracentral and peripheral vision. With its
exceedingly high sensibility for form we have little to do here, but it
should be borne in mind. More important for the present purpose are
its high photopic chromatic sensibility, and its very low scotopic sensi-
bility.

It has already been mentioned that for white light achromatic
scotopic sensibility is proportional to the area stimulated (p. 52 ;
see also Section VI, Chap. i). For the fovea, in order to produce
a luminous sensation, the total quantity of light, i.e., the product of
the area and the illumination, must attain a certain value, and this
value is constant for a given condition of adaptation.

Since the colourless interval increases in dark adaptation owing to
lowering of the general threshold, the chromatic threshold remaining
almost or quite constant {v. p. 60), and since dark adaptation increases
the foveal sensibility little or not at all, one might expect a very great
diminution or even absence of the photochromatic interval at the fovea.
Charpentier^ demonstrated a photochromatic interval for colours at the
fovea, and Koster^ and Tschermak^ confirmed the results, thus differing
from A. E. Fick^, Parinaud^, and Konig and v. Kries. If it exists for

1 Konig, p. 353. ^ ^rch. cCOpht. xvi. 337, 1896 ; La Lumiere d /(.*■ Couleur«, p. 2U(i.

3 Arch. J. Ophth. XLi. 4, 13, 1895.

^ Arch.f. d. ges. Physiol, lxx. 320. 1898. =^ Ibid, xuii., 48L 1888.

« Ann. d'ocul. cxii. 228, 1894 ; Arch. d'Opht. xvi. 87, 1896.

P. 0. V. 6

82 COLOUR VISION

the fovea it is much less than for the periphery. The point is one
having a very practical bearing and some experiments made by Gotch^
may be quoted.

" If the eye is fixed in the dark room on one small spot of light (A)
and a second small coloured light (B) is flashed out at some little distance,
then the sensation produced by A varies with the position and hue of B.

"KB is spectral green or blue, and is some degrees away from A
(A being fixed by the visual gaze and thus in the centre of the field of
vision), B gives rise to a sensation of rather dazzling white. This is
especially the case when the light itself is of very small angular area.
The sensation continues when B is moved nearer to A, and tends to mask
any recognition of green or blue colour. In my own case such colour
recognition may not occur with the Board of Trade light green light
until this is focussed on the retina within 1|° to 2° from the centre when
the dazzle disappears and the colour becomes quite plain.

" If B is red, then neither the colour nor the light itself is recognised
when it is so situated as to be focussed on the retina over 8° or 10°
outside A. In my own case with a dim Board of Trade light red, the
limit of failure of colour recognition was a little under 6|°. Beyond
such recognition limiting distance there was no sensation of white
dazzle, but a barely perceptible dull grey spot was sometimes seen
instead of the red light. Further out this light became quite invisible.
When brought as near as 6° it always showed as a distinct red spot,
and continued so however close it was brought. There was no simul-
taneous white dazzle to mask the hue ; thus the light, if visible was
easily recognised.

" A number of experiments on individuals ascertained to possess
normal colour vision support the view that, in the dark-adapted eye
red light is recognised as red over an area whose radius is three or four
times that observed with green light ; yet the red light is not seen at
all outside this larger area. On the other hand green (or blue) light,
whilst it is only recognisable as green over the much more restricted
central area, is seen as a bright light of a dazzling white type over a very
extensive area.

" As illustrations of such visual phenomena in connection with what
is termed ' dark adaptation perimetry,' I append a few experimental
results obtained from ten different observers, all with normal colour
vision.

^ Report of Departmental Committee on Sight Tests, Appendix 3, 152. 1912.

Red

Light (X G563)

Green

Light (\ 51 G9)

With

in 6°

or 7°

Within

U°

or 2°

7J°

» sr

»2r

9°

., 10°

2h°

„Sh°

6°

„7°

2°

„2h°

„8°

2°

„r

8°

„y°

2°

„2r

8°

»n°

2J°

„r

»?

8i°

„ 91°

2i°

.,3°

6°

„7r

2°

,,2^

8°

„9°

2°

„2i°

THE MACULA LUTEA AND FOVEA CENTKALIS 83

Number of Angular Distances from Centre within which the Colour

Observer of a Spectral Light 30" Diameter was recognised

1
2
3
4
5
6
7
8
9
10

" It may be added that with the red spot of light any recognition of
light immediately beyond the sensitive limit given above was very
doubtful or non-existent, although with practice a dull grey spot can
be perceived for a short distance. On the other hand, with the green
spot of light there was, from 3° outwards, most definite recognition of
light over a very large area, the appearance being a white or bright
dazzle. This was also present with blue light, but the hue was unrecog-
nisable as regards both blue and green. Yellow light also gave the
same bright dazzle ; it was often blended with a reddish sensation if it
lay from the centre about 3° or 4°.

" The recognition of small areas of red or green by the dark-adapted
eye is, as regards colour, thus only possible when these are focussed near
the fovea, i.e., lie near or at the centre of the field of vision. This is
particularly the case with the peripheral vision of green ; presumably
these rays, by exciting rods, evoke a sensation of white, which has a
dazzling effect and masks the true colour. Such white dazzling sensa-
tion is still present, but to a less degree, as the green light approaches
the centre of the field of vision, but it ceases when this light is 1° to 2°
from the centre. According to v. Kries the explanation is the differential
stimulation of both rods and cones, that of the rods giving rise to the
dazzling white sensation in addition to the colour sensation produced
by the stimulation of the cones."

Although the fovea is night-blind compared with the periphery it
is capable of some degree of dark adaptation. It is necessary to have
very strong preliminary light adaptation, as from looking at the clear
sky, in order to demonstrate it (Nagel and Schafer^). Tschermak^
also obtained foveal dark adaptation and thought that it was slower

^ Ztsch. f. Psijchol. u. Physiol, d. SinnKSorg. xxxiv. 271; 1904.
- Arch.}, d. ges. Physiol. Lxx. 297, 1898.

6—2

84 COLOUR VISION

than peripheral, but he used too large test objects and thus exceeded
the foveal limits (Nagel). More recently a slight increase in the foveal
sensibility on dark adaptation has been proved by Wolfflin^ and by
Dittler and Koike'^. In the experiments of the latter one eye only was
dark-adapted and the luminosities of the binocular double images of
the illuminated object were compared. The adaptation at the fovea
is so slight that one would not expect to obtain Purkinje's phenomenon
there under ordinary circumstances. The demonstration of its absence
is beset with difficulties — accurate foveal fixation, comparison of bright-
ness of very small areas, etc. — and opinions are therefore divided.
Tschermak^, Koster* and Sherman^ are in favour of its presence, but
their methods were less satisfactory than those of Konig^, Lummer'^
and Nagel and v. Kries**. v. Kries and Nagel have proved fairly con-
clusively that Purkinje's phenomenon is absent over a central field
not greater than 2°, i.e., not exceeding the rod-free area. The longer
the dark adaptation the better was the delimitation of the area. If a
red and a blue spot are fixed with this area, the red being brighter
than the blue, then on diminishing the intensity of the light the relative
luminosity remains the same. On the slightest deviation of the eyes,
however, the blue at once appears the brighter and less saturated.
Hering^ found that small fields of red and green, equally bright when
fixed centrally, become of different brightness when fixed eccentrically
(" Purkinje's phenomenon by change of position in the visual field
without change of the intensity of the light "), and he also stated^"
that the deviations from Newton's law " become less marked the
smaller the area of the retina stimulated, and are absent when the field
is made sufficiently small."

1 Arch.f. Ophth. Lxxvi. 464, 1910. ~ Ztsch. f. Si7inrsphi/sloL XLVI. 166, 1912.

3 Arch.f. d. ge.s. Physiol, lxx. 297, 1898. * Arch. J. Ophth. xli. 4, 1, 1895.

» Wundt's Philo.-i. Stud. xin. 1898. « Konig, p. 338.

^ Verhandl. d. Deutschen. physik. Gcscllschft. vi. 2, 1904.

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxili. 167, 1900; v. Kries, loc cit. ix. 81,
1896 ; Arch.f. Ophth. xlii. 3, 95, 1896 ; CcntralU.f. Physiol. 1896.

9 Arch. f. d. ges. Physiol, lx. 533, 1895. " Ibid. Liv. 277, 1893.

8ectio:n y

TEMPORAL EFFECTS
CHAPTER I

RECURRENT VISION ; THE TALBOT-PLATEAU LAW ;
THE FLICKER PHENOMENON

We have already had evidence (p. 57) from stimulation of the dark-
adapted retina by light of low intensity that two mechanisms are
involved. One, which may be called the " scotopic mechanism," in
the condition of maximum dark adaptation, responds to light of low
intensity by a colourless light sensation, no matter what the nature of
the light stimulus so long as it be an " adequate " stimulus {v. p. 19).
The other, which may be called the " photopic mechanism," responds to
light of higher intensity (which in itself induces a relatively high degree
of light adaptation) by a light sensation of greater intensity and greater
complexity, the sensation being that of white or colour according to
the nature of the stimulus. It may at once be admitted that theoretical
conceptions of definite mechanisms subserving these diverse though
allied functions are here introduced. They will be allowed to intrude
as little as possible in the description of the facts, but the comprehen-
sion of the description is facilitated by permitting their use.

The sensational response to a single short-lived stimulus is not a
single, equally short-lived light sensation. Except in the case of a very
feeble stimulus it is "a series of pulses of sensation of diminishing
intensity rapidly succeeding one another^." Attention was early called
to these recurring responses by C. A. Young in 1872^. He found that
when a discharge from a powerful electric machine momentarily illumi-
nates a room the objects may be seen, not once only, but two, three or
four times in rapid succession, although the spark is single. He called

1 MoDougall, Brit. JI. of Psychology, i. 78, 1904.

- Nature, v. 512, 1872 : Philos. Mag. XLin. 343 ; SiU. J I. iii. 262.

COLOUR VISION

the phenomenon " recurrent vision," and the observation was con-
firmed by Shelford Bidwell^. An allied phenomenon was described by
Purkinje^, rediscovered by A. S. Davis^, and is commonly called
"Bidwell's ghost"; Hamaker^ called it the "satellite", v. Kries "the
following image." The ghost is seen in typical form when a light of
moderate brightness is moved across a dark field of view at a moderate
speed while the eye remains at rest, appearing as a second dimmer
image following the primary image after a short interval, usually
about -^sec, of darkness. The subject has been specially studied by
Bosscha^, Charpentier^, Hess', v. Kries^, Samojloff^ Munki", Exner^i,
and McDougalli2.

Fig. 34.

Fia;. 35.

Fig. 34. Appearance of a radial slit 2° in width and 7 cm. in length, with its mid-point
15 cm. from the centre of the disc, rotating at the rate of one revolution per 3" before
a glass illuminated by four acetylene-gas jets. (McDougall.)

Fig. 35. Appearance with slightly increased speed of revolution — " Charpen tier's bands."
(McDougaU.)

' Proc. Roy. Soc. Land. lvi. 1.02, 1894 ; Curioftitict of Light and Sight, London. 1899.

2 Beobachtungen zur Physiol, d. Sinne, ii. 110. 1825.

3 Lond. and Edin. Philosoi^h. Mag. xi.iv. 526, 1872.

* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxi. 1, 1899.

5 Arch.f. Ophth. xl. 2, 22, 1894. « 4^g^_ ./^ physiol. iv. 541, 1892 ; vi. 677, 1896.

' Arch. f. d, ges. Physiol, xlix. 190, 1891 ; Arch. f. Ophth. XL. 2, 259, 1894 ; XLiv. 3,
445, 1897 ; Li. 2, 225, 1900 ; Arch.f. d. ges. Physiol, ci. 226, 1904; ovii. 290, 1905.

8 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xii. 81, 1896 ; Arch.f. Ophth. XLii. 3, 1896 ;
Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxv. 239, 1901 ; xxix. 81, 1902 ; Arch. f. d.
ges. Physiol, cm. 167, 1904.

* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xx. 118, 1899.

1" Ibid. xxra. 66, 1900. " Arch.f. d. ges. Physiol, cm. 1905. ^- Loc. cit.

RECURRENT VISION

The Primary Image. With momentary stimulation by a stationary
light McDougall found that a brief stimulus of low intensity provokes
a single brief pulse of sensation, and that a stimulus of higher intensity
provokes two, three, four or more pulses according to its intensity,
and that the extrafoveal region, being more sensitive under dark adapt-
ation than the fovea responds to a stimulus of any given low intensity
with a rather longer series of pulses than is provoked by the same
stimulus when applied to the fovea. If the stimulus is given by a radial
slit in a rotating disc the image is spread out into a sector, varying in
width according to the rapidity of rotation. This is due to the per-
sistency of the sensation beyond the time of stimulation. The sector is
not of uniform brightness, but is marked with alternate light and dark
bands which vary in intensity and width according to speed of rotation
and so on. These fan-like bands are commonly known as " Charpentier's
bands\" though they had been previously described by Bidwell (1894).

Fig. 36. Diagram illustrating the primary response to a single momentary stimulus.
The series of pulses of sensation commences at a and lasts through a period of about
f sec, represented by a — i. The height to which each curve rises represents the
intensity of the pulse of sensation. The effects of a momentary stimulus of less
intensity are represented by a part of the series of curves, e.g., e — i. The overlapping
of the curves represents the fact that the intervals between the pulses of sensation
are not quite dark. (McDougall.)

The Secondary Image. The above is not the only response. In the
case of momentary stimulation by a stationary light a secondary image
of much less luminosity seems to appear at a great distance at the
moment in which the pulsating image comes to an end, and rushing up
towards the eye to .stop suddenly, filling the position of the primary

I Arch, de Physiol, iv. 541, 1892 ; viii. 677, 1896

88 COLOUR VISION

image. The secondary image fades gradually and slowly. In the case
of the travelling object the secondary image appears abruptly at a brief
interval after the last bands of the primary image, and fills a part or
the whole of the track of the image. If the object light is coloured and
of high intensity the secondary image is in both cases tinged with the
same colour ; but if the object light is coloured and of low intensity the
secondary image is grey in the case of blue and green light, but is absent
in the case of pure red light. The secondary image is the ordinary
positive after-image {vide infra).

The interval between the primary and the secondary images is not
always completely dark. As the rate of movement of a travelling
object light is increased dim grey bands appear in the interval. They
are dimmer and broader than the leading bands and differ markedly
in quality, the leading bands being yellowish-white, these others a
neutral grey. The interval-bands show the following peculiarities :
(1) they are brightest when the object light is green, absent when it is
red : (2) they are absent where they cross the fovea, even with a green
object light; (3) they have the neutral ghostly quality, inclining
towards blue, characteristic of the scotopic spectrum ; (4) they are
absent when the eye is light-adapted.

It is probable therefore that the primary response is a response
of the photopic mechanism, any response of the scotopic mechanism
being obscured by the preponderant photopic effect. On the cessation
of the photopic response the scotopic response becomes manifest under
favourable circumstances. There is reason to believe that the latent
period of the photopic reaction is shorter than that of the scotopic by
about j-g sec.

The secondary image is similarly complex. With an object light
of low intensity it is a pure scotopic reaction, as shown by its grey
quality and its absence with a pure red object light. With a brighter
object light it is a combined photopic and scotopic reaction, the photopic
preponderating with strong lights, as shown by the colour and saturation
of the secondary image.

McDougall has further shown that the bright initial reactions which
constitute the primary image exert an inhibitory effect upon the im-
mediately succeeding reactions. BidweWs gJiost is the last of the series
of pulses of sensation, the intermediate members being thus inhibited.
In its typical form it is a pure scotopic reaction, but that it is not
necessarily so is shown by the fact that it can be obtained by a pure
red object light of high intensity and does not then jump the fovea.

RECURRENT VISION 89

I have hitherto followed McDougall's excellent paper. It will be
readily appreciated that since the response differs in the number and
character of the pulses of sensation according to the intensity and nature
of the light stimulus, and since the stimulus acts upon two mechanisms
of different intensity-response and different latent period, the nature
of the phases of the response vary greatly according to the conditions
of the experiment.

Fig. .37. Appearance of a narrow slit, 2° to 5^ in width, in a large disc, rotating at one
revolution per 3", before a milk-glass illuminated with one acetylene-gas burner, as
seen by the light-adapted eye which has been kept covered for about three minutes
and then fixed upon any point in or near the path of the moving slit — " Bidwell's
ghost." (McDougall.)

It is not surprising, then, that under appropriate conditions v. Kries
found the following series of events : (1) a primary image ; (2) a short
dark interval ; (3) a secondary image, complementary in colour to
the primary (the satellite or ghost) ; (4) a second dark interval ; (5) a
tertiary image (Snellen, Bosscha), faintly tinged with the colour of
the primary ; (6) a third dark interval. The secondary image in this
series follows the primary by | — | sec. It is absent with foveal fixation.
It increases in brightness and extent in the early stages of dark adapta-
tion, but is absent after prolonged dark adaptation. It is also absent
with red light and its brightness corresponds to the scotopic value of
the exciting light (v. Kries). Much discussion has arisen around the
colour of the secondary image. According to Charpentier it is violet
with low, and colourless with high intensity. Hess describes it as
faintly tinged with the same colour as the primary i. With regard to

1 Cf. v. Kries, Ztsch. /. Pmjchol u. Physiol, d. Sinnesorg. xii. 81, ISflO : Hess, Arch f.

90 COLOUR VISION

the tertiary image its hue is best appreciated when red is chosen as the
stimulus. With increasing dark adaptation the tertiary gains in bright-
ness but loses in chromatic value. There is some difference of opinion
as to whether the tertiary image can be seen at the fovea.

Hamaker described the secondary image as coloured with the
complementary colour and lasting | sec, the tertiary as homochromatic
and lasting several seconds. Dark adaptation had little influence. The
secondary image was the better seen for all colours ; the tertiary better
for red and yellow than green and blue. With stationary light he
found the following phases : (1) the primary image ; (2) a dark interval,
often absent ; (3) the secondary image ; (4) a dark interval (| sec.) ;
(5) the tertiary image ; (6) a dark interval, often absent ; (7) with
stimulus of 1 to 4 sees, a quaternary image, accompanied by shortening
of the tertiary. The quaternary is a true negative after-image, of
complementary colour and surrounded by a bright halo. With moving
light he found the following phases : (1) the primary image {e.g., 3°),
passing into (2) a short homochromatic or white tail (6°) ; (3) a dark
interval (10° — 15°) ; (4) the complementary, coloured or colourless
secondary image (satellite, 3°, absent with red light) ; (5) a dark interval
(50°) ; (6) the tertiary, faintly coloured homochromatic image (about
360°). As regards the region of retina stimulated, with stationary
light, red and green gave a very marked complementary secondary
foveal image, which was absent for yellow and blue ; the tertiary image
was absent with foveal stimulation. With moving light the fovea gave
no secondary and apparently also no tertiary image.

The " action-time " of a light stimulus (McDougall), i.e., the least
time during which a light of given intensity must act upon the retina
in order to excite the most intense sensation it is capable of exciting,
has been studied by Swan^, Exuer^, KunkeP, Charpentier'*, Martins^,
and McDougall^. It is a necessary preliminary to the proper under-
standing of flicker phenomena.

It may be accepted that the sensation curve rises rapidly to a
maximum and falls gradually. The rapidity of the rise varies with

Ophth. XLiv. 3, 445, 1897 ; SamojlofE, Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xx. 118,
1899 ; Hamaker, loc. cit. xxi. 1, 1899; P. Miiller, Arch. f. d. yes. Psychol, xiv. 358, 1909;
Kinoshita, Ztsch. /. SijDiesphysiol. xixn. 420, 434, 1909; Hering, Arch. f. d. grs. Physiol.
cxxvi. 604, 1909; Dittler and Eisenmeier, ;6/(7. cxxvi. 610, 1909.

1 Trmis. R. S. Edin. Ii. 230, 1849 ; xxn. 33, 1861.

2 Sitz. d. Wiener Akad. Lvm. 2, 601, 1868.

» Arch.f. d. ges. Physiol, ix. 197, 1874. « C. r. soc. de hiol iv. 1887.

^ Btitr. z. Psychol, u. Philos. i. 2, 1902. « Brit. Jl. of Psychol, i. 151, 1904.

RECUKRENT VISION 91

the intensity of the stimulus, and the rate of diminution of the action-
time with increase of intensity of the light diminishes as the intensity
increases, so that with light of ordinary, fairly high, intensity it is very
small. It has been pointed out above that when the duration of stimula-
tion is very short the response is a series of pulses of sensation. If the
stimulus be other than very brief the sensation shows no such pulses.
McDougall has shown that with a light whose action-time is
61'^(yfi^7y sec.) the multiple character of the response persists until the
duration of its action exceeds its action-time by about IG*^, and ceases
altogether when the duration of the action of the light exceeds its
action-time by about 40"^. As regards the variation of the intensity
of the sensation with the duration of the stimulus when that duration
is less than the action-time McDougall concludes that it follows the
"photographic law," i.e., for such duration the intensity of the sensation
varies directly with the duration of the action of the light. In mathematical
terms the law may be stated thus : Sec I . St, where S is the intensity
of the sensation, I the intensity of the stimulus, and 8^ a small increment
of time. This law is a corollary of the Talbot-Plateau law {vide infra),
and was assumed by Talbot. With very dim lights of scotopic value
the action-time is probably slightly greater than 200"^ or i sec. The
practical conclusion — that the dimmest light visible to the peripheral
retina of the dark-adapted eye, i.e., the dimmest light perceptible under
the most favourable conditions, must be allowed to act for a period
of not less than 200"^ or i sec. in order to be perceptible — is of great
importance in the construction of lighthouse flash-lights and so on.
Its practical importance is increased, as well as the theoretical signifi-
cance, by McDougall's observations which show that the action-times
of red, green and blue lights are the same for lights of equal intensities,
i.e., intensities which excite sensations of equal luminosities.

Briickner and Kirsch^ have investigated the chromatic action-time
or the minimum time during which a coloured light must stimulate
the retina in order that the colour may be discriminated {Farbenzeit-
schwelle or specifische Zeitschivelle). They found that the time varies
with the intensity of the white light acting upon the retina both before
and after stimulation with the coloured light and is nearly proportional
to the brightness of this " grey " sensation. The chromatic action-time
follows Weber's law, at any rate within certain limits. It varies
inversely as the width of the pupil and the size of the retinal area
stimulated, but not proportionally. The intensity of the following

^ Ztsch.f. SinnespTiysiol. xlvi. 229, 1911.

92 COLOUR VISION

white stimulation appears to have a greater effect than that of the
preceding white stimulation, though the authors can give no explanation
of the fact. If a colour stimulation follows instead of white, the effect
on the action-time depends largely upon the similarity or dissimilarity
between this light and that which is being tested.

THE TALBOT-PLATEAU LAW,

When periodic excitations follow each other with sufficient rapidity
the resulting sensation is one of continuous light of uniform brightness.
Talbot^ and Plateau^ investigated the relationship of the brightness
of the individual periodic stimuli and of the resultant sensation. Their
conclusions are usually known as the Talbot-Plateau Law, which states
that the resultant impression is the mean of the periodic impressions,
i.e., the resultant brightness is that which would have arisen if the
amount of light intermittently reaching the retina had been uniformly
distributed over the whole period of stimulation, v. Helmholtz confirmed
the law for ordinary physical intensities of light. Fick^ found some
deviations, low intensities giving a continuous sensation brighter than
the intermittent. 0. Clrlinbaum^ also found deviations for high in-
tensities, the intermittent light being brighter than the continuous.

The accuracy or otherwise of the Talbot-Plateau Law is of consider-
able importance in the investigation of colour vision, for the principle
of the episcotister depends upon its accuracy. Abney finds that the
adjustable sectors, i.e., those which can be altered during rotation, are
only available for accurate measurement when the angles of aperture
lie between 180° and 10°, chiefly owing to the errors in reading being
proportional to the angles of aperture. With fixed sectors angles of
2° or even 1° can be used. It is not certain, however, that the method
is reliable for very low intensities of light, and for these the annulus
{v. p. 5) is to be preferred. The experiments of Abney, Lummer and
Brodhun^, Hyde^ and others show that the law applies accurately over
a wide range of physical intensities. Hyde's experiments were very
carefully conducted, the probable errors of measurement being under
0"1 per cent. The average deviation of the observations for any given
angular aperture of the sectors was in no case as large as 0*2 per cent.

1 Load., Edin. and Dublin Phil. Mag. v. 327, 1834.
- Ann. d. Phys. u. Chem. xxxv. 457, 1835.

* Arch.f. Anat. Phyxiol. u. iviss. Med. 754, 1803.

« J. of Physiol, xxii. 433, 1898. ^ Zhch.f. Imtrumentenkunde. xvi. 209 1896.

* Bull, of the Bureau of Standards, Wasliington, ii. 1, 1906.

RECURRENT VISION 93

It has been seen that McDoiigall arrived at the conclusion that
the relationship Sec I . St is probably correct. Viewed from the purely
physical standpoint this relationship and hence the Talbot-Plateau law
are such as might be anticipated^. From the physiological standpoint
one might rather have anticipated that the resultant sensation from
intermittent stimuli would follow the analogy of muscle tetanus,
i.e., that the application of new stimuli in the course of the curve of
sensation produced by the primary stimulus would suffice to prolong the
sensation curve at or near the maximum of the primary curve. It
appears therefore that the superposition of fresh stimuli during the
progress of the sensation curve of the primary stimulus produces
positive or inhibitory effects of such a nature that the resultant sensa-
tion curve shows rapid oscillations about a mean sensation intensity.
If the oscillations are sufficiently rapid a continuous mean sensation
results, in accordance with the Talbot-Plateau law. If the oscillations
are less rapid the sensation of flicker is felt. The analogy to muscle
tetanus is therefore not one to be pressed, but physiologists will have no
difficulty in finding analogies in the domain of neurology to the in-
hibitory effect of superposed stimuli. One may direct attention
particularly to Sherrington's work^.

THE FLICKER PHENOMENON.

If the oscillations produced by intermittent stimulation are not
sufficiently rapid to cause complete fusion, a sensation of flickering is
felt. If black and white sectors are rotated with gradually increasing
velocity there is first separate vision of the individual sectors. This is
followed by a peculiarly unpleasant coarse flickering, which passes into
a fine tremulous appearance, after which complete fusion occurs. Care-
ful observation reveals further interesting, though complicating details.
At a certain stage there is a peculiar glittering, the brightness being
greater than that of the continuous sensation after complete fusion.
Briicke^ found the maximum with 17 "6 stimuli per second. It may be
explained by an absence of those inhibitory effects referred to above,
or by reinforcement of each white stimulus by the recurrent image of
the preceding sector (Briicke), or by temporal induction {vide infra),
or by a combination of such causes, v. Helmholtz noticed that at one

1 Cf. V. Kries, in Nagel's Handb. d. Physiol, d. Mcnschcn, p. 231.

- The Infegraiive Action of the Nervous System, London, 1906; ci. also McDougall,
Brain, xxvi. 153, 1903.

3 Sitz. d. k. Akad. d. Wissensch., Wien, XLix. 2. 128, 1864.

94 COLOUR VISION

stage the forward edge of the white sectors appears reddish, the back-
ward edge bluish. Fechner^, by a suitable arrangement of black and
white sectors obtained a pattern of yellow and blue. Subsequent
observers have succeeded in eliciting all gradations of colour. This
phenomenon is most easily seen in Benham's top^. It is probable
that successive induction (Section V, Chap, ii) plays a prominent part
in these phenomena, and they are clearly of great importance in the
theory of colour vision. Exner^, Burch* and others have made
important observations on this subject, using coloured lights.

More important for our present purpose is the relationship between
flicker and the conditions of stimulation — intensity and nature of
stimulus, rapidity of stimulation, adaptation of the retina, and region
and area of the retina stimulated. Various methods of producing
intermittent stimulation other than by the usual rotating pigment-
coloured discs, have been devised by Rood^, Whitman^, v. Kries',
Simmance and Abady^, Kruss^, Wild^", Ives^^, and Watson^^, and the
subject has been investigated not only by them but by others, notably
by Ferryi^, Sherrington^^, Haycraft^^, 0. Griinbaum^^, Polimanti^'',
T. C. Porter^s, Alleni^, Kennelly and Whiting^o, DowSi, Tufts^a,
Millar^^, and Morris Airey^*.

As regards the intensity of the stimulus, more rapid stimulation
is required for complete fusion with increased intensity. Up to a
certain point with alternate darkness and light as the intensity is

1 Ann. d. PhijK u. Chem. xlv. 227, 1838.

- Bidwell, Proc. Roy. Soc. lx. 368, 189«; lxi. 268, 18D7 ; Percival, Trans. Ojihth.
Soc. XXIX. 119, 1909.

3 Arch.f. d. gc.s. Physiol, i. 375, 1868. ^ J. of Physiol, xxi. 431, 1897.

s Atner. J. oj Sc. (3) XLVi. 173, 1893; (4) viii. 194, 1899; Science, vii. 757; vm. 11,
1898.

6 Phys. Rev. m. 241, 1895.

' In Polimanti, Ztsch. f. Psychol, u. Physiol, d. iSinnesorg. xix. 203, 1899.

* Proc. Phys. ifioc. xix. 39.

9 Phys. Zeitung, ni. 65 ; Jl. f. Gas u. Wass. XLvn. 129, 1904.

1" London Electrician, 1909 ; The Illuminating Engineer, i. 825, 1908.

" Philos. Mag. 1912. i- In Abney, p. 107.

" Amer. J. of 8c. (3) xliv. 193, 1892. " J. of Physiol, xxi. 33, 1897.

" Ihid. XXI. 126, 1897. " Ibid. xxn. 433, 1898. i' Loc. cit.

i« Proc. Roy. Soc. Lond. lxiii. 347, 1898 ; lxx. 313, 1902.

" Phys. Rev. XI. 257, 1900 ; xv. 1902 ; xxviii. 45, 1908.

20 The Illuminating Engineer, New York, ti. 347, 1907.

21 Proc. Phys. Soc. xx. 644, 1907; xxn. 58, 1910; Philos. Mag. 120, 1906; 58, 1910;
The Electrician, lviii, 609, 1907.

22 P%5.jBei;. XXV. 433, 1907. "^ T he I Hum inatingEngineer,NewYork,i\. 7 Hi), IQOd.
24 J. hist. Eke. Engs. xliv. 177, 1910.

RECURRENT VISION 95

increased in geometrical proportion the rapidity of stimulation increases
in arithmetical proportion. Griinbaum found that with high intensities
the law fails, just as he found the Talbot-Plateau law to fail. If light
alternates with less light the point of fusion is lowered by the diminution
of the differences of intensity.

Filehne^, Schenck^ and others found that with rotating discs the
number of sectors influences the result. The more numerous the sectors
the greater the number of rotations necessary per second for fusion. The
difference is less marked when the disc is viewed through a slit (Schenck)^.
Griinbaum worked out the relations and found the explanation in
successive contrast (vide infra). Sherrington with coloured discs found
the results markedly affected by simultaneous contrast {vide infra).
We may therefore conclude that " the point of fusion of intermittent
stimuli, so as to produce a continuous sensation, depends, not on the
phvsical intensities of the stimuli, but on their physiological intensities,
as determined by the condition and nature of the stimulated retina "
(Rivers)^.

This fact is further borne out by the areal and regional differences.
Exner^ found that the duration of the sensory process decreased in
arithmetical proportion as the size of the retinal image increased in
geometrical proportion ; and Charpentier^ similarly found that increase
in the size of the area of retina stimulated raised the point of fusion.
Exner found that the fusion frequency was less for the fovea than for
a region 1"33 mm. outside it, and all agree that flicker persists longer
in the peripheral than in the central areas of the retina. Bellarminoff'
found differences between the nasal and temporal areas.

T. C. Porter's researches have increased the knowledge of flicker
phenomenon materially. Using black and white sectors illuminated
by pure spectral lights from the interference spectrum of lime-light,
he found that the speed at which flicker was abolished for yellow was
nearly double that for violet ; green and the last distinctly visible
red occupying the mid-position. Having regard to the increase in
speed with increasing intensity of the light, he concluded that as the
retinal stimulus increases in intensity the sensation produced retains
its maximum for a shorter and shorter time. With constant illumina-
tion, altering the relative sizes of the white and black sectors, he found

1 Arch.f. Ophth. xxT. 2, 20, 1885. ^ Arch.f. d. ges. Physiol, cxn. 1, 190G.

« Ibid. LXiv. 165, 1896. « In Schiifei's TeM Book of Physiology, ii. 1072, 1900.

5 Silz. d. k. Akad. d. Wisscnsch. Wicn, Lvm. 2, 601, 1868.

6 Arch. d'Opht. x. 340, 1890. ^ Arch. f. Ophth. xxxv. I, 25, 1889.

96 COLOUR VISION

that at the point of fusion the effective stimulus at any point of the
retina is to the maximum stimulus as the angle of the bright sector to
360°. The fact that the colour stimulus requires a finite time to produce
its maximum effect was confirmed by this method. The period during
which the sensation remains undiminished appears to decrease as the
time of stimulation increases, though within narrow limits of variation
one of these quantities is nearly inversely proportional to the other.
He found that the relation between the intensity of illumination (/)
and the number of revolutions {n) per minute at which a disc half white
and half black must be run in order that the flicker may just disappear,
the distance of the eye remaining constant, is

n = k . log I + k'

k and k' being constants. He therefore proved the geometrical-
arithmetical relationship between intensity and rapidity of stimulation.
The relation was found rigidly true for illuminations from 0"25 metre
candle to 12,800 times this value. For intensities below 0'25 metre candle
the constant k suddenly changed its value to practically half its former
value. We have here further evidence of the duplex mechanism
involved, the higher value of k applying to the photopic, the lower to
the scotopic mechanism. T. C. Porter also proved that the duration of
the undiminished sensation produced by different spectral hues depends
solely on the luminosity of the colours and not on their wave frequency.

.Ives's researches have elicited several highly important results and
are specially valuable owing to the great care taken to secure accuracy
in his methods. The flicker method can be applied in two ways. In
one, that used by Haycraft, Ferry and Allen, the critical frequency
of alternation of the lights is measured, i.e., two lights are regarded as
being of the same luminosity when the flicker produced by rapid alterna-
tion of each with black disappears at the same speed of alternation.
The other method is that employed by T. C. Porter and generally used
in flicker photometry. By it two lights are regarded as being of the
same brightness when no flicker results on alternating one with the
other, the speed of alternation being such that the slightest change
of intensity of either light causes flicker. Ives found that the second
or ordinary method is more sensitive than the equality of brightness
method used by Abney and others {v. p. 44). The results are reproduced
with much greater constancy, since in the latter method psychological
factors influence the judgment in arriving at the results. On decreasing
the illumination the maximum shifts towards the blue (Purkinje's

RECURRENT VISION 97

phenomenon) by the equality of brightness method ; by the flicker
method it shifts towards the red. On the other hand, decrease of the
area stimulated at low intensities shifts the maximum of luminosity
towards the red by the equality of brightness method, towards the blue
by the flicker method. Ives found the relative positions of the two kinds
of spectral luminosity curves generally different. They differ most in
position at low illuminations with large areas ; least at high illumina-
tions with small areas. The mean curves of several observers show
close agreement in the position of the maxima and the shape of the two
curves at high intensities, but the areas of the curves are not equal.
At low illuminations all observers agree in showing the Purkinje and the
reversed Purkinje effects.

Haycraft, by the critical frequency method, obtained a pronounced
Purkinje shift at low intensities. Ives obtained the reverse effect,
except at very low intensities (0"5 metre-candle), when he confirmed
Haycraft's result. Ives sought an explanation in Porter's change in the
logarithmic rates at which critical frequency varies with the illumination.
In Porter's equation

n = k . log I + k'

k has a different value above and below 0*25 metre-candle. If the
critical frequencies are plotted against the logarithms of the illumination
for white light two straight lines of different slope, which meet at about
0"25 metre-candle, are obtained. The reversed Purkinje effect occurs
above, the true Purkinje effect below this point.

When separate colours are investigated and plotted in the same
manner, a set of straight lines of differing slope results. The most
remarkable curves are those for red (650 ^/x) and blue (480 /xju). The
former maintains its direction unchanged ; the latter suddenly changes
from a diagonal to a horizontal, i.e., the critical frequency becomes a
constant, independent of the (low) illumination. The curves for other
colours take an intermediate course. Hence Porter's law for white
light holds good for different colours if the values of the constants are
changed.

i^==/iC,.log^, + /iC/,

where F is the critical frequency, S^ is the slit-width, K^ is a constant
involving the relationship between critical frequency and intensity of
radiation for the individual eye for the colour in question and for the
size of the area stimulated, and K^' is a constant involving the quantity
of energy emitted by the source, the dispersion, etc. of the instrument,
p. c. V. 7

98 COLOUR VISION

and the sensibility of the observer's eye to flicker for different colours
at a given speed. The Purkinje effect, and its reversal above 0*25
metre-candle, follow at once from these facts.

In the flicker phenomenon with colours two causes of flicker are at
work, the colour element and the luminosity element^. Of these colour-
flicker ceases first. Hence the total flicker effect may be the resultant
of the two flicker sensations. Ives has shown that the flicker photometer
is largely influenced by the critical frequency phenomenon, but that
it does not obey the simple law which would follow were it a mere dove-
tailing of two pure flickers.

Allen and others^ found that the peripheral retina is more sensitive
than the fovea to flicker, as might be expected from its high sensitiveness
to movements. Ives, however, found that this result is only true for
momentary observation. Adaptation or fatigue sets in very rapidly
and then the periphery becomes less sensitive. The fovea is more
sensitive to red flicker, the periphery to blue, and this difference is more
striking at low intensities (Dow, Ives).

If the comparison light is coloured or the stimulating area is sur-
rounded by white instead of black the equality of brightness method
produces irregular and unsystematic shifts and distortions of the spectral
luminosity curves, possibly owing to the increase in distracting psycho-
logical factors. Such changes produce no alteration in the luminosity
curves by the flicker method. A curve almost identical with the flicker
curve can be obtained by the equality of brightness method if it is
built up of small steps of slight hue- difference and with small areas.
By this so-called " cascade " method the differences in hue are made
so small that they do not disturb the judgment of brightness.

When the areas of the luminosity curves by different methods are
compared it is found that the visual acuity method {v. p. 44) gives
a curve many (about five) times as great as the equality of brightness,
the flicker and the critical frequency curves, which agree much more
nearly. The enormous area of the visual acuity curve is due to the
chromatic aberration of the eye. It has been shown by BelP and
Luckiesh^ that the resolving power of the eye is much greater for
monochromatic than for complex light of the same hue. For a method
of measurement to be accurate it should conform to two axioms : things
equal to the same thing are equal to one another ; and the whole is

^ Cf. Liebermann, Ztsch.f. Sinnesphysiol. xlv. 117, 1911.

2 Cf. Lolimann, Arch. f. Ophfh. Lxvm. 395, 1908.

3 Eke. World, LVii. 11G3, 1911. « Ibid. Lvm. 450, 1911.

RECURRENT VISION

equal to the sum of its parts. Most physical measurements conform
to these axioms, but it cannot be assumed that the same agreement
holds good for measurements involving qualitative differences, such
as those of colour. The reliability of a method of heterochromatic
photometry must be judged according to the following criteria. First,
the shape of the luminosity curve must not be altered by change in
the reference standard. Second, the sum of the measurements of the
brightness of the parts of the spectrum must be equal to the brightness
of the recombined spectrum.

€^

"^=^~Ci

\ \

//
//

\
\

FULL LINE- EQUALITY OF BRIGHTNESS BY SMALL STEPS

DASHED LINE - " , " " "wHITE COMPARISON EIELD

CIRCLES - FLICKER METHOD
1 1 1 1 1 1 1

Fig. 38. Photopic luminosity curves taken by the flicker and equality of brightness
methods. Abscissae, wave-lengths from 520 ^/a to 640 a'M ; ordinates, arbitrary
scale. (Ives.)

So far as the position of maxima and the general form of the curves
are concerned all the methods agree fairly well. Comparison of the
different methods shows that the areas of their curves differ most for
the visual acuity method as compared with the other three. These
latter do not agree well. Of them, the equality of brightness method
gives greatest variations, the critical frequency method smaller, and
the flicker method least. The crucial test has been applied to the flicker
method by Whitman^, Tufts^ and Ives. Of these the last named is by
far the most accurate. The method consists essentially in measuring
the luminosities of the parts of the spectrum against a standard which

^ Phys. Rev. m. 241, 1896.

2 Ibid. XXV. 433, 1907.

7—2

100

COLOUR VISION

is identical with the source of light of the spectrum. Ives obtained
a remarkable confirmation of the accuracy of the flicker method, and
arrived at the conclusion that it surpasses all other photometric methods
in sensitiveness and accuracy.

Finally, Ives measured the luminosity curves of eighteen observers
with normal colour vision at 25 metre-candles illumination of a mag-
nesium oxide surface. The average curve deduced from these experi-
ments is shown in Fig. 39 and is compared with Konig's curve as
reduced by Nutting^ and with Thiirmel's curve^.

■40 -42
Fig. 39.

•44 46 id -50 -52 34 -56

•58 -60 -«

•64 66

•68

•70 -ri/i

Average photopic luminosity curve of 18 observers.

Konig's photopic luminosity curve (equality of brightness method).

Thiirmel's photopic luminosity curve (flicker method). (Ives.)

In conclusion it may be stated that in the case of colours it is
generally agreed that the fusion frequency depends solely on the
luminosities of the alternating fields, and the coincidence of the equality
of brightness curve with the flicker luminosity curve (Figs. 38, 39) bears
out the assumption. We may at any rate assume with confidence
that both the methods measure the " something " (p. 44) which we
call brightness or luminosity.

^ Bull, of the Bureau of Standards, vii. 2, 235.
2 Ann. d. Physik. xxxm. 1154, 11)10.

SUCCESSIVE INDUCTION OR AFTER-IMAGES 101

CHAPTER II

SUCCESSIVE INDUCTION OR AFTER-IMAGES

After the stimulation of an area of the retina with light, the
withdrawal of the stimulus does not result in the obliteration of all
visual sensation. The succeeding sensations vary according to the
nature and intensity of the primary stimulus, the condition of adapta-
tion of the retina, the region of the retina stimulated, etc. If a second
stimulus is applied to the same area of the retina during the course of
the after-effects of the primary stimulus the resulting sensation is
modified by these after-effects. The effect produced by the secondary
stimulus can be measured by comparing it with that obtained from
a retinal area not previously exposed to the primary stimulus. The
stimulus applied to such a resting area is called the comparison
lighfi.

The sensations obtained from the persistence of the physiological
processes set up by a primary stimulus, whether altered by the effects of
succeeding stimuli, or unadulterated, i.e., observed under the condition
of complete exclusion of fresh stimuli, are commonly called " after-
images." After looking for from 20 to 40 seconds at a white spot of
light and then directing the gaze upon a white surface a black spot
surrounded by a bright halo is seen. This is the " negative after-image."
If the spot or light is coloured the after-image is seen to be tinged with
the complementary colour. This is the " complementary after-image."

If care be exercised a different phenomenon will be observed. Direct
the eyes towards a bright spot or light, but keep them carefully covered
with the hand for a minute or so. Remove the hand rapidly but with-
out any sudden or violent action and quickly replace it, so that the
object is momentarily seen. If the experiment is properly carried out
the spot will be seen as a persistent after-image in its original brightness
and detail. This is the " positive after-image." If the object is
coloured it is seen tinged with the same colour ; this is the " homo-
chromatic after-image."

Although in ordinary circumstances negative after-images are more
easily obtained than positive McDougall^ has shown that under suitable

^ In Germany the effect of the pi'imary stimulus is called "tuning" the retina
(Hering). The jirimary stimulus is called the "retuning light" (das umsl immende
Licht); the secondary stimulus the '-'reacting light" (das reagirende Licht).

2 Mind, X. N.S. 74 sqq., 1901.

102 COLOUR VISION

conditions the reverse is the case. If the primary stimulus is an area
of white light which is sharply defined and surrounded by a dark back-
ground the negative after-image, bordered by a halo as described,
is observed. If however the transition from the illuminated area to
the background is made gradual a positive after-image is almost
invariably obtained. McDougall used a ground glass disc, 12 cm. in
diameter, illuminated from behind. On the far side about a dozen
sheets of white paper were pasted, each with a circular hole in it con-
centric with the edge of the disc. Of these holes the smallest was
2 cm. in diameter, and each of the others was about 1 cm. larger in
diameter than the preceding one. Such a " shaded disc," when
illuminated, showed a central evenly lighted circle, 2 cm. in diameter,
surrounded by a zone, 5 cm. in breadth, in which the brightness
diminished regularly to the periphery, where it became negligible.
The shaded disc gives with white light a positive after-image without
any halo. On the other hand with colours, though the after-image
is often homochromatic in the first one or two seconds, with most
intensities of light it is approximately complementary through almost
the whole of its course. In the case of red and green especially it is
very constantly the rule that red predominates in the after-image of
green, and vice versa. Sufficient attention does not appear to have been
paid hitherto to this fundamental difference between the relation of
black to white and that of green to red and blue to yellow in such after-
images from " shaded " lights.

Though positive and negative after-images are opposed there
is no discontinuity between them. If the positive after-image is
developed and the eyes are then uncovered and directed towards a
uniform moderately illuminated field the negative after-image at once
appears. If the illumination of the field is suitably chosen no after-
image is seen. The nature of the after-image depends therefore upon
the nature and intensity of the primary stimulus and upon the nature
and intensity of the secondary stimulus. Great diversity of after-images
may therefore be obtained. The after-image of a white spot may be
coloured^ ; that of a coloured spot may not be accurately the comple-
mentary of the primary stimulus. Anomalous colouration may be due
to light passing through the sclerotic and the iris, or to abnormal
conditions of the retina (Hilbert^). Moreover, Burch^ has brought

^ Aristotle; Goethe, Farbenhhre (1819), Eastlake's trans, p. IG, 1840.

2 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. iv. 74, 1893.

3 Proc. Roy. Soc. Land. B. lxxvi. 212, 1905.

SUCCESSIVE INDUCTION OR AFTER-IMAGES 103

forward evidence to show that the after-effects of a primary stimulus
are much more prolonged than has been generally recognised, and these
later after-effects may materially alter the character of the after-image.
So complex indeed are the effects that the greatest care should be
exercised in making theoretical deductions from after-images.

With primary stimuli of considerable intensity a very short exposure
is necessary to induce the after-image, which may be so strongly
developed as to nullify successive stimuli to the same part of the
retina for a considerable time ("positive scotoma"). W. Tschermak
stated that primary stimuli with coloured lights of such low intensity
as to be below the chromatic threshold were followed by coloured
after-images, but it was not found to be so by Titchener and Pyle^.

Analogy with other physiological processes would lead us to the
conclusion that the positive after-image results from a persistence of
those processes which have been set in action by the primary stimulus,
and that the negative after-image is the expression of a diminished
excitability of the stimulated area to fresh stimuli (Fechner, v. Helmholtz).
The fact that a stimulus usually gives rise to a rhythmical response, as
has already been shown (p. 87), makes the periodical variations in the
after-image, which were first described by Plateau, intelligible. Under
suitable conditions, with the eyes motionless, the negative image
disappears and reappears at intervals of three or four seconds, sometimes
alternating with positive images. Sudden movements of the eyes,
distraction of the attention, and other like influences abolish the after-
image ; hence they are seldom noticed in every-day life.

Moreover with prolonged or strong stimuli the condition of altered
sensibility of the affected part of the retina may persist materially
longer than the apparent after-effect as shown by the negative after-
image. When the latter seems to have quite disappeared if the eye is
thoroughly darkened the affected portion of retina gives rise to a sensa-
tion of blackness which is more intense than that derived from the
surrounding areas.

It might seem that the effect of a single isolated stimulus, without
any secondary stimulus, was a simple matter. This is, however,
by no means the case. The negative after-image of a white object
appears as a black spot in the midst of a less black field. There is there-
fore a sensation of blackness which is blacker than that obtained from
the eye when all external light is excluded. The eutopic sensation when
the eyes have long been completely excluded from light is variously

1 Proc. Amer. Philo.-i. Soc. XL. 300. 1908.

104 COLOUR VISION

described by different people, but all agree that it is not an impenetrable
darkness. There are waves or points of light, and the general sensation
is one of very dark greyness. So far as negative after-images are
concerned this " light chaos " or " light dust " acts as a grey surface
on which the black image is projected. Many explanations of this
" intrinsic light " have been suggested. That endogenous stimuli are
the cause can scarcely be doubted, but whether these are primarily
retinal or of central nervous origin remains Uncertain^, though it is now
generally agreed that they are central. The question is of theoretical
importance and will receive further treatment in Part III.

McDougalP found certain features which, he claims, are quite
constant in the after-images of coloured lights. They are as follow :
(1) The after-images show in nearly every case a play or succession of
colours, in which each of the three simple colours, red, green, and blue,
makes itself felt in some phase. (2) The brighter the coloured light
fixated, the brighter are the colours of the after-image, and the more
keen is the antagonism between the three simple colours, so that these
colours fuse less than in the after-images of duller colours. When the
primary stimuli are very bright the three simple colours tend to appear
pure and saturated in turn in a recurring scale, the unchanging phases
of pure colour being separated by periods of struggle between the fading
and the succeeding colour, just as is the case in the after-image of
bright white light. (3) In the case of fixation of one of the three simple
colours, red, green, and blue, the lower intensities are followed by after-
images in which the other two colours predominate, more or less fused,
i.e., the after-image is predominantly complementary ; while when the
colour fixated is very bright the first phase of the after-image is usually
homochromatic and of considerable duration. The like holds good for
the compound colours, but in their case the phases of the after-image
tend to be rather more varied. (4) The order of occurrence and the
duration of the different phases vary readily with slight variations in
the conditions. (5) With any given light, the vividness and duration
of the after-image, observed in the dark, increase with increase of
duration of fixation of the light from about 10 sees, to 90 — 100 sees.,
but with further increase of the period of fixation, the duration and
vividness of the after-image diminish, so that after very prolonged
fixation the after-image is either dull or of short duration or is not seen
at all.

As with white light, a sharply bounded patch of coloured light,

1 See V. Helmholtz, 3rd ed. p. 12 sqq. « Mind, x, N. S. 1901.

SUCCESSIVE INDUCTION OR AFTER-IMAGES 105

fixated for more than a few seconds, gives rise to an after-image sur-
rounded by a halo. If the coloured light is not very bright, and the
fixation not very prolonged, the halo is of the same colour. The longer
fixation is continued, the brighter and the less saturated is the colour of
the halo, until after prolonged fixation it becomes white or even tinged
with the complementary colour, and is so much brighter than the after-
image itself as to inhibit it partially or wholly. The colours of the
after-image and its halo tend to be complementary to one another,
for during the observation of an after-image in the dark the conditions
are very favourable to contrast effects (See Section VI).

The effects of a secondary stimulus on a previously stimulated
area are of great importance in considering the facts of colour vision.
We have seen that the primary stimulus alters the excitability of the
retina so that it is lowered for a succeeding stimulus of the same nature.
So far at any rate as colours are concerned we may carry the matter
further and say that it is raised for a secondary stimulus of the com-
plementary colour. If a complementary after-image of any spectral
colour be obtained and the secondary stimulus be this complementary
itself, a sensation of this complementary colour is produced which far
exceeds in purity and saturation any such colour which is found in
nature or in the spectrum. If for instance a complementary after-
image of green obtained from a suitable purple field be compared with
a patch of the corresponding spectral green, it will appear of extra-
ordinary brilliancy and saturation.

The question therefore arises whether these after-effects seriously
complicate the equations of colour matches. It may be said at once
that so far as the peripheral retina is concerned they do, because we
have already seen that the periphery values are not the same as the
foveal values and they are far more susceptible to variations in adapta-
tion. With regard to the fovea, however, it is found that all colour
matches still remain valid, no matter what kind of light may have
previously stimulated the retina. Thus a match of monochromatic
yellow with a mixture of red and green remains a match after
previous illumination with yellow and blue. If yellow has been
used, both become paler ; if blue, both become more saturated
yellow; but they still match. This law has been systematically in-
vestigated by Biihler^ under v. Kries' direction^. It may be stated

1 Diss. Freiburg, 1903.

^ See also v. Kries, Arch. f. Anat. 503, 1878 ; and Dittler and Orbeli, Arch. f. d. ges.
Physiol, cxxxii. 338. 1910.

106 COLOUR VISION

at once that it also holds good for the ordinary types of colour blind
(dichromats)^.

The law has, however, been denied by Hering^ and Tschermak^,
and Watson* has shown that it is at best an incomplete statement of
the facts. He confirmed the fact that colour matches remain valid
after previous stimulation with another light. He obtained the fol-
lowing results :

Width of " green " slit
Resting Previously stimu-

Character of match retina lated retina

Correct 22-7 23-5

Toolittle green 20-5 110

Too much green 25-0 27'2

In each case white was matched by a mixture of spectral red, green
and violet lights by altering the width of the slits through which the
coloured light proceeded. When the match was correct for the resting
eye it was found to be also correct for the eye which had been previously
stimulated with a colour, e.g., red. If now the slits through which the
red and violet lights passed were kept constant, while the green slit
was altered, being reduced until the deficiency of green was just observ-
able and opened until the excess of green was just perceptible, it was
found that the range through which the green slit could be altered
while still preserving a correct match with the white light was much
greater for the eye previously exposed to red light than for the eye
not thus previously stimulated. As shown above, the range between
a perceptible excess and a perceptible deficiency of green was 4*5 for
the resting eye and 16'2 for the previously stimulated eye.

These experiments therefore show that the statement that " all
colour matches still remain valid," though true, is not the whole
truth, and is indeed misleading. After previous stimulation the range
of intensities which give a valid match is much wider than for the
resting eye.

If we accept the law of the validity of optical equations irrespective
of previous stimulation, we can obtain a relation between the changes
in appearance of various lights caused by previous stimulation. If
R is the measure of the stimulus, the sensation will be aR, where a. is
the measure of the retinal excitability for the particular stimulus R.

^ V. Kries and Nagel. Zf.^ch. f. Psychol, u. Physiol, d. Sinncsorg. xii. 1, 1896 ; xxni.
161, 1900.

2 Arch.f. d. ges. Physiol i.iv. 309, 1893. » Ibid. lxx. 297, 1898.

* Abney and Watson, Proc. Roy. Sac. Land. A. lxxxix. 1913.

SUCCESSIVE INDUCTION OR AFTER-IMAGES 107

Further, if Li at one retinal point gives rise to the same sensation as
Lo at another, previously stimulated by a different light, and M\ at
the first point gives rise to the same sensation as Mi at the second,
then L] + M\ will have the same action at the first as Li + Mo at the
second. As a special example, 'projjortiofial alteration of intensity of
two stimuli acting on retinal areas which have been previously exposed
to dift'erent excitations will produce the same alteration in sensation.
This is the " Law of Coefficients " (v. Kries^), adumbrated by Wirth^
as the Fechner-Helmholtz law. From the law it follows that if the
change from normal produced by stimulation with three different
lights or light mixtures is known the change produced in any other
light mixture can be deduced.

The law applies only to the photopic condition. It is therefore
least subject to deviation for foveal values. For peripheral values,
which are so subject to scotopic variations, and for stimuli of low in-
tensity, which so readily induce the scotopic condition, it ceases to be
valid. Moreover, with feeble stimuli the endogenous stimulus of the
intrinsic light becomes measurable relatively to the exogenous stimuli.

When the primary and secondary lights are identical the result is a
gradual diminution of luminosity (local adaptation, Hering). Such a
diminution in differences of brightness occurs when two similar coloured
fields of unequal luminosity are fixed for a considerable time. Besides
the alteration in brightness there is also a diminution in saturation
with coloured lights, and further, though it is scarcely noticeable with-
out a comparison light, there is also a change in hue, Voeste^ found
that a yellow of wave-length 560 //./i, a green of 500 /x/x, and a blue of
460yLiyLt,show no appreciable change of hue. Hues between 500 and 560yu./Lt
change towards 560 yti/x, those between 500 /a^u, and 460 ^jx towards
460 /x/i. on prolonged fixation. These facts are of importance in the
equality of brightness method of estimating luminosity matches. In
Abney's* method " the angles of the sectors are rapidly altered from
" too light " to " too dark " and back again, and the range of angle
is gradually diminished until the observer sees both to be equally
bright." The change in brightness on prolonged fixation constitutes
one of the difficulties of this method with unintelligent examinees,
and is an argument in favour of the flicker method.

If the primary stimulvs is white and the secondary stimulus coloured

1 Nagel, p. 211. 2 Wundt's PMlos. Stud. xvi. 4 ; xvn. 3 ; xvni. 4.

^ Ztsch.f. Psychol, u. Physiol, d. Sinne/forg. xviii. 257, 1898.
* Abney, p. 88.

108 COLOUR VISION

the exposure to white changes the chromatic stimulus values. Thus,
using a comparison light, and experimenting with rotating discs v. Kries^
found that if the coloured sectors were made the same both for the
stimulated and the comparison area there was never even an approximate
match ; and if, keeping the same coloured sectors, the white sectors
were chosen such that the luminosities were equal, the colours seen by
the stimulated area were of far too low saturation. In order to produce
a match it was necessary for the amounts of coloured light in the
secondary and comparison to be about 3 : 1 (blue sectors 270° : 97° ; red,
270° : 84° ; yellow, 270° : 97°). These experiments have been adversely
criticised by Hering^ and diverse results have been obtained by Dittler
and Richter^.

Dittler and Richter arranged that two contiguous areas of the
retina were stimulated with homogeneous blue lights so that an exact
match was obtained. The stimulus was then cut off from one area and
the other was stimulated with a binary white light obtained by mixing
the blue with its complementary colour. The previous blue matches
were then replaced and it w^as now found that the blue of the stimu-
lated area was much more saturated than that of the resting area.
Increase of intensity of the blue of the stimulated area failed to re-
establish a match, but addition of white light effected a much better
match.

// the 'primary stimulus is coloiired and the secondary stimulus
white the chromatic excitation causes the white to be coloured
with the complementary colour. This is the ordinary complementary
after-image, a red, yellow, green, or blue object giving a blue-green,
blue, purple, or yellow after-image respectively when a white or grey
field is fixed. If the sensation is matched with a comparison light on a
neighbouring retinal area it can be shown that it is nearly if not quite
as saturated as the spectral colour, even if the exciting light is of
moderate intensity and has acted for only 30 to 40 seconds. Exact
coincidence with the complementary colour of the exciting light is not
generally found**.

The colour of the resultant sensation is not a mixture of the colour
which would be observed with the darkened eye without any secondary

^ Berichle d. Freiburger Naturf. Gesellschaft, 1894.
- Arch. f. (1. ges. Physiol, xciv. 533, 1903.

^ Ztsch. f. Sinnesphysiol. xlv. 1, 1910; Dittler and Orbeli, Arch. f. d. ges. Physiol.
cxxxn. 338, 1910.

« Cf. Tschermak, Ergeb. d. Physiol, ii. 2, 7G3, 1903.

SUCCESSIVE INDUCTION OK AFTER-IMAGES 109

stimulus plus the white light of the white or grey surface. If it were,
after stimulation with blue, for example, the yellow of the after-image
would always be compensated by mixing the same amount of blue
with the secondary light, so that a white after-image would result.
By the coefficient law, with increasing intensity of the white light
the amount of blue added should increase proportionately, and this
is found to be the case (v. Kries). The complementary after-image is
therefore not the result of a mere additive process, but is a genuine
qualitative change in the white light, though it may also involve a
change of intensity.

If the primary stimulus is coloured and the secondary stimulus also
coloured various coloured after-images are obtained. It has already
been mentioned that if the secondary stimulus is the complementary
colour of the primary the resulting sensation is that of the extremely
saturated complementary colour. This is specially true of the red-
yellow end of the spectrum and for secondary lights of high intensity.
It may indeed be far more saturated than any spectral colour, so that
it is impossible to obtain a comparison light which will match it.
This is a fact of profound theoretical significance, since it shows that
we are capable of experiencing visual sensations which cannot be
elicited by the application of any known physical stimulus to the
resting eye.

If the secondary stimulus differs from both the primary and its
complementary the most diverse results may be obtained, but as a
rule the complementary colour of the primary stimulus predominates.
After green yellow appears reddish orange, blue purple, and so on.
After red, yellow (589 /xyu) matched greenish-yellow (556 fi^) ; after
green, 517 fi/u, matched 565 /^/z (Hess^).

Attempts have been made by v. Kries^ for white, and by Schon^
for coloured lights, to obtain the time relations of after-images. The
sensation derived from the secondary light is matched at rapid intervals
with a comparison light. The results are of little value owing to rapidly
changing adaptation and the difficulty of avoiding parafoveal stimula-
tion. V. Kries* also studied the time relations of recovery from the
primary stimulus. It is unknown whether the effect passes off as a
smooth curve (Fechner, v. Helmholtz, v. Kries) or in rhythmical waves

1 Arch. f. OpMh. xxxix. 2, 45, 1893. Cf. A. W. Porter and Edridge-Green, Proc. Roy.
Soc. Lond. B. lxxxv. 434, 1912; Edridge-Green, loc. cii. lxxxvi. 110, 1913.
3 Arch.f. Ophth. xxni. 2, 1, 1877. 3 Ibid. xx. 2, 273, 1874.

* loc. cit.

110 COLOUR VISION

(Plateau, Hess^). From analogy the latter is the more probable, but
the question requires further elucidation^.

The fading of after-images. The observations recorded above deal
with successive induction resulting from short exposures. There are
a number of careful observations by some of the older authors,
notably Plateau^, Fechner*, Seguin^ and v. Helmholtz^, which are of
importance in the theory of colour vision. They deal with the subjective
impressions which occur during the fading of the after-images of a light
stimulus.

If the primary stimulus be of sufficient intensity the passage of the
positive into the negative or complementary after-image is accompanied
by a series of colour sensations. Of particular interest for our present
purpose is the subjective development of colour sensations from stimulation
with white light. The series varies according to the intensity and
duration of the primary stimulus. Momentary excitation of white
light produces an after-image which " passes rapidly through greenish
blue (green, Seguin) into beautiful indigo blue, later into violet or
rose-red. These colours are bright and clear. Then follows a dirty
or grey orange, during which the positive after-image generally changes
to a negative, and in the negative image this orange often becomes
a dirty yellow-green'^." When the negative after-image has developed
it appears dark against the background of the " intrinsic light "
[v. p. 104). If white light of low intensity is now admitted the after-
image passes on into the later coloured phases, and retreats again if
the white light is again diminished. Thus, if the image when the eye
is covered is blue the introduction of dim light causes it to pass through
rose-red to a negative yellow. If the eye is again quickly covered
the blue reappears. If the image in darkness is rose-red, a weak white
light turns it yellowish-red, and so on. If the positive image has com-
pletely disappeared from the darkened eye a dim white light develops
a grey or green-grey negative after-image surrounded by a rose-red
halo.

If the primary stimulus is more intense or acts for a longer time
colouration may commence before obscuring the eye (Fechner). It is
first yellow, then blue-grey or blue, without passing through green,

1 Arch. /. (I. (jes. Physiol, ci. 226, 1904.

' Cf. Fick and Gurber, Arch. f. Ophth. xxxvi. 2, 245, 1890; Fick, loc. cit. xxxvra. L
118, 1892 ; Hering, loc. cit. xxxvra. 2, 252, 1892; Fick, loc. cit. xxxviii. 3, 300, 1892.
3 Essai dfune Theorie gen. etc. Bruxelles, ] 834. * Ann. d. Physik, l. 220, 1840.

* Ann. de Chimie, 3rd series, xli. 415, 1850.

• 3rd ed. n. p. 208. '^ v. Helmlioltz, loc. cit. ii. p. 208.

SUCCESSIVE INDUCTION OR AFTER-IMAGES 111

then red- violet or red. The yellow phase is shortest. After longer and
stronger stimulation by white light the after-image in the darkened eye
passes through white, blue, green, red, blue, and on exposure to dim
white light blue-green and yellow. Different authors give different
phases with stronger excitation, but blue always starts the series.
The after-images from direct exposure to sunlight are still more
complicated^.

The fading phases of after-images have been more recently in-
vestigated by Miss Washburn^ and McDougalF. The latter has shown
that it is unnecessary to use stimuli of excessive strength. He found
that after excitation with white light a very constant feature was the
tendency of the three simple colours to follow one another in a recurring
cycle of the order green, red, blue, green, red, blue. If the white
light be only moderately bright, red is usually the first colour to appear
and is then succeeded by green, which persists to the end. After
brighter light green comes first, and is succeeded by red, then blue,
and then green again. If the light is still a little brighter, McDougall
describes the green as being " usually mixed with red from the first,
i.e., it appears yellow with a red edge." If the light be very bright then
the first phase of the after-image is white or bright blue of very low
saturation ; the blue soon passes through blue-green to green, which is
then followed by the cycle of pure colours.

According to McDougall " an important feature of the after-images
of bright white light is that, after a first short period in which two colours
fuse to give yellow, or, as is the case after the brightest lights, all three
fuse to give white, the colours that in turn occupy the area of the after-
image, alone and unchanging for considerable periods, are red, green,
and blue only, and these are in every case of exactly the same colour-
tone although varying in brightness in different cases and in different
stages of one after-image. The red is a rich crimson red, decidedly
less orange than the red of the solar spectrum, the blue is a rich ultra-
marine, and the green a pure green having no inclination towards blue
or yellow^." McDougall states that "they are the purest, richest,
most saturated colours that I have ever experienced, and I believe that
in this way, and this way only, one may experience absolutely simple,
i.e., unmixed and fully saturated colour-sensations."

The fading phases of the after-images froyii coloured lights are also
not limited to the homochromatic positive and the complementary

1 V. Helmholtz, loc. cit. n. 211. 2 Pgychol. Rev. vii. 1900.

3 31{nd. X. N. S. 235, 1901. * Cf. Part III.

112 COLOUR VISION

after-images. As the former passes into the latter in the case of a strong
primary stimulus the image does not simply become paler, but un-
saturated hues appear. The homochromatic hue of the positive image
first disappears and the image resembles that of the after-image of a
white object in which the rose-red phase predominates. " Then the
complementary colour of the negative after-image gradually developes,
but it may be already visible before the positive image has become
negative ; it may therefore appear brighter than the dark background.
I think that the appearance of the complementary colour may be
attributed to the fact that the positive image, which at this phase has
become feeble and white, is superposed upon the negative and comple-
mentary images induced in the intrinsic light chaos by the fatigue of
the eye. It is clear that by such a superposition after exposure to red
the positive white and the negative blue-green can together give a
greenish white positive image. These positive complementary images
are mentioned by several observers (Purkinje^, Fechner-, Briicke^)*."
It is an instructive experiment to view the spectrum momentarily and
then to observe the phases of the after-images.

Burch^ has adduced strong evidence to show that the duration of
after-image effects is much longer than has been commonly supposed.
He has observed changes occurring in the subjective sensations for as
long as two hours. As with all subjective observations of this kind
generalisations from them are to be accepted with caution.

It is unnecessary to pursue this complicated problem further.
The older experiments, especially Fechner's with coloured stimuli, were
mostly made with impure lights. The matter requires further investiga-
tion with homogeneous lights and due regard for adaptation, etc.

CHAPTER III

THE EFFECTS OF " FATIGUE

The term " fatigue " has been avoided in the previous consideration
of the effects of retinal stimulation. With our increased knowledge of
adaptation it has become difficult to estimate the importance of fatigue
and further discussion must be postponed until we deal with the theory

1 Zur Physiol, d. Sinne, n. 110. 1819. « Ann. d. Physilc, l. 21.3, 1840.

3 Wiener Denkschr. m. 12, 1850. * v. Helmholtz, loc. cit. ii. 213.

5 Proc. Roy. Soc. Lond. B, lxxvi. 212, 1905.

THE EFFECTS OF "FATIGUE" 113

of colour vision. Earlier authors, including v. Helmholtz, and most
physicists make free use of the conception of fatigue to account for
successive induction, but the facts, and especially those of simultaneous
contrast {vide infra), negative so simple an explanation. There is,
however, a group of phenomena associated with prolonged or intense
stimulation to which the term may be fittingly applied, though even in
this case it should be done " without prejudice." These phenomena
have been studied particularly by Burch^. By flooding the eye with
bright sunlight which had passed through a lens of 2 inch focus and a
suitable colour screen or by a similar process with a spectrum of wide
dispersion Burch was able to study the after-effects of fatigue for the
various colours.

After fatigue for red, scarlet geraniums appear black, calceolarias
and sunflowers various shades of green, and marigolds green shaded
with black in the parts that are orange to the normal eye. Purple
flowers, such as candytuft and clematis look violet, and pink roses
bright sky-blue. Short exposures — a few seconds to two or three
minutes^suffice, and the effect is transient, passing off in about
ten minutes.

After fatigue for violet, violet wools look black, purple flowers
crimson, some blues greenish, green a richer hue. A noticeable effect
is the tinging of all objects which do not reflect violet with that colour,
and the same applies to " dazzling " with other colours.

Fatiguing with green makes the landscape look like a picture painted
with vermilion, ultramarine and flake-white, variously blended. The
foliage is reddish-grey or bluish-grey, blue flowers are dirty blue,
red flowers are impure red, and every colour but green is tinged with
green.

Fatiguing with jmrple {i.e. red and blue) makes everything look
monochromatic green. All red, purple or blue flowers look black, and
green looks a quite unnaturally brilliant green. The red fatigue passes
off first, the observer then being in a condition of violet fatigue, which
passes off in 15 — 60 minutes.

Fatigue of one eye with purple and of the other with green produced
a very weird and exaggerated stereoscopic effect.

In fatiguing the eye with the colours of the spectrum comparatively
simple changes, differing in degree but not in kind, occur with four
regions, viz. the regions which give the sensations of pure red,

^ Phil. Trans. Roy. Soc. Land. B, cxci. 1, 1898; Proc. Roy. Soc. Lond. lxvi. 216,
1900.

P. c. V. 8

114 COLOUR VISION

unmixed green, unmixed blue, and pure violet. The changes are as
follow :

(1) All direct sensation of the fatiguing colour is abolished, not
merely from the corresponding part of the spectrum, but also from those
regions in which it overlaps other colours. It is rather the sensation
of the particular colour that is weakened than the sensitiveness of the
eye to a particular part of the spectrum. Thus, in red fatigue, the red
end is shortened, but the green appears with full intensity at the
D line and reaches almost as far as the C line. In green fatigue the
green is replaced by red and blue, which not only meet but overlap.

(2) The fatiguing colour produces a homochromatic after-effect
like a luminous fog, by which the hue of all the other colours is modified
if they are relatively weak, but which is unnoticed if they are bright.
" A strong light not only fatigues the eye, but dazzles it ; that is to
say, the sensation of light persists after the source of light has been
withdrawn " (Burch). Each " dazzle-tint " has its own rate of develop-
ment and of subsidence, and each is independent of the rest. Red passes
oft" first, then green, then blue, and last violet. The positive after-effect
of light between the C and D lines is at first red, but it comprises two
dazzle-tints — red and green, of which red soon vanishes, leaving green.
The dazzle-tint becomes unnoticeable long before the colour sensation
is fully restored.

(3) Fatigue with one colour has no effect upon the intensity of the
remaining colour sensations.

Fatigue with intermediate portions of the spectrum produces inter-
mediate effects. Thus fatigue with orange or yellow affects not only
the appearance of the orange or yellow, but that of the red and green
also ; similarly with the blue-greens and the indigo.

By fatiguing with red for 30 seconds and then finding pure yellow,
fatiguing with E for 30 seconds and then finding the beginning of blue,
fatiguing with F for 30 seconds, and then finding the beginning of
violet, and again working backwards on the same plan, it has been
possible to map out the range of the colour sensations. The averages
of 70 people with normal colour vision give the extent of

red = 760-6 to 551 /x/x.

green = 593 to 484 „

blue = 517 to 443 „

violet = 454 to 397-5 „

THE EFFECTS OF "FATIGUE" 115

" In reality each colour sensation extends so much further that red
and blue overlap, and so do green and violet, the only colours which I
have not proved to do so being red and violet " (Burch).

From the data in Clerk-Maxwell's papers and from his own observa-
tions Burch estimates the transition point between

Maxwell, red and green, 583"7 ; Burch (average of 70 persons) 573 ;

,, green and blue, 500*3 ; Burch (average of 70 persons) 500*3 ;

„ blue-indigo = 449"8 ;
Burch, blue and violet = 448.

Pure colours — -

Maxwell— red = 630-9 ; green = 529-7 ; blue = 457-4.

Burch (average of 70 persons) — red = 625 ; green = 525 ; blue = 471 ; violet = 415.

Burch therefore thinks there are usually four colour sensations —
red, green, blue, and violet. He is unable to detect the existence of a
separate yellow by this method.

It should be pointed out that these are facts independent of any
theory of colour vision, but it must be remembered that the fatiguing
lights were of great intensity and might well produce changes which
are quasi-pathological.

Edridge-Green and Marshall ^ controvert some of Burch's statements,
especially those relating to fatigue for the D line. Their experiments
differ from his in that the intensity of the fatiguing light was much less.
They find that after exposure of the eye to the sodium flame for from three
to fifteen minutes and then looking at the spectrum the yellow is entirely
obliterated and only a faint band of orange separates the red and green.
If the eye is still further fatigued this also is obliterated, the red and
green meeting. The red looked rather more purple, the green bluish.
The blue and violet appeared diminished in intensity. There was no
shortening of the red end of the spectrum.

It should be noted in this connection that Burch does not deny the
existence of a yellow sensation in some people, but states that he finds
no evidence of it himself. On the other hand Edridge-Green and
Marshall's experiments do not produce nearly so complete a fatigue of
the sensations — whether true yellow (according to them) or red and
green (according to Burch) — as in Burch's experiments.

A. W. Porter and Edridge-Green^ have also investigated the effects
of fatigue with spectral lights. The eyes were fatigued for about 20 sees.
with monochromatic light and then fixed a spectrum of weaker intensity

1 Tram. Ophlh. Soc. xxix. 211, 1909.

2 Proc. Roy. Soc. Lond. B. lxxxv. 434, 1912.

8—2

116 COLOUR VISION

so that the after-image formed a band across the middle of the spectrum.
After fatigue with 654 — 675 /x/x the extreme red was slightly diminished,
there was no change in the orange, yellow, or green, and the blue and
violet became darker and bluer. If only red and orange were viewed
the red disappeared and the orange remained. The sodium flame
appeared unchanged (Porter), slightly greener (Edridge-Green). After
fatigue with orange, 619 — 631 /x/u,, the dark blue after-image was seen
right across the spectrum except in the region of the orange, which
appeared unaffected. If only red and orange were viewed the red was
replaced by the green-blue after-image, whilst the orange appeared
unaffected. After fatigue with orange-yellow, 585 — 595 /x/x, the purple
after-image appeared as if painted over the spectrum, the red being
affected most. Fatigue with yellow-green, 545 — 550 /x/x, gave a similar
result, with least effect in the orange. Fatigue with blue-green, 496 —
500 /x/A, gave the same effect,a purple (not red) after-image, with orange
least affected : the sodium flame appeared unchanged (Porter), slightly
redder (Edridge-Green). After fatigue with blue, 478 — 480 /x/x, there was
no change in the red and orange, and the reddish purple after-image
was seen over the rest of the spectrum. With blue, 445 — 455 /x/i, the
after-image was yellow-green (Porter), orange (Edridge-Green) ; the
violet and blue were cut off, red and green became yellower ; if only
green and red were viewed the former was obliterated, the latter un-
affected. With violet, 425 — 436 /x/x, the after-image was green ; the red
and orange were unchanged, the green became yellow-green, and the
violet and blue appeared green.

The authors found that ' ' the stability of the after-image was remark-
able ; it did not change colour, and was not influenced by subsequent
light falling on the retina when this was not of too great intensity."
Thus, if after viewing the reacting spectrum the eyes were turned to
the dark screen the after-image appeared as a uniform dark band,
darker than the screen. In contradiction to previous observers the
authors found that " the complementary to the exciting light is never
strengthened in the spectrum on the screen by the after-image." They
also found no change in colour of the after-image as it fades.

Wanach^ found that the eye fatigues more quickly for some
spectral colours than for others, and least for the more refrangible.
Macular fatigue lasts longer than peripheral.

The consideration of Abney's researches on colour fatigue will be
postponed (Part III).

^ Ztsch. /. Sinnesphysiol. XLni. 443, 1908.

SECTION VI

AREAL EFFECTS.
CHAPTER I

THE LOCAL QUANTITATIVE EFFECT

Some effects of the size of the area stimulated have already been
touched upon {v. pp. 51, 79, 95). They are of considerable importance
and demand more detailed consideration.

With regard to foveal vision it may be said that given a sufficiently
intense illumination for a given condition of adaptation a mathematical
point of light will be visible. Its image on the retina is always a diffusion
area, the central parts of which afford at least a minimal effective
stimulus if they anywhere impinge upon a retinal cone. If the effective
area is so small as to occupy an interconal space a slight movement of
the eye must be predicated in order that the point may be visible. It is
probable that for a given condition of retinal adaptation a subminimal
stimulus for a single cone may become a minimal or eft'ective stimulus
if spread over several cones.

Ricco^ conducted a very careful series of experiments bearing upon
this point. The experiments were carried out in six different ways^.
He found that at the threshold of sensibility the quantity of light
entering the eye is constant, or, in other words, the light intensity and
the area of the retinal image are reciprocal functions, or the product
of the area into the light intensity is constant. In terms of the visual
angle, the law is that the minimum visual angle varies inversely as the
square root of the light intensity, or the product of the minimum visual
angle and the square root of the light intensity is constant. The limit
of the law is determined by the size of the foveal region ; it ceases to be
accurate for visual angles above 40' to 50'.

1 Ann. di Ottal. vi. 1877.

2 Parsons, Roy. Lond. Ophth. Hasp. Rep. xix. 1, 114, 1913.

118 COLOUR VISION

Charpentier^ published a series of experiments which confirm and
extend those of Ricco. He used small bright squares up to 12 mm.,
viewed at 20 cm. distance. Below 2 mm. the smaller the surface the
greater the minimum illumination necessary for perception. Two
millimetres at this distance correspond to about 0"17 mm. on the retina,
i.e., about the size of the fovea. For larger areas the area has no effect.
It follows, therefore, that in order to produce a luminous sensation at
the fovea the total quantity of light, i.e., the product of the area and
the illumination, must attain a certain value, and that that value is
constant for a given condition of adaptation. " The fovea centralis
forms a sort of autonomous territory, in which the luminous excitation
diffuses itself, and which always requires a certain quantity Of light
to be set in activity." Charpentier showed, in answer to criticisms by
Leroy, that the diffusion could not be accounted for by irradiation due
to dioptric aberrations.

Asher^ found that for the range of light intensities used by him up
to a visual angle of 2' to 3' the apparent size depends entirely upon the
quantity of the light. According to him, therefore, vision of objects
subtending angles up to this size is a function purely of the light sense
and not of the form sense. The earliest observations bearing upon this
aspect of the subject were by Volkmann (1863) and Aubert (1865).
Aubert used lines 2 mm. wide and 50 mm. long, and determined the
distance the lines had to be apart in order that the interspace might
look the same as the breadth of the lines. He found that when the
breadth of the lines was varied by Volkmann's macroscope so that they
subtended visual angles of from 10" to 45" the angular distance apart
of the lines varied from 104" to 112" for black lines on a white background,
and from 140" to 153" for white lines on a black background. Asher
used small black, white, and grey squares and rectangles of paper and
determined the distance at which a difference of size could be detected,
the light intensities of the papers being calculated by the colour top
and by Hering's polarisation photometer. He found that visual angles
between 23" and 78" might be increased by 25" to 100" (average 58")
under the given differences of light intensity (from 360 to 6) before a
difference in size could be detected. At a great distance from the
different sized objects they appear either of equal size and brightness,
or, if the objective brightness of the smaller is much greater than that
of the larger, of equal size but unequal brightness, the smaller being the

1 Compt. rend. xci. 1880 ; Arch. d'Ophf. n. 234, 487, 1882.

2 Ztsch.f. Biol. XXXV. 394, 1897.

THE LOCAL QUANTITATIVE EFFECT 119

brighter. >^metimes the smaller appeared the larger. On approaching
the objects the difference in apparent brightness increased whilst the
sizes remained equal. Then followed a stage in which the difference in
apparent brightness diminished, the sizes remaining equal ; both might
even appear of the same brightness. On still further approximation
the difference in brightness again became manifest, and simultaneously
or shortly afterwards the larger object showed indefiniteness of the
edges and greater apparent size. In many cases the size and brightness
remained the same until the difference in size became distinguishable.

Asher's explanation is as follows : So long as the objects subtend
so small a visual angle that they cover a single sensibility area^ they
appear equal, since the same quantity of light acts upon the sensibility
area. With increase of the visual angle the influence of aberration
becomes manifest. The larger object has a larger light area which is
larger than a sensibility area, but the periphery of the light area has so
low an intensity that the effective light area is not larger than a sensi-
bility area. The smaller object, on the other hand, has a smaller light
area, the effective part of which, however, is as large as a sensibility
area. The ordinates corresponding to an effective brightness may extend
farther from the centre for the smaller than for the larger object, so
that the smaller may appear the brighter. So long as the relationship
of the objects is such that the sensibility areas of both cover one or
an equal number of sensibility areas they must appear of equal size,
though the aberration areas may be very different. The conditions
of light and contrast may easily be such that the smaller sensibility
area may belong to the larger aberration area.

Asher denies that any proof has yet been given that it is possible to
produce a retinal image so small as to stimulate only one cone.

Schoute^ does not agree with Asher that it is impossible to stimulate
a single cone, and he holds that for single cone images the impression
of size is dependent solely upon the product of the area into the light
intensity. If only one cone is stimulated the object always appears

1 Mach (1866) first distinguished between the physical distribution of light over the
area of a retinal image and the physiological distribution of brightness over the same
area. If the retina be imagined flattened out and ordinates erected upon it, the lengths
of which correspond with the intensities of light at the given spots, the area obtained by
joining the summits of the ordinates will give an area representing the light intensity, or
more briefly a " light area." If the ordinates represent the apparent brightness of the
light at the spots as seen by the observer, the area will represent the sensibility and is
briefly termed a " sensibility area."

2 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xix. 252, 1899; Ztsch. f. Augenhlk. vm.
419, 1902.

120

COLOUR VISION

round, but differences in size are still appreciable. If the objects are
of equal size the brighter appears the larger, i.e., the apparent size
varies with the light intensity. Schoute, like v. Helmholtz, attributes
this to psychological causes, i.e., to an error of judgment.

Loeser^ has entirely confirmed Ricco's law for foveal vision. The
law is stated in one form thus : The product of the minimum visual
angle and the square root of the light intensity is constant. Loeser's
results are shown in the following table :

)istance of

Diameter of

Visual angle.

Sq.

rt. of light

Product

object, E

object, D

D/E

intensity, J

DJ/E

mm.

200

2-5

0-87

2-18

>»

14-0

1-75

1-27

2-22

8-5

1-06

^•4

2-5

9«

0-63

3-45

2-26

The photochromatic interval {v. p. 60) can be demonstrated by
altering the area of the retina stimulated. Thus Bonders^ found that
in full daylight the hue of intensely coloured papers on a dark back-
ground could be distinguished when they subtended a visual angle of
0"7 minute (1 sq. mm. at 5 metres distance).

The dependence of the discrimination of hue on visual angle is
readily demonstrated with relatively unsaturated colours. Coloured
objects on a white background appear dark or grey under the smallest
angles at which they are visible. It is possible to select a grey
background of such a luminosity that the colourless interval is
abolished, i.e., as the visual angle is increased the colour of the object
is recognised as soon as the object becomes visible.

Charpentier^ determined the absolute and the chromatic thresholds
from the area stimulated for the fovea of the dark-adapted eye. Since
there is little adaptation at the fovea, this factor is of relatively slight
importance. He obtained the following results, the measurements
being the diameters of the diaphragm of the photometer.

Colour (sunlight spectrum)

Extreme red
Orange
Yellow
Green
Blue . .

1 Hirschberg's Festschrift, 1905

2 Ann. d'ocul. LaXIX. 1878.

Absolute

Chromatic

Ratio

Threshold

05 mm.

1 mm.

0-9 „

21 =,

5-5

3-1 „

9-6

0-3 „

4-2 „

196

0-3 „

7-5 „

625

Feilchenfeld and Loeser, Arch. f. Ophth. lx. 97, 1905.
3 La Lumiere ct les Couhurs, pp. 213, 238.

THE LOCAL QUANTITATIVE EFFECT 121

Conversely, diminution of the area stimulated causes the colour to
lose in apparent intensity — red less than green, green less than blue.
The law associating area stimulated and intensity is therefore not so
simple as that for white light.

Abney^ in his experiments on the extinction of colour and light,
i.e., on the point in the diminution of the intensity of light which just
causes, first the colour, and then the light to become invisible, made a
series of investigations on the influence of the area stimulated. He
found, as was to be expected from the results of previous observers,
that the smaller the disc the less reduction in intensity of the ray was required
to extinguish it and the same ratio existed between the extinction of the
different colours. Plotting curves with aperture diameters in powers
of 2 as abscissae and logarithms of light intensities as ordinates, with
apertures less than 1| inches diameter the curves become straight lines,
all of which are parallel. Hence " from that point the intensity of a
light which will be just extinguished with a certain diameter of aperture
may be increased 10 times and yet be invisible when an aperture with
one quarter of that diameter is employed ; if the intensity of the light
be increased 100 times, we have only to diminish the diameter of the
aperture to y'^ and it will again disappear, or if to -^}-^, the light may be
increased 1000 times." When the angidar aperture exceeds 4° apparently
the upper limit is reached, all extinctions being the same beyond it.

With regard to the point of extinction, Abney^ says : " The light
from a square, or a disc, or an oblong, just before extinction, is a
fuzzy patch of grey, and appears finally to depart almost as a point.
This can scarcelv account for the smallest width of an illuminated
surface determining the intensity of the light just not visible ; but it
tells us that the light is still exercising some kind of stimulus on the
visual apparatus, even when all sensation of light is gone from the outer
portions. The fact that the disappearance of the image takes place
in the same manner whether viewed centrally or excentrically tells us
that this has nothing to do with the yellow spot, or fovea, but is probably
due to a radiation of sensation (if it may be so called) in every direction
on the retinal surface. Supposing some part of the stimulus impressed
on one retinal element did radiate in all directions over the surface of the
retina, the effect would be greatest in the immediate neighbourhood,
and would be inappreciable at a small distance, but the influence
exerted upon an adjacent element might depend not only on its distance,
but also upon whether it was or was not itself excited independently.
1 Abney, pp. 169, 174. 2 Ibid. p. 177.

122

COLOUR VISION

Following the matter out further we should eventually arrive at the
centre of an area as the part which was the recipient of the greatest
amount of the radiated stimuli, and consequently that would be the
last to disappear. With a slit aperture the slit is visible till extinction
is very nearly executed, but it finally merges into a fuzzy spot at the
moment before it finally fails to make any impression of light."

Fig. 40 shows that there is an angular aperture or size of retinal
area stimulated at which any ray will be extinguished both for colour
and light at the same time. The aperture is largest for red.

-a -J -4 -5 -6

Diameter of apertures xrv powers of z.

-7

Fig. 40. Extinction of colour and light curves with different areas of stimulation. SSN
44 = 548-l/iM; SSN 50-0 =i) line: I and II, different observers and different in-
tensities. (Abney.)

Abney has also compared the luminosity of the spectral light from
apertures of different size. Fig. 41 shows the curves for scale numbers

THE LOCAL QUANTITATIVE EFFECT

123

27"3 and 50'6 (yellow D line, 589 ju/x), the logarithms of the annulus
values of the two apertures being plotted as abscissae and ordinates.
For points on the curve the luminosity of the large aperture is equal
to that of the small, equality being obtained by means of the annulus.
If the luminosity of the spots of light were always equal, irrespective
of size, the inclination of the curves would be 45°.

'•J

•J

•

+ /

0 I a

Log readings with large aperture

Fig. 41.

Equality of brightness curves for different sized apertures.
SS:S 27-3 =575 M/" ; SS'S 50-6 =Z> line. (Abney.)

The relation of area stimulated to intensity for the jperi'pliery of the
retina has been studied by Piper^, Loeser^, Henius^, and Fujita*.
Piper gives the following table for the dark-adapted periphery :

Area

100

x^Area

or Angular

size

315

Relative

Product of angular

Threshold

stimulation

size

and threshold

value

10-0

100

2-94

3-4

9-3

1-96

5-1

9-8

102

9-8

10-2

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxn. 98, 1904.

2 Hirschberg's Festschrift, 1905 ; Feilchenfeld and Loeser, Arch.f. Ophth. hx. 97, 1905.

Ztsch. f. Sinnesphysinl. xLiii. 99. 1008.

* Ibid. XLHi. 243. 1908.

124 COLOUR VISION

There is therefore obviously not the same relationship between area
and threshold intensity that appertains to the fovea. There is, how-
ever, a relationship between the square root of the area (which is
equivalent to the visual angle subtended by circular objects) and the
threshold intensity. Piper states the law thus : For the dark-adapted
periphery the stimulation value of a luminous surface is proportional
to the square root of the area of the retinal image ; or in other words,
the product of the threshold value and the square root of the area of
the retinal image is constant. Henius and Fujita showed that the law
is valid only for mixed white light and objects subtending a visual
angle up to 10°. Above 10° the sensitiveness of the retina increases
much more slowly than the visual angle increases, and still more slowly
for red light. Fujita showed too that the law is not valid below 1°.

For the light-adapted periphery Fujita showed that there was
no simple relationship between area and threshold intensity. For small
objects, however, subtending less than 2° the threshold value diminishes
with the visual angle, but not so quickly as for the fovea.

Owing therefore to the great difference between the photopic and
scotopic periphery threshold values, especially for relatively large
luminous areas, the condition of adaptation is of great importance,
especially as it itself changes with the actual stimulation.

The relationship of the region of the retina and the size of the area
stimulated respectively to the sensations derived from colourless
mixtures and equations have been studied by v. Helmholtz^, v. Frey
and V. Kries^, and Hering^. Hering observed colourless mixtures of
complementary monochromatic lights of a given area and studied the
effects of altering the size or shifting the fixation point. A mixture of
red and blue-green became redder and darker on diminution, greener
and brighter on augmentation of the area. The foveal colourless
mixture became greener and brighter on direct fixation ; the peripheral
colourless mixture became redder and darker on central fixation. A
colourless mixture of yellow-green and violet became yellow-green on
diminishing the area, rose-red on increasing the area or observation
by indirect fixation, but the effects were much less than in the first case.

Hering found that colourless equations with spectral lights become
invalid, both with light and dark adaptation, if the area is increased or
fixation changed from direct to indirect (contrast p. 69). So too in a
match between a mixture of spectral red and blue-green (A) and yellow

1 1st ed. p. 301. 2 jirch. f. Physiol. 330, 1881.

3 Arch. f. d. ges. Physiol. Liv. 277, 1893.

THE LOCAL QUANTITATIVE EFFECT 125

and blue (B), A becomes green and brighter, B lilac and darker.
These changes are probably due to macular absorption. Tschermak^,
for example, was unable to distinguish definite variations in parafoveal
matches on increasing the eccentricity of fixation, thus confirming
earlier results by Hess and v. Kries. Under such conditions, however,
he found slight changes on increasing the size of the area stimulated,
and these were of the same kind as those found by Hering.

The effect of the size of the area stimulated on the photochromatic
interval at the periphery and on the size of the field of vision has already
been discussed (p. 79).

CHAPTER II

SIMULTANEOUS CONTRAST OR SPATIAL INDUCTION

We have seen (Section V, Chap, ii) that stimulation of a retinal area is
followed at a certain interval by sensations, potential or kinetic, of an
opposite nature, a phenomenon known as " succedaneous " or " succes-
sive contrast." There are numerous facts which tend to show that the
sensation resulting from stimulation of a retinal area is modified by
the condition of the surrounding areas, and per contra that stimulation
of a retinal area itself causes a change in the condition of excitability
of the surrounding area. This reciprocal interaction of retinal areas,
or, in terms less open to criticism, of contiguous or nearly contiguous
areas in the field of visual sensations is called " simultaneous contrast."
The term contrast (Chevreul) in each case indicates opponent activities.
Less open to objection is the term " induction " (Briicke), though this
term has unfortunately been used in a more restricted sense by Hering.
He uses " simultaneous induction " to connote change during fixation,
" successive induction," change occurring after removal of stimulation.
We shall use " induction " in the broadest sense, temporal induction
being any change in a visual area due directly to stimulation (and there-
fore including adaptation), and spatial induction being any change in
other areas resulting from the primary stimulation. To avoid circum-
locution we shall speak of " retinal " areas, without prejudice as to the
exact site at which the change manifests itself.

Spatial induction manifests itself by changes both in luminosity
(or brightness) and hue. A patch of grey paper on a white background

1 Arch. f. d. ges. Physiol. Lxxxn. 559, 1900.

126 COLOUR VISION

looks darker than a patch of the same paper on a black background
(Goethe). Innumerable experiments have been devised to demonstrate
luminosity (or brightness) contrast. Especially instructive are those
with Hering's double-room or double-screen method, in which an aperture
in a screen is variously illuminated from behind, whilst the illumination
on the front of the screen can also be varied independently at will.

It has been recognised since the time of Leonardo da Vinci that
coloured surfaces are altered in hue by their surroundings, and vice versa.
Briicke called a coloured surface surrounded by white the inducing
colour. It must, however, be well borne in mind that the phenomenon
is reciprocal, no matter what the luminosity or hue of the two fields
may be. Tschermak^, who has written an excellent resume of our
knowledge of contrast, enumerates five methods of demonstrating colour
contrast.

(1) The background method was early studied by Chevreul^. In
its simplest form white, grey, black, or coloured patches or strips are
laid on coloured backgrounds. The contrast is accentuated by placing
tissue-paper over both^. Maxwell's top can also be used (Dove,
V. Helmholtz, H. Meyer, Hering^, and others), or a projection apparatus
(Rollet).

(2) The mirror method, used by Goethe, is familiar in Ragona
Scina's well-known experiment, which has been modified and improved
by Hering''^,

(3) The method of coloured shadows dates from Goethe, and has
been much used, notably in recent years by Hering*" and Abney for the
detection of colour-blindness.

(4) The double-image method was introduced by v. Helmholtz'^
and modified by Hering^.

(5) Negative after-images have also been used by Hering^.

Chevreul, Fechner, and Wundt^° showed that actual contiguity of the
reacting surfaces was not essential. Aubert^^ showed that weak and
localised stimulation altered the excitability of the whole retina. The

1 Ergebnisse d. Physiol, ii. 2, 726-798, 1903.

^ De la Loi du Contraste simultane des Couleurs, Strassburg, 1839 ; new ed. Pa.ris, 1890.
3 Joh. Miiller, Handb. d. Physiol, n. 1837 ; H. Meyer, Ann. d. Physik. xcv. 170, 1855.
« Arch. J. d. rjes. Phy.^iol. xli. 1, 1887. * Ibid. xli. 358, 1887.

« Ibid. XLii. 119, 1888. ' 1st ed. p. 406; 2nd ed. p. 559.

* Arch.f. d. ges. Physiol, xlvii. 236, 1890 ^ Sitz. d. Wiener Akad. 1872-4.

" Philos. Stud. IV. 112, 1887. " Physiol, d. Netzhaut, 1865.

SIMULTANEOUS CONTRAST OR SPATIAL INDUCTION 127

extent of the background, as well as its luminosity and colour, modifies
the luminosity and saturation of the contrast colour.

The contrast effect is not uniform over the whole surface, but is
most marked at the edges. A distinction is therefore sometimes made
between " surface-contrast " {Fldchenkontrast) and "border-contrast"
{Randkontrast). The disparity diminishes on prolonged fixation (" simul-
taneous induction," Hering). Border contrast is easily demonstrated
with black and white, or coloured sectors on a rotating disc (Mach^,
Aubert, v. Helmholtz), or by the shadow method (Goethe, A. Fick^).
It is visible by momentary stimulation and is reversed in the negative
after-image (Mach) : in the latter case it may be more obvious than in
the primary image. With colour-contrast the saturation is greater at
the borders than in the middle.

Contrast effects are diminished by separation of the contrasting
fields, and are therefore seen better with small than large fields. A
narrow black line between the fields diminishes contrast markedly
(v. Helmholtz), and the effects are greatest at contiguous contours
(Mach).

Jurin (1783) and Brandes (1827) attempted to explain simultaneous
contrast by successive contrast, due to slight movements of the eyes.
Though this factor is a frequent complication and requires special heed
to its elimination it is not the cause (Fechner^, Hering). With the
greatest care in controlling the movements of the eyes and keeping
accurate fixation, simultaneous contrast manifests itself immediately.
Moreover it occurs with colours of such weak saturation that no
coloured after-image is induced. Aubert, Mach, Meyer'* and Rollet^
have shown that it occurs on momentary stimulation.

Attempts have been made to obtain quantitative measurements of
simultaneous contrast by means of a comparison field by Lehmann^,
Ebbinghaus'^, Hess and Pretoria and others.

For black and white or pure luminosity (brightness) contrast
Ebbinghaus found that the increase in brightness of a patch on a darker
background is proportional to the difference of the two light intensities,
irrespective of their absolute values. Hess and Pretori found that the
apparent luminosity of a small bright surface on a dark background

^ Sitz. d. Wiener Aknd. Lii. 2, 303, 1865.

2 Hermann's Handh. d. Physiol, iii. 1, 3, 1879.

3 Ann. d. Physik xliv. 513, 1838 ; L. 193, 427, 1840.

^ Amer. J. of Sc. XLVi. 1, 1893. ^ Sitz. d. Wiener Akad. lv. 424, 1867.

« Wundt's Philos. Stud. iii. 497, 1886.

' Sitz. d. Aknd. d. Wiss. Berlin, 994, 1887. » Arch. f. Ophth. xl. 4, 1, 1894.

128 COLOUR VISION

remains constant when the increase of illumination of the surface
bears a constant ratio to the increase of illumination of the background.
The contrast effect therefore varies directly with the inducing stimulus
and is independent of its absolute value. Ebbinghaus and Hess and
Pretori agree therefore as to increase of brightness by contrast or
contrast-brightening. Hess and Pretori find that the law also applies
to contrast-darkening, therein differing from Ebbinghaus, but the results
of the latter are probably due to error in technique. Kirschmann^
found that the amount of contrast varies as the square root of the area
of the inducing field, i.e., the relationship between the area and the
intensity of the inducing light is reciprocal.

For colour-contrast Fechner showed that the brightness of a grey
background must be appropriately chosen. Under optimum conditions
an inducing colour of very slight saturation suffices to induce marked
contrast. The statement, however, that simultaneous contrast is most
evident with slight differences of colour between the reacting fields
(v. Helmholtz) is not true (Fechner, Rollet). Hering^ showed that
with optimum black-white background conditions contrast increases
with the saturation of the coloured inducing field, an observation
confirmed by Pretori and Sachs^. Kirschmann (1892) had previously
arrived at the same result, but found that the increase was not
proportional, but probably a logarithmic function. Colour-contrast,
as might be expected, is accompanied by change in luminosity, and
this subject has been exhaustively studied by Pretori and Sachs under ,
Bering's guidance. Their results will be considered in Part III.

The most important element in colour-contrast from our present
point of view is the opponent or complementary effect, early noticed
by Briicke*. Under ordinary circumstances the induced colour is not
the precise complementary of the inducing, as was known to Goethe,
and has been fully investigated by Hering^. Various physical factors
conduce to the effect, such as the variable nature of "white" light
and " grey " surfaces, macular pigmentation (Hering, Sachs), the pig-
ment of the lens (Hering), the brown pigment of the retinal epithelium
(Tschermak), and the reddening by the blood of the light which passes
through the sclerotic (Hering, Hess). These do not suffice to explain
all the facts. There are other, physiological, factors, such as the

1 Wundt's Philos. Stud. vi. 417, 1890 ; vn. 362, 1891 ; Amer. J. of Psychol, iv. 4, 74,
542, 1892.

2 Arch.f. d. ges. Physiol. XLi. 1, 1887 ; XLii. 117, 1888.

3 Ibid. LX. 71, 1895. * Ann. d. Physik, lxxxiv. 418, 1851,
* Zur Lehre vom Lichtsinne, 1876.

SIMULTANEOUS CONTRAST OR SPATIAL INDUCTION 129

previous stimulation of the eye (Hering, Hess^, Tscherniak) and fatigue
(Tschermak and Krause). The discrepancy between the induced and
the complementary colour is said to be absent when the eye is dark-
adapted (Mayer, Kuhnt^). There are also psychological factors, which
form at any rate the most probable explanation of simultaneous con-
trast over colour-scotomatous areas (Tschermak^) and over the blind
spot. The entoptic visibility of the blind spot* is itself evidence of
contrast, as are also the effects of stimulation of the optic nerve by
the constant current (G. E. Miiller^).

Simultaneous contrast is under ordinary conditions limited to the
eye stimulated (Hering^), but binocular contrast can also be proved to
occur'^.

1 Arch.f. Opkth. xxxv. 4, 1, 1889 ; xxxvi. 1, 1, 1890.

2 Loc. cit. xxvn. 3, 1, 1881.

3 Arch.f. d. gca. Physiol, i.xxxii. 559, 1900.

* Brewster; Puikinje; Aubert ; v. Helmholtz; Cliarjjentier, Compt. rend, cxxvi. 1()34,
1898.
— ^ Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xiv. 329, 1897.

« Loc. cit. I. 18, 1890.

' Fechiier ; H. Meyer ; v. Helmholtz ; Hering, in Hermann's Hundb. d. Physiol, in.
i, (JOO, 1879 ; Ebbinghaus, Arch.f. d. ges. Physiol. XLVi. 498, 1894 ; Chauveau, Compt. rend.
cxiii. 1891 ; I). Axcnfeld, Arch. ital. de Biol. xii. 28, 1889 ; xxvii. 103, 1897 ; Burch, Jl. of
Physiol, xxv. 1900.

P. C. V.

SECTION VII

THE EVOLUTION OF COLOUR VISION
CHAPTER I

INTRODUCTION

A priori we should expect some light to be thrown upon the fully
developed colour sense of man by a knowledge of the stages through
which that colour sense has evolved. The sources of our information
on the evolution of colour vision are few and the methods of in-
vestigation difficult and arduous. Only recently have the researches
been carried out in a scientific manner and yielded valuable results.

Positive evidence is derived from three chief sources. In the first
place we naturally appeal to the visual sensations of lower animals.
These are extremely difficult to investigate since we are almost wholly
dependent upon observation of motor responses which the animals
make to various light-stimuli, though some deductions can be made
from the structure of the visual organs. In the invertebrata little can
be done beyond recording the phototropism of the animal, i.e., its
attraction or repulsion by lights of different wave-length and intensity,
as exhibited by its movements towards (positive phototropism) or away
from (negative phototropism) the light^. As we ascend the animal
scale the increase in complexity of the nervous system and of the visual
organs is accompanied by a corresponding increase in complexity of
the motor responses, associated with a greater difficulty in their inter-
pretation. On the other hand, as we descend the animal scale from
man there is an unwarranted tendency to interpret the apparently
purposeful responses of the animal in an anthropomorphic manner
which is not necessarily justified on neurological and psychological
grounds. For example, we have little knowledge of the psychology

^ Mast, Light and the Behavior of Organisms, New York, 1!)11.

INTRODUCTION 131

of the lower mammal, with its less highly developed nervous system.
The temptation to interpret such an animal's actions in terms appropriate
only to the human mind has proved very great and has undoubtedly
given rise to error in the past. Our deductions must of necessity be
anthropomorphic, since such terms as visual sensation, attraction, repul-
sion, pleasure, pain, and so on, have no meaning for us except in so
far as these processes form a part of the contents of our own minds^.
Yet it should be a guarded anthropomorphism, neither exaggerating
the psychological elements nor flying to the impossible antithesis of
imagining that the anthropomorphism can be eliminated by a new
terminology.

In the second place a study of the colour vision of primitive races
may throw some light on the evolution of visual sensations. It may be
that some primitive races are in a condition of arrested development —
of vision, as of other faculties. We have only just crossed the threshold
of this part of the investigation and it is to be hoped that no time will
be lost in carrying it forward, lest the material for the research be
obliterated by the march of civilisation.

The third source of information is the development of visual sensa-
tions in the infant. It is generally admitted that " ontogeny is a
compressed phylogeny," — that each individual passes rapidly through
the same stages of development which have marked the upgrowth of
the race. Here again, little progress has been made, and the investiga-
tion is arduous and full of pitfalls.

Besides these main sources there are others of less security. We
are familiar with congenital defects of vision, and it may be that some
of them are atavistic, that development has become arrested at a stage
which corresponds with an earlier stage in the development of the race.
Some arguments too may be derived from the careful study of normal
colour vision, but the evidence derived from both these sources is too
uncertain to be of much value.

CHAPTER II

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION

I shall briefly review the comparative psychology of vision in verte-
brates only, laying particular stress upon points of theoretical interest.

I Cf. Washburn, The Animal Mind, New York, li)OS.

9—2

132 COLOUR VISION

The most recent and most exhaustive experiments have been made
by Hess^.

Mammals. Graber^ experimented on nine dogs by his " preference
method." A j^oodle and a fox terrier showed definite preferences.
When black and white were presented white was chosen 56 times,
black 4 : when light red and blue, blue 53 times, red 7 ; when the same
and a much darker blue, blue 32 times, red 28. The results may have
been due to preference for the brighter colour. Lubbock^ experimented
with a poodle. Three pairs of cards coloured blue, yellow, and orange,
were used. One card from each pair was placed on the floor and the
remaining card of one of the pairs shown to the dog in the hope that
she would learn to select from the three cards before her the one that
was of the same colour as that held up. Training for ten weeks proved
entirely without success. Lubbock himself remarks that negative
results prove nothing as to the colour vision of the animal. Himstedt
and Nagel* taught a poodle to fetch coloured balls. On being ordered
to fetch " red " it brought first scarlet or bright red, then orange, but
only the latter when there were no more conspicuous red balls left.
Balls coloured with Bismarck brown were a cause of difficulty to it.
" Blue and grey were obviously quite different to it from red, even in
all degrees of brightness of each."

Elmer Gates^ experimented with dogs and food receptacles of
various colours, including grey ; only receptacles of one particular
colour actually contained food. In some experiments coloured metal
plates were laid on a passage, certain colours being connected with a
battery, so that if the dog stepped upon them it received a shock.
Gates concluded from his researches that dogs could distinguish not
only colours but relatively fine difl^erences of hue. The description is
so meagre that it is impossible to determine whether due precautions
were observed.

Samojloff and Pheophilaktowa*" tested whether a dog could be taught
by much training to distinguish between different colours as well as
between different brightnesses. Three boxes were placed in a row ;

1 Vcrglcichende Physiolofjie des Gcsichisinnes, Jena, 1912 (Bibliography) : " Birds " —
Arch. /. Augenhlk. lvii. 4, 298, .317, 1907; Lix. 2, 142, 1908 : " Fishes "—Zoc. cit. lxiv.
Erganzungsheft, 1, 1909; Arch. f. d. ges. Physiol, cxxxiv. 1910; cxlii. 1911; Zool.
Jahrb. 1912; " Eeptiles and Amphibia" — Arch. f. d. ges. Physiol, cxxxii. 255, 1910.

- Grv.ndlinien zur Erforschung des Helligkeits- u. Farhciisiiines der Thiers, Prag, 1884.

' On the Senses, Instincts, and Intelligence of A)nmah, p. 280, Londoji, 1888.

* Festschrift d. Univ. Freiburg, 1902. = The Moni.sf, p. 574, 1895.

8 Centralhl. f. Physiol, xxi. 133, 1907.

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 133

each had a disc in front, 25 cm. in diameter, of which two lield arev
papers and one a saturated green. The " green " box contained a
biscuit. After training it was found that out of fifty papers, ranging
from black to white there was no grey which, when presented to the
subject for discrimination from the standard green, yielded a percentage
of errors greater than 3L Later tests showed that such discrimination
only occurred under certain conditions. If the grey was presented
as a square and the green as a circle, green was chosen : but if the green
was a triangle or square, the grey square was invariably chosen. Hence
the discrimination of a colour from a series of greys is only possible
when the conditions have been thoroughly learnt.

Nicolai^, did not succeed in teaching his two dogs to distinguish
between red and green bowls. Colvin and Burford^ experimented in
the same manner with three dogs, a cat, and a squirrel, food being
placed in red receptacles amongst similar empty receptacles of other
colours. The colour was distinguished in 87 '3 per cent, of trials, the
squirrel responding best. Kinnaman^ attempted in a series of similar
feeding experiments to eliminate the ambiguity of discrimination of
brightness and colour-differences. Monkeys were tested with glass
tumblers covered wdth papers of different colours, and when it had been
shown that they were able to identify a vessel of a particular colour
as associated with food, the power of distinguishing the colour from
greys of the same brightness was tested. Kinnaman came to the
conclusion that the capacity to distinguish colours as such in monkeys
was undoubted.

J. B. Watson^ criticising Kinnaman's results, says that the use of
coloured papers can never give a satisfactory test of colour vision in
animals. He himself used a spectrometer apparatus which illuminated
a screen w^ith two monochromatic red and green patches. Food boxes
w^ere placed beneath the patches, one being empty, the other containing
a grape. If the red box were opened the grape was obtained ; if the
green, the monkey was pulled back. In early tests with red and green
the animals failed to react to red. Blue-vellow discrimination arose
more rapidly than red-green in all cases (three monkeys). The experi-
ments were early vitiated by the onset of position habits.

Yerkes^ made very exhaustive experiments on the Japanese dancing

1 .//./. Psychol, u. Neurol, x. 1907.

- Psychol. Rev., Psychol. Monographs, xi. 1. 1909.

■■' Amer. J. of Psychol, xiii. 98, 173, 1902.

* ./. ofComp. Xeur. and Psychol, xix. 1. 1909.

5 y/,p Dancing Mouse, New York. 1902.

134 COLOUR VISION

mouse. The method consisted in teaching the animals to associate one
of two differently illuminated compartments with a disagreeable electric
shock. Light blue and orange, green and red, violet and red, were
distinguished even when their luminosities were considerably varied.
The possibility that these discriminations were made from brightness
rather than colour was not wholly eliminated. Yerkes concluded, how-
ever, that the mice have a certain degree of ability to distinguish red,
green, and violet as colours. No ability to discriminate green and blue
was shown unless there was a great difference in brightness. There
is evidence that the red end of the spectrum is much darker to the
mouse's than to the human eye. Waugh^ on the other hand found that
his mice could only distinguish red from white and light greys with great
difficulty.

Davis and Cole^ experimented on racoons by the feeding method
and found that while discrimination of black from white, yellow, red,
blue or green appeared easy, that of blue from yellow and red from green
was difficult. Davis thought it probable that the animals were colour-
blind, but Hess does not agree with this conclusion.

Washburn and Abbott^ made attempts to discover the brightness
value of red for the light-adapted eye of the rabbit. They arrived at
the conclusion that this animal can discriminate between a saturated red
and a grey paper, and that the discrimination is based upon luminosity
rather than colour. Red has a low stimulus value for the rabbit. The
experiments furnish no evidence that it sees red as a colour, but do not
prove that it does not do so.

An elaborate series of researches has been carried out in Pawlow's
laboratory^ with a view to establishing " conditional reflexes." The
salivary secretion was used as the indicator. An indifferent stimulus,
e.g., a strong electrical stimulus to the skin, is at first applied simul-
taneously with the placing of food or acid in the mouth. After a short
time the salivary secretion is called forth by the application of the
indifferent stimulus alone. Orbeli^ studied the effects of coloured
optical stimuli. After exposure of a red square on a white screen
reflex salivary secretion was obtained. The same recurred when other
colours were exhibited. If red and grey squares were exhibited suc-
cessively and the dog fed only after exhibition of the red square the

^ J. of Comp. Neur. and PsycJtoI. xv. 549, 1P05.

" Cole, he. cit. xvii. 211, 1907. ^ J. of Animal Behavior, ii. 145.

« Pawlow, Brit. Med. Jl. ii. 473, 1913.

^ Comptes rend dc la Soc. med. russe, St Petersburg, 1907 ; Arch, des Sc. hiol. xiv.
1908.

THE COMPARATIVE PSYCHOLOGY OF COLOUR VLSION 135

salivary reflex eventually followed the red stimulus only. It was found,
however, that the reflex in dogs was conditioned by the change in
intensity of the light, not by variations in the wave-length. The experi-
ments are not of a nature to demonstrate colour-blindness in the dogs.

Kalischer^ has published interesting experiments on dogs. His
animals were placed in a dark chamber which could be illuminated by
means of light from a lamp, transmitted through coloured glass. On
the appearance of red light the dog was given food, which was, however,
withheld during the period when light was excluded. When a perfect
reaction habit had been formed, the experiment was varied sometimes
by transmitting the light through blue, instead of through red glass,
though the subject was still only fed when a red light was exposed.
In order to render the comparison of brightness between the different
lights more difficult, their successive exposure was always followed
by a dark interval. At first the subjects reacted in a similar
manner to either coloured light, but after some further training the
difference was duly learnt, and eventually discrimination became so
perfect that no matter how the brightness of the red or the blue light
might be changed the reaction was almost invariably correct.

Observations were also made with green, yellow and red-violet lights,
but discrimination was found to be the most definite when red and blue
were used ; red and yellow proving very difficult to learn, and red with
red-violet almost impossible. Colvin and Burford likewise found that
when their subjects were required to discriminate between red and violet,
only a relatively low percentage of right choices was obtained.

In later experiments Kalischer left his subjects untested for two
or three days, and then presented a blue or a green light previous
to the exposure of the food-colour, red. Though these experiments
were repeated many times and under varied conditions, the animals
never failed to react correctly to the food-colour when it appeared.

It is of importance to note that not only did the animals learn to
discriminate, during the course of the experiments, between different
colours irrespective of the degree of their luminosity, but that in addition
the behaviour shown on the appearance of each colour was characteristic,
e.g., on the exposure of a blue light the subject would appear frightened
and quickly withdraw his head from the food ; whereas when a green
or a yellow light was exposed great hesitation was shown, the head was
allowed to fall gradually nearer and nearer towards the meat as though
to seize it, and was then suddenly jerked away.

1 Arch.f. Anat. p. 316, 1909.

136

COLOUR VISION

The conclusions which Kalischer draws from his experiments are as
follow :

(a) That there is no doubt of the ability of dogs to perceive
differences of hue as well as differences of brightness.

{b) That whereas such colour discrimination is cortical, brightness
discrimination is to some extent sub-cortical, reactions to changes of
illumination being found to occur even after extirpation of the visual
cortex.

By far the most exhaustive experiments on dogs have been carried
out bv Miss E. M. Smith^. The apparatus used is shown in Figs. 42 and

o
o

r-t

home-box

h k\

trial-box

o
o

^ ■window-door ^

Fig. 42. Ground plan of experimental dog box. (E. M. Smith.)

43. It will be noticed that the " home box " is separated from the
" trial box " by a partition in which there are five glass screens Sj.^.
Cards of various colours could be introduced into these screens. The
dog was placed in the home box and let into the trial box through Sj,
which was then closed. The preliminary " general training " con-
sisted first in teaching the dog to pass through the screens. These

1 Brit. Jl. of Psychol, v. 119, 1912.

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 137

were left transparent and a biscuit was placed within view in the home
box. Next, cards of medium grey were placed in S,., S:i, S4, and S5.
Finally, only one shutter contained a grey card. The dogs were thus
taught always to go to the opaque screen in order to get through into
the home box and receive the reward.

The various test series were then commenced. (1) "Colour prefer-
ence series." Maximum blue, red, yellow and green cards were put in
four screens and the frequency with which each was selected was noted
in 70 tests. For one dog the numbers were red 32, blue 26, yellow 8,
green 4. The red and blue were much darker to the human eye than
the yellow and green. (2) " Approximate brightness value series."

f- ^■^....■v -J^.'. ^ 'V — ■■■■"■■;^-^'

fl t f I //

?■* ^ i I //

i s L I I * /

ii . . . ■.-^. $t . . . .1. .....t^ > .. 4,/ ...

Fig. 43. Elevation of experimental dog box. (E. M. Smith.)

Red, blue, yellow, green, and 14 shades of grey were used. First, the
particular colour was presented simultaneously with the grey which was
most remote from it in brightness. When the animal had given proof
of its ability to discriminate between these two the next grey, differing
only slightly less in brightness, was substituted. Second, all the greys,
in the same order as before, were presented on the same day for dis-
crimination from the standard colour, each grey being exposed 5 times
in succession. Next, the colour and the greys were presented in
irregular order. It was found that in the case of the most intelligent
dog the average percentage of right reactions was much higher for green
and yellow than for blue and red : further the first colour tested, blue,

138 COLOUR VISION

had the smallest, and the last, green, the largest percentage of right
cases. To test if this were due to general improvement from increased
practice rather than specific qualitative differences the " repeat bright-
ness value series " was given. In this series all four colours were re-
tested with additional precautions. (3) " Hue discrimination series."
(a) Coloured cards and equivalent greys were exhibited in pairs : and
(6) two coloured cards were shown simultaneously, such that (o) the
pairs were equal in brightness and saturation ; {/3) the standard colour
was slightly or obviously darker than the other ; (7) the standard colour
was slightly or obviously lighter than the other. (4) " Grey difference
threshold series." (5) " Colour threshold series."

Enough has been described to show that the tests were exhaustive,
and elaborate efforts were made to determine the nature of the dis-
crimination evinced. Perhaps the most serious objection which can be
brought against the experiments is their extreme elaboration. Anyone
who has had experience of testing the colour vision of untrained
human observers knows the frequency with which luminosity and colour
matches are confused. How much more this source of error is likely
to be present in dogs needs only to be mentioned. It was allowed for
to a certain extent by a graduated method of marking. As Miss Smith
says, " In view of their relative difficulty, sensory discrimination tests
on the higher vertebrates might aptly be compared to complicated
intellectual tests on man, in which the subject, unable to grasp completely
the point at issue, has to rely on a series of sporadic gleams."

Only a summary of the chief results can be given here, the reader
being referred for details to the admirably thought out original paper.
In favour of the view that dogs are only able to perceive differences of
brightness are (1) the result of the " colour preference series " ; (2) the
greater the difference in brightness between a standard colour and grey,
the more definite is the discrimination ; (3) the length of time required
to establish a successful discrimination habit between blue and red ;

(4) the fact that whereas change of colour frequently appeared to pass
unnoticed, a marked change of brightness often caused confusion ;

(5) the fact that green and yellow were not at all, or only with the
greatest difficulty, discriminated from white ; (6) the fact that during a
prolonged series increased confusion and uncertainty in discrimination
were shown from day to day.

In favour of the view that dogs possess rudimentary colour dis-
crimination are (1) the decreased time for training required by each
new colour in the " approximate brightness value series " ; (2) the unique

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 139

physiological effects produced by the difEerent colours even when of
approximately the same brightness (greater affective value of red
compared with blue, etc.) ; (3) the evidence afforded by the " approxi-
mate brightness value series " that on the whole no grey was found that
was indistinguishable from the standard colours employed, training
being still imperfect ; (4) the difficulty and confusion of the " grey
difference threshold series " as compared with the parallel " approximate
brightness value series " ; (5) the results obtained from the " hue
discrimination series " ; (6) the difference in the approximate thresholds
of the four colours and grey as determined by the " colour threshold

series."

Whereas the facts recorded in the first group of items of evidence
are not incompatible with the assumption that dogs possess some sort
of colour vision, those in the second group are quite irreconcilable with
the view that the subjects are limited to brightness vision.

Miss Smith draws the following conclusions : (1) That while evidence
has been obtained to show that some dogs possess a rudimentary power
of colour discrimination, such discrimination is highly unstable and
cannot be supposed to play any part in the animal's normal existence.

(2) That the colour sense is, as shown by the high colour threshold,
very weak.

(3) That colour discrimination, even where clearly established, may
be readily inhibited by differences of luminosity or position.

Piperi investigated the electrical reactions of dogs, cats and rabbits.
In dogs and cats he found the reactions in both light- and dark-adapted
eyes to correspond with the achromatic scotopic values for man. In
rabbits the strongest photopic value was 570 /x/z, scotopic 540 ixfi.

Hess tested a monkey in the following mannner. Grain was scattered
over a black surface, upon part of which a spectrum was thrown. The
monkey gathered all the grains from the extreme red to the extreme
violet, leaving those unilluminated. When the animal was dark-adapted
and the intensity of the light diminished until the grains were visible
to the dark-adapted experimenter only in the yellow and green, only
these grains were gathered. Hess concluded that the extent of the
visible spectrum is the same for the monkey as for man, and that it is
brightest for the dark-adapted monkey in the region where it is also
brightest for the dark-adapted man.

Hess also studied the effects of different spectral lights and the lights
transmitted by coloured glasses on the pupil reactions of cats and rabbits.

1 Arch. f. A)iat., Suppl. ]fl0.5.

140 COLOUR VISION

It was shown by Sachs^ that in man the extent of the pupil reaction
varies with the subjective luminosity of the light. Similarly AbelsdorfE^
showed that the maximal pupillomotor effect for the light-adapted eye
was at 600 /x/x, for the dark-adapted eye at 540 [xjjl. Hess found the
photopic and scotopic values for cats and rabbits about the same as for
man.

It appears therefore that mammals have the same spectral limits
as man ; that they have a similar capacitv for dark adaptation ; and
that their luminosity curves are similar.

Birds. Lloyd Morgan^ experimented with chickens. A chicken
which picked out bits of yolk of hard-boiled egg from the white was given
pieces of orange peel, which it found distasteful. Afterwards for some
time it was suspicious of yolk. On the other hand after having learned
to avoid bad-tasting black and yellow caterpillars, it did not object to
wasps, but probably points of difference other than colour were apparent
to it.

J, P. Porter* investigated the colour vision of the English sparrow and
other birds by feeding experiments. A number of glasses of like size
and shape were covered inside and out with coloured papers, including
dark and light grey. The glasses were placed in a row on a board, and
food was always put in the same coloured glass, the position of which
was changed. The sparrow and cowbird learned to pick out the right
vessel. Rouse^ investigated pigeons by Graber's method of allowing
a choice between compartments illuminated by different coloured glasses.
The pigeons showed a preference for green and blue. A greater rapidity
of breathing was recorded under green and blue lights than under
others.

Hess experimented with birds of diurnal habits (chickens, pigeons)
and nocturnal habits (owls) in the same manner as for mammals.

For diurnal birds corn or rice grains were scattered over a matt
black surface, part of which was illuminated by a spectrum. They
usually pecked at the grains in the orange and red first, then those in
the green and yellow, and some in the blue-green. Those in the green-
blue, blue and violet were left untouched. Dark-adapted chickens

1 Arch.f. d. ges. Physiol, lii. 79, 1892 ; Arch.f. Ophth. xxxix. 3, 1893 ; Ztsch.f. Psychol
u. Physiol, d. Sinnesorg. xxn. 388, 1900.

2 Zisrh. f. Psychol, u. Physiol, d. Sinnesorg. xxi. 451. 1900; Arch. f. Aiigenhlk. XLi.
155, 1900; Arch.f. Anal. u. Phy.<^iol. .561, 1900.

3 Habit and Instinct, Tjondon, 1896.

■* Amer. J. of P.^ychol. xv. 313, 1904; xvii. 248. 190G.
* Harvard Psych. Studies., ii. 580, 1906

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION lil

generally pecked somewhat farther towards the blue, but seldom in
the blue and violet. When the spectrum was reduced until it was almost
colourless for the dark-adapted human eye they pecked chiefly or solely
in a region corresponding to the yellow and orange-yellow, i.e., rather
red-wards of the human scotopic maximum luminosity. Chickens
48 hours after hatching behaved in the same way as older birds. Hawks
{Falco tinnunculus), fed with pieces of meat, showed the same shortening
of the violet end of the spectrum. All the birds first pecked in the region
which is brightest for man, and then sought regions of less luminosity.
If half the field were illuminated with light transmitted through a red
glass and the other half with blue light, light- or partially dark-adapted
birds pecked first in the red half, turning to the blue half only when
the red was cleared of grains. By suitable alterations of the strengths
of the lights, the blue half was left untouched, even though the red grains
appeared to the human eye very dark red and scarcely visible whilst
the blue were clearly seen. Blue grains, however, were as eagerly pecked
as red if the light were made sufficiently strong. Dark adaptation
increases the sensitiveness of birds' vision very markedly. The " peck-
ing-limit " is for them only slightly, if at all, higher than the extrafoveal
threshold visibility of the grains for a 1 — 2 hours' dark-adapted human
eye. Adaptation appears to be slower in hawks and buzzards than in
the human eye.

Nocturnal birds were investigated in the same manner. If an owl,
which has perched for several hours in sunlight, is examined with a
moderately bright spectrum illuminating white pieces of meat, these
are snapped up first in the red, yellow, green, or blue-green. The
spectrum extends further violet-wards for nocturnal than for diurnal
birds, but is still much shortened at this end. When thoroughly dark-
adapted owls snap at meat in the blue and violet; they snap first in that
part of the achromatic spectrum which is brightest for man, or slightly
farther towards the violet. Dark adaptation causes a rapid rise of
sensibility during the first half-hour and the maximum sensibility
appears to be much greater than for man.

The pupil reactions of birds are somewhat complicated. The
maximum constriction for diurnal birds (fowl and pigeon) is in the region
of the orange and yellow, diminishing gradually towards both ends of
the spectrum, more rapidly towards the violet the less the dark adapta-
tion. For the dark-adapted eyes of nocturnal birds the maximum is
in the yellow-green and green, falling slowly towards the violet end.

It appears therefore that diurnal birds see the spectrum shortened

142 COLOUR VISION

towards the violet end, much as it would be seen by the human eye
through a reddish yellow glass. The shortening is similar but rather
less for nocturnal birds. Both types possess a power of dark adaptation
the same as or very similar to that of man.

Reptiles. Hess investigated several varieties of tortoise, and found
that their vision nearly resembles that of birds. They have the same
shortening of the spectrum towards the violet end. More remarkable
is the fact that they possess a high degree of dark adaptation, the
increase in sensibility being seemingly the same as that of the human eye
looking through a suitable orange-tinged glass. It may be mentioned
here that these animals are generally admitted to possess no rods or
visual purple in their retinae.

Hess noticed that the only animals which he examined and found
to possess a shortened spectrum towards the violet end were such as
possess coloured oil globules in their cones. At the same time he points
out the difficulties in accepting this fact as a complete and satisfactory
explanation.

Amphibia. Yerkes^ caused a green frog to go through a simple
labyrinth leading to a tank of water. At the point where the first
choice between two paths occurred a red card was placed on one side
and a white on the other. When the frog had learned to take the
correct path the cards were exchanged and the confusion of the animals
showed that they discriminated between the cards, though it may have
been a luminosity- rather than a colour-discrimination. Loeb^ found
that frogs showed negative phototropism, and preferred red to blue light.
Torelle^ found that two species of frog showed positive phototropism,
associated with a tendency to prefer blue to red light. The phototrop-
ism persists when the animal is blinded, although in the normal animal
the eyes are involved in the reaction since it occurs when the skin is
covered and the eyes left intact*. Cole^ found that the phototropism
of Rana clamata differs according to the surrounding temperature.
Pearse^ found some amphibia positively, and others negatively helio-
tropic.

1 Harvard I'.sijch. Studies, i. 570, 11)03.
- Der HcUotropismuH der Tie.re, Wiirzburg, 1890.
3 Amcr. J. oj Phymol. ix. 466, 1903.

* Koranyi, Centralbl.f. Physiol, vi. 6, 1892 ; G. H. Parker, Amcr. J. of Physiol, x. 28,
1903.

5 J. ofComp. Neur. and Psychol, xvii. 193, 1907.

" Proc. of the Amcr. Acad, of Arts and Sc. xlv. 16 J, 1910.

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 143

Hess examined an American salamander {Diemictylus viridescens),
the ordinary toad {Bufo vulgaris), and the African spurred frog {Xenopus
Mulleri). He found that the spectrum does not appear shortened to
them. He also found that their range of adaptation appeared to be
the same as for the human eye.

Fish. Graberi tested two species of fish, but no convincing proof
of their powers of colour discrimination were obtained. Bateson^ placed
food on coloured tiles and found that fish picked it ofE most readily from
white and pale blue, and least readily from dark red and dark blue, a fact
which can be explained by luminosity differences. Zolotnitzki^ fixed
dark red Cheironomus larvae and white, green, yellow, and red pieces
of wool of the same size and shape as the larvae to the wall of the
fish tank. The fish showed decided preference for the red, swam past
the white and green, while some of them paused at the yellow. Washburn
and Bentley^ investigated the chub {Semotilus atromaculatus). Two
dissecting forceps were used, alike except that to the legs of one were
fastened, with rubber bands, small sticks painted red, while to those of
the other similar green sticks were attached. The forceps were fastened
to a wooden bar projecting from a wooden screen, which divided the
circular tank into two compartments, and hung down into the water.
Food was always placed in the red pair of forceps, which were made
frequently to change places with the green ; and the fish was caused
to enter the compartment half of the time on one side, and half of
the time on the other. This was to prevent identification of the food
fork by its position or the direction in which the fish had to turn. The
animal quickly learned to single out the red fork as the one important
to its welfare, and in forty experiments, mingled with others so that the
association might not be weakened, where there was no food in either
fork, and where the forceps and rubber bands were changed so that no
odour of food could linger, it never failed to bite first at the red. More-
over, the probability that its discrimination was based upon brightness
was greatly lessened by using, when experimenting without food, a
different red much lighter than that in the food tests. The fish success-
fully discriminated red from blue paints in the same way, and it was

^ Grundlinien znr Eijurschuufj des HelUgkeUs- u. Farbcn/iiimes dcr Thierc, Prag and
Leipzig, 1884.

- Jl. of the Marine Bwl. Afi.soc. U. K. i. 225, 1887.

3 Arch, de Zool. exper. ix. 1901.

* J. of Comp. Near, and Psychol, xvr. 11.3, I'JOG ; Waslihuin, The Animal Mind,
New York, 1908.

144 COLOUR VISION

afterwards trained by putting food in the green fork, to break the earlier
association and bite first at the green.

Reighard^ fed grey snappers {Lutianus griseus) with small Atherinae
which were artificially coloured with seven hues. On simultaneous
presentation of white and blue the white was generally seized first, with
blue and light or dark red or yellow the blue first ; no difference was
apparent between blue and green. If Cassiopea tentacles were fastened
to the Atherinae the snappers soon learnt to avoid them. After they had
learnt to avoid red Atherinae with tentacles it was found that they also
rejected red ones without tentacles, but that they still took those stained
any other colour.

The researches of Zolotnitzki, Washburn and Bentley, and Reighard
are consistent with the view that the fish are totally colour-blind.

Hess examined a number of species of fish and found that they
appeared to behave exactly as if they had the visual perceptions of a
totally colour-blind man (see Part II). In the light of the spectrum
young Atherinae rapidly congregated in the yellow-green and green,
i.e., the brightest part of the human achromatic scotopic spectrum.
Far fewer were found in the yellow and scarcely any in the red ; there
were rather more in the blue. If the spectrum were displaced the fish
rapidly rearranged themselves in the same fashion. By moving a card
along the spectrum so as to intercept certain rays the fish could be
driven up to the blue or violet ends. If the red-yellow and green-blue
regions were intercepted the fish spread in both directions towards the
ends. If only the red and yellow-red and the blue and violet were
exposed many more fish collected at the violet than at the red end
though to the human photopic eye the red is much brighter. Just as
to the human achromatic scotopic eye, the red end was shortened so that
the extreme red produced no more effect on the fish than complete dark-
ness. By lighting one half of the basin with homogeneous light and the
other with equivalent mixed light which could be varied in intensity it
was possible to get the fish to arrange themselves evenly in the two
halves. In this manner the luminosity values of different parts of the
spectrum were worked out and it was found that the luminosity curve
agreed very well with that of the achromatic scotopic or totally colour-
blind human eye. Hess was also able to measure the effects of dark
adaptation. He found that after 15 — 20 minutes in the dark the sensi-
bility of the fish for light was many hundred times as great as im-
mediately after transference from light to dark. By using red and blue

^ Canicgio Inst., Washington, ii. 257, l'JU8.

THE COMPARATIVE PSYCHOLOGY OF COLOUR VISION 145

lights of suitable intensity he was able to prove that the Purkinje
phenomenon is not shown by fishes.

Owing to the forward movement of the pigment in the pigment
epithelium of fishes when exposed to light they possess a certain amount
of physical adaptation. Owing to the absorption by the pigment they
are in the photopic condition less sensitive to the violet end of the
spectrum.

Hess used various coloured imitation baits against different back-
grounds, and showed that their visibility to the fish depended upon
their luminosity and not upon their colour. If the brightness of the
grey background corresponded to the achromatic scotopic brightness
of the bait it was not seen.

Hess's results have been criticised by Bauer^, v. Frisch^ and Franz^,
without, however, seriously shaking his position.

Amphioxus. The response of amphioxus by movements to light
show that the curve of stimulus values of different homogeneous lights
agrees nearly, if not completely, with the luminosity curve for fishes and
for the totally colour-blind man, and that the adaptative changes of
sensibility to light in amphioxus with its primitive visual organs, nearly
resemble those of higher vertebrates.

CHAPTER III

THE COLOUR VISION OF PRIMITIVE RACES

Attention was first drawn to the colour vision of primitive people
by Gladstone, in 1858. In his Studies on Homer and the Homeric Age
he drew attention to the vagueness of the colour terminology of Homer,
and concluded that the ideas of colour at that time must have been
different from our own. Ten years later Geiger* came to the conclusion
that the colour sense of the ancients must have been defective. Not
only in Greek literature, but also in the Indian Vedic hymns, in the
Zendavesta, in the Norse Eddas, and in ancient Chinese and Semitic
writings there is evidence of the paucity of colour terms, especially in

1 Ce7itralbl. f. Physiol, xxm. 1909 ; Arch.f. d. ges. Physiol, cxxxni. 7, 1910 ; oxxxvii.
1911.

2 Verhandl. d. Deutsch. Zool. GeselUch. 220, 1911.

^ Inter nat. Rev. d. ges. Hydrobiol. u. Hydrogr. 1910.

* Contributions to the History of the Development of the Human Race, p. 48, 1880.

P. c. V. 10

U6 COLOUR VISION

the case of names for green and blue. Geiger advanced the view that
the sensation of red had been evolved first, then yellow and green, and
finally blue. Magnus^ came to the same conclusions from still more
extensive researches, and Gladstone^ returned to the subject in the same
year.

These views were strongly criticised by Grant Allen^. It was shown
that modern poems showed similar peculiarities, red occurring much more
often than blue in Swinburne's Poems and Ballads and in Tennyson's
Princess. La Fontaine used an epithet for blue once only in all his
poems (Javal*).

Philological evidence on matters of this kind is notoriously open to
doubt, but it is probable that the opposition to the views of Gladstone,
Geiger, and Magnus was carried too far. The examination of two parties
of Nubians, who were travelling in Germany in 1877, by Virchow and
others did much to bring about this result. It was found that they
used the same word for blue as for black and other dark colours, yet they
sorted coloured papers and wools correctly.

The use of pigments by the Egyptians, Assyrians, Greeks and others
has also been brought forward in opposition to Geiger's views. Green
and blue pigments were used by the Egyptians long before the time of
Homer, and green and blue beads are found in the prehistoric Egyptian
tombs. Benzaky^ has collected the evidence from ancient monuments
and considers that it conclusively disproves any colour defect in the
Egyptians and Greeks. As Rivers points out, however, the colour sense
of the Egyptians — and they appear to have had two names for green
and one for blue — has no direct bearing on that of the Greeks, who may
have remained in a state of arrested development. Moreover, even in
Egypt human statues with blue hair have been found, and in the
Acropolis there may be seen a blue bull, a blue horse, and a man with
blue hair and beard, all dating from later than the time of Homer^.

Magnus' pointed out that the same defect of terminology for green
and blue which characterises ancient writings still exists among many
primitive races.

^ Die geschichtlich? E nhuickelung des Farbensinves, Leipzig, 1877.
' Nineteenth Century, ir. 366, 1877.
^ The Colour Sense, London, 1879.

* Bull, de la Soc. d' Anthropologie de Paris, xii. 480. 1877.

* Du Sens chromatique dans Vantiquite, Paris, 1897.

^ Cf. E. A. Gardner, Handbook of Greek Sculpture, p. 28, 1902.

^ Untersuchungen u. d. Farhensinn d. Naturvolker, Jena, 1880 ; Ueber ethnol. Unter-
such. d. Farbensinnes, Breslau, 1883.

THE COLOUR VISION OF PRIMITIVE RACES 147

By far the most important contributions on the subject are those
of Rivers^.

In the Torres Straits expedition Rivers examined the colour vision
of two tribes of Papuans, of natives of the island of Kiwai, and of
members of several Australian tribes. The languages of these people
showed different stages in the evolution of colour terminology, which
correspond in a striking manner with the course of evolution derived
from ancient writings by Geiger. Some Australian natives from the
district of Seven Rivers used only three colour epithets ; one for red,
purple, and orange ; another for white, yellow and green ; and a third
for black, blue, indigo and violet. In Kiwai there was a very definite
name for red, and a less definite for yellow ; greens were called by the
same names as white and black or light and dark ; blue, violet, and
black had the same name ; other Kiwaians had a name for green which
was also applied to blue and violet. The natives of Murray Island and
of the Island of Mabuiag showed two further stages in the evolution of
colour language.

In these four stages the lowest possesses only a definite term for
red apart from white and black. In the next stage there are definite
terms for red and yellow, and an indefinite term for green. In the third
stage there are definite terms for red, yellow, and green, and a term for
blue has been borrowed from another language ; while in the highest
stage there are terms for both green and blue, but these tend to be
confused with one another. The series corresponds with the order of
the natives in general intellectual and cultural development.

The absence of a definite term for blue is very common. In some
the word for blue and black is the same, e.g., Kiwaians, Hovas, Bushmen,
and many Australian and Melanesian tribes. In many others the word
for blue and green is the same, e.g., in African and South American races
and in Jamaica. The absence of a term for brown is also common in
Australian and Melanesian languages, as well as in Tamil, Singhalese,
Eskimo, Welsh, etc., a brown object being called red, yellow, or dark.

Rivers tested 150 natives of Murray Island with Holmgren's wools
and failed to find any who confused red and green, but blue and green
and blue and violet were constantly confused. Examination with
Lovibond's tintometer showed a relatively greater sensibility to red and
less to blue than in the European.

1 Po}). Sci. Mo. Lix. 44, 1901 ; Jl. of the. Anthropological Ins. xxxi. 220, I'JOl ; Bcp. of
the Cnmhridge Expedition to Torres Straits, n. 1, 1901 ; "Observations on the Senses of the
Todas," Brit. Jl. of Psychol, i. 321, 1905; The Todas of the Nilgiri Hills, Cambridge, 190(3.

10—2

148 COLOUR VISION

Rivers suggests that the insensitiveness to blue may be due to
greater macular pigmentation. The natives were able to recognise blue
readily on the peripheral retina.

Rivers tested 50 natives of Upper Egypt with Holmgren's wools,
Nagel's cards, and Lovibond's tintometer. He found much confusion
in their colour names, but general agreement with the characteristics of
primitive colour nomenclature in other parts of the world. There was a
very definite word, ahmar, in use for red, but it was also applied to
colours containing a red element, such as orange, purple, violet and
brown. A somewhat less definite term, asfar, was employed for yellow,
also for orange and brown and occasionally for green and faint red.
The word for green, akhdar, was still less definite, being very often
applied to blue, violet, grey and brown. There was no definite
word for blue. The word azraq, usually regarded as the Arabic
equivalent of blue, was never used by these people for light blue, and
was applied by them more frequently to black than to an indigo blue.
The nearest approach to a word for blue was Idhdni, milk colour, which
was, however, often used also for green, grey, and brown. Azraq and
iswid (black) were used indiscriminately for black, blue, and violet, and
also for dark brown. The Arabic term for brown, asmar, was never
once used for this colour though occasionally applied to blue and grey.
Over twenty different names were used for brown papers and wools,
but generally ahmar or asfar.

Of 80 natives of Upper Egypt tested by Randall-Maclver and
Rivers four were certainly colour-blind (" red-green blind "), making
the characteristic mistakes with Holmgren's wools (see Part II).
Others made the same kind of matches as the Torres Straits' natives,
i.e., they behaved normally with the red, pink, and yellow test wools,
but compared green with blue, and blue with violet. Two distinct
tendencies were also noted, viz., to match according to saturation
rather than according to colour tone, and to put together wools which
would be called by them the same name. The colour thresholds for
red and yellow were low, that for blue much above the European
average.

Amongst the Todas there was a definite word for red which also
meant blood. Orange was often called " blood " or " earth." Yellow
was called drsena. probably a borrowed word. Green had many
names — " leaf," " moss," including the Tamil name and nil, blue,
and Mg, dark or black. Blue and indigo were generally called ml, a
word used by all the Dravidian races of Southern India, but sometimes

THE COLOUR VISION OF PRIMITIVE RACES 149

hag or hdthiti, black. The nomenclature for violet and brown was very
indefinite. There was a definite name for white, and greys were called
white or black, or occasionally " ashes," a common term for this colour
in Southern India.

Rivers tested 503 Todas with Holmgren's wools : 43 were found to
be definitely colour-blind. The normal individuals matched the wools
well, but most confused red and pink, green and blue, violet and blue,
and there was a general tendency to confuse faintly coloured wools.
One confused blue with grey and brown, and two blue with brown.
The defects of this kind were less marked than among the Uralis
and Sholagas examined in India^ and the Egyptians. All showed a
tendency to discriminate greens, blues, and violets less definitely than
reds and yellows, indicating that the deficiencies in nomenclature for
the former group of colours were accompanied by a certain degree of
deficiency in their discrimination.

Quantitative observations on the colour thresholds were made with
Lovibond's tintometer. The table gives the results obtained by Rivers
in five different communities.

No. ex-
amined. Red M.V o- C. v. Yellow M.V. <t C.V. Blue M.V. <r - O.V.

Todas .. 47 321 18] 24-3 755 29-2 15 0 213 731 53-3 207 234 440
Uralis and Shola-
gas .. 14 311 13-5 16-6 53-4 264 79 87 33-1 664 15-5 17-6 264
Egyptians 26 28-7 142 15-8 55-3 260 lO'S 123 475 854 34-3 43-4 509
Murray Islanders 17 17-6 7-5 8-9 506 26-8 9-9 11-6 430 60-0 165 20-2 33-6
English .. 41 27-5 17-9 21-7 79-0 16-7 7-2 100 601 30-8 11-2 14-9 48-4

In this Table the average thresholds, expressed in units of the
instrument, are given in the columns headed by the names of the colours.
The figures under the heading M. V. give the mean variations of the
results of the individuals of each group from the average result of that
group. Under a are given the standard deviations ^ of the individual
results from the average ; under C.V. the coefficients of variation as
worked out from the formula

o-xlOO
Average '

This table shows that the Todas have the highest thresholds for
red and yellow obtained in any community hitherto tested. The

1 Bull. Madras Gov. Museum, v. 3, 1903.

^ I.e., ./ — , where 25 is the sum of the squares of the individual differences 5j , 5-2 , . . ,

from the mean, and n is the number of individuals.

150 , COLOUR VISION

differences, however, between the Todas on the one hand and the
Sholagas and Uralis and Egyptians on the other are very slight and
within the limits of the probable error.

More important is the difference between the Todas and the Papuans
of Torres Straits. There is no great difference between the thresholds
for yellow of the two communities, but the thresholds for red differ
greatly, the dift'erence, 14-5, being considerably more than three times
the probable error of the difference^, which is 2-8.

This difference is probably connected with the fact that among the
Papuans red-green blindness does not occur, while among the Todas it
reaches the highest frequency recorded in any community. The
difference seems to show that even in those Todas who were able to pass
the tests for colour-blindness, there was some deficiency in the sensibility
for red as compared with the Torres Straits islanders.

For blue the Todas have a very distinctly higher threshold than the
English observers, though lower than the other savage or barbarous
races tested. The difference between the figures for the English and
the Toda observers is enormously greater than the probable error of the
difference, which is only 2-8. The Todas confirm the conclusion pre-
viously reached by Rivers, that the defective nomenclature for blue
which is so generally found among races of low culture is associated
with a certain degree of defective sensibility for this colour.

Rivers made observations on the colour fields of the Papuans and
the Todas. The former showed colour fields of considerable size, and
differences in the limits for different colours were clearly demonstrated.
The Todas had very small colour fields, so small that it was impossible
to detect any difference in size. Only in one man did the field for blue
seem larger than those for other colours. Since the Todas were more
intelligent than the Murray Islanders it is probable that the small fields
point to a definite sensory defect.

McDougall^ suggests that the colour names of the Homeric Greeks
and uncultured peoples may be explained by environment. The superior
power of red to strike the attention may be due to the fact that red
objects in nature are rare, while blue and green are constantly spread
out in large tracts of sky and sea and foliage. Further, while many
of the rare red objects (blood, fruits, animals, ironstone) are of practical
importance, and while the abundant green objects are presented in

^ I.e., J'zJ+%^ where 2^ and 2,, are the probable errors of the two figures to be
"ompared,

2 Brit. Jl. of Psijchol. 11. 349, 1908. .

THE COLOUR VISION OF PRIMITIVE RACES 151

endless variety of tones, shades and shapes, many of which call for
the exercise of discriminatory perception, the widespread blue surfaces
of sky and sea are commonly of uniform tone, and in hardly any circum-
stances is the discrimination of blue tones of practical importance to
men of the lower cultures.

The Egyptian peasants examined by Rivers w^ere, however, well
acquainted with blue objects and many were wearing blue garments.
McDougall attaches much importance to the undoubted greater affective
value of warm colours as compared with cold tones, and holds that
this factor, rather than defective sensibility for blue and violet, explains
the defects in nomenclature and in colour matching. He suggests the
view that primitive vision corresponded to our sense of grey, that our
senses for blue and yellow became differentiated as the affections
produced by the light of the two ends of the spectrum, and that at a
later period the senses of red and green became differentiated in a similar
way from the sense of yellow. The facts of colour-blindness and the
distribution of colour-sense in the periphery of the retina (as generally
accepted) fit well with this scheme of development.

Myers^ holds that " language affords no safe clue to sensibility.
A colour name occurs when it is needful. Where it is needless, it
will not be formed, be the sensibility to that colour ever so great. If
we are to gauge the colour sense of a people by colour nomenclature,
nearly every primitive people must be dubbed ' brown-blind ' or
' brown- weak,' inasmuch as it is very rare to find a special word for
brown.

" The ' red-blindness,' which occurs among the colour-blind, and in
the intermediate zone of the normal retina, is strong evidence against
the view that red is the first colour-sensation phylogenetically acquired.
The one prominent fact, which stands out clearly in this discussion, is
that both primitive peoples and infants are attracted most by red and
next by yellow ; this fact. being manifest among the former in their
nomenclature and among the latter by their readiness to grasp objects
of these colours. I suggest that this superior attractiveness of red is a
fundamental characteristic far too deeply and immutably ingrained to
be attributable, as McDougall so ingeniously suggests, to the greater
utility or rareness of red and yellow objects or to the relative unattrac-
tiveness of the broad surrounding expanse of uniformly blue sky, blue-
green sea and green foliage. The excitatory action of red is manifest
in organisms lower than man."

1 Brit. J I. of Psychol, u. 361, 1908.

152 COLOUR VISION

CHAPTER IV

THE DEVELOPMENT OF COLOUR VISION IN THE CHILD

Darwin^ first pointed out that the power of distinguishing colours
is a very late accomplishment in childhood ; he found that his children
were unable to name colours correctly at an age when they knew the
names of most common objects. Preyer^ made many investigations
on one child. Various methods were attempted without very encourag-
ing results. Up to the third year it was found impossible to hold the
attention of the child sufiiciently long to obtain concordant results by
such methods as sorting out coloured wools, cards, etc. or naming different
colours. In 1897^ Preyer drew attention to a more successful device
for naming colours. Children of an early age show no facility for
comprehending abstract terms. It was found, however, that interest
could be awakened by association of different colours with concrete
objects. Thus Mrs Stanley Hall's son^ in the forty-sixth week of life
called all black objects " Kitty," because a black cat was so named.
The method does not seem to have been much used and could only be
employed at a period when probably the colour sense is already fully
developed.

Garbini^ made observations on 600 children. Both Preyer and
Garbini agree that the child is unable to distinguish colours until to-
wards the end of the second year, and that red is distinguished and
named correctly at an earlier age than blue. Garbini emphasises the
fact that the power of distinguishing colours develops earlier than that
of naming them, and these early experiments give evidence of little
beyond that fact, for the methods were not suited to determining the
earliest age of colour discrimination at all accurately.

Methods better adapted to this purpose have been employed by
Baldwin^, Marsden'^, Shinn^, McDougalP, Myers^^ and Valentine^^.

1 Kosmos, I. 376, 1877. ^ j)i^ g^^ig ^^g Kindes, Leipzig, 1881.

3 Ztsch.f. P.-iyckol. u. Physiol, d. Sinnesorg. xiv. 231, 1897.

■* The Child-Study Monthly, ii. 460, 1897.

^ Arch, per V Antro-pologia e la Etnologia, xxiv. 71, 193, 1894.

* Mental Development in the Child and the Race, Chap. ni. 3rd ed. London, 1906.
' Psychol. Rev. x. 37, 1903.

* The Development of the Senses in the First Three Years of Childhood, 148.
9 Brit. J I. of Psychol, n. 338, 1908. " Ibid. ii. 353, 1908.
" Ibid. vj. 363, 1914.

DEVELOPMENT OF COLOUR VLSION IN THE CHILD 153

Baldwin's method consisted in placing small squares of coloured
blotting paper upon a rod within reach of the child, one at a time,
and recording for each colour the ratio of the number of times the piece
was seized to the total number of presentations of that colour. He found
that "the colours range themselves in an order of attractiveness, i.e., blue,
red, white, green, and brown " and that pieces of newspaper were as
attractive as any of the colours and even more so. The ratios of
acceptances to presentations of the colours were : blue 0"766, red 0'714:,
white 0'636, green 0'6, brown 0"5. McDougall points out that the
number of experiments with white was only eleven, and though the
figures would perhaps suggest some colour appreciation if the experi-
ments had been more numerous they hardly can be claimed, as they
stand, as evidence in that direction.

Marsden employed Baldwin's method and obtained similar results.
In 175 presentations the acceptances were : newspaper 130, yellow 127,
red 122, blue 121, white 113, grey 114, black 114, brown 107. It is to
be noted that as white, grey, and black were taken ^\\i\\ almost the
same frequency, brightness appears to have no influence ; yet of the
colours yellow was taken most often and brown least. McDougall
considers that the results obtained by Baldwin and Marsden " cannot
be held to show that colour vision was present, or to throw any light
on the state or on development of the colour sense " in the subjects.

McDousall's method was a modification of Baldwin's. It consists
essentiallv in presenting two similar objects of different colours simul-
taneously, and recording the number of acceptances of each. For the
first child coloured flowers were used, and later balls of worsted 1 inch
in diameter and pieces of crinkled paper of low and high saturation.
With flowers the results during the twenty-first and twenty-second
weeks were : during the first two days red was taken 19 times, white
9 times : later, red 82 to white 33 ; blue 17 to white 13. With worsted
and paper between the twenty-third and twenty-ninth weeks the results
were :

R_B R— G B— G R— W G— W B— W R— Grey G— Gy B— Gy W— Gy
30—26 32—16 32—31 36—21 35—15 32—22 30—4 25—7 24—5 38—15

An improved method was used for another child. The objects presented
in pairs to this child were balls about 1 inch in diameter, each consisting
of a pill box containing a pea and embedded in a loose sheath of knitted
wool. The woollen thread was prolonged to form a plaited cord about
three inches in length. The balls were red, green, blue, yellow, white

154 COLOUR VISION

and grey, the grey being decidedly brighter than the colours. Pairs
were dangled before the child. The results between the eighth and tenth
months were :

R— B R— G B— G R— W Y— W Y— R Y— B

14—13 6—9 0—2 4-0 6—2 6—2 5—3

R— Gy B— Gy G— Gy Y— Gy
22—12 27—11 6—3 19—5

The most favourable time for experimenting by this method occurs
about the sixth month and lasts two months ; at later periods both
objects are often grasped simultaneously with the two hands. Probably
good results might be obtained in precocious children in the fifth month.

The experiments indicate that the power of appreciating red is fully
developed at the end of the fifth month ; that red, green, and blue are
appreciated during the sixth month, since they are decidedly preferred
to white, and still more decidedly to grey -of equal brightness with the
colours. They also indicate that in the sixth month no one of these
three colours is markedly preferred to the others ; but there was a
faint indication that during the fifth month blue is less appreciated than
red.

Myers used wooden cubes (" bricks "), each measuring 33 x 20 x 15
mm., painted uniformly in a different shade of grey or colour. A pair
of these bricks were placed before the child on a table covered with
black velvet. Each time the child picked up a particular colour, e.g.,
red, she was rewarded by being given a taste of honey, syrup, or sugar.
Experiments between the twenty-fourth and twenty-sixth weeks of life
failed to show the development of any association between colour and
reward. In later experiments the reward was given if either brick was
grasped, and from January (thirth-seventh week) to May a pair of grey
bricks, one lighter than the other was used. Light grey was selected
101 times, dark grey 75. A vivid yellow and an intensely white brick
were then used, yellow being selected 27 times, white 8. In May a
saturated blue brick was chosen 8 times w^hen presented with white ;
but later white was often chosen, so that in 78 times blue was selected
35 times, white 43. These results show the influence of novelty of colour.
Yellow, however, was chosen with increased instead of diminished
frequency at successive sittings.

Myers concludes that at a very early age, probably long before the
sixth month, infants are susceptible to relatively small differences of
brightness ; that at this age reds and yellows are distinctly preferred
to other colours and to colourless objects of far greater brightness ;

DEVELOPMENT OF COLOUR VLSION IN THE CHILD 155

and that novelty may play an important part in determining the infant's
choice of colour. He is strongly of opinion that such experiments do
not afford evidence of the course of the development of colour sense.
He says, "It is true that the positive results of such experiments may
be significant. When, for example, a child shows a distinct preference
for yellow, presented with white, that is a clear indication that yellow
has a different effect on him from white. And we are doubtless justified
in assuming that this difference is not merely an affective and physio-
logical one, producing greater pleasurable excitement in the infant and
determining the choice of the brick which is grasped, but that it is
also of sensational significance, that is to say, the visual sensation
excited by yellow is different in the infant from that excited by white.

" I think it highly probable that the primary physiological basis of
colour vision is completely installed before the infant has reached the
stage when he can successfullv dift'erentiate from one another all the
various colour sensations which such an apparatus permits him to receive ;
just as, in foetal life, he is provided with lungs before he can make use
of them. If this be true, then it follows that the gradual differentiation
of the colour sensations from one another is a process distinct from the
developing constituents of the peripheral cerebro-retinal apparatus."

Valentine commenced his experiments on a child at the age of three
months, before grasping was sufficiently developed to be of use. The
method he adopted was to measure the time the child looked at either
of two coloured wools held before him for two minutes at a time. The
colours used were black, white, red, yellow, green, blue, violet, pink and
brown. The following table gives all the scores (in seconds) of each
colour, together with the scores against them.

Totals
A

For Agst.

Totals
B

For Agst.

Percentage

scores
A B

Yellow

106

133

525

137

282

79-3

85-4

White

104

r)3

153

188

680

246

487

108

73-4

81-8

Pink

102

476

186

280

72-2

80-9

Red

242

283

153

45 3

32-8

Browii

151

275

276

37-8

190

Black

149

263

152

109

35-7

58-2

Green

165

421

276

28-2

16-3

Blue

122

300

106

28-9

14-5

Violet

447

287

9-7

3 0

Totals 137

246

186

283

275

263

421

300

447

col

umi

n A

all s

core

s of

3" a

nd 0

ver .

are r

ecko

ned.

column B

156 COLOUR VISION

only scores of 8" and over are reckoned. That is, when the child
only looked at a colour 7" or less continuously, that particular score was
ignored.

It would be unfair to draw up an order merely upon the total scores
of each colour. For such conditions as the child's mood, or the light,
may have been exceptionally favourable on those days when some
particular colour, say pink, was used, and exceptionally unfavourable
on the days when some other colour was used. Thus green in the table
has 165 to its credit — about the same as brown (151). But far more
seconds were scored against green (421) than against brown (275). We
therefore get a fairer basis of comparison by adding the score of green
(165) to the score against green (421), which gives us the total scored
during the experiments in which green was used, viz., 586 seconds :
and then finding what 'percentage of this total was scored by green.
Such percentages are given in the above table.

From these results two kinds of inferences may be drawn, — as to
colour preferences, and as to the development of the colour sense. We
cannot of course infer from the absence of preference between two colours
the absence of any difference of sensation : but from evidence showing
that one colour is markedly preferred to another we can infer that the
colours are sensed as different colours, unless the preference can be
ascribed to differences in brightness.

Brightness certainly plays an important part, for the three first
colours on the list are by far the brightest. That it is not the only
factor is shown by the percentage scores for violet (9"7 %), red (45*3%),
and brown (37 "8 %), which were of equal brightness : similarly for red
(45-3 %), blue (28-9 %), and green (28-2 %), which were alsoof equal
brightness. It is of course possible, but not probable, that the luminosity
values for the infant are different from those for the adult. The experi-
ments of Valentine confirm those of McDougall and Myers that novelty
has a great attraction for the child.

When the child was seven months old Valentine commenced experi-
ments by the grasping method, using the same wools except that brown
was omitted and a grey of the same brightness as the red, blue, green
and violet was introduced. In 36 experiments, involving 360 choices,
yellow still held the first place, but red and pink were now almost
bracketed second. The most striking difference apparent was the drop
in the comparative attractiveness of brightness. White was now only
on a level with violet, green, blue, and black, and at least not more
attractive than a dull grey. Pink also scored only about the same as

DEVELOPMENT OF COLOUR VISION IN THE CHILD 157

red. Valentine regards the grasping method as more open to objection
than has been thought, owing to the eagerness to grasp anything and to
the change which occurs in the favourite hand for grasping.

When the child was eight and a half months old the " grasp and
reward " method, suggested by Myers, was tried, the reward being
given only when a particular colour was chosen. There was some
indication that an association of reward with blue was set up when it
had to be discriminated from green, but less when it had to be discrimi-
nated from red, probably owing to the antagonistic effect of the greater
attractiveness of red.

Valentine's results are summarised thus :

1. There is good evidence that at the age of three months an infant
may experience the sensations of red, yellow, brown, green and blue.

2. In the case of the child investigated the order of preference of
the colours used was as follows :

,, (white) , (brown) (blue) . ,

vellow i . 1 ^ red i , , , M r violet

( pmk J ( black J (green )

3. The order of preference seems to be partly determined by bright-
ness, but cannot be explained entirely by reference to brightness or to
novelty.

4. The order of preference is partly determined by the relative
powers of the various colours as stimuli to the organism.

5. At seven months the same infant still liked yellow best of all
the colours used, and then red, and pink. By this time the comparative
attractiveness of white had decreased, being no greater than that of
violet or even grey.

6. There was perhaps a trace of association between the grasping
of the blue wool and the idea of a reward, when blue and green wools
were offered to the child. The lack of more definite association can be
ascribed to the difficulty of establishing any association of such a nature
at this age, and need not be attributed to failure to discriminate blue and
green.

PART II

THE CHIEF FACTS OF COLOUR BLINDNESS

CHAPTER I

INTRODUCTION : COLOUR NAMES

It has long been known that certain persons show peciiKarities of
colour vision distinguishing them from the normal^. The colour blind-
ness of the chemist, John Dalton^, first led the attention of scientists
to the analysis of the sensations of the colour-blind. In 1807 that
remarkable genius (" bewundernswiirdiger Forscher," v. Helmholtz),
Thomas Young, discussed Dalton's case in his Lectures on Natural
Philosophy. So great was the stir produced by Dalton's defect that
colour blindness was long known as Daltonism. In 1810, the great
poet Goethe referred to these abnormalities of colour vision in his book
on the Theory of Colours^. An enormous literature on the subject
sprang up during the nineteenth century*.

During this period two types of colour blindness were fairly clearly
distinguished. They had this in common' that in them the whole gamut
of colour sensations could be referred to a function of two variables.
Since normal colour vision is a function of three variables and these
types of colour blindness display colour systems which are functions
of two variables the normal are conveniently classed as trichromats, these
types of the abnormal as dichromats.

As already mentioned the dichromats can be divided into two groups,
and to these a rare third group has since been added. The names given
to the groups varied according to the theoretical predilections of the
writer. Thus, v. Helmholtz called them red-blind, green-blind, and

1 Turberville, Phil. Trans. Roy. Soc. Lond. 1684 ; Huddart, op. cit. Lxvn. 1, 14, 1777 ;
Whisson, op. cit. LX^^^. 2, 611, 1778 ; and others.

2 Lit. and Phil. Soc. of Manchester, 1794; Edin. J. of Sc. ix. 97, 1798 — reprinted in
Edin. J. ofSc. v. 188, 1831.

' Zur Farbenlrhre ; translated by Eastlake, London (Murray), 1840.

■* V. Helmholtz, 2nd ed. BibliograiAy, pp. 1176-1198, complete to the end of 1894.

INTRODUCTION: COLOUR NAMES 159

blue-blind, and this terminology has generally been adopted by phy-
sicists. Hering, on the other hand, regarded the first two groups as
variants of a common class, the red-green-blind, the third group being
due to abnormality of the mechanism subserving blue-yellow sensations.
V. Kries introduced terms for the three groups which were merely
descriptive and were prejudiced by no theory. I shall adopt his terms,
viz., protanopes, deuteranopes, and tritanopes, corresponding respectively
with V. Helmholtz' red-, green-, and blue-blind.

In 1881 Lord Rayleigh^ made an important discovery. He found
that many people with apparently normal colour vision require different
amounts of red or green in their colour mixtures from the majority.
His observations were confirmed by Bonders^ and have since been
verified. As the colour system of these people is, like that of normal
people, a function of three variables, they may be conveniently termed
anomalous trichromats.

Later researches tend to show that these do not constitute the only
type of abnormality of the trichromatic system.

All the varieties hitherto mentioned are abnormal from birth. The
defect is congenital and incurable. The statistics are very unreliable.
In its grosser forms (dichromats) amongst civilised races it is said to
affect about 4 per cent, of the male population, and 0'4 per cent, of the
female ; but the tests from which these statistics are derived were often
crude. There is good evidence to show that colour blindness is hereditary
and that generally it is transmitted through the female, who is herself
not usually ali'ected^. An unaffected male never carries colour bhndness,
but an affected male sometimes transmits to his son ; consanguinity
of parents — by intermarriage of cousins — is rare^. Transmission
through several branches of a familv is not uncommon, and several
siblings, usually of course male, of a childship are often affected. Nettle-
ship has published pedigrees of colour-blind families in which females
were affected^. Statistics are not available for any precise estimate of
the slighter defects of colour vision (anomalous trichromats), but they
are probably widespread and certainly occur in colour-blind childships.

Differing from all these groups there are people who apparently see

all parts of the spectrum of one hue, the parts differing only in luminosity.

These are the total colour-blind or monochromats .

1 Nature, xxv. 64, 1882. ^ Arch. f. Anaf. 518, 1884.

' Cf. pedigree in Nettleship, Trans. Ophth. Soc. xxvm. 248, 1908 ; also bibliography
in Parsons, Pathology of the Eye, iv. 1413, 1908.

* Nettleship, Trans. Ophth. Soc. xxix. p. Ix, 1909.

* Ibid, xx^^. 251, 190(3.

160 COLOUR VISION

Colour vision is often affected in disease of the eyes or brain^. I do
not propose to discuss acquired colour blindness, though some few
references to it may be necessary.

The outstanding characteristic of all known abnormalities of colour
sensations is that the abnormal people see fewer hues than the normal.
The great difficulty in arriving at a true conception of the relationship
of any abnormal colour system to the normal is psychological. No
individual can judge with certainty the sensations of any other
individual. We in general assume that another person has the same
sensations as ourselves until, either by accident or research, we discover
differences between their judgments of external stimuli and our own.
Investigation of the sensations produced by various stimuli, through
the mediation of aft'erent nervous impulses, is similarly extremely
difficult in lower animals. We have to judge for the most part by motor
responses which may or may not be directly contingent to the afferent
impulses, and which are in any case particularly liable to misinter-
pretation. The investigation is rendered easier in man owing to the
faculty of speech. Amongst the majority of mankind, however,
multitudinous nuances of sensation, perception, and conception are
often conveyed by the same word. The higher the intellectual status
the greater is the discrimination of these shades of feeling and thought
and the more elaborate is the terminology employed to express them.
Education is, or should be, largely concerned with the equipment of the
individual with a sufficiently comprehensive and finely discriminating
vocabulary.

So far as these considerations apply to colour vision the majority
of people are equipped with a vocabulary of colour names, such as white,
black, red, yellow, green, blue, violet, purple, which are in common use
and are used by most people in such a manner that no glaring discrepancy
is noticed between the sensations of the various individuals as thus
expressed. The more minutely trained observers have added to this
vocabulary subsidiary terms which express the finer nuances of colour
perceptions, such as orange, indigo, mauve, puce, drab, and so on.
Unfortunately increased complexity of nomenclature has not been
accompanied pari passu with accurately defined discrimination of
meaning. Consequently many such terms are used in a loose and ill-
defined manner, and it would be quite unreasonable from the use made
of the terms to deduce differences of perception which may or may not
exist amongst the various individuals.

1 Kollner, Die Storungen des Farbensinnes, Berlin, 1912.

INTRODUCTION: COLOUR NAMES 161

A fortiori such deductions are most untrustworthy amongst those
whose colour vision is defective. Just as a person whose stereoscopic
vision is defective may learn to estimate distances and judge solid shapes
by accessory aids, such as apparent size, distinctness, shadows, the
correlation of tactile sensations, and so on, and may even be wholly
unconscious of his defect and of the greater endowment of the majority
of his fellow-creatures, so too the colour-blind. His vocabulary of
colour terms is often as full and comprehensive as that of his fellows in
a similar walk of life. Experience teaches him that these terms are
applied with a general consistency to the various objects of every-day
life. These objects are generally well-defined to his senses by other
criteria than colour, such as shape, size, and general relationship to other
objects. He may therefore pass through life without being conscious
of any inferiority of perception as compared with his fellows, and he
may also fail to make any glaring mistakes, such as would convey to
others a suggestion of his deficiency. Hence if we wish to analyse his
sensations and to arrive at some idea of their divergence from ours we
must take care to remove those accessory aids upon which he uncon-
sciously or consciously relies. Because he calls a cherry red it does not
follow that his sensation of red agrees with ours. We know indeed that
in many cases the hue of the cherry is very little different to him from
that of the cherry leaves ; yet he never calls a cherry green. If we show
him small spots of coloured light he may or may not give them the names
which we regard as correct. The fact that he is often right proves that
his accessory aids are more complex than those already mentioned.
It is our main object to study these accessory aids so that we may
eliminate them and place him hors de combat.

It will suffice for our present purpose if it is clearly understood that
the names which the colour-blind use for various colour sensations must
not be relied upon as accurate criteria of their sensations as cotnpared
with the normal. On the other hand, to the practised examiner these
names are full of suggestion and will often indicate the lines which the
examination should pursue. When we deal with practical tests for
colour blindness, the main object of which is to determine whether the
colour-defective are dangerous for certain occupations, the problem is
quite different and the colour names used by the examinee are them-
selves indications of his fitness or unfitness.

With regard to what is commonly known as " colour ignorance "
it is found that there are verv few civilised people who do not know
the fundamental terms for colours, whereas many people are ignorant

p. c. V. 11

162 COLOUR VISION

of the terms for the nuances of colour, such as mauve, puce, and so on.
The latter are, however, quite unnecessary for colour testing, and we
may conclude that, with the exception of the monochromats, the great
majority of people know that the terms red, yellow, green, blue, violet,
and purple are applied to different colour sensations. Amongst the
uneducated, and even amongst those otherwise well educated but un-
practised in describing differences of colour, there are many individuals
in civilised races who from inaccuracy or carelessness apply generic
colour terms erroneously — thus, it is quite common for blue-greens to
be described as blue or green, violets and blue-purples as blue, deep
orange as red and so on. This is still more common amongst primitive
races, who, however, often have the excuse of an actual paucity of
colour terms in their vocabularies. It is important that defective
terminology shall not be interpreted as the expression of defective colour
perception without adequate confirmatory evidence.

We must conclude then that we can have no accurate idea of the
actual physiological sensations or of their psychological counterparts
in the colour-blind. We soon discover, however, that he often mis-
names colours, and that if we compare the effects of two stimuli they
are often dissimilar to him when they match to us, and vice versa.
Thus, with many colour-blind we can find a certain intensity of mono-
chromatic blue-green light which exactly matches for him a certain
intensity of monochromatic yellow ; they are obviously quite dis-
similar to the normal. We shall therefore commence by investigating
the laws of colour mixture for the colour-blind.

CHAPTER II

DICHROMATIC VISION

The Mixture of Pure-Colour Sfitnidi. We have seen that within
a certain range which includes all ordinary conditions of colour vision
and subject to certain limitations of stimuli before referred to, every
conceivable light or light mixture gives rise to a sensation which can be
accurately matched by the sensation produced by a suitable mixture of
only three lights (p. 38). The vision of dichromats can similarly be
expressed in terms of two Hghts. Thus, two monochromatic lights can
be found, such that, by mixing them in various proportions, the mixtures
will match every part of the spectrum as it appears to the dichromat,

DICHROMATIC VISION 163

and also the unanalysed white light. It is found that there are two
groups of dichromats, as was first pointed out by Seebeck^, agreeing
amongst themselves but differing from each other in the proportions
of the two mixed lights required for the matches. These are the
protanopes and deuteranopes, of whom the latter are more common
(23 : 11, Nagel).

Now, since white Hght can be matched by the dichromat with a
suitable mixture of two monochromatic lights, and since all spectral
colours can be matched by mixing the same two colours in various
proportions, it follows that there is some spectral colour which will
match white. This point is called the neutral 'point of the dichromatic
spectrum.

Konig^ first made systematic examinations of the colour-mixtures
of the colour-blind with spectral colours. He was followed by v. Kries
and Abney^.

The principle of the method adopted by Konig had been previously
applied on a less extensive scale by Bonders and van der Weyde*.
Konig and Dieterici divide the dichromatic spectrum into three regions
as compared with the five regions of the trichromatic spectrum. If
ij and Z3 are two spectral lights from the end regions, and L^ is any
spectral light from the middle region then

where a and 6 are two coefficients determined experimentally. In other
words any spectral colour can be matched by the mixture of appro-
priate quantities [a and h) of two colours selected from the end regions
of the spectrum. If for L^ a red (645 /x/x) is chosen, and for L^ a blue
(460 jx^i or 435 /z/x), then Konig and Dieterici found that for protanopes
no blue is necessary to match any colour from about 550 /^t/x. to the end
of the spectrum on the red side, hence 6=0. Similarly no red is
needed to match any colour from about 460 /x/x to the violet end, and
a = 0.

A more complicated method, ensuring greater accuracy, was to
divide up the spectrum into fractions, determining the matches for

^ Ann. d. Physik, xui. 177, 1837.

^ Konig and Dieterici, Sitz. d. Akad. d. Wiss. Berlin, 805, 1886 ; Ztscli. f. Psychol, u.
Physiol, d. Stmiesorg. iv. 241, 1892 ; Konig, Brit. Assoc. 1886.

' Much of Abney's work covers the same ground as that of v. Kries. Since, however,
it is expressed throughout in terms of the Young-Helmholtz theory and was inspired by
that theory detailed consideration of it will be left until Part III.

* Arch. f. Ophth. xxvin. 1, 1, 1882.

11—2

164

COLOUR VISION

each part and correlating the results mathematically. The principle
is the same.

The average of the results, which did not differ materially inter se,
is shown in Fig. 44, where the abscissae are wave-lengths of the matched
spectral colours and the ordinates are arbitrary units of intensity.
Wi is the deuteranopic red or " warm " curve, Tf 2 tlie protanopic red
curve, and K the blue or " cold " curve common to both. H is the
monochromatic curve of which more will be said later.

18
16
14
12
10
8
6

^ N

— 1

Fig

.720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380

a B C D E b F G H

44. Equal-area gauging curves for dichromats and nionochromats {H).

Ifj , deuteranopic " warm " curve ; W2, protanopic " warm " curve ; K, " cold '
curve common to both types of dichromats. Abscissae, wave-lengths of the
interference spectrum of sunlight ; ordinates, arbitrary scale. (Konig.)

It is to be noted that the curves are so drawn that the areas enclosed
between each curve and the axis of abscissae are all equal. They
therefore represent the relative magnitudes of the stimulus values of the
given elements — red or blue — in the unanalysed light from which the
spectrum is derived {v. Part III, Section III, Chap. 11). They do not
establish any quantitative relationship between the stimulus values for
different individuals, since these are qualitatively of a different order.

V. Kries^ used as his gauging lights a red (589-2 /x^u, for protanopes,
645 /x/x. for deuteranopes) and a. blue (460-8 ^a^u.), and determined the
relative amount of each which must be mixed in order to match exactly
the spectral colours.

1 V. Kries and Nagel, Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xii. 1, 1896 ; v. Kries
op. cit. xm. 241, 1897.

DICHROMATIC VISION 165

The method of procedure was as follows, v. Helmholtz' spectro-
photometer was used, one half of the field being illuminated with the
homogeneous light, the other half with the mixture. In this apparatus
the lights are derived from two sources. The mixture is obtained
from one source by forming two spectra with the aid of a doubly
refracting prism, so that these spectra are polarised at right angles
to each other. The intensities of the lights in both halves of the field
are altered by varying the widths of the slits transmitting the lights
and by means of Nicol prisms.

Suppose that a deuteranope is being examined and that the mixing
lights are 645 yn/x and 460-8 /x/x. One half of the field is illuminated
with a homogeneous red, say 670-8 /x/x. With this light a perfect
match, both in colour and brightness, can be made with 645 ju/x merely
by altering the intensity and without adding any of the 460-8 /x/x light.
The red or warm {W-) value of 670-8 fxfM is expressed in arbitrary units,
which depend upon the width of the slit, which transmits this light
from the second source of light, when the two exactly match. Let us
suppose that it equals 33 under the given conditions.

A second wave-length is then chosen, e.g. 656 /XjU, the width of the
slit which transmits this light from the first source of light being kept
unchanged. We now find that the 645 /x/x slit must be opened more
in order to establish a perfect match, say to 48. For each succeeding
wave-length this slit must be opened wider until about 600 /x/x is
reached. Beyond this point the slit-widths must be reduced.

At about 540 /x/x it is found that no perfect match can be made
with the 645 /x/x light alone. For example, 536 /x/x appears to the
deuteranope less saturated than 645 /t/x. It is necessary to add a
small quantity of light of shorter wave-length. On adding the appro-
priate amount of 460-8 /x/x light a perfect match, both in colour and
brightness, is obtained. This light, 536 /x/x, therefore possesses both
a red or IF-value and a blue or cold {K-) value, and these are recorded,
viz.. If -value 41, Z-value 6-3.

The readings are not quite so simple as would appear from this
description. For example, it is found necessary to alter the width of
the slit which transmits the homogeneous light, otherwise the second
slit has to be opened so wide that the two spectra derived from it are
impure. Moreover the relative amounts of 645 /x/x and 460-8 /x/x lights
mixed, as well as the brightness of the resultant mixture, are altered
by rotation of the Nicol prism, and corrections have to be applied
according to the angle of rotation.

166

COLOUR VISION

If however the apparatus is kept as constant as possible and the
same procedure is adopted in each series of observations concordant
results are obtained froro the same individual and comparable results
from different individuals, v. Kries and Nagel compare directly the
results obtained from deuteranopes, using the mixtures of 645 /x/a and
460-8 jjLjx lights, with those obtained from protanopes, using mixtures

200

190

180

170

160

150

140

130

120

1 10

100

1
^

Xy\

•i

■. '\

■

'■.

: 1

'\-.

/ /

\ \

•

(
(
1

■ /
» /

/ /
/ /
f /

V-

St.,

•^

■\

'••..

"v.

..^

■^=:

^iii:^

1 3

10 11

13-5

16-5

19-;

en en

x»

O O

Ol Ol

Fig. 45. Gauging curves for dicliroinats : " warm " curves. <S and M, two protanopes ;
N and St, two deuteranopes. Abscissae, arbitrary scale of prismatic spectrum of
gaslight, indicating certain wave-lengths ; ordinates, arbitrary scale, (v. Kries.)

of 589-2 iJi^i and 460-8 {xp, lights. While it is legitimate to choose two
mixing lights of any wave-length so long as one is chosen from each of
the two end regions of the spectrum, the curves in all such cases having
the same characteristic form, the units are referred to different standards
and it is a somewhat doubtful assumption to suppose that the results
are directly comparable.

DICHROMATIC VISION

167

The red gauging values for two protanopes {S and M) and for two
deuteranopes {St and N) are shown in Fig. 45. The corresponding
blue values are shown in Fig. 46. The abscissae are the monochromatic
colours of the spectrum, the ordinates the intensities in arbitrary units

It will be noticed that Konig's blue curve extends red-wards as
far as 600 /x/x, whereas v. Kries and Nagel's extends only to 536 fi/j,.
The difference shows the great importance to be attached to adaptation.
Every care was taken in v. Kries' experiments to make the matches
with good light adaptation ; no such minute precautions were taken

i-w*^

err

^-^

'*.- "'

'__*■_!

■^

v.-

<-■'

•

^■''

*
y

-'if

•

•
^

.-—

•

■^.

'm.

16-5

18
■f-

19-5

•IS.

ot>

i>-

Ol
CO

24-7

Fig. 4G.

Gauging curves for dichroraats: "cold" curves,
as in Fig. 45. (v. Kries.)

Same observers

by Konig and his subjects. The latter are indeed chromatic scotopic
curves, in which there is some dark adaptation combined with a large
residual photopic condition. Another factor, however, is the much
greater luminosity value of red compared with that of blue {vide infra).
Hence the arbitrary units adopted by v. Kries do not signify equal
stimulus values of red and blue for the same unit-number. In their
gaslight dispersion spectrum one part of lithium red has about the
same luminosity as 8 — 12 parts of blue (480 ^/x) for the deuteranope.
In V. Kries' units one part of red is equivalent in luminosity to about

168 COLOUR VISION

20 parts of blue. Hence 6 or 10 parts of blue will have relatively-
little stimulus value when mixed with 30 to 35 parts of red, whilst a
trace of red will manifest itself by definitely increasing the brightness
when mixed with 60 parts of blue.

The red curves show distinctly the difference in the two groups
of dichromats. No blue is necessary for the match until beyond 530 ju/x.
The red protanopic maximum is at 571 jUju, ; the sharp fall towards
the red end shows the relatively low stimulus value for the long-wave
light. The deuteranopic maximum is at 603 jm/x, and the curve does
not fall to so low a value at the red end. Rivers^ has therefore intro-
duced the terms scoterythrous and photerythrous for protanopes and
deuteranopes respectively. The advantage of these terms is that they
are purely descriptive of observed facts and are quite independent of
any theory. Unfortunately they have not been generally adopted,
and it has been thought best in this book to use v, Kries' terminology.
The protanopes are relatively more sensitive to the short-wave light,
the deuteranopes to the long-wave. In order that a yellow and a red
may look equally bright a protanope will require a much more intense
red than a deuteranope. This fact cannot be explained on physical
grounds, such as macular pigmentation, since these rays are not
appreciably absorbed by the yellow pigment.

Further to prove that the fact is true of the two groups v. Kries
made thirteen equality of brightness observations on 9 deuteranopes
and eleven on 11 protanopes, using the red lithium line (670-5 /z/x) with
the yellow sodium (D) line (589-2 /x/x). The protanopes required the
following quantities of red : 214, 213, 211, 205, 196, 198, 210, 200, 210,
203, 235 : the deuteranopes required : 36-5, 36-3, 36-3, 36-5, 38-4, 37-3,
37-0, 37-0, 37-8, 37-0, 36-9, 38, 40. The protanopes therefore require
on an average five times as much red as the deuteranopes in order
that the red may appear as bright as the standard yellow. These
observations confirm earlier results obtained by Bonders^.

The low stimulus value of red for the protanopes accounts for the
so-called " shortening of the red end of the spectrum " in this class of
dichromats. The limit of visibility of the red end, however, varies
with normal people, and depends upon the intensity of the light,
whether the whole spectrum or only the isolated colour is observed,
and so on. Too much stress has often been put upon this point.

The blue curves show a general agreement, but more individual

1 Schafer's Text Book of Physiologij, 1091, 1900.

2 Arch.f. A licit, u. Physiol. 528, 1884.

DICHROMATIC VISION 169

variation, which is to be explained on physical grounds. This is the
part of the spectrum where macular pigmentation makes itself most
felt. Moreover, here too adaptation is most likely to introduce
complexity. It is probable that there is no crucial difference between
the blue stimulus values of the two groups.

Macular pigmentation also probably accounts for the difference
between the two red curves (Fig. 45) beyond 552 /x^u,. The ratios of
the red of one observer to that of the other throughout the spectrum
agree well with the determination of the absorption of the yellow pig-
ment by Sachs. Similarly the ratio of blue to red in two persons
gave the same value (0'3) as for the normal (v. Kries). The estimation
of macular absorption is much easier in dichromats than in normal
trichromats.

Macular pigmentation is a serious complication in the accurate
determination of the neutral point. This point, where the mono-
chromatic colour appears colourless or white to the dichromat, should
be situated w^here the W- and /i-curves intersect, and hence it should
be situated rather nearer the violet end in protanopes than in deuter-
anopes. If, however, there is much macular absorption the light will
look yellower and the neutral point will be displaced slightly towards
the red end. Indeed, the neutral point of a protanope with densely
pigmented macula may be nearer the red end than that of a deuteranope
with slightly pigmented macula, v. Kries has made the following
estimates, specially directed to elucidate this matter.

Wave length of homogeneous match
Nature of light

Daylight — reflected from magnesium oxide coated sur-
face
Light reflected from clouds — weakened by ground glass
:, „ „ „ weakened by smoked glass

These estimates were made with a comparison white light.

The existence of a neutral point shows that dichromats receive
the same sensory impression from what normal colour-sighted indi-
viduals call white, and also from a certain homogeneous spectral light
and from the mixture of red and blue which to them matches that
homogeneous light. The homogeneous light appears to the normal a
highly saturated bluish-green ; the red-blue match appears a red-
purple. If we suppress nuances it may therefore be said that one of
the chief characteristics of both groups of dichromats is that they
confuse red and green. But this is a very imperfect expression of

Deuteranope

Two protanopes

499 mm

498 fill 490 n/i

499 ,,

497 „ 489 „

495 „

494 „ 486 „

170 COLOUR VISION

the facts, for owing to the very low stimulus value of red for the
protanope he requires much more red in his red-blue match of a blue-
green homogeneous hue than does the deuteranope for the same amount
of blue. Hence a protanope confuses a slightly bluish red (in the
physical sense) with a dark green, e.g., scarlet with olive green, whereas
a deuteranope matches a much bluer red with a green which is of about
equal brightness to the normal. " The red which appears to match
a given green differs markedly both in colour-tone and intensity in
protanopes and deuteranopes " (v. Kries).

We have considered the conspicuous difference in dichromatic
vision from trichromatic vision. Its conspicuous similarity in an
important respect was pointed out by Seebeck in 1837, viz., that all
colour mixtures which appear equal to the normal eye, also appear
equal to the colour-blind. This statement is too general, and the
following is more accurate, all colour equations valid for normal vision
are also valid for dichromatic vision, or colour matches which are valid
for the trichromatic are also valid for the dichromatic eye. If this
statement can be fully substantiated it follow^s that the dichromat
possesses no variable which the trichromat lacks, but lacks a variable
which the trichromat possesses. In other words, dichromatic vision
IS, Q> reduction form (v. Kries) of normal vision, and not a fundamentally
different kind of vision.

V. Kries^ has made a very extensive series of observations of the
equivalence of trichromatic and dichromatic matches, and they
strongly support the validity of the law. Especially is this the case
for the red end of the spectrum, where macular pigmentation is least
disturbing and where the typical differences in the two groups of
dichromats are most obvious. Monochromatic yellow is matched by
a red (670-8 yLfx) and yellow-green (550 /z/x) mixture. As the ratio of
red to green is altered a suitable intensity of yellow can always be found
which will match the mixture. The match for the protanope, however,
is not valid usually for the deuteranope. For the former a very red
mixture matches a dark yellow and a very green mixture a bright
yellow. The deuteranope finds the mixture too bright in the first
case and too dark in the second. When the mixture is such that it
exactly matches the yellow for both dichromats it also matches for
trichromats, and that with extraordinary exactitude. In fact when
we succeed in finding matches with which both protanopes and

1 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xiii. 274, 1897.

DICHROMATIC VISION 171

deuteranopes agree trichromats will also agree with them. The follow-
ing table (v. Kries) bears out this statement.

Ratios of red (670 8 mm) to yellow-green (550 ntj.)
in mixtures wliich match the homogeneous light are

AVave-length
homogeneous

of
liglit

for deuteranopes
too dark

for protanopea
too light

for trichro
valid

639 ^M

0012

0026

0016

625 „

0038

0-062

0-044

613 „

007

0-12

0-09

589 „

0-22

0-49

0-33

569 „

1-00

300

1-34

The stimulus values of the red and green can indeed be calculated
from the matches for the normal. The latter are given on p. 34 and
are repeated in the next table. We see that the normal requires
88-5 units of standard red to match, both in colour and brightness,
670-8 /xjLt, and 71 units of standard green to match 550 /j-fx. A certain
deuteranope required 33 and 64 units respectively for the same matches.
Now for the normal trichromat,

Qk = Qr + Qg

where Qk is the quantity of homogeneous light, Q,. the quantity of red
(670'8/i/x), and Qg the quantity of green (552 /iyu) to make a perfect
match.

But in the method used, i.e., with the spectrophotometer, Q^ belongs
to one spectrum, and Qr and Qg belong to two spectra derived from
another source of light. By making r=\, we obtain the value of Q,- in
terms of the Qa spectrum, and similarly for g = \.

Now, if observations under identical conditions by different observers
are directly comparable,

Q,. = 88"5 ; Qg=l\ for the normal trichromat.

Q;. = 33 ; Qg=Qi for the deuteranope.

Therefore, in the intensity scale of the deuteranope

Qa = Q'r + Q'g

Where ^'' = 8^5' ^^^^ Q'^-^^-

For example, the normal trichromat requires 202 units of red and
67 units of green to match 591 /iyu., or

Q,,, = 202Q, + 67Qg.

We shall expect the intensity value of 591 [xn for the deuteranope to be

202x33 67x64 ,^,

-^^^+-Ti"=i^^-

172

COLOUR VISION

V. Kries found the observed value to be 137, and so on, as in the follow-
ing table.

Wave-length

of homo-
geneous light

Quantities of
670 8 HM and 552 HM
in normal match
670-8 fi/j. 552 MM

Stimulus value
for deuteranope
Calculated Observed

Stimulus value
for protanope
Calculated Observed

670-8

88-5

—

4-9

628

251

106

107

28-8

38-5

615

276

126

147

54-2

603

270

145

151

591

202

135

137

108

105

581

123

114

124

117

113

571

110

103

137

126

561

111

106

552

—

101

From this table we also see that the stimulus value of the yellow-
green (552 fMfju) is to that of the red (670-8 fiiA.) as 101 : 4-9 or about 5 : 1
for the protanope, whereas for the deuteranope it is only as 64 : 33 or
about 2:1.

The luminosity of the spectrum. The luminosity curves of dichromats
were investigated by Hillebrand (1889)^, who found the protanopic
maximum at 560 — 658 /x/x, and by Konig and Ritter (1891)^. The
following table gives the values for two trichromats and three
dichromats at the intensity value H.

Trichromats

Dichromats

Deuteranope

Protanopes

Wave-length

Konig Kottgen

Brodhun

Ritter

670

0-855 1-120

0-540

0-0518

0-071

650

2-381 2-137

1-368

0-155

0-183

625

3-460 3-413

2-630

0-493

0-517

605

3-650 3-247

3003

0-996

0-976

690

3-030 2-645

2-539

1-389

1-370

575

2-358 1-923

2-183

1-615

1-477

555

1-695 1-389

1-661

1-412

1-339

635

1- 1-

520

0-554 0-553

0-576

0-606

0-700

505

0-224 0-250

0-225

0-314

0-492

490

0-0994 0-092

0-0846

0-152

0-250

Figs. 47 and 48 show the curves.

Ritter's protanopic curve for the lowest intensity A is almost
identical with Konig's. Konig thus early demonstrated the identity
of the protanopic achromatic scotopic and the normal achromatic

1 Sitz. d. Wiener Akad. xcviii. 3, 70, 1889.

- Konig, p. 144.

DICHROMATIC VISION

173

scotopic luminosity curves. As the intensity increases the divergence
increases. At the highest intensity the protanopic maximum is at
about 560 nix as compared with the normal 615 fxfji. Brodhun's
deuteranopic maximum is almost identical with the normal. The
deuteranopic achromatic scotopic curve is also nearly identical with
both the normal and the protanopic, having a maximum at 535 /x/z.

Konig emphasises the fact that nothing as to heterochromatic
luminosity values can be deduced from the equivalence of colour
matches of two individuals, as of course follows directly from the
validity of such matches for both normal and dichromatic vision
{v. p. 170).

Light intensities FG H
E
D
C
B
A

670 650 625 605 590 575 555 535 520 505 490
B C D B F

470

4 50

4 30

Fisr.

47. Protanopic luminosity curves for different intensities of light, A being the
lowest, H the highest. Abscissae, wave-lengths of the prismatic spectrum of gas-
light ; ordinates, arbitrary scale. (Konig.)

Konig's curves agree well w^ith Abney's^ (Fig. 49). His normal
maximum is 585 /xjli, protanopic 559*6 /x/x, and deuteranopic 599-6 juju.

Polimanti^ obtained similar results with the flicker method (Figs. 50
and 51). The curves show the general resemblance of the flicker
periphery luminosity values. The normal maximum is 589 jtx/x, the
protanopic 565 /x/x, and the deuteranopic 606 /x/x. It is noteworthy, that
though Polimanti found his flicker and periphery values appreciably
different from Nagel's (deuteranope), v. Kries found no appreciable
difference between his own and Nagel's periphery values. However this
may be explained it is certain that the divergence of the deuteranopic
luminosity curve from the normal is less than that of the protanopic.

Watson^ has also published a series of observations by the flicker

^ Abney, p. 279; Proc. Roy. Soc. Lowl. A, Lxxxni. 1910.
^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xix. 263, 1899.
* Proc. Roy. Soc. Land. A, Lxxxvm. 404, 1913.

174

COLOUR VISION

method, which agree with those already mentioned. We may more
conveniently discuss his results at a later stage (Part III).

The spectrum as seen by the dark-adapted eye. Tschermak^ has
stated that the process of adaptation varies according to the type of

/-~>,

_ >

3-8-

3-6-

(

" Trichromats

3-4-

f \

\
\

~*'*'^^""""

3-2-

j 1
1 i

Deuteranope\

") f Dichromats

\ Protanopes )

1 i

\ \

3--

/ /

\ \

, . . . . Rfinhfnoco t'rflllP9 nffhf* rnrrntt

2-8-

/ /
/ /

■•. \

of the Spectrum

2-6-

/ / /

2-4-

1
1

;\A

i '

\\\

2-2-

/.' /"

V •'

^■^■

•. \

f \

^ ^\

1 1

\ A

V A

1-8-

i i

v>\

1-6

/ ,.

1-4-

1-2-

•'if •'

1 /

1
j

•V.

■■.'A

/.'

"'^-'--'•^ **\

V--

[ '

1
^

'/^

0-8-

0-6-

■;
j

0-4-

X. N.

0-2

..•/
"•

1 f'l

•

670 650 625 605 590 575 555
B C D

535 520 505
E

490

Fig. 48. Photopic luminosity curves (intensity H) of trichromats and dichromats.
Abscissae, wave-lengths of the prismatic spectrum of gaslight; ordinates, arbitrary
scale. (Konig.)

colour blindness, being normal in deuteranopes and much below normal
in protanopes. Nagel and v. Kries, however, are fully convinced that

^ Arch. f. d. ges. Physiol, lxx. 297, 1898 ; Ueber ■physiol. u. j)ath. Anpassung des Auges,
Leipzig, 1900 ; Ergeb. d. Physiol, i. 2, 700, 1902.

DICHROMATIC VISION

175

adaptation occurs exactly as in the normal in all types of colour
blindness.

It has already been mentioned that the achromatic scotopic
luminosity curve is identical with the normal in all cases of colour
blindness.

With regard to Purkinje's phenomenon, owing to the relatively
far greater ratio of the achromatic scotopic values of the yellow-green
region of the spectrum to the red as compared with normal vision all

■?'/

in.(t

_Li^

1 1

^- 1

■i^^

100

60 S

50 a

10 15 20 25 30 35 40 45 50

Scale of Priam^atir Spectrum.

Fig. 49. Photopic luminosity curves of trichromats and dichromats. (Abney.)

the effects are accentuated. This fact was early brought out by
Preyer^ and van der Weyde^ and more conclusively by Konig^. Thus
K5nig found a change of maximum brightness from 605 /xjn to 535 /x/x
for the deuteranope Brodhun and from 575 fx/x to 535 [xfi for the
protanope Ritter on diminution of the intensity of the light, associated
of course with increasing dark adaptation. Konig and Tonn made a

1 Arch, f.d.ges. Physiol. XXV. 31, IS^^- " Arch. f.Ophth.xxvni. 2, 1,1882.

3 Konig and Brodhun, Sih. d. Akad. d. Wiss. Berlin, 311, 1887 ; Brodhun, Ztsch. f.
Psychol, u. Physiol, d. Sinnesorg. v. 323, 1893 ; Tonn, op. cit. vn, 279, 1894.

176

COLOUR VISION

number of matches between a mixture of two spectral lights and mono-
chromatic light of an intermediate wave-length, and observed the efiect
of diminishing the intensity of the light, v. Kries and NageP took
greater precautions as to the condition of adaptation and found that
a scotopic colourless homogeneous blue-green possesses a six or seven
times higher scotopic value than the photopic equivalent red-blue
mixture. The greatest difference is shown between homogeneous reds
and yellow-greens. The latter, from 544 /x/x violetwards, give with red

.^.

100

^'■^

f/i

/
/
/
/ /

'; 1

/
-I
f

/ j

1
i

1
1

i 1

; 1

VS.
N >

■v

■■-'\

30
on

■I

>>.

i
/ /

'-^

10
0

■r

664
1 1 —

642

624

606

^ 1 —

589

565

553

— 1 —

543

526

509

Na-Q. Na-^-bN&-y /Va-0-5 Na.

/V«+4

/Va+i A'a+1'5 Ha-^1 Na*3

Fig. 50 Photopic luminosity curve of a protanope (flicker method).

Periphery himinosity curve of the same protanope.

Photopic luminosity curve of a trichromat (flicker method).

Periphery luminosity curve of the same trichromat. (Polimanti.)

exactly equivalent photopic values, whereas the achromatic scotopic
value of the yellow-green is more than 100 times that of the photopic
equivalent red (642 /x/x). These remarks apply to the deuteranope
and are illustrated in Fig. 52. Similar though less marked differences
occur in protanopes.

The differences to which reference has already been made (Part I,
Section IV, Chap, i) between the photopic and scotopic periphery
values of the trichromat are accentuated in the dichromat. Colours

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xii. 1, 1896.

DICHROMATIC VISION

177

which, when viewed by peripheral parts of the retina, accurately
match under light adaptation and with high intensities of light, may
possess very unequal achromatic scotopic values, and the inequality
is much greater with dichromatic than with trichromatic vision.

Regional effects. As might be expected the dichromat, like the
trichromat, sees colours best with foveal stimulation. For certain
conditions, indeed, his powers of discrimination with central vision

120

t1 0

100

■7

.-^

/
/ /

'tS;

■ I'

: T'

^!>.

'^>

ill

"-«^

'I
/

^•N.

>>s

""■■■■•.

^ 1

*'s.

687

I— 1 —

664

1 1 1

G42

624

606

— '

589

1 1 1

565
L— + —

563

543

1 •

520

50 9.

t^a-lb Na.-'2N^.-■\■^Na-^ Na.-0-b Na.

;7a+1 //SH ; 5 A;2+2

A£+a

;Va+4

Fig. 51.

Photopic luminosity eurve of a deuteranope (flicker method).

Periphery luminosity curve of the same deuteranope.

Photopic luminosity curve of the same trichromat as in Fig. 50

(flicker method).
Peripliery luminosity curve of the same trichromat. (Polimanti.)

are markedly superior to those of the normal. Thus, in the protanope
a yellow-green light (545 jit/x) possesses at least 100 times the achromatic
scotopic value of its photopic red (670 ju/x or higher) match, whereas
in the normal the maximum ratio is 6:1. He is, therefore, in a
peculiarly advantageous condition for determining whether the fovea
is sensitive to changes of adaptation. Nagel^, a deuteranope, was

^ V. Kries and Nagel, op. cit. xxin. 161, 1900.

P. c. V.

178

COLOUR VISION

unable to discover any difference produced by dark adaptation in
colour matches with small central fields. A greenish yellow spot on a
red background appeared to match the background both for the light-
adapted and dark-adapted eye so long as it was fixated exactly. On

— "

—

300

1 ^

—

— '

74.7

2 C

5 (

5 7

8 f

) 1

0 1

1 19 1

q.«=

^ 1

fi.e;

R 1

q.p

1 o

n o

q '

05 O 0> CTJ O CT> Cl

ocntocDCico-. — _._.

oi oi CI o en

05 Ol -t* CO
■fw O)

■ft
■ft

Fig. 52.

Scotopic values of the homogeneous lights.

Scotopic

values of the photopic equivalent red (642 ^/u) — blue (460-8 mm) mixture. Abscissae,
wave-lengths of the prismatic spectrum of gaslight ; ordinates, arbitrary scale,
(v. Kries and Nagel.)

slight deviation (1° to 2°) of the dark-adapted eye the spot appeared
much brighter than the background. The scotanopic foveal area
subtends a visual angle of about 1-5° (Nagel), but its exact dimensions
are difficult to measure.

DICHROMATIC VISION

17&

With regard to the periphery of the retina v. Kries^ compared a
long series of normal colour matches with those of dichromats. His
results have been confirmed by van der Weyde^. He found that whilst
the deuteranopic matches agreed well with the normal, the protanopic
matches were very different. The difference can be shown by the
different brightness of various coloured lights without any comparison
light (Levy^, Schenck*). The divergence is most marked for colourless
red-blue mixtures. Such a mixture which matches a grey background
for the normal also matches for the deuteranope within the somewhat
wide range of experimental error for such observations. For the
protanope such a mixture looks distinctly blue and much too dark ;
his match contains less blue and much more red.

100
90
80
70
60
50
40
30
20
10
0

--''"' ^^

7 -' ^

Z ^' ^ ^v_

V ^ ^-. -

t / -4- -^^it X

/ -' H^

/ /'' ^r^

.680 651 629 608 589 573 55£

530

513

Fig. 53. Photopic perij)heral luminosity curves for the normal trichromat ( )

and for the protanope ( ). Abscissae, wave-lengths of the prismatic spectrum

of gaslight; ordiaates, arbitrary scale, (v. Kries.)

Dichromats have a monochromatic peripheral zone as have trichro-
mats. The normal and protanopic periphery values for this zone are
shown in the curves in Fig. 53. The diminished sensitiveness for light
of long wave-length and relatively increased sensitiveness for light of
short wave-length in protanopes is clearly shown.

The deuteranopic periphery values indicate that the summit of
the curve is rather red-wards of the normal. Angier^ claims to have
proved more marked differences from the normal in this group of
dichromats, but as Frl. v. Maltzew^ has shown considerable individual
variations in normal trichromats the question must be left open.

^ Zisch. /. Psychol, u. Physiol, d. Sinnesorg. xv. 266, 1897

2 Onderzoekingen, Utrecht, iv. 3, 2, 1882.

3 Ztsch. f. Psychol, w. Physiol, d. Sinnesorg. xxx^^. 74, 1904.

* Arch. f. d. ges. Physiol. cx\Tn. 174, 1907.

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvn. 401, 1905.

* Ztsch. f. Sinnesphysiol. xun, 76, 1908.

12—2

180 COLOUR VISION

Areal Effects. NageP, who was a deuteranope, found that he was
only completely dichromatic for small fields. With larger fields,
involving the extra-foveal regions, he became partially trichromatic.
By fatiguing his eyes with red and orange lights he became completely
dichromatic. His perception of red was much better than that of
green, possibly due to red not causing strong contrast effect. Good
dark adaptation had the same effect as fatiguing with red. He found
a similar condition in 30 other dichromats. Stilling^ regards Nagel's
observations as erroneous.

The Colour Sensations of Proteinoses and Deuteranopes. Abney has
given illustrations of the names applied by dichromats to the various
parts of the spectrum. Since they have only two true colour sensations
the distinctions are based chiefly upon variations in luminosity, and
are therefore often contradictory. Most observers think that the two
sensations experienced correspond most closely to normal yellow and
blue^. Uncomplicated cases of unilateral congenital colour blindness
would afford valuable evidence, but most such cases recorded are of
doubtful value. One case for example was regarded by Holmgren*
as a typical protanope with shortening of the red end of the spectrum,
but V. HippeP found that he could see the rubidium line y. Both agree
that his colour sensations were yellow {D line) and blue (indium or
caesium line). Holmgren recorded a second case and Becker^ a third.
Hayes'' has recorded a case of a woman with partial protanopia in one
eye and normal vision in the other. So far as I am aware no other
congenital cases have been recorded, and pathological cases are value-
less for the purpose under consideration.

Tritanopia. Cases of tritanopia or so-called blue-blindness are rare
and mostly due to disease. Since the defect causes little risk of con-
fusion of red and green lights it is not of much practical importance.
It is, however, of considerable theoretical importance, and a bibliography
of the literature on the subject is appended. Goethe^ referred to the
condition as "akyanoblepsia^." Konig^" investigated five pathological

1 Arch.f. Anat. u. Physiol. 543, 1907; Zhch.f. Sinnesphy-siol. xliv. 5, 1909.
^ Ztsch.f. Sinnesphysiol. xliv. 371, 1909.
' Pole, Phil. Trans. Boy. Soc. Land, cxlix. 322, 1859.
* Centralbl.f. d. med. Wiss. 898, 913, 1880.

5 Arch.f. Ojihfh. xxv. 2, 205, 1879 ; xxvi. 2, 176, 1880 ; xxvn. 3, 47, 1881.
« CentralU. f. Augenhlk. 353, 1888. ' Amer. J. of Psychol, xxn. 369, 1912.

« Zur Farbenlehre, 1798. » Konig, p. 4.

" p. 396 ; Sitz. d. Akad. d. Wiss. Berlin, 718, 1897.

DICHROMATIC VISION 181

cases. The subjects match yellow (566 — 570 jxfx) with unsaturated
complementary blue. They confuse blue-green with blue, greenish
yellow with grey and rose-purple, yellowish green with bluish violet,
orange with reddish purple. They appear to be a reduction form of
trichromatic vision. Tritanopes call the red end of the spectrum
red, the violet end green or blue. Vintschgau and Hering's case
was congenital, but showed changes in the red and green, thus
approximating to total colour blindness. There was a neutral band
in the yellow (598- -568 /x/x), but the violet end also appeared
colourless, though of good luminosity and without marked shortening.
Yellow and blue could be matched with grey, but a match could not
be made between red and green. The most saturated green was at
532 /xfx, and the site of maximum brightness was between 558 fifj, and
565 /x/x. In Bonders' case the spectrum was shortened at each end
and the neutral band in the yellow occupied one-third of the spectrum.
Tritanopia appears to be commonest in cases of detachment of the
retina. It is simulated in cases of jaundice and sclerosis of the crystal-
line lens, these being due to absorption by yellow pigment. The same
ambiguity may arise in people with unusually dense macular pig-
mentation^.

Bibliography of Cases of Tritanopia

Stilling. Klin. Monatshl.f. Augenhlk. xm. Supij]. 2, 41, 1875 ; xiv. Suppl. 3, 1, 1875.

Cohn. Studien uber angeb. Farhenhlindheit, 139, Breslau, 1879.

Donders. Ann. d'ocul. xxxiv. 212, 1880.

Holmgren. Med. Zentralbl. xym. ; Centralbl. f. Augenhlk. v. 476, 1881.

Hermann. Inaug. diss. Dorpat, 1882.

Wundt. Phil. Studien, vm. 173, 1892.

Vintschgau. Arch. f. d. ges. Physiol. XL\Tn. 431 ; Lvn. 191, 1894.

Hering. Arch. f. d. ges. Physiol. Lvn. 308, 1894,

Konig. Sitz. d. Akad. d. Wiss. xxxiv. 718, 1897 (Pathological).

Piper. Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvin. 155, 1905 ^ Pathological).

Levy. Arch. f. Ophth. Lxn. 464, 1906.

CoUin and Nagel. Ztsch. f. Sinnesphysiol. xli. 2, 74, 1906 (Pathological).

Schenck. Arch. f. d. ges. Physiol, cxviu. 161, 1907.

Krienes. Ztschr. f. Augenhlk. xx. 392, 1908.

Kollner. Die Stdrungeii des Farbensinnes, Berlin, 1912 (Bibliography),

^ Cf. Abney, p. 343 ; Proc. Roy. Soc. Loud. xlix. 1891 ; Hess, Arch. f. Anat. lxi. 29,
1908.

182 COLOUR VISION

CHAPTER III

ANOMALOUS TRICHROMATIC VISION

Slight variations in the colour matches made by people with
apparently normal colour vision are commonly met with, and varia-
tions occur in the same individual under as nearly as possible identical
conditions at different times of examination. Setting aside physical
causes, such as differences in macular pigmentation, pigmentation of
the lens, etc., ample reasons for slight variations will be found in
successive induction, fatigue, the psychological state, and so on.
There yet remain, however, cases which cannot be explained on any
such grounds. The majority on exhaustive examination show inter-
mediate grades linking them with the normal on the one hand and the
dichromats (protanopes and deuteranopes) on the other. Konig called
this form of colour vision anomalous trichromatic vision. As Green-
wood saysi, " It is not improbable that these abnormal trichromatics
are extreme variants oi a frequency system representing the whole
range of visual types. The matter can only be settled when the
quantitative mixing ratios for a definite match have been determined
on a large number of persons taken at random ; we may then find
that the results are in accordance with some well-known frequency
distribution, the normal equation representing the modal value."

We owe the discovery of probably the two largest classes of these
anomalous trichromats to Lord Rayleigh^. They approximate to the
protanopes and deuteranopes. Lord Rayleigh found that if homo-
geneous yellow (Z) line, 589 ju^u,) is matched with a mixture of homo-
geneous red {Li line, 670-8 /x/z) and homogeneous green {Th line, 535 /xju.)
some persons require much more red, others much more green than
the normal. The latter are the more numerous class and may be
called partial deuteranopes {deuteranomal, Nagel). Of them, Bonders^
found four cases amongst 60 people examined ; Konig and Dieterici^
3 amongst 70. Many cases have been thoroughly investigated by
Hering^, Lotze^, v. Kries'^, and Abney and Watson. Of the partial

^ Physiology of the Special Senses, p. 137.

2 j^Tature, xxv. 64, 1882 ; Brit. Assoc. Rep. 728, 1890.

» Arch f. Anat. u. Physiol. 520, 1884.

* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. iv. 293, 1892.

^ Lotos, VI. 1885. * Dissertation, Freiburg, 1898.

' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xix. 64, 1899.

ANOMALOUS TRICHROMATIC VISION 183

protanopes {profanomal, Nagel) Bonders and Konig do not record any
case, V. Kries one only^, whereas NageP, Guttmann^, and Abney and
Watson record a considerable number. The two last have exceptional
opportunities as examiners for the Board of Trade. Their results,
of which only a very small proportion have been published, will be
discussed later (Part III).

v. Kries and his pupils made a series of investigations on the
ratio of green to red in the matches with intermediate homogeneous
lights for normal and anomalous trichromats. The quotients of long
series of average ratios is given in the following table, I for v. Kries
(normal) and Lotze (abnormal), II, for Halben (normal) and Lotze
(abnormal), III, for Polimanti (normal) and Zehender (abnormal).

jgeneou

3 light

Quotients
I II

Ill

Homogeneous
light

Quotients
(Levy)

628 mm

4-51

4-35

4-82 ■

625 Mu

0019

615 „

3-74

—

613 „

0123

603 „

3-15

3-7

417

601 „

0-230

591 „

314

— •

—

589 „

0-278

581 „

2-68

3-92

579 „

0-262

571 „

2-48

—

569 „

0-249

561 „

2-15

2-6

2-98

559 „

0-176

552 „

2-12

—

2-09

550 „

0-080

The table shows conclusively that the anomaly is the result of a
physiological condition, not of physical conditions, such as macular
pigmentation as suggested by Hering*. If it were due to the latter
cause all the quotients of any series would be the same. On the
contrary the quotients of partial deuteranopes diminish 23ari passu
with the diminution in wave-length of the comparison homogeneous
light.

The divergence is even more marked for the partial protanope
(Levy) ; here the quotients also vary with the wave-length, increasing
up to 589 [xfji and then diminishing.

If the results are compared with Breuer's^ for macular pigmentation
we find that his ratio of green to red for central fixation (1° field) was
I'l times that for paracentral fixation (3° field, 3° — 6° paracentral).
The difference therefore between normal and anomalous trichromats

^ Levy, Dissertation, Freiburg, 1903.

2 Klin. MonatsU. f. Augenhlk. xlii. 356, 1904.

3 Ztsch.f. Sinnesphysiol. XLn. 24, 250, 1907 ; XLni. 146, 199, 255, 411, 1909
« Lotos, \^. 142, 1885.

^ Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xm. 464, 1897.

184 COLOUR VISION

equally manifests itself with paracentral fixation. There might,
however, be more widespread dense macular pigmentation or pigmen-
tation of the refractive media. This possible factor can be tested by
comparison of the achromatic scotopic values of spectral lights for a
normal and an anomalous trichromat with a fixed blue (460 (jlix)
comparison light. The ratios of the quantities of blue to match the
homogeneous lights for Halben (normal) and Lotze (partial deuteranope)
were

Wave-lengths . .

591

581

571

561

552

544

536

529

Ratios . .

103

1-49

1-31

110

1-24
(0-99)

1-18

0-90

109

The difference is therefore not due to more extensive pigmentation.

As might be expected the luminosity curves of the anomalous
trichromats show corresponding differences from the normal (Watson^).
It has already been mentioned {v. p. 179) that v. Maltzew^ found
individual variations in trichromats, and these formed a complete
series between the trichromats and complete deuteranopes by way
of the partial deuteranopes. The protanopes and partial protanopes
showed a quite different luminosity distribution. In red-green mixtures
the red component is much more powerful than the green. She con-
cludes that the differences in colour matches between the normal and
the anomalous have a different cause from the individual variations
of the normal. The latter correspond with the luminosity differences,
whereas the former do not.

Guttmann^ has made important contributions to the knowledge
of anomalous trichromatic vision. He is himself a partial deuteranope.
He finds that anomalous trichromats show seven chief differences from
normal trichromats.

(1) For partial deuteranopes the discernible difference in hue is
smaller in the yellow and greater in the green than for the normal.
The difference in the green is not marked, but that in the yellow
is very appreciable. Guttmann's mean error {v. p. 19) at 589 /xfi
was from 12 to 13 /x/x, whereas that for the normal under like conditions
was 1 to 2 fx^.

(2) The anomalous deuteranopes are more dependent on intensity.

1 Proc. Roy. Soc. Land. A, Lxxxvm. 1913 ; see Part III.

2 Ztsch. f. Sinnesphysiol. xun. 76, 1908.

3 Neurol. Centbl. 491, 1904; Ztsch. f. Sinnesphysiol. xlh. 24, 250, 1907 ; XLm. 146,
199, 255, 1908-9; Munch, med. Wochenschr. 566, 1910 ; Ztsch. f. Sinnesphysiol. l\ti. 271,
1910.

ANOMALOUS TRICHROMATIC VISION 185

They can only distinguish colours at the optimum intensity, and their
optimum is narrower than the normal and of greater intensity.

(3) They are more dependent on luminosity differences, which
they are expert in translating into hue, thus showing a specially well
developed capacity for distinguishing differences in brightness of
coloured objects.

(4) They need a considerably larger area of stimulation or visual
angle for the accurate perception of hue, a fact confirmed by Nagel^.
Diminution in field causes a marked diminution in their capacity for
distinguishing differences in hue.

(5) They need longer time for the correct perception of colours,
a fact of great practical importance in certain occupations.

(6) Fatigue for colour, and with it defective discrimination, occurs
more rapidly than in the normal,

(7) They have a very marked increase in simultaneous contrast
effects as compared with the normal. Contrast depends to an excep-
tional extent upon the luminosities of the contrasting fields, and the
anomalous trichromatic colour thresholds are largely dependent upon
it. NageP also drew attention to the acuteness for contrast of the
partial deuteranopes. He found that a red and a yellow light, side
by side, of equal luminosity to the normal appeared different to them,
and they often called the yellow greenish or green, whereas in the
absence of the red light the yellow was correctly named.

Guttmann found his own periphery values agreed with the normal.
Like Donders, he calls anomalous trichromatic vision a " weakness "
of colour vision, a term to which Nagel objects. Guttmann found that
-the " extreme anomalous dichromats " nearly resemble complete
dichromats, and show diminished differential sensibility for hue,
rapid fatigue and increased perception of simultaneous contrast
effects. He also found the after-images of anomalous dichromats
agreed with those of the dichromats, but only with homogeneous lights.
With pigments the after-images appeared to resemble those of the
normal, and no accurate deductions could be drawn without comparison
lights, since the naming of colours is very liable to lead to mistakes.

Koffka^, himself a partial protanope, confirmed Guttmann's general
conclusions for this group. They show the same greater sensitiveness

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxix. 1905.
2 Klin. Monatshl. f. Augeiihlk. XLn. 356, 1904.
' Ztsch. f. Sinnesphysiol. XLm. 123, 1908.

186 COLOUR VISION

to simultaneous contrast effects, but these are spread over the whole
spectrum with the exception of yellow (compare the partial deuter-
anopes). The contrast-responding field is equal in luminosity to that
of the contrast-exciting field. Contrast extends over a larger area and
commences more rapidly than in the normal. It is strongest in the
green, and is greatest for small fields except towards the violet end.
Colour-fatigue sets in more quickly with them, as with the partial
deuteranopes. The two groups are thus easily confused. The normal
variations in the Rayleigh equation — matching D light with mixed
thallium- and lithium-line lights — are more restricted in one group
of partial deuteranopes than in the partial protanopes.

It has been found that the partial dichromats vary relatively
little in their individual matches for the sodium line. If, however,
they match a wave-length materially shorter than 589 /z/x their matches
become very indefinite and variable. Considerable quantities of red
can be added without upsetting the match. On the other hand, for
536 /x/x for example, the red can be reduced almost to zero without
the fields ceasing to be at any rate nearly equal. These are extreme
cases of partial deuteranopia, and they therefore show very defective
sensibility for colour difl'erences in the neighbourhood of 570 — 535 /x/x.
Similarly there are extreme cases of partial protanopia in which pure
yellow and especially lights of somewhat greater wave-length appear
almost the same as the spectral red.

CHAPTER IV

MONOCHROMATIC VISION

There are certain persons who appear to have no perception of
colour ; their vision is monochromatic. The condition is congenital,
and is known as total colour blindness or achromatopia. The subjects,
apart from the colour defect, usually have very bad central vision
(less than -f^), marked photophobia, and nystagmus. Several members
of a family (11 groups) may be affected either with the same condition
or less commonly with other forms of dyschromatopia. Consan-
guinity (five cases) and heredity appear to be factors in the incidence
of the disease. Males are afi^ected about twice as often as females.
Nettleshipi has published 10 pedigrees containing 34 cases affected :
18 males, 15 females, and 1 of unrecorded sex.

1 Trans. Ophlh. Soc. xxix. cxci, 1900.

MONOCHROMATIC VISION 187

Grunert^ has collected all the recorded cases up to the date of his
paper, adding four others. Cases have since been recorded by Bj errum^,
Ronne^, Hessberg^, Kollner^, Juler^, and Gertz'^.

The total of published cases is 84 (Nettleship, 1909). Refractive
errors are common, generally moderate myopia ; correction produces
little improvement of vision. One case recorded by v. Hippel and
Uhthoff^ could see Rontgen rays, but this observation has not been
confirmed and was negatived in Ronne's case. The only case with
good central vision was Frau Prof. R., examined by Raehlmann^,
but this case is peculiar in other respects. Obj ective causes for defective
central vision have been found occasionally : — macular changes^",
pallor of the discs^^, moderate albinism^^, and corneal nebulae^^.
In the majority of cases the ophthalmoscopic appearances are quite
normal. In Konig's experiments^* on the influence of intensity of
illumination on visual acuity, as the intensity rises the curve shows a
well-marked angle where the intensity changes from scotopic to pho-
topic. In the wholly colour-blind Konig found that this angle was
absent, the scotopic part of the curve being continued in the same
direction without interruption until an intensity was reached which
caused dazzling and diminished visual acuity. In monochromatic
vision moreover the usual diminution in acuity in the parafoveal region
is not noticed.

Photophobia and nystagmus are almost always present. The
former increases with the intensity of the light and is absent with low
illuminations. Bright light causes an unpleasant sensation of a luminous
cloud before the eyes. Central vision is also slightly improved by

1 Arch.f. Ophth. LVi. 1, 132, 1903. ^ Hospiialstidende, 1145, 1904.

3 Ibid. 1230, 1906 ; Klin. Monatshl. f. Augenhlk. xliv. Beilageheft, 193, 1906.

* Klin. Monatshl. f. Augenhlk. xl\ti. 2, 129, 1909.

^ Ztsch. f. Sinnesphysiol. xlhi. 409, 1909.

« OpJithal. Rev. 65, 1910. ' Arch.f. Augenhlk. Lxx. 202, 228, 1911.

» Bericht. d. Ophth. Gesellschft. Heidelberg, 150, 158, 1898.

^ Wochenschr. f. Therapieu. Hygiene d. Auges, ii. 165, 1899 ; Ztsch. f. Augenhlk. ii. 315,
403, 1900.

" Nettleship, St Thomas's Hosp. Rep. x. 37, 1880; Uhthoff, Ztsch. f. Psychol, u. Physiol,
d. Sinnesorg. xx. 326, 1899; Nagel, Arch. f. Augenhlk. xliv. 153, 1901; Hess, Ztsch. f.
Psychol, u. Physiol, d. Sinnesorg. xxix. 99, 1902 ; and Grunert, loc. cit.

" Landolt, Arch. d'Opht. iv. 211, 1893 and Grunert, loc. cit.

12 Uhthoff, op. cit, xxn. 1, 200 and Konig and Dieterici, the same case as Uhthoff's,
Sitz. d. Akad. d. Wiss. 805, 1886.

" Raehlmann, Arch.f. Ophth. xxn. 29, 1879.

1* Sitz. d. Akad. d. Wiss. Berlin, 559, 1897 ; see also Parsons, Roy. Loud. Ophth. Hosp.
Rep. XIX. 2, 283, 1914.

188 COLOUR VISION

lowered illumination. Uhthofi found the maximum at 12 metre-
candles ; the normal maximum is 30 metre-candles or over. The
scotopic visual acuity resembles the normal, and the light sense of the
totally colour-blind, whenever it has been tested, has been found good.
No objective cause can be found for the photophobia. The unpleasant
sensation experienced in a bright light is due rather to the interference
with vision than to any painful sensation. Thus Nagel and May's
patient^ could look at an Auer burner without discomfort ; she was,
however, practically blind for several minutes after doing so. Simi-
larly, examination with Thorner's ophthalmoscope and dilated pupil
caused no discomfort, but great diminution of vision followed for a
quarter of an hour.

The nystagmus is not found quite so often as the amblyopia and
photophobia. Amongst Grunert's last 23 cases it is not mentioned by
Fukala^, nor in two cases recorded by Hess. In Nagel's case it was
absent in one eye. In v. Hippel's first case it was present, but had
disappeared four years later when examined by Hess and Hering^.
Grunert noticed diminution in one case in the course of eleven years.
These were all elderly people, whereas most of the recorded cases were
children. The nystagmus, sometimes rather slow in the condition of
rest, becomes very rapid, with short excursions, on fixation. There is
a striking resemblance between the nystagmus and that met with in
miners*. Many cases showed divergent strabismus, and when absent
it could be elicited by eliminating the tendency to binocular vision, as
on shutting the eyes or going to sleep. The defect of foveal vision
renders binocular fixation difficult and the eyes readily take up a
position of rest, which is usually one of divergence.

The field of vision is generally normal, though Grunert found ring
scotomata in one of his cases. Of greater theoretical interest is the
frequency of an absolute central scotoma. Owing largely to the
accompanying nystagmus its presence is very difficult to demonstrate,
and certainly undue importance has been attached to it since Mrs Ladd-
Franklin (1892) and Konig (1894) hypothecated it on theoretical
grounds. Nearly 30 cases have been carefully examined as to
this point. Central scotoma was absent in 14 cases, published by
V. Hippel (1), Pfluger (1), v. Kries (1), Hess (5), Grunert (2), Juler (3),

^ Ztsch. f. Sinnesphysiol. XLn. 69, 1907.

- Klin. Monatsbl. f. Augenlilk. xxxvi. 175, 1898.

2 Arch. /. d. gcs. Physiol. Lxxi. 105, 1898.

* Llewellyn, Miners' Nystagmus, London. 1912.

MONOCHROMATIC VISION

189

and Gertz (1). Gertz used a special method and was convinced that if
there were a central scotoma it was less than 50'. There was a central
scotoma in nine cases: Konig (1), Nagel (1), Uhthoff (3), Grunert (3),
and Hessberg (1). Bjerrum and Ronne were unable to decide with
certainty; in Bjerrum's case the blind spot could not be demonstrated.
The positive cases include those with macular changes and albinism,
but several had quite normal fundi. Whether all cases have an
absolute central scotoma or not, it is certain that foveal vision is
very defective even as compared with parafoveal, as is admitted by
Hess.

300

280

260

240

220

200

180

160

140

120

100

,._

■>

^ ^

-* — 1

ji— i

—

r — 1

JTrn

0 1 2 3 4 5 6 7 8 9 10111213-5 1516-5 18.19-5 2122 2324-7 262728 29 30 31

CT Cl 01 o> o

^ CJi -^ ^5 —

o fn »o o) CJ1

Oi c-i tn
O lO CJ
CO — -

c^ m cji oi m

-. _. ro

^ o

■t'

*■

-fc

Fig. 54. Achromatic scotopic luminosity curve of a deuteranope.

Luminosity curve of a monochromat. Abscissae, wave-lengths of the

prismatic spectrum of gaslight ; ordinates, arbitrary scale, (v. Kries.)

The spectrum appears to the totally colour-blind as a monochromatic
strip, which is usually described as grey. No colour distinctions are
made, but the luminosity varies in different parts. The part which
appears to them brightest is what the normal call green, and when the
luminosity curve is worked out it is found to agree in a very remarkable
manner with the normal achromatic scotopic luminosity curve. This
fact was first discovered by Hering and Hillebrand^ and has received
conclusive confirmation, notably in the researches of Konig and

^ Sitz. d. Wiener Akad. xcvin 70, 1889.

190

COLOUR VISION

Dieterici^ and Konig and Ritter-, Hering^, v. Kries'*, Nagel and May^,
and Abney and Watson. The maximum luminosity with the gaslight
spectrum is about 536 /x/x, with sunlight about 527 ju/x. The red end is
shortened, but not the violet (Figs. 54 and 55).

Fig. 55. — Achromatic scotopic luminosity curve of a deuteranope.

Luminosity curve of a monochromat. Abscissae, wave-lengths of the

prismatic spectrum of the Nernst light ; ordinates, arbitrary scale. (Nagel and
May.)

Whereas, however, increasing the intensity of the light shifts the
maximum for the normal towards the yellow (see Section III, Chap, ii),
it merely increases the luminosity for monochromatic vision. The
curve is also independent of adaptation. We have already drawn

1 Sitz. d. Akad. d. Wiss. Berlin, 805, 1886 ; Ztsch. f. Psychol, u. Physiol, d. Sinnesorg.
IV. 241, 1892.

2 Konig, p. 144. ^ Arch. f. d. ges. Physiol, xlix. 598, 601, 1891.
* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xiii. 239, 1897.

8 Ztsch. f. Sinntsphysiol. xlii. 69, 1907.

MONOCHROMATIC VISION 191

attention to the similarity of the scotopic luminosity curve and the curve
of bleaching values for different monochromatic lights for the visual
purple (p. 55).

T. C. Porter's and Schaternikoff's experiments, especially the
latter, (p. 95), have shown that the rate of change of fusion frequency
of intermittent stimulation of the retina is lower for achromatic
scotopic than for photopic vision. Uhthoff ^ examined one of his cases
and found that the totally colour-blind reacts like the normal with
achromatic scotopic vision. A rotating disc with black and white sectors
ceases to flicker with an intensity of light even above the normal
photopic threshold whilst it is still flickering to the normal eye. Nagel
has confirmed this observation.

The facts of monochromatic vision are inconsistent with the view
that it is a simple reduction form of trichromatic vision, though
some cases have been regarded as of this nature. The theoretical
explanation will be discussed in Part III, Section I.

Bibliography of Cases op Monochromatic Vision

Huddart. Phil. Trans. Roy. Soc. Lond. Lxvn. 260, 1777.

Schopenhauer (1812). Griesebachsche Ausgabe, vi. 81, 1875.

Rosier. Obs. sur la 'physique, etc. xm. 87, 1779.

d'Hombres-Firmas. C. r. acad. des sci. n. 1849.

Rose. Arch. f. Ophth. xn. 2, 98, 1860.

Galezowski. Du diagnostic des maladies des yeux par la chromatoscopie retinienne, Paris,
1868.

Bonders. Klin. Monatsbl. f. Augenhlk. 470, 1871.

Raehlmann. Arch. f. Ophth. xxn. 29, 1879 ; Zisch. f. Augenhlk. n. 315, 403, 1900.

Cohn. Studien uber angeb. Farbenblindheit, Breslau, 1879.

Becker. Arch. f. Ophth. xxv. 2, 205, 1879.

Nettleship. St Thomas's Hasp. Rep. x. 37, 1880 ; Roy. Lond. Hasp. Ophth. Rep. xi.
373, 1887.

Magnus. Centralbl. f. Augenhlk: iv. 373, 1881.

Landolt. Arch. d'Opht. i. 114, 1881 ; xi. 202, 1891.

Scholer and Uhthoff. Bcitrdge z. Path. d. Sehnerven, BerUn, 1884.

Dor. Rev. gen. d'Opht. iv. 434, 1885.

Konig and Dieterici. Sifz. d. Akad. d. Wiss. Berlin, 805, 1886; Uhthoff, Arch. f.
Ophth. xxxn. 1, 200, 1886 : Konig, Arch. f. Psychiat. und Nervenkr. xxi. 284, 1889 ; in
Konig, p. 206.

Kressig. Mitt. a. d. ophth. Klinik zu Tubingen, n. 332, 1890.

Qucrenghi. Ann. di Ottal. 35, 1891 ; Ann. d'ocnl. cvi. 333, 1891.

Hering. Arch.f. d. ges. Physiol. XLix. 563, 1891.

V. Kries. Zisch. /. Psychol, u. Physiol, d. Sinnesorg. xm. 241, 1897.

Colburn. Amer. J. of Ophth. xiv. 237, 1897.

1 V. Kries, Zisch. f. Psychol, u. Physiol, d. Sinnesorg. xxxn. 113, 1903.

192 COLOUR VISION

A. V. Hippel. Festschrift, Halle, 1894 ; Bericht. d. Ophth. Gesellschf. Heidelberg, 150,
1898.

Hess and Heriiig. Arch. f. d. ges. Physiol, lxxi. 105, 1898.

Pfliiger. Bericht. d. Ophth. Gesellschf. Heidelberg, 166, 1898; Internal. Congress,
Moscow, 315, 1898.

Fukala. Klin. Monaishl. f. Augenhllc. xxxvi. 175, 1898.

Uhthoff. Bericht. d. Ophth. Gesellschf. Heidelberg, 158, 1898 ; Ztsch. f. Psychol, u.
Physiol, d. Sinnesorg. xx. 326, 1899 ; xxvn. 344, 1902.

Hess. Arch. f. Ophth. Li. 225, 1900 ; Ztsch, f. Psychol, u. Physiol, d. Sinnesorg. xxix.
99, 1902.

Abney. Proc. Roy. Soc. Loud. Lxvi. 179, 1900.

Nagel. Arch. f. Ophth. XLiv. 153, 1901.

Pergens. Klin. Monatsbl. f. Augenhlk. XL. 2, 46, 1902.

Grunert. Arch. f. Ophth. lvi. 1, 132, 1903.

Wehrli. In Nagel's Jahresbericht f. Op)hth. xxxiv. 92, 1903.

Bjerrum. Hospitalstidende, 1145, 1904.

Piper. Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvrrr, 155, 1905.

Ronne. Kliii. Monatsbl. f. Augenhlk. xliv. Beilageheft, 193, 1906.

Nettleship and Holmes Spicer. Trans. Ophth. Soc. xxvrn. 83, 1908.

Nettleship. Trans. 02)hth. Soc. xxix. cxci, 1909.

Hessberg. Klin. Monatshl. f. Augenhlk. XLvn. 2, 129, 1909.

KoUner. Ztsch. f. Sinnesphysiol. xlhi. 409, 1909.

Juler. Ophth. Rev. 65, 1910.

Gertz. Arch. f. Augenhlk. lxx. 202, 228, 1911.

PART III

THE CHIEF THEORIES OF COLOUR VISION

SECTION I

GENERAL REVIEW
CHAPTER I

INTRODUCTION

In the previous parts I have endeavoured to set forth the best-
established facts of normal and abnormal colour vision. There are
other equally well established facts, but they are so intimately associated
with the theories which we have now to consider that it has been thought
better to defer consideration of them. We have already, however,
accumulated sufficient to warrant an attempt at this stage to classify
them. Such as have been mentioned form the basis of the chief theories
of colour vision and will enable the reader to obtain a grasp of the
significance of these theories.

No theory has ever been propounded which suffices to explain all
the facts of the department of science to which it refers. If it succeeds
in offering a complete solution of all the problems it ceases to be a theory
and merits at least as secure a position in the domain of knowledge
as the facts themselves. As McDougall^ says, " the most fruitful
hypothesis ever developed is perhaps that of an ethereal medium to
which an impossible and inconceivable combination of properties is
assigned by physicists." Theories must be judged solely according to
their efficacy as working hypotheses. In so far as they serve the
purpose of sign-posts, pointing out the paths of future research, so far
are they of value. " Sterile theories easily relinquish immortality.
Fruitful theories hand down their immortal part to their children, while
their epheiueral shell falls to pieces " (Hering)^.

1 Brain, xxiv. 611, 1901. 2 L^iog^ n. f. i. 15, I88O.

P. c. V. 13

194 COLOUR VISION

The task which we have set before us is that of discovering in the
realm of vision " what psychical event a and what physical stimulus a
are universally connected in the order of nature, and of finding the
law by which a undergoes a definite change and becomes /3, when a by
a change equally definite (but definable only by a physical standard and
not a psychical one) becomes b (Lotze^)." In the accumulation of the
facts already set forth we may discern two distinct methods which I will
term the synthetic and the analytic. In every case there is a stimulus
which excites and a sensatipn of which the individual is conscious.
In most cases — one might say in all — both are complex. In the
investigation two methods may be adopted. In the first the stimulus
is reduced to its simplest form and the resulting sensation observed.
The sensation is evoked by the simplest possible physical stimuli :
hence the designation synthetic. In the second method the sensation
is reduced to its simplest form and the varieties of stimuli which
elicit this simple sensation are classified and analysed. This is the
analytic method. A priori one is inclined to attach more significance
to the results obtained by the synthetic method. It is the method
which prevails in the exact sciences, chemistry, physics, and so on.
Our sensations are notoriously liable to play us false. They are complex
and they are modified by all the other sensations to which the individual
is subject at the moment of observation. It is impossible to nullify
all the conflicting sensations and it is easy to overlook those potent to
mislead. On the other hand the problem itself deals with the sensation
and its modifications, and it may therefore be reasonably urged that
the analytic method of attack is of the essence of the problem. In
the exact sciences the sensations involved are generally simple and
well-defined and their modifications either complicate the observations
little, or are so disturbing as to indicate their own corrective. They
are one cause of " errors of observation," and the manner in which
these are minimised in the exact sciences is familiar to all.

The two methods are not antagonistic but complementary, and as
such should lead to identical results. That they do not is the fault of
ignorance and is not due to any innate defects. If we had certain
knowledge of all the effective factors in the physical stimuli and of all
the efiective factors in the resultant sensations and of all their quantita-
tive and qualitative relations, we should arrive at consistent results.
But that is a truism. It is the role of physiology to deal with com-
plex relations of this nature, and especially the role of physiological

^ Metaphysik; Bosanquet's translation, 295, 1894.

INTRODUCTION 195

psychology. A more material analogy may be drawn from metabolism.
Here we have more or less accurate information as to the nature and
constitution of the food material supplied to an organ or to the body
and as to the nature and constitution of the secreted or excreted material.
The intermediate steps are full of gaps. In the domain of the special
senses physiology is the realm of the intermediate steps, physics and
psychology the termini. It is natural that the physicist should adopt
the method characteristic of his branch of science, the psychologist
that characteristic of his own. But if we survey the advance of psy-
chology in recent years we cannot but be struck by the fact that it
has been largely due to the adoption of the synthetic method. Experi-
mental psychology and the elaboration of " psycho-physical laws " are
results of this tendency^.

It appears to me, therefore, that more credence is to be attached
to the results of the synthetic method. They alone are submissible
to approximately accurate quantitative estimates, and it is only by
quantitative estimates that the facts can be conclusively correlated.
We thus obtain a group of correlations which themselves attain the
dignity of precise knowledge. Outside this aristocracy of facts is a vast
multitude of undigested facts. Either they have not yet been sub-
mitted to accurate quantitative relationship or are as yet incapable of
so being. In so far as they fall into line with precise correlations they
are of confirmatory value. In so far as they are antagonistic they are
impotent to destroy these correlations, but ofier a well-stocked field
for further research.

Not that they themselves are incapable of correlation, but it is
qualitative in nature and therefore less precise. Always remembering
their relative smaller value as coins of the realm these qualitative
correlations are well worthy of extended study.

We have therefore in quantitative relations a criterion which can be
applied to the grouping of the facts which we have already brought
forward. If we rapidly survey the sections of Part I we shall find that
the quantitative relationships are best established for luminosity and
colour with the photopic fovea and for luminosity with the achromatic
scotopic eye. When we consider peripheral vision, temporal, and areal
effects, both for photopic and scotopic vision, the quantitative relations
are far less well established. The same grouping applies to Part II.

Bearing in mind the relative precision of our knowledge of the
various facts we can at once hypothecate certain relationships with

^ Cf. Myers, Experimental Psychology, Chap, i, London, 1909.

13—2

196 COLOUR VISION

greater or less accuracy. First, there is the divergence between photopic
and achromatic scotopic vision, a divergence so striking as almost
necessarily to hypothecate diverse mechanisms. Second, there is the
outstanding triplex relationship between colour and colour-less sensa-
tions that three colour-components suffice to arouse almost the whole
gamut of colour and colour-less sensations. Third, — and here the relative
precision of our knowledge is less — there is a certain opposition evinced
by certain colour sensations to each other, between black and white,
and between the complementary colours. Observations by the analytic
method particularly indicate that there are specially differentiated
opponent activities between certain particular colour sensations, such
as those of red and green, yellow and blue.

CHAPTER II

HISTORICAL REVIEW OF MODERN THEORIES
OF COLOUR VISION

In 1866 Max Schultze^, as the result of anatomical researches, human
and comparative, came to the conclusion that the rods were the more
primitive organ of vision and were concerned with the perception of
light, but were incapable of initiating impulses leading to colour percep-
tion. The cones he regarded as more highly differentiated and capable
of initiating impulses leading to both light and colour perceptions.
The only arguments available to him were (1) the parallel diminution
of visual acuity and colour sensations in indirect vision and of the
cones in the periphery of the retina : (2) the absence of cones in certain
animals of nocturnal habits (bat, hedge-hog, mole, eel, etc. ; relative
paucity in the owl) and the paucity of rods in certain animals of diurnal
habits (lizard, snake) {v. p. 10).

The discoveries of the visual purple by H. Mliller (1851) and that it
was bleached by the action of light by Boll (1876) were followed by the
exhaustive researches of Kiihne (1878). They led Klihne to the con-
clusion that we could not only perceive the spectrum by means of the
rods and the visual purple, without the intervention of the cones, but
that it would under these circumstances appear grey and colourless as
it does to the totally colour-blind. He hypothecated the existence of
other visual substances to account for the colour phenomena of vision

1 Arch f. milcr. Anat. n. 1866 ; iii. 371, 1867.

HISTORICAL REVIEW OF MODERN THEORIES 197

and thought that one such was the so-called visual yellow. Haab^
elaborated similar suggestions on the basis of Schultze's theory and
Hering's theory.

Charpentier (1877 sqq.) distinguished " perception lumineuse brute "
or " vision diffuse " from " vision nette," the former with vague localisa-
tion, the latter with sharply defined. The " photaesthesic elements "
initiate impulses which mutually modify each other as in the inter-
ference phenomena of light and thus the two apparatus, each capable
of initiating only colourless perceptions alone, become capable of giving
rise to colour-perceptions. He did not attempt to define the exact
locality of the apparatus ; it is supposed to be partly peripheral or
retinal, partly central or in the nervous system. He conjectured that
diffuse vision is initiated by the rods and visual purple, but only in
achromatic scotopic vision, since the visual purple can itself be viewed
entoptically^.

Parinaud^, in ignorance of Schultze's theory, elaborated much the
same views with more extensive material. He attributed the disease
known as night blindness -(often badly termed hemeralopia) to defects
in the rods and visual purple'*, and first demonstrated that central
vision is intact in this complaint. He asserted the failure of adapta-
tion at the fovea in 1884, having previously in 1881 pointed out that
the photochromatic interval is absent here. These facts, confirmed
by the night blindness of fowls and pigeons owing to absence of visual
purple, led him to the conclusion that the cones, which are alone present
in the fovea, are incapable of dark-adaptation, whereas achromatic
scotopic vision is a purely extrafoveal phenomenon and is the function
of the rods and visual purple. This is the " theory of two retinas."
Colour-perception according to Parinaud is a purely cerebral function.

Similar views were expressed by Liesegang^ and by Parinaud's
pupil Weiss^. Berry'^ suggested that the pigment epithelium is the
seat of colourless light sensations.

Konig^ also regarded the visual purple as the excitant of the rods
(KUhne) and with low intensities of light stimuli the basis of achromatic
scotopic vision. He considered the visual yellow, the first product of

^ Correspondenzbl. f. Schweizer Aerzte, ix. 1879.

^ C. r. soc. de biol. 1890 ; C. r. acad. des sci. 1891.

3 C. r. acad. des sci. 1881, 1884, 1885; Ann. d'ocul. lxxxv. 113, 1881; cxii. 228,
1894 ; La Vision, Paris, 1898.

* Arch. gin. de med. 1881. s photogr. Arch. 1891.

6 Rev. gin. des sci. 1895. ' Ophth. Rev. 1890.

8 Sitz. d. Akad. d. Wiss. Berlin, 1894.

198 COLOUR VISION

bleaching of visual purple with higher intensities, as the basis of the blue
sensation, which is therefore also carried out by the rods and is absent
from the fovea.

V. Kries^, also independently of any knowledge of Schultze's paper
and apparently of Parinaud's writings, elaborated a similar theory, the
duplicity theory (DupUzitdtstheorie), which has received wide accepta-
tion. It will be considered in Section II.

The physical basis of colour vision, as exemplified in the wonderful
researches of Sir Isaac Newton, opened up the ground for the modern
theories. Newton himself compared the simple colour notes of the
spectral scale to the tones of the musical scale. This idea was elaborated
by Hartley, Young, Drobisch, and others, but was soon found wanting^.
Attempts to explain the facts of colour-mixtures directly from the
undulatory theory of light by Challis, Grailich, and later Charpentier
have also proved inadequate^.

More recently Hartridge^ has attempted to give a purely physical
explanation of the sensation of yellow. Yellow (650 [xfu. — 560 [jl[jl) is
accurately matched by a suitable mixture of red (740 /x/x — 690 /^/a) and
green (550 ju./x— 510 yu/x) if a small quantity of white light is added to
the yellow. By drawing on mathematical paper two sine curves of
equal amphtude, which have periods corresponding approximately to
the mean wave-lengths of red and green components of this compound
yellow, it will be found that their summation gives a curve which
approximates closely to a sine curve of wave-length 620 /x/x. It shows
beats at every 2000 ^/x ; at the nodes the amplitude is almost zero
whereas at the antinodes it is nearly double that of either of the curves
of which it is compoimded. By compounding a red and green wave
motion one has obtained a yellow wave motion, the mean wave-length
of which approximates closely with that found by experiment.

If this hypothesis were accurate the combination of the same red and
green, polarised at right angles to each other, should fail to arouse the
sensation of yellow ; but such is not the case. Further, although the
compound curve obtained by combining the sine curves corresponding
to the red and green has between any two consecutive minima a period
equal to that of the yellow light, yet at each minimum the curve has

^ Especiallj' in Bericht d. naturf. Ges. zu Freiburg i. B. ix. 2, 1894 ; Arch. f. Ophth.
XLn. 3, 95, 1896 ; Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. ix. 81, 1896.
2 V. Helmholtz, 3rd ed. n. 97.

' Ibid. n. 129 ; Charpentier, La Lmniere et les Couleurs, Chap, xii, Paris, 1888.
* Proc. of the Physiol. Soc, J. of Physiol. XLV. p. xxix, 1913.

HISTORICAL REVIEW OF MODERN THEORIES 199

an abrupt change of phase of half a period (Fig. 56). Hence if a
disturbance of this character were to act on a system of which the
natural period corresponds to that of the yellow light there would be
little response, because the disturbance produced during the interval
between two consecutive minima would be neutralised by the disturbance
produced during the next interval. An illustration will make the point
clear. If a succession of correctly timed small impulses be given to a
heavy pendulum the pendulum will in time be set in violent motion.
If, however, after giving, e.g. four of these timed impulses the timing
is suddenly altered by half a period the new impulses will tend to check
the motion already communicated, and after four impulses the pendulum
will be brought to rest^.

Fig. 58. Combination of two sine curves.

In 1801, in the Bakerian Lecture before the Royal Society, Thomas
Young2 propounded the theory which now commonly bears the name
of the Young-Helmholtz Theory. His third hypothesis of " the theory
of light and colours " was that the sensations of difierent colours depend
upon the different frequencies of the vibrations of the light which falls
upon the retina. Since it is well nigh inconceivable that each sensitive
point of the retina possesses innumerable parts capable of responding
to the individual vibrations, he concluded from Newton's researches
that three parts would suffice to account for all the colour sensations,
including white. He chose three chief colours, red, yellow, and blue,
the wave-lengths of which were in the ratios 8:7:6. Each afiected a
corresponding part of the sensitive mechanism. In 1802, after Wollas-
ton's description of the spectrum, he chose red, green, and violet, which
he confirmed on experimental grounds in 1807. He considered that
the simplest conception of the fundamental " parts " was three kinds
of nerve fibre. Excitation of the first aroused the sensation of red,
of the second green, and of the third violet. The first type of fibre was
most excited by lights of long wave-length, the third by those of short
wave-length, and the second by those of intermediate wave-length, but

1 I am indebted to Prof. W. Watson, F.R.S., for this explanation.

2 Sec Roxj. Lond. Ophth. Hosp. Rep. xix. i. 1, 1913.

200 COLOUR VISION

light of all kinds excited all three kinds of fibres, though in varying
degrees.

Young's hypothesis received scant acknowledgment, largely owing
to his resuscitation of the undulatory theory of light, which was regarded
as a grave heresy by the majority of scientists, who firmly adhered to
Newton's corpuscular theory. It remained unheeded until the middle
of the nineteenth century, when v. Helmholtz^ and Clerk-Maxwell^
adopted it as the explanation of their experiments on the mixture of
colours. Grassmann^ simultaneously enunciated the laws of colour
mixture which bear his name.

An essential feature of the Young-Helmholtz theory is the additive
process, especially as applied to colourless light sensations. Addition
of physical stimuli entails additive physiological reactions resulting in
compounded psychical end results. Goethe^ approached the subject
from the psychologist's point of view and quickly accumulated facts
which were difficult to explain on the Newtonian basis. Such are
particularly the phenomena of after-images and simultaneous contrast.
The addition theory seemed incapable of supplying a satisfactory
explanation^. The claims of four fundamental colours as opposed to
three were emphasised by Aubert^, and this psychological point of view
became crystallised into the theory of opponent colours which bears
the name of Hering'.

The clash of the contending theories led to modifications of each.
The psychophysical difficulties led Bonders^, Ad. Fick^ and v. Kries to
limit what the latter calls the components theory (the Young-Helmholtz
theory) to the peripheral link in the visual path. v. Kries has elaborated
this view in the theory of zones.

Hering, regarding the question from the psychological point of view,
and endeavouring to bring the facts into a universal common relation-
ship with other physiological processes, emphasised the complementary
and opponent characters of certain fundamental colour sensations.
From the psychological point of view black, white, red, green, yellow,
and blue are fundamental sensations (Mach^°). All other hues appear to

1 Miiller's Arch. f. Anat. 1852. * Trans. Boy. Soc. Edin. xxi. 1855.

' Ann. d. Physik Lxxxix. 1853. * Zur Farbenlehre, 1810.

* See, however, McDougall, iyifra.

' Physiologie dcr Netzhaut, Breslau, 1865; Graefe-Saemisch, Handb. d. ges. Augenheil-
kunde, 1st ed. n. 1876.

' Sitz. d. Wiener Akad, 1872-1874 ; Zur Lehre vom Lichtsinne, 1876.

8 Arch. f. Ophth. xxvii. 2, 155, 1881 ; xxx. 1, 15, 1884.

» Arch. f. d. gei. Physiol, xvn. 152, 1878 ; XLin. 441, 1888.

" Sitz. d. Wiener Akad. loi. 2, 320, 1865.

HISTORICAL REVIEW OF MODERN THEORIES 201

our consciousness as mixtures of these sensations. They may be divided
into two groups, the tone-free or colourless (black and white) and the
toned or coloured. The tone-free are simple sensations, neither contains
any element of the other. The toned are also, inter se, simple sensations
in the same meaning of the term. Both groups may be divided into
three opponent pairs, black and white, red and green, yellow and blue.
So much for the fundamental psychological conceptions.

From the physiological point of view these opponent sensations can
be hypothetically brought into line with other physiological processes.
We can imagine three substances of unstable chemical constitution :
when white is seen the black-white substance undergoes dissimilation
{D) or katabolic change, when black the same substance undergoes
assimilation (^4) or anabolic change. The sensation is dependent upon
the ratio oi D : A, not upon the absolute intensity of either. Similarly
red is seen when the red-green substance undergoes the Z)-process,
green when it undergoes the ^-process ; and yellow when the yellow-
blue substance undergoes the Z)-process, blue when it undergoes the
^-process.

Several difficulties are at once apparent. First, while we readily
accept the proposition that katabolic changes arouse sensations it will
not be so readily admitted that anabolic changes can produce opponent
sensations, or indeed any sensations at all. Though this may affect
the physiological analogies of the theory it is not of fundamental
importance to the theory as a theory of colour vision. Opponent
chemical processes on the analogy of oxidation and reduction, or
electrical on the analogy of positive and negative electricity, and so on,
would serve the purpose equally well and avoid the difficulty.

Second, — what must, however, be conceded as a great difficulty of
all theories of colour vision, — black and white occupy a peculiar relation-
ship as compared with the others. Black is on this theory the result
of a physiological process excited by an internal stimulus and not the
result of mere absence of stimulation. Moreover, whereas we can pass
continuously from black to white, or vice versa, through innumerable
gradations of grey, there is no such uniform passage from red to green
or from yellow to blue. We can indeed pass by innumerable gradations
from red to yellow, yellow to green, green to blue, and blue to red,
respectively, but there are no intermediaries between red and green,
and yellow and blue, corresponding to grey between black and white.
Further, the fundamental toned colours differ in brightness and darkness,
and this may be regarded as an inherent black-white element in the

202 COLOUR VISION

colours. Hering has therefore found it necessary to hypothecate a
white valency for the toned colours, thereby introducing a complication
which mars the symmetry of his design.

The peculiar position of black and white in the gamut of colour
sensations early made itself felt. Woinow^ distinguished between the
white which is a mere sensation of light and is due to stimulation of
the rods, the binary red-green and yellow-blue whites, and the quaternary
white which is a combination of the two binaries. Preyer^ also dis-
tinguished between the whites due to stimulation of the chromatogenous
elements, the cones, and that due to the photogenous, the rods.
Konig^ distinguished between the simple white of rod stimulation and
the trichromatic white of the cones, v. Kries expressly adopts this
view and thinks that " it would perhaps be more correct not to say that
the rods initiate colourless sensations, but that they initiate a sensation-
effect which is variable in only one sense."'* There is certainly a
peculiarity, which may be described as a bluish greyness, about the
scotopic colourless sensation which distinguishes it from photopic
" white."

Evidence derived from the colour-blind led Young in his sole
reference to the subject^ to ascribe Dalton's defect to absence or paralysis
of the fibres which were supposed to initiate the red sensation. Konig^
adopted the explanation brought forward by Ad. Fick and Leber'^ that
so-called partial colour-blindness (dichromatism) is due to coincidence
of two of the elementary-sensation curves, v. Kries^ adopted v. Helm-
holtz' reduction theory or congenital absence of one of the fundamental
components. Both Konig and v. Kries accepted Liesegang's view^
that total colour-blindness (monochromatism) is due to rod- or visual
purple-vision. Hess and Hering^° adduce against this view the complete
analogy in the normal and totally colour-blind dark-adapted eyes of
the low central and increasing peripheral threshold values.

Later years have brought forth a crop of theories, mostly modifica-
tions of those already mentioned. The more important will be briefly
mentioned in the following pages.

1 Arch.f. Ophth. xxi. 1, 223, 1875- ^ Arch. f. d. ges. Physiol, xxv. 31, 1881.

3 Sitz. d. Akad. d. Wiss. Berlin, 1894.

* Ztsch. /. Psychol, u. Physiol, d. Sinnesorg. ix. 87 note, 1896.
^ Lectures on Natural Philosophy, ii. 315, 1807.

* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. iv. 241, 1892.
' Arch. f. Ophth. xv. 3, 26, 1869.

8 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xiii. 241, 473, 1897.

» Photogr. Arch. 1891. " Arch. f. d. ges. Physiol. Lxxi. 105, 1898.

SECTION II

THE DUPLICITY THEORY

The duplicity theory (v. Kries) states that achromatic scotopic
vision is carried out through the mediation of the rods alone, the
cones being the organ of photopic vision. It is not definitely stated
that rod activity is in abeyance in photopic vision, but there is some
evidence in favour of this view and in any case the influence of rod
vision in the light-adapted eye may be regarded as slight.

It has already been necessary for the sake of clearness to point out
the great probability of two separate mechanisms being involved in
these two very different types of vision {v. p. 57). Broadly speaking
vision with the dark-adapted eye, i.e., scotopic vision, is monochromatic
or tone-free. Vision with the light-adapted eye, i.e., photopic vision,
is polychromatic or toned. In the former the threshold stimulus
intensity is low ; in the latter relatively high. We have hitherto
stated the facts and attempted to regard them impartially. It will
now be instructive to review them briefly in the light of the duplicity
theory.

So far as anatomical considerations are concerned the distribution
of rods and cones in the retina is of prime importance. Rods are absent
from the fovea, an area subtending a visual angle of 50' — 70', and
also over a surrounding zone, the whole subtending a maximum of 3°.
At the same time it should be pointed out that the structural peculi-
arities of the rods and cones are not precisely differentiated. The
macular cones are elongated and more rod-like than those found in
peripheral parts of the retina^. This merging of structural peculiarities
is even more marked in the retinae of lower animals^, and while on the
whole later researches have confirmed the observations of Max Schultze
{v. p. 196), precise anatomical diversity of the two types of neuro-
epithelium has been somewhat discounted. Moreover, if we regard the

1 Cf. Greeff, in Graefe-Saemisch, 2nd ed. Bd. i. Abt. 2, Kap. V.

- Cf. Putter, in Graefe-Saemisch, Bd. n. Abt. 1, Kap. x ; Franz, in Oppel's Lehrbuch d.
vergleich. mikr. Anat. d. Wirbeltiere, vn. Jena, 1913.

204 COLOUR VISION

rods as the more primitive type of visual neuroepithelium, as we are
probably justified in doing, the persistence of recognisable rod attributes
in the cones, even if modified, differentiated, and rendered more complex,
might well be expected. Apart therefore from the difficulties of isolating
the physiological results of excitation of the rods from those of excita-
tion of the cones it may be anticipated that the latter cells will retain
some measure of the functions which are in the highest degree charac-
teristic of their prototypes. Hence, if it should ever be conclusively
proved that the rod-like foveal cones of the human eye possess some
trace of visual purple and are endowed with some slight degree of light-
adaptation it would not be surprising ; neither, on the other hand,
would it militate seriously against the view that the rods and cones
have become essentially diverse in function.

Beyond the rod-free central area the cones diminish continuously
in number and the rods correspondingly augment in passing towards the
periphery in all directions.

The chief characteristic of central vision in the photopic condition
is its great acuity as compared with peripheral vision. This acuity is
most marked in the form sense, and the rapid diminution in " visual
acuity " in passing from the centre towards the periphery is strong
evidence in favour of the view that the cones are the essential retinal
end-organs concerned in the discrimination of form. A striking feature
of the vision of lower animals not endowed with a specially differentiated
fovea is the remarkable acuteness in the perception of the movement
of objects. This feature is also prominent in human peripheral vision,
though it is certainly equally, and perhaps more highly, developed in
foveal vision. Thus Ruppert^ found that while visual acuity and
ability to perceive movements both diminish in passing from the
centre towards the periphery the former diminishes more rapidly than
the latter. On teleological grounds the perception of movement, a
function specially associated with the light sense as opposed to the
form sense, must be regarded as primitive ; and we have here, if such
be needed, an example of the persistence of this primitive attribute in
the more highly differentiated cones.

When the field of vision of the photopic eye is further investigated
it is found that the perception of colours requires an increasingly intense
stimulus in passing from the point of fixation towards the periphery.
The complicated details of peripheral colour vision have already been

^ Ztsch. f. Sinnesphysiol. xlh. 409, 1908 ; cf. Easier, Munch, med. Woch. p. 1904,
1906.

THE DUPLICITY THEORY 205

discussed, and though in photopic vision they afford little incisive
evidence in favour of the duplicity theory they cannot be regarded as
seriously discounting it. It is certain that the almost, if not complete,
colour blindness of the extreme periphery cannot be attributed to mere
paucity of cones. The great difference in luminosity values with different
lights, in spite of the tone-free perceptions to which they give rise, is
against such a view. We must therefore conclude that the scanty cones
of the periphery are incapable of arousing colour perceptions, or that
there is some block in the conduction of such impulses from this region.

The duplicity theory depends therefore for its support chiefly
upon the facts of achromatic scotopic in relation to those of photopic
vision, and in this relationship the evidence is abundant and confir-
matory.

On this theory the rods are the end-organ of achromatic scotopic,
the cones of photopic vision. In other words, the rods possess a very
high degree of adaptation, the cones little or none. The photopic
apparatus is isolated in the fovea, but the scotopic apparatus is nowhere
completely isolated. The threshold values for the cones, however, are
considerably higher than for the rods, and hence in complete dark
adaptation the rods become almost, if not wholly, isolated physiologically.
In dark adaptation, with low intensities of light, vision is carried out
through the rods alone (achromatic scotopia). As the intensities are
increased the liminal stimulation values for the cones are exceeded and
vision is carried out by both rods and cones (chromatic scotopia). At
still higher intensities vision is carried out chiefly or wholly by the
cones (photopia), but whether the rod effects are added to the cone
effects or are abolished is as yet uncertain.

If the rods are the organ of achromatic scotopic vision, the visual
purple, which so far as has yet been proved is present in them only,
attains a new significance. The early researches of Kiihne and Sewall^
sufficed to show the slight effect of rays of long wave-length upon this
substance, and they found that the maximum efl'ect is in the green,
not in the part of the spectrum which is brightest to the photopic eye.

Kottgen and Abelsdorff^ in more recent researches showed that the
curve of absorption values of the visual purple of mammals, birds and
amphibia with the interference spectrum of the Auer light has its
maximum at about 500 /x^u, of that of fishes at about 540 /x/x. These
results do not agree fully with Kiihne's, and further researches on human

1 Untersuchungen, Heidelberg, ni. 221, 1880.

2 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xii. 161, 1896.

206 COLOUR VISION

visual purple are desirable. Judging by the position of the maximum
it should approximate more nearly to the type found in fishes. It
should be borne in mind that visual purple has not been isolated in a
pure state, free from hasmoglobin.

Trendelenburg's curve for bleaching values of the frog's visual
purple agrees more closely with the achromatic scotopic luminosity
curve of the human eye (Fig. 1).

There can be no doubt that the visual purple is of fundamental
importance in scotopic vision and that its accumulation is the basis
of dark adaptation. It follows that the relation of the achromatic
scotopic luminosity values of two lights, e.g. a green which is strongly
absorbed and an orange which is weakly absorbed by the visual purple,
should depend upon the concentration of visual purple in the rods,
i.e., it should vary with the degree of dark adaptation. Stegmann^
has shown that such is the case. If a luminosity match is made between
green and orange with intensities at which they appear colourless after
5-15 minutes' dark adaptation, and the lights are again compared after
much more prolonged adaptation the orange appears much brighter
and must be diminished to about three-quarters of its former intensity
to restore the match. It is to be noted that the change is in the
opposite direction to that of the Purkinje phenomenon.

Sachs^ showed that the pupil-reactions vary with the luminosities
of the lights, independently of their colour, both in the normal and
colour-blind. Abelsdorff^ confirmed these observations and showed
further that maximum pupil-constriction occurs in nocturnal animals
from green light, whereas in animals of diurnal habits the maximum is
obtained from yellow light.

Similar, but more reliable and much more striking results, have been
obtained from the electrical reactions, especially in day- and night-birds
by Himstedt and NageH and Piper ^.

Vision of lights of low intensities with the dark-adapted eye is char-
acterised by (1) the absence of colour sensations, (2) a greatly increased
sensitiveness for lights of low intensity, (3) a relatively increased

1 Dissertation, Freiburg, 190].

2 Arch. f. d. ges. Physiol. Lii. 79, 1892 ; Arch. f. Oj)hth. xxxix. 3, 108, 1893 ; Ztsch. f.
Psychol, u. Physiol, d. Sinnesorg. xxii. 388, 1900.

' Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxii. 81, 451, 1900 ; Arch. f. Augenhlk.
XLi. 155, 1900 ; Arch. f. Anat. u. Physiol. 541, 1900.

* Ber. d. naturf. Ges. Freiburg, ii. 1901 ; Ann. d. Physik, iv. 1901 ; in v. Helmholtz,
3rd ed. n. p. 328.

* Arch. /. Anat. u. Physiol. 543, 1904.

THE DUPLICITY THEORY 207

sensitiveness for rays of short wave-length as compared with those
of long wave-length, red showing no appreciable increase. The
third characteristic is the cause of the Purkinje phenomenon. The
characteristics of rod vision will therefore be (1) total colour blindness,
(2) maximum sensitiveness in the situation of the green of the photopic
spectrum, and shortening of the red end, (3) a very high degree of
adaptation.

The theory explains satisfactorily the apparent deviations from
Newton's law of colour-mixtures described by the earlier investigators.
Of these, Hering^ and v. Kries and Brauneck^ held that colour equations
were independent of the intensity of the light. Konig and his pupils^
disagreed with this conclusion, and v. Kries found it necessary later to
modify his opinion^. The photopic values of the matches are essentially
cone-values. At lower intensities rod- and cone-values are mixed, and
at the lowest only rod- values persist. The results of Konig and his
pupils are partly vitiated by lack of attention to adaptation, but more
to wandering fixation and the use of objects whose retinal images
surpass the rod-free limits. The striking examples brought forward by
Ebbinghaus and Mrs Ladd-Franklin {v. p. 60) are easily explained by
the duplicity theory, and indeed strongly support it. v. Kries^ extended
these observations and explained the anomalous results of other observers.
He showed that it was theoretically possible to obtain a photopic
" white " equivalent to a scotopic " white," i.e., a " white " which
would lose only in subjective intensity on diminution of the physical
intensity. The difference in rod-values between a homogeneous scotopic
white and a mixture- white are so slight in normal colour-vision that
the results are ambiguous ; they are much more conclusive in dichromats.
The theory also accounts for the " wandering of the neutral point "
in dichromats on altering the physical intensity of the light. In most
of the observations referred to above macular pigmentation is a disturb-
ing influence (cf. Hering). In general form it may be stated that
matches valid for high intensities become invalid for low intensities and
dark-adaptation in the sense that the mixture which possesses the
greater rod-value exhibits the greater luminosity in tone-free scotopic
vision (v. Kries). The regional peculiarities of scotopic as compared
with photopic vision afford very strong evidence in favour of the

1 Lotos, vu. 1886 ; Arch. f. d. ges. Physiol, liv. 277, 1893.

2 Arch.f. Physiol. 79, 1885.

" Brodhun, Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. v. 323, 1893 ; Tonn, op. cit. vn.
279, 1894.

« V. Kries, op cit. ix. 81, 1896, etc 6 /jj^^ j^. gl, 1896

208 COLOUR VISION

duplicity theory. The " night blindness " of the fovea, with its almost
if not quite complete absence of dark adaptation, the absence of
Purkinje's phenomenon on direct fixation with sufficiently small areas
of stimulation, the almost if not quite complete absence of a photo-
chromatic interval at the fovea, and so on have already been sufficiently
discussed, but should be referred to again in the light of the theory.

The difference between the peripheral and the achromatic scotopic
luminosity curves, the former having the same character as the foveal
luminosity curve, shows, as has already been mentioned, that the
duplicity theory cannot be held to account for the peripheral total
colour blindness of the photopic eye.

Temporal effects, such as those of recurrent vision, are so compli-
cated that the support they give to the theory is equivocal. MacDougall,
as has been seen, brought forward evidence to show that the " ghost "
is not necessarily, as was thought, a purely scotopic image, though it
is so in its typical form. The change in the fusion frequency in passing
from lights of low to lights of high intensity, as shown particularly in
T. C. Porter's experiments {v. p. 95), and confirmed in those of Ives {v.
p. 96), Dow, Schaternikof!^ and v. Kries^, demonstrate the change over
from the scotopic to the photopic mechanism. Particularly noteworthy
is the sudden change in value of the constant in Porter's logarithmic
equation, which is exactly parallel to the change in the constant for
Konig's logarithmic equation for visual acuity^ and is attributed to the
same cause. The agreement in the intensity, about 0"25 metre-candle,
at which the sudden change in the curve takes place, is striking. There
is also some evidence to show that the latent period of excitation of the
scotopic apparatus is appreciably longer than that of the photopic
apparatus.

The experiments of Piper, Loeser, and Henius and Fujita {v. p. 123)
show that in the areal effects of the periphery as compared with the
fovea at low intensities summation of stimuli plays a much greater
role in the scotopic than in the photopic apparatus.

Certain cases of abnormal vision afford valuable evidence in support
of the duplicity theory. Of these, total colour blindness or mono-
chromatic vision may be explained as due to functional abeyance of
the cones, and night blindness to functional abeyance of the rods.

1 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxix. 242, 1902,

2 Ibid. XXXII. 113, 1903.

3 Koiiig, p. 378 ; see Parsons, Roy. Lond. Ophth. Hosp. Rep. xix. 2, 283, 1914 ;
Cf. Bloom and Garten, Arch. f. d. ges. Physiol. Lxxn. 372, 1898.

THE DUPLICITY THEORY 209

Total colour blindness, as we have seen, is characterised by luminosity
values which correspond with those of the normal achromatic scotopic
eye. Comparison of the curves (Figs. 54, 55) shows their almost complete
identity. Whereas, however, the normal achromatic scotopic curve is
valid only for low intensities of objective lights the achromatopic curve
is valid for all intensities, though with the higher intensities photophobia
occurs. The process of dark adaptation pursues the normal course in
the totally colour-blind. Exposure to bright light causes prolonged
diminution of vision, and recovery follows the course which would be
expected if it were dependent upon restoration of the visual purple.

All totally colour-blind people have marked diminution of central
visual acuity, and in many cases there is undoubtedly a central scotoma.
The curve of visual acuity with gradually increasing light-intensities
shows no sudden bend, as in normal-sighted people, but continues in
the same direction as for lower intensities until photophobia sets in
(Konig). The reduction in central vision tends to absence of binocular
fixation and divergence of the optic axes. We may conclude that the
foveal neuroepithelium, if it retains its function at all, approximates
in character to that of the normal rods.

Uhthoff and v. Kries^ have shown that the flicker phenomenon in
the totally colour-blind follows the same laws as in normal achromatic
scotopia.

Nagel^ has tested a monochromat with matches between orange-
yellow (600 nfi) and greenish-blue (490 fjufx) {vide supra). A monochromat
can make such a luminosity match at ordinary intensities of light, where-
as a trichromat must be dark-adapted and the intensities must be very
low. With the photopic eye the intensity {i.e., slit- width of the spectro-
photometer in millimetres) was 0-44. After one hour's dark adaptation
it was 0-8. Compared with the achromatic scotopic trichromatic, the
achromatopic photopic retina contains less visual purple : hence the
trichromatic achromatic scotopic value for greenish-blue light is lowered
by absorption relatively to the achromatopic.

Night-blindness is found in several diseases of the eye, most commonly
in retinitis pigmentosa and allied conditions, least adulterated by other
pathological symptoms and signs in congenital night blindness. The
last-named is congenital and hereditary^,

^ V. Kries, Ztsch. f. Psychol, u. Physiol, d Sinnesorg. xxxii. 113, 1903.
2 V, Helmholtz, 3rd. ed. n. p. 328.

^ Nettleship, Trails. Ojthth. Soc. xxvii. 1907 ; Parsons, Pathology of the Eye, iv. 1400,
1908.

P. c. v. 14

210 COLOUR VISION

Parinaud^ regarded night blindness as strongly supporting the
views which he propounded and which have since become incorporated
in the duplicity theory. Night blindness may be looked upon as the
obverse of total colour blindness. In the latter rod-vision is isolated,
in the former rod-vision is seriously disturbed or in abeyance.

In night blindness dark adaptation may be almost abolished or
much slower than normal ; in high degrees it is both slowed, and
diminished quantitatively, i.e., the higher degrees of sensitiveness of
the retina are never reached and the highest possible are reached only
after prolonged exclusion of light from the eye^. In low degrees central
vision is normal in the light-adapted condition, and may be so in high
degrees of the disease, e.g. in retinitis pigmentosa. Colour vision is
normal with the exception of occasional diminution of sensibility for
blue lights. In retinitis pigmentosa the field of vision is diminished,
often almost down to the fixation point, and dark adaptation may be
completely absent.

Purkinje's phenomenon is much less marked in the night-blind.
If a red and a green are chosen, which are equal in luminosity for the
normal and night-blind in light adaptation, and the room is then
darkened the green becomes much brighter than the red in the course
of a few minutes for the normal, but only after a long time, if at all,
for the night-blind. Quantitative experiments show that for lights of
short wave-length the increase in luminosity after half-an-hour's dark
adaptation is 10 to 100 times as great for the normal as for the night-
blind. With stimulation areas exceeding the foveal limits the sensi-
bility for mixed white lights increases much more rapidly for the normal
than for the night-blind on dark adaptation, but if both are tested with
red lights there is very little difference. This is further proof of the
diminution in the appreciation of Purkinje's phenomenon by the night-
blind.

The temporal effects in the night-blind have not been investigated
as thoroughly as could be wished, and the results are not concordant.

The condition is often, in fact generally, partial. It is not sur-
prising therefore that cases occur in which Purkinje's phenomenon
occurs and which show some degree of dark adaptation, colourless
interval for red pigments and so on. Hess^ has laid great stress upon

^ Arch. gen. de mM. 1881 ; C. r. acad. des sci. 1881 ; La Vision, Paris, 1898.

^ Heinrichsdorff, Arch. f. Ophth. lx. 405, 1905; Messmer, Ztsch.f. Sinnesphy^iol. xixi.
83, 1907; Lohmann, Arch. f. Ophth. lxv. 3, 1907; Stargardt, op. cit. Lxxni. 1, 77, 1909;
Behr, op. cit. Lxxv. 201, 1910 ; Wolfflin, op. cit. lxxvi. 464, 1910.

3 Arch. J. Avgenhlk. lxii. 50, 1908 ; LXix. 205, 1911.

THE DUPLICITY THEORY 211

these cases and regards them as seriously opposed to the duplicity
theory. Taken alone the phenomena associated with night blindness
are by no means unadulterated evidence in favour of the theory, but
viewed in conjunction with the other facts already discussed they are
on the whole confirmatory and highly suggestive. Moreover it is by
no means certain that in diseases accompanied by night blindness the
cones remain unaffected. Except in congenital night blindness it is
almost certain that they do not.

The pathology of retinitis pigmentosa^ shows that the condition
is primarily due to vascular degeneration in the choroid, whereby the
chorio-capillaris is destroyed. The pigment epithelium and the outer
layers of the retina are dependent for their nutrition upon the integrity
of the chorio-capillaris. From its proximity the pigment epithelium
must suffer first. The production of visual purple in the rods is un-
doubtedly bound up in the integrity of the pigmented epithelial cells,
so that primary disorder of the functions of the rods might be confidently
anticipated on pathological-anatomical grounds. It is the rule in most
nutritional disorders for the most highly differentiated and complex
structures to suffer first. That there is good evidence of a survival of
the cone-functions after destruction of rod-functions is evidence of an
intermediary process such as that described.

It must be admitted that the researches of Hess on the comparative
physiology of vision show that little support to the duplicity theory can
be derived from lower animals. He found {v. p. 132) that all classes
of vertebrates possess good powers of dark adaptation, including even
tortoises, which possess neither rods nor visual purple.

The arguments against the duplicity theory have been collected by
Siven^. He lays stress upon the peculiarity of the achromatic scotopic
" white " or grey, which he calls blue or violet. He thinks that the
rods are chiefly concerned in the perception of lights of short wave-
length. He finds the strongest support of his view in the colour-fields,
which are admittedly difficult to reconcile with the duplicity theory.
He admits the absence of Purkinje's phenomenon at the fovea, and says
that when colour is no longer perceived centrally it can still be elicited
from the periphery, presumably however only by lights of short wave-
length. His experiments in conjunction with Wendt on the effects of
santonin poisoning and the yellow vision of jaundice afford him additional

1 Parsons, Pathology of the Eye, n. 602, 1905.

2 Siven and Wcndt, Skand. Arch. f. Physiol, xiv. 196, 1903 ; Sivcn, op. cit. xvn. 306,
1905 ; XIX. 1907 ; Zisch.f. Sinnesphysiol. xlh. 224, 1907 ; Arch, of Ophth. xlh. 2, 1913.

14—2

212 COLOUE VISION

arguments, and he regards Hess's observations on birds^ as also con-
firmatory.

Baiier^ has brought forward arguments to show that the role of the
visual purple is not limited to the condition of dark adaptation. Since
Klihne's researches it has been known that it is difficult to bleach the
visual purple completely so long as the retina is in contact with the
pigment epithelium. Bauer found that though frog's visual purple was
rapidly bleached when exposed to bright sunlight, the retina again
became red after several hours in spite of continuous exposure. He
concluded that increased destruction of the substance is associated under
physiological conditions of exposure to bright light by correspondingly
increased production, and that therefore the visual purple plays an
important part in photopic as well as in scotopic vision.

The clear explanation which the duplicity theory affords of the
chief differences between human photopic and achromatic scotopic vision
and of the peculiarities of the intermediate stage, scotopia with only
moderate dark adaptation (chromatic scotopia), tempts one to carry
the theoretical considerations farther than is perhaps warranted.
Dark adaptation appears to be directly associated with the visual
purple. On teleological grounds extreme sensitiveness to light and
shade are most needed in the dusk, and while the loss of visual acuity
for form is a serious disadvantage the gain in concentration is a counter-
balancing merit. The rods are more intimately connected with each
other by nervous paths than the cones (Ramon y Cajal), so that relatively
widespread impulses become summated in the resultant psychological
impressions. An even less complete insulation of the rods may be
brought about by dark adaptation, possibly through the retraction of
pigment in the retinal pigment cells. On the other hand the photopic
position of the pigment tends to protect the more sensitive rods with
their contained visual purple and to isolate the less sensitive cones.
The role of the pigment cells in dark adaptation has already been the
subject of conjecture^, but it must be remembered that the wandering
of the pigment has not been conclusively proved to occur in warm-
blooded animals.

1 Arch.f. Angenhlk. Lvn. 298, 317, 1907 ; lix. 143, 1908.
* Arch. f. d. ges. Physiol, cxli. 479, 1911.

3 Exner, Sitz. d. Wiener Akad. xcvm. 3, 1889 ; Nagel, in v. Helmholtz, 3rd ed. n.
331.

SECTION III

THE THREE-COMPONENTS THEORY (YOUNG-HELMHOLTZ)

CHAPTER I

STATEMENT OF THE THEORY

The Yoiing-Helmlioltz theory of colour sensations is based upon the
facts of the mixture of pure-colour stimuli (see Part I, Sec. II, Chap. iii).
We found that within a certain range, which includes all ordinary
conditions of colour- vision, and with certain well-defined exceptions
{v. p. 36), every conceivable light or light-mixture gives rise to a sensa-
tion which can be accurately matched by the sensation produced by a
suitable mixture of only three lights. If we choose three actual wave-
lengths, R, G, and F, on the colour-diagram and join them by straight
lines, we find that part of the curve is outside the triangle thus obtained.
This means that there are certain spectral colours which cannot be
obtained in their full spectral saturation by the mixture of R, G, and V.
If we choose a point on the curve outside the triangle, e.g., GBl, we can
by experiment obtain an equation of the following form :

aR + ^GBl ^yG + eV.

We thus find an expression for GBl

^GBl = yG + eV - aR

which represents the unmixable colour in terms of R, G, and V.

In order to avoid negative quantities we must assume the existence
of colour-sensations which lie outside the colour-diagram. The fact
that, by previously stimulating the eye with the complementary colour,
we can obtain a colour-sensation which in saturation far exceeds that
obtained by viewing the spectral colour without such previous excitation
{v. p. 109), affords some evidence that this assumption is not unwarranted.

214 COLOUR VISION

The assumption is minimised by describing a triangle around the
diagram which will only just succeed in including every part of the
diagram. Such a triangle is seen in Figs. 8 and 9 (pp. 39, 40). We
can then obtain a universal equation for any spectral colour, F, viz.

in terms of the three theoretical sensations, R, G, and V, in which
X, y, z are all real positive quantities.

The objective quantities of light in this equation come into con-
sideration here only as sensation-stimuli, and as such have a physically
measurable value. If we make a further assumption, viz., that the
physiological processes which accompany sensations have a definite
quantitative relationship to the physical stimuli, we may say that
X, y, and z are respectively the red, green, and violet values of the light
F in terms of the fundamental colours R, G, and V ^.

It also follows that in the sensory process there must be three
corresponding kinds of activity, which coexist side by side without
interfering with each other, and upon which every variety of colour
sensation depends.

Let us suppose that there is some method whereby we can determine
three measurable quantities, (f), x^ ^ representing three physiological
processes, which taken together completely define the character of the
visual sensation. We should then be able in every case to find by
observation the relationship between cf), x, »A ^^^ *^® values x, y, z of the
incident light. We should then have cf), x? ^ represented as three
functions of x, y, z and conversely x, y, z as three functions of cf), x, ^^
That is,

</•=/! i^, y, z), x=f2 (x, y^ 2), j/r = /g {x, y, z) and

Since no two different groups of values for x, y, z give the same sensation,
i.e., the same values of cf>, x, "A, therefore x, y, z can be expressed solely
by (/), X, ^- These values of the x, y, z functions of <^, x, ^ {'^'-^-ifi^f^ifz')
are therefore quantities which depend only upon the character of the
sensation, and moreover possess a certain individuality, since each can
be aroused, exist and again disappear in the nervous apparatus inde-
pendently of the other two and unaffected by them. This independent
existence is, however, exactly what we are in search of when we speak
of elements, ingredients, or components of the sensation. If we therefore

1 See V. Helmholtz, 2nd ed. p. 341.

STATEMENT OF THE THEORY 215

denote the function of </>, x^ 'P which represents x by R, and the correspond-
ing other two functions by G and V, then these quantities, R, G, V, are
to be denoted as components of the colour sensation, and similarly
any linear function of them (aR + 6G + cV) may also be thus denoted.
From the mathematical elaboration of these assumptions v. Helm-
holtz stated the Young-theory in the following form :

" (1) In some part of the conducting nerve substance, under the
influence of coloured light, three different, independent, and mutually
unopposed elementary activities arise ; we will them call the elementary
stimulations. Their amount is directly proportional to the correspond-
ing colour-values, x, y, z of the objective light ; they correspond to the
R, G, V of the above description.

" (2) All activities passing further towards the brain, as well as
the sensations actually entering into consciousness under the given
conditions of the reacting brain, are only actions of the three elementary
stimulations, R, G, V, and in amount are functions, cf), x^ ^> of those
elementary stimulations.

" (3) Either the elementary stimulations themselves or three
mutually unopposed actions dependent upon them are conducted inde-
pendently to the central organ."

Putting these conclusions in simpler language and in their most
general form, R, G, V, are any three points so situated that, when joined,
the triangle thus constructed completely encloses the colour-diagram
of the given spectrum. In this manner positive values are ensured.
From observations on colour-mixtures with the given spectrum we can
construct valency curves which represent the stimulation values of any
spectral light for each of the three components, R, G, V, of the resultant
sensation. Thus

F = xR + yG + zV

means that the light F is matched by a mixture of x parts of R light,
y parts of G light, and z parts of V light, R, G, V being the physical
stimuli in the mixture. If these physical stimuli act respectively upon
the physiological counterparts or elements of sensation, R, G, V, then
xR, yG, zV clearly represent the strengths with which the light F acts
upon the R, G, V elements, i.e., they are its R, G, V valencies or values.
The valency curves are therefore nothing more than the gauging
curves of the spectrum to which reference has already been made
{v. p. 39). But it has been shown that every gauging value belonging to
one such group of curves must always be a linear function of the three

216 COLOUR VISION

gauging values belonging to any other group of curves. In any given
spectrum, therefore, the R, G, V values must be some linear function
of the three empirically observed gauging values {R, G, V). The
three gauging curves therefore represent the relative values of the three
sensations for each light throughout the given spectrum.

The simplest concrete conception of the bases of the three sensation
elements or ingredients is that there are three components which are
counterparts of the physical stimuli. We shall at present use the term
component in the broadest sense : it may represent a chemical, an
electrical or some other process acting upon different substances or
nerve-fibres and giving rise to nervous activity, but the general state-
ment of the theory necessitates no such concrete conception.

In this, its most generalised form, the Young-Helmholtz theory
explains satisfactorily the sensations resulting from colour-mixtures in
the normal visual system, since it is indeed founded upon them.

It also explains satisfactorily the gross divergences from the normal
system. Thus dichromatic vision is due to absence of one of the theo-
retical components ; in the protanopes this is the R component, in the
deuteranopes the G component, and in the tritanopes the V component.
Indeed, the determination of the absent component follows mathe-
matically from the correlation of the facts of colour-mixtures in dichro-
mats. In their colour-diagram all the colours which appear to them to
match must lie upon a straight line, since the line joining any two points
representing homogeneous colours contains all the points representing
the colours which can be mixed from those homogeneous colours.
Similarly the mixtures of any of these colours with any other colours
lies on a series of straight lines, v. Helmholtz has shown that all these
lines either meet in a point or are parallel^. The point of intersection
corresponds to a colour which has no stimulus value for the dichromatic
eye. It is generally called the Null-point (Fehlpunkt) of the system.

Conversely, the normal colour diagram can be constructed from the
combined protanopic and deuteranopic observations (Fig. 57). It
cannot be expected to coincide precisely with that of a single individual
on account of differences in macular pigmentation, variations in the
spectra used, and so on, but it shows a remarkable similarity^.

Anomalous trichromatic vision may be regarded in various ways
on this theory. The simplest explanation is that it is a reduction system

^ V. Helmholtz, 3rd ed. p. 123 ; Greenwood, Physiology of the Special Senses, p. 153.
2 Greenwood, loc. cit. p. 150 ; v. Kries, in Nagel's Handb. d. Physiol, d. Menschen, in.
p, 161.

STATEMENT OF THE THEORY 217

in which the stimulus values for any light of one of the components is
uniformly less than normal. Partial protanopia is then a diminution
of sensitiveness of the R component, partial deuteranopia of the G
component. Other modifications are theoretically possible and are
included in the general form that if in the normal system ^ = /i {x, y, z)
and so on, then in the anomalous trichromatic system (f)' =/i' {x, y, z)
and so on. This generalisation is, however, too vague to be of much
practical value, and there are other possibilities arising directly out of

V.515

Fig. 57. Normal trichromatic colour diagram constructed from combined protanopia
and deuteranopic observations. A, null-point of protanopes; B, null-point of
deuteranopes. (v. Kries.)

the Young-Helmholtz theory, such as a shift of one of the curves {vide
infra), which account for at least one group of the anomalous trichromats.
Monochromatic vision may be explained on the Young-Helmholtz
theory by the identity of all three components. In this case

4> -fi (»' 2/> 2) = X =/2 i^> y,^) = ^ =h {^. y^ 2)-

The facts do not support this view, and are much more satisfactorily
explained by the duplicity theory. At the same time there are some

218 COLOUR VISION

observers, e.g., Schenck, who regard monochromatic vision as divided
into various types, some of which are due to modifications in the cones
and must therefore be correlated in some such manner as that suggested
with the three-components theory.

Difficulty is experienced when the attempt is made to explain the
phenomena of peripheral vision and of induction by the theory. It is
at once clear that neither can be explained as evidence of any reduction
system. Fick^ suggested that the reaction-values of the components
were not the same in all parts of the retina. The suggestion is inadequate ;
on the one hand because it implies a simple reduction system and this
fails to account for the phenomena, or on the other hand because it is
too general to be of practical value. Schenck's attempt to correlate
the facts with a modification of the trichromatic theory will be described
later. The relationship of normal and deuteranopic peripheral colour
vision to central colour vision approximates more closely to a reduction
system than the corresponding relationship in the protanopic system.
In all, however, the modifications produced by areal effects negative so
simple an explanation and afford some evidence in favour of v. Kries'
theory of zones.

The greatest difficulty, however, is experienced in explaining the
facts of induction. In general terms it is not difficult to conceive a
diminution in response of the components in one direction, associated
with an increase in another, after previous stimulation. We might thus
account for the increased response to the complementary after stimula-
tion with a given light. Indeed, such a view falls in well with other
physiological findings, so admirably elaborated in Sherrington's work.
If all spectral lights act upon all three components, then the increased
response to the complementary after previous stimulation with a colour
can be explained, and this was the view adopted by v. Helmholtz. It
lands us, however, on the horns of a dilemma, for the facts of dichro-
matic vision — and the same is true of trichromatic {vide infra, Chap, ii) —
show that lights of greater wave-length than about 550 /x/x do not act
at all upon the V component, since no standard blue has to be mixed with
the standard red in order to match colours in this part of the spectrum.
Yet the saturation of yellow (589 /x/x) is undoubtedly increased by
previous stimulation with the complementary blue. v. Kries has shown
that this effect is not to be explained by any alteration of the intrinsic
light of the eye, but is caused by a quantitative change in the response
to the secondary light. If yellow light acts only on the red and green

1 Arch. f. d. ges. Physiol XLvn. 274, 1890.

STATEMENT OF THE THEORY 219

components we cannot explain on the theory the increase in saturation
which follows previous activity in the blue components. Hess refuses
to admit that the facts can be brought into line with the theory. We
must therefore accept the theory as explaining satisfactorily either the
phenomena of after-images or those of dichromatic vision, but not both.
The large mass of accurate evidence which has now accumulated, chiefly
from the observations of Abney and v. Kries and their fellow-workers,
showing that all spectral lights do not act upon all the components
makes it impossible to accept the explanation as it stands for successive
induction. Moreover, successive induction is a complex condition allied
to simultaneous induction, and it must be admitted that the facts of
simultaneous contrast cannot be explained directly by the trichromatic
theory alone.

With regard to spatial induction v. Helmholtz made no serious
attempt to correlate the facts with the theory. In his opinion the
phenomena of simultaneous contrast are purely psychical and are
explained as "illusions of judgment." The arguments which can be
advanced in favour of this view do not concern the three-components
theory, and it is only by modifications of the theory, which will be dealt
with at a later stage, that any more purely physiological explanation, on
the three-components' basis, of spatial induction can be advanced.

Finally, it may well be asked whether there is any direct positive
evidence of the existence of independent visual components answering
to the theoretical components. It is in this respect that the electrical
researches of Gotch, Himstedt and Nagel, and others, and the fatigue
experiments of Burch are so valuable. They offer no crucial proof,
but they are suggestively concordant with the view that such com-
ponents exist. It has already been mentioned that Burch is inclined
to predicate four. Theoretically this is of no importance, though it
complicates matters and is unnecessary for the explanation of the facts
of colour-mixtures, etc. It does not in any way detract from the
support which Burch's experiments afford of an additive theory de-
pendent upon processes occurring in certain components.

As is usually the case, the three-components' theory becomes less
plausible the more concrete the form it takes. We have seen that
Young suggested three different types of nerve-fibre, others have
suggested three substances which undergo chemical change, others
again have suggested electrical changes. Each such hypothetical
fibre or substance, etc., leads inevitably to further hypothetical con-
ceptions of the details of the processes. These become the subject

220 COLOUR VISION

of controversy and the whole theory is thrown into disrepute, the
cardinal fact being overlooked that they are in no integral sense germane
to the theory.

It may, however, be interesting to record some of the suggestions
which have been made.

As regards the three fundamental component sensations Konig's
experiments led him to regard them as a red just outside the spectrum
and rather more purple than the spectral colour, and a blue at 470 fifji
slightly more saturated than the spectral blue. v. Helmholtz, on
the basis of Konig and Brodhun's observations on the sensitiveness for
discrimination of hues in the spectrum {v. p. 30), chose a carmine-red,
bluer than spectral red, a yellowish-green between 540 /x/a and 560 /x/x,
and an ultramarine blue, all much more saturated than the spectral
colours.

As regards the retinal bases for the theory, Konig as already
mentioned regarded the visual purple as the basis of colourless scotopic
vision. With greater stimulus intensities the visual purple is trans-
formed into visual yellow, which forms the basis of the fundamental
blue sensation. He placed the substrata of the red and green sensations
in the pigment epithelium. If this were true, foveal vision should be
monochromatic in protanopes and deuteranopes. He regarded the cones
as a purely dioptric mechanism.

CHAPTER II

RESEARCHES BASED UPON THE THEORY

I, Normal Colour Vision

The first researches based on Young's theory were made by v. Helm-
holtz (1852-3)^. From them he elaborated the three-components or
Young-Helmholtz theory. He was followed by Clerk-Maxwell (1855-6)^.

Clerk-Maxwell placed three slits in the spectrum of daylight at the
following places : (1) in the red {R) between the Fraunhofer lines C
and D, twice as far from the latter as from the former ; (2) in the
green (G) near E ; (3) in the blue (J5) between F and G, twice as far
from the latter as from the former. Lights from these slits were
mixed in the proportions to match a comparison white {W) derived

^ References in v. Helmholtz, 3rd ed., ii. p. 137.
- Scientific Papers, Cambridge, 1890.

RESEARCHES BASED UPON THE THEORY

221

from the same source of light. From the sUt-widths he thus obtained
a standard equation :

He then obtained 14 other equations for W, in each case mixing three
Hghts from different parts of the spectrum in suitable proportions. By-
eliminating W from each of these equations he obtained the sensation-
values of 14 positions in the spectrum in terms of the sensation- values
of the three standard colours, R, G, B. Fig. 58 shows the curves plotted
from these values. It will be noticed that the R and B curves pass
below the base line. His standard colours were therefore not chosen
so as to eliminate negative values.

Red. Blu.e.

Fig. 58. Clerk-Maxwell's sensation curves. The dotted line is the algebraical sum of
the ordinates at each point ; it is not a true luminosity curve. Abscissae, wave-
lengths of the prismatic spectrum of sunlight ; ordinates, arbitrary scale. (Clerk-
Maxwell.)

Papers followed by J. J. Miiller^, Preyer^ and Bonders^, but no
other researches on the three " sensation curves " were made until
Konig embarked upon his observations in 1883, continuing them until
his death in 1901. Almost simultaneously, Abney commenced the work
which he has recorded in a series of papers, culminating in his Researches
in Colour Vision (London, 1913), and which is happily still in progress.
Certain points on the sensation curves have also been worked out inde-
pendently by F. Exner^.

1 Arch.f. Ophth. xv. 2, 208, 1869. " Arch.f. d. ges. Physiol, i. 299, 1869.

3 Arch. f. Ophth. xxiii. 4.. 282, 1877 ; xxvn. 1. 155, 1881 ; xxx. 1, 15, 1884 ;
Onderzoek. i. Lab. Utrecht. 1882.

* Sitz. d. Wiener Akad. cxi. i ; a, 1902.

222

COLOUR VISION

Konig's experiments on normal colour vision were carried out in
conjunction with Dieterici^ and an abstract was communicated to the
British Association in Birmingham in 1886. The curves were calculated
from a vast number of matches made with Helmholtz' spectrophoto-
meter. An objection to this instrument is that the variations in in-
tensity are made by a polarisation method and account is not always
taken of the appreciable polarisation of the light by the prism of the
spectroscope itself. The observations and calculations were of the
same kind as those made for dichromatic systems (v. p. 163), but were
necessarily of a more complicated nature. Fig. 59 shows the curves

720 700 680 660 640 620 600 580 560 540 5 20 500 480 460 440 420 400 380

a B C P E b F G H
Fig. 59. R, G, and V. sensation curves. K, Konig's G-curve; D, Dieterici's G-curve;
G-curve of an anomalous trichromat. Abscissae, wave-lengths of the inter-
ference spectrum of sunlight; ordinates, arbitrary scale. (Konig and Dieterici.)

referred to the interference spectrum of sunlight. Konig's and Die-
terici's R and F curves coincide ; the G curves are slightly different.
The distortion of the G and F curves between 535 /x/x and 475 ju,^ is due
to absorption by the macular pigment.

By far the most exhaustive experiments have been made by Abney,
and they will be discussed in greater detail. He has adopted another
method of determining the three sensation curves, dependent upon the
luminosities of the colours. His results confirm and correct in detail
the curves obtained by Konig. The principle of the experiments is as
follows^.

^ Konig and Dieterici, Sitz. d. Akad. d, Wiss. Berlin, 1886 ; Ztsch. f. Psychol, v. Physiol.
d. Sinnesorg. iv. 241, 1892 ; m Konig, pp. 60, 214.

2 Watson, Proc. Roy. Soc. Lond. A, lxxxyui. 404, 1913.

RESEARCHES BASED UPON THE THEORY

223

"We have already described the methods of obtaining the luniinosity
curve of the spectrum. In it the abscissae are wave-lengths of the
spectrum, the ordinates luminosities in arbitrary units, the maximum
brightness being 100.

Suppose that when making a set of observations we start with the
movable slit at the extreme end of the spectrum and determine the
intensity iVi of the white which appears of the same brightness as the
colour and then move the slit towards the blue through a distance equal
to its width and again determine the intensity Wg of the white, and so on
throughout the spectrum. We should in this way determine piece by
piece the brightness of the whole spectrum, and the sum Wj^ + w.2 + w^ -}- ,
etc., could be taken to represent the total brightness of the whole

700

600

400

Fig. 60. R, G, and B, sensation curves. These are Konig and Dieteriai's curves cor-
rected to new determinations of the points of section, n, h, c, d. Abscissae, wave-
lengths of the interference spectrum of the arc light ; ordinates, arbitrary scale.
(F. Exner.)

spectrum. If now the slide carrying the slit were removed, so that the
light corresponding to the whole spectrum were allowed to fall on the
same portion of the screen, thus forming white, and we now determine
the intensity W of the comparison white which is equal in brightness to
this recombined spectrum, then it has been shown by Abney, Tufts,
Ives and others that

y^ ^ w^ + W2 + w^+ , etc.

That is, the luminosity of the recombined spectrum is equal to the sum
of the luminosities of its parts.

Now the sum w-^^ + Wo-\- w^-\- , etc., is proportional to the area

224 COLOUR VISION

enclosed by the luminosity curve. Hence the area of the luminosity
curve represents the total brightness of the light which is formed into
the spectrum and is, therefore, a constant whatever the condition of
the vision of the person who makes the observation. This is at once
apparent if we remember that if the brightness of the whole recombined
spectrum is compared with the comparison white, since these whites
are derived from the same source and must, therefore, have exactly
the same composition, a setting which appears correct to one person
must also appear correct to any other, whatever the differences which
may exist between their vision.

On the Young-Helmholtz theory the sensation produced by light
of any colour is the sum of the sensations due to stimulation of the
three component mechanisms. Konig and others obtained their curves
for the three components by matching light of one wave-length with a
mixture of lights of two other wave-lengths. Abney, using a similar
method, adopted the principle that the sum of the ordinates of the
luminosity-values of each component for any given wave-length is
equal to the ordinate of the luminosity curve of the spectrum for that
wave-length. He then proceeded thus^ :

The red sensation can be perceived in purity at one end of the
spectrum. From the darkest red to a point near the C line, a little
above the red lithium line, the colour is the same, though, of course the
brightness varies, but the brighter red colour can be reduced so as to
form an exact match with the dark red, and no mixture of any colours
will give a red of the description we find at the end of the spectrum.

At the violet end of the spectrum we also find that the colour is the
same throughout, from the extreme visible limit to a point not far
removed from G, but it is not for this reason to be accepted that the
colour is due to only one sensation. It might be due to two or three
sensations if they were stimulated in the same proportions along that
region, and if the identical colour could be produced by the combination
of other colours. Experiment shows that a combination of two colours
will under certain conditions make violet, and that instead of a simple
sensation of violet we have in this region a blue sensation combined
with a large proportion of red sensation. If we know the percentage
composition of the violet mixture we may provisionally use this part
of the spectrum as if it excited but one sensation, and subsequently
convert the results obtained with it into the true sensations. Thus
in calculating the percentage of red in any colour, that existing in

1 Abney, Phil. Trans. Boy. Soc. Land, cxcin. 259, 1899 ; ccv. 333, 1905.

RESEARCHES BASED UPON THE THEORY

225

the provisional violet sensation would have to be added to it, and the
same amount be abstracted from the violet to arrive at the true blue
sensation. The green sensation would remain unaltered.

Having at one end of the spectrum a pure red sensation, and at the
other mixed sensations, due to the stimulation of the red and a blue
sensation, it remains to isolate the green sensation. Owing to the over-
lapping of the curves in the green of the spectrum, due to the fact that
this region stimulates all three of the sensations, the effect of the pure
green sensation is never experienced by a normal eye. In any colour
where the stimulation of all three sensations occurs there must be
always an admixture of white light, and we have to search for that point
in the spectrum where white alone is added to the green sensation.

The following diagram, Fig. 61, will show some variations in com-
position of a colour that may be met with. The provisional use of a

G
A

G
C

Eig. 61. Diagrams illustrating Abney's method of determining the
normal sensation curves.

violet sensation will not alter the argument, since, as before said, we
may replace it by blue and red sensations. The different figures are
purely diagrammatic. They are constructed on the supposition that
equal heights of line above the base line show the stimulation necessary
to give the effect of white light. The scale applicable to each of the
three lines is necessarily quite different in the scale of luminosity ; that
of the violet in particular is very greatly exaggerated.

A, B, and C represent colours each containing a sensation of white.
Let the stimulation of the sensations be represented by vertical lines.
In A we have the red and green sensations of equal heights, but V is less.
Drawing a horizontal line through c, aR, bG, and cV, represent equal
stimulations, which make white, leaving da and eb equal. We thus
have a colour which is made up of a mixture of R and G sensations
{RS, and GS), together with white. Now equal stimulations of RS and

p. c. V.

226 COLOUR VISION

GS, we shall see later, give the sensation of yellow. If we place a slit
in the violet and move another slit along the less refrangible part of
the spectrum, we shall find a place where this colour and violet together
make a white, the slits being opened or closed to make the match. This
position, then, is that in which the red and green sensations are equally-
stimulated, and answers to ^. In 5 we have a green and violet with
equal ordinates and a deficiency of red. If we place a slit in the red
and move another about in the green, we shall find a colour which with
the red makes white. This position, then, will have an equal stimulation
of green and violet. This gives another fixed point. The next point to
determine is shown diagrammatically by C, which illustrates the green
we have to look for, mixed only with white. This is more difficult to
find, as it would require a purple to be added to make a match with the
white, and this does not exist in the spectrum. Suppose we mix A with
B, we get a diagram of the kind shown in the fourth diagram. There
are equal reds and violets stimulated, but a larger stimulation of green
sensation. This gives a colour paler than the spectrum colour, but
still a green which can be matched. There are also other plans,
dependent on trial and error, for fixing this point.

When the observations for obtaining the fixed points have been
made it will be found that the complementary of the violet is at scale
Qumber {SSN) 48-7 (577-2 /x^u), that of the red at SSN 34-6 (500 /x/x),
and that where green is mixed only with white is at SSN 37-5 (515 /x/x).
These are therefore the points of intersection of the three sensation
curves (Fig. 64).

It is next necessary to ascertain the amount of white in the green
at SSN 37-5. One slit is placed in this situation and another at SSN
59-8, the position of the red lithium fine. The luminosities of these
colours, with equally wide slits, are taken by comparison with another
light, such as yellow or white. They are found to be 39-2 and 9-4
respectively. A patch of yellow light from a second spectrum, derived
from the same source of light, is placed beside the mixture of red and
green. In order to obtain a colour match it is necessary to add white
to the homogeneous yellow. From the slit widths and the luminosity
of the added white an equation in terms of luminosity is obtained :

a (yellow) + b (white) = c (red) + d (green)
or a (yellow) = c (red) + d (green) — b (white).

Since the red contains no white the percentage of white in the green

RESEARCHES BASED UPON THE THEORY

227

is ^xlOO. It was found to be 69 per cent. The percentage of green

sensation is ^ x TOO.

Having obtained the percentage of white in the green the percentage
sensation composition of other colours in terms of luminosity can be
readily found ^. Fig. 62 shows the percentage sensation curves.

From the data obtained by these experiments the analysis of the
sensation-values can be carried further by calculation. Having deter-
mined the luminosity values of the different parts of the spectrum by
heterochromatic photometry (585/zju. = 100) the luminosity-values of

100
90
60
70
CO
30
40
30
20
10

^ ^

.^"^

___

^ ^

t--^.

____—-

^^•Jt

^v^^

— -^^

,>^

'"^"r..

..^-^^

^ Li
1

] i

^^iSiia

^S^SCi/

1. . . .

IS 20 25 30 35 40 46 50

Scale of Prismatic SpecfruTn.

fig. 62. Percentage of the B. G, and B sensations in the spectrum colours in terms of
luminosity. Abscissae, wave-lengths of the prismatic spectrum of the arc light
ordinates, arbitrary scale. (Abney.)

the R, G and V sensation can be readily obtained from their percentage-
values. Further it is found that the violet, which has hitherto been
used as a simple sensation, is made up of 72 per cent, of red sensation
and 28 per cent, of blue sensation. The R, G, and V values can there-
fore be transformed into fundamental R, G, and B sensation-values,
B being the fundamental blue sensation. We thus obtain the R, G, and
B percentage-values, and from them, by calculation, their luminosity-
values.

The luminosity curves of the spectrum and of the three components
are shown in Fig. 63. For any wave-length the sum of the ordinates of
the three sensation curves is equal to the ordinate of the total luminosity
curve at that point. When white light, i.e., light of all wave-lengths,
enters the eye the effect produced on the three components will be

1 Abney, p. 235.

15—2

228

COLOUR VISION

proportional to the areas of the three sensation curves. The relative
areas of these curves for the electric arc are RS 579, GS 248, BS 3-26.

When considering the matches made between different lights it is
often convenient to adopt a different scale, viz., one in which the areas
of the three sensation curves are equal to each other. With this scale
equal ordinates of the three sensation curves correspond to a mixture
which will appear white to the normal eye. If we multiply the
ordinates of the green sensation curve by 579/248 = 2"21, and those of
the blue sensation curve by 579/3-26 = 117, the green and blue sensation
curves will have the same area as the red sensation curve.

100
90
80
70
60
60
40
30
20
10

C-

)

f^-""-

r>%

\Ofj_

SEN

SAT

ro/v

^i;:::

64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24
7o|oo" ' I lesjool I ' I 6o|oo • ' ' ' 5o[oo I I i I 5o[oo*u ' • I '"

Fig. 63. Abney's E, G, and B sensation curves in terras of luminosity. The sums of
the ordinates at any point are equal to the ordinate of tlie photo pic luminosity
curve. (Watson.)

In the curves in Fig. 63 the source of light was the crater of
the arc light with a horizontal positive pole. The source of light
in Abney's original investigations was the crater of the arc light with
sloping carbons, and the corresponding factors are 2-3 and 190.

The luminosity curves are therefore brought to equal areas by
multiplying the GS and BS luminosity- values by 2-3 and 190 respectively,
thus giving the curves shown in Fig. 64, where equal stimulation of all
three components, i.e., equal ordinates, give the sensation of white.
Sir William Abney was the first to work out the colour sensations of
the normal trichromat exhaustively in this manner.

RESEARCHES BASED UPON THE THEORY

229

These results afford an explanation of the change in hue which
occurs when white light is added to the spectral colours. When this is
done the red becomes pinker, the scarlet orange, the orange yellow,
the yellow green. At SSN 48-7 (577-2 /x/a) in the yellowish-green, no
change in hue occurs. Beyond this point, the green becomes yellowish,
the blue shows little change, and the violet becomes nearly salmon-pink.
Now SSN 48-7 is the point where the red and green sensation- values
are equal to their sensation-values in the white light. It therefore
seemed probable that the change in hue in lights of medium and long
wave-length was due to the addition of the red and green sensation-
values of the white light, the value of the blue sensation being so small

-1

{ I

K 1

t ^r

1, ...

Fig. 64. Abney's R, G, and B equal-area sensation curves. The sums of equal ordinates
of the three curves at any point represent the sensation of the unanalysed white
light. (Abney.)

as to be negligible. This conjecture was fully borne out by experiment,
the matches being identical with those calculated, within the range of
experimental error. SSN 48-7 is easily found, since it is. the comple-
mentary colour of the pure blue of the spectrum.

In the colour triangle (Fig. 9) a mixture of white with any spectral
colour is represented by a point (a) on the line joining W with the
given point on the curve, the position of a being determined by the
relative amounts of the colour and white. The change in colour due
to the admixture with white is found by joining the points B and a and
producing Ba to meet the curve. The mixture will match the spectral

573

503

496

450

577-2

515

500

577

508

494

475

230 COLOUR VISION

colour which is represented by the point of intersection with the curve.
That this should be the case shows that the blue element is practically
in abeyance.

In experiments in matching white with mixtures of spectral rays
it was found that about 2-8 per cent, of red or green could be added
without being perceived.

If we examine the sensation curves shown in the previous diagrams
we shall find that, besides the maxima of each sensation, there are
important points where the curves intersect. We will call the point
where the R and G curves intersect a, R and B, b, and B and G, c.
These points have been determined by Konig, Abney, and Exner^, and
their values are as follows :

Konig (sunlight)
Abney (arc light)
Exner (arc light)

d is the complementary colour of a. From these data Konig and Exner
deduced the fundamental colour sensations : R, a purplish red (com-
plementary to 494 /XjU. (Exner); G, 505 fx/x (Konig), 508/z/la (Exner);
B, 470 fxfjL (Konig), 475 />t/x (Exner) ; all, however, more saturated than
the spectral colours.

We have here the explanation of the variations in the discrimination
sensibility for hues in the spectrum {v. p. 30). In the equal area curves,
e.g. Fig. 64, equal ordinates at any point make white. If we subtract the
white in those parts of the spectrum in which all three sensations are
stimulated, we can obtain the ratios of the ordinates of the sensation
curves to each other throughout the spectrum. These ratios will vary
in different parts. In regions where the variation occurs most rapidly
we should expect the hues to change most rapidly. Calculations made
by Steindler on this basis from Exner's values show that the positions
of maximum change are at 500 /i^u., 570 — bdOfxfx, and 635 fifi, of minimum
change at 470 /x/x, 530 fif-t, and 625 /x/x. These calculations agree remark-
ably well with the results of direct observation, which show that the
maxima of discrimination sensibility for hues were at II, 492 /x/x,
III, 581 /x/x, and IV, 635-5 /x/x, the minima being at 458 /x/x, 533 /x/x, and
627 /x/x {v. p. 31).

This very striking confirmation of the Young-Helmholtz theory was
first pointed out by Konig^.

The curves in Fig. 65 are derived by calculation from the same

1 Sitz. d. Wiener Akad. cxi. iia, 857 1902. ^ Konig, p. 106.

RESEARCHES BASED UPON THE THEORY

231

numbers which gave the curves in Fig. 62 1. Thev are useful for fore-
telling the results of fatiguing the eye for different colours. They are
constructed on the basis that equal stimulations of all three components
give rise to the sensation of white. If the eye is fatigued with white
light the effect will be to tire each equally, and therefore the same
white seen by such an eye will appear darker. H the eye is fatigued
for SSN 48-6, RS and GS are equally fatigued. Suppose they are
fatigued so that the ordinates of the curves are reduced to one-half, and
then a colour, about SSN 42, of which the normal composition is IRS
to 2GS, is observed. The effect will be to make the new relationship
^RS to IGS. Since the proportion remains the same no change in hue

^\^

,<f

Gret

n Se,

^ff/S

'T—

-y

' \

•^

■^

"-*

\I00

60 ^
;d

70 e

•>

60 _>

0
SO 3

ao S

50 I

20 "^

20 22 24 26 26 30 32 34 36 36 40 4! 44 46 *8 SO 52 S4 S6 Se 60

Scale of Spectrum SSN.

Fig. 65. Percentage of the R, G, and B sensations in the spectrum colours when e(|ual
stimulations of the three sensations give rise to the sensation of white. (Abney.)

will be observed. Suppose, however, a colour about SSN 60, where
there is no GS is observed. The only effective fatigue will be for RS,
and the red will merely appear darker.

One more example. Suppose the fatiguing colour is about 42,
where the ordinate of GS is twice that of RS, and suppose the fatigue
reduces the ordinates to one half. They are then RS = \ and GS = |.
The eye then observes SSN 50, where RS is 54-5 and GS 42-5. As

observed RS will be ^ = 13-6, and GS will be -^ = 21-25, i.e., as

1 : 0-64. This is about the ratio found at SSN 44. Therefore the
yellow at SSN 50 will appear to the fatigued eye as a green of SSN 44,

1 Abney, p. 240.

232 COLOUR VISION

Experiments made by Abney support these conclusions. He has
shown how the factor of fatigue can be arrived at and how luminosity
curves can be obtained for the fatigued eye. He did not employ the
high intensities used by Burch.

II. Dichromatic Vision

Young himself^ suggested that Dalton's colour blindness was due
to absence of the red component, v. Helmholtz, Donders, Konig, and
others found that this hypothesis accounted well for the facts, and
further, that the second great type of colour blindness, now known as
deuteranopia, could be attributed to absence of the green component.
The rare cases of tritanopia were attributed to absence of the blue
component.

If the red component is absent it will be seen from Fig. 64 that
from SSN 65 to 60 there will be no sensation of colour, nor indeed of
light. The red end of the spectrum will be shortened.

From SSN 60 to 50 the green component only is present. It will
give rise to a sensation much exceeding in purity that of the normal
unfatigued eye. The sensation will differ only in intensity, not in hue,
just as in the red of the normal eye between SSN 65 and 60. As has
already been pointed out we have no certain knowledge of the actual
sensation which the protanope is conscious of, and there can be little
doubt that it is very different from what we call " green," though he
may call it green and often does ; yet he often, too, calls it red.

At SSN 50 the third component is stimulated. At SSN 34-6 (500 /z/x)
the green and blue sensation curves intersect. As the ordinates are
equal he will have a sensation similar to that of the trichromat when all
three components are equally stimulated, i.e., he will have a sensation
of the same nature as that which he obtains from white light, and he
will therefore probably call it white. At any rate he will match it with
the white of the combined spectrum if the intensity is suitably arranged
for him. In other words SSN 34-6 is his neutral point {v. p. 163).
Between SSN 50 and 34-6, then, his colour-sensation, whatever it is,
is becoming less saturated. Beyond SSN 34-6 his colour sensation
becomes more and more saturated, until at SSN 16, where the green
component ceases to be stimulated, it reaches its full spectral saturation,
and remains the same, dift'ering only in intensity, to the end of the
spectrum.

1 Lectures, ii. 315, 1807.

RESEARCHES BASED UPON THE THEORY 233

If the green component is absent the visible spectrum will be the
same length as to the trichromat. From SSN 65 to 50, and from
SSN 16 to the end of the spectrum he will have isolated pure sensations,
"red" and "blue" respectively. At SSN 37-5 (515 /^^la), where the
two equal-area curves intersect, he will see " white " : this is his neutral
point, and we see that it is rather nearer the violet end than in the
protanope.

In some parts of the spectrum it is easier for the dichromat to measure
the luminosities of spectral lights than for the trichromat. In fact near
the neutral points it is a simple matter of homo- as opposed to hetero-
chromatic photometry. Theoretically, the protanopic plus the deuter-
anopic luminosity curves should equal the trichromatic plus one blue
luminosity curve. The extra blue curve makes very little difference,
as it is relatively very small compared with the other two {v. Fig. 63).
Moreover, its effect is further diminished by macular absorption. As
already mentioned, we have no means of making absolute measure-
ments, but the relative values hold good.

We should expect the discrimination sensibility for hues in the
spectrum to be very defective in dichromats. The first experiments
of this nature were made by Brodhun^, himself a deuteranope. His
curve is given in Fig. 4. We see that he has only one maximum,
at 500 /x/Li, near the F line, but here the curve is very sharp and the
value of SA is very low, so that his discrimination sensibility is very
great at this point.

Steindler^ examined three protanopes and five deuteranopes (Figs.
66, 67). She found that SA was much larger than for the normal in
all cases. The deuteranopes showed only one maximum, the average
being at 503 [xfi. The protanopes showed two maxima, at 500 /x/a and
598 /x/x. On the same basis of calculation as for trichromats {v. p. 230)
the maxima for deuteranopes should be at 500 /a/x and 635 /x/x.
The first position agrees well with that found experimentally ; at
635 jjifi the luminosity was too low for deuteranopes for accurate obser-
vations to be made. The calculated maxima for protanopes were
500jLt)M and 600 /Xju, which agree excellently with the observations.
Tritanopes are so rare that they could not be examined ; their theoreti-
cal maxima are at 540 /x/x and at 620 /x/x.

Konig^ calculated that the number of hues which a deuteranope
could discriminate in the spectrum was 140, as compared with the normal

1 Ztsch. f. Psuchol. u. Physiol, d. Sinnesorg. in. 89, 1892.

2 Sitz. d. Wiener Akad. cxv. ii a. 115, 1906. 3 Konig, p. 368.

234

COLOUR VISION

160-165. The number seems high, especially when one considers that
the integration interval is much less. It is explained by the fact that
between the lines E and F deuteranopes possess a much greater dis-
crimination sensibility than normal trichromats {v. Fig. 4). Konig
thought that the number for protanopes was probably about the same
as for deuteranopes.

Fig. 66. Curve of discrimination sensibility for hues of tlie interference spectrum of the
arc light for a protanope. # Neutral point. Abscissae, wave-lengths ; ordinates,
differences of wave-length (5X) capable of being discriminated. (Steindler.)

Liebermann and Marx^ have compared the discriminative sensibility
for hue of a protanope and trichromat by the method of mean error.
They found that of the protanope much inferior. At 509-6 /x/x, near the
neutral point, the mean of 50 observations gave 8A = 3-6 /xju, for the
normal, 16-2 ju,^ for the protanope.

^ Ztsch. J. Sinnesphysiol. xlv. 103, 1911.

RESEARCHES BASED UPON THE THEORY

235

Bruckner and Kirsch^ found that the chromatic action time (v.
p. 91) for deuteranopes, especially for red and green, is considerably
greater than for normal trichromats. No experiments of this nature
have been made upon protanopes.

^00

1 , ,

n-o

\ '■ '

to-o

li I

ji /

C-0

\ ^ /

<rO

\ /

5<?

1 1

500

600

Fig. 67. Curves of discrimination sensibility for hues of the interference spectrum
of the arc light for two deuteranopes. x Neutral point. (Steindler.)

III. Anomalous Trichromatic Vision
Approximate Dichromatism

When the anomalous trichromats were discovered by Lord Rayleigh
the natural assumption was that the two groups correspond to the
protanopes and deuteranopes, but that the defect is partial instead of

1 Ztsch.f. Sinnesphysiol. XLVi. 229, 1911.

236 COLOUR VISION

complete {approxnnate dichromalism). Konig and Abney worked on
this theory and have brought forward much evidence in its favour.
There is, however, another possibility, viz., that there is a displacement
of the normal sensation curves. If, for example, the green sensation
curve is of the same form and magnitude as in the normal, but is dis-
placed towards the red end of the spectrum, the colour sensations of
the individual will be abnormal. It is clear that it is possible to have
a combination of the two characteristics, shift of one or more curves
combined with reduction of one or more fundamental sensations. Of
cases bearing out this conjecture we have at present no evidence, but
there is good evidence that both reduction cases and cases in which
one curve, otherwise normal, is shifted, actually occur. In this chapter
we shall consider the cases of approximate dichromatism only.

If, for example, the red sensation is defective, the ordinate of the
red sensation curve for any given wave-length will be less than the
corresponding ordinate for the normal, e.g., one-half. Then for all
other wave-lengths the ordinates of the red sensation curve are also
half those of the normal curve. Hence it follows that the area of the
red sensation curve must be half the area of the corresponding normal
curve.

We have no method of measuring in absolute units the sensation
produced when light of a given intensity stimulates the retina. Con-
sequently we are unable to determine whether the maximum ordinate
of the sensation curves is the same for all persons. What is actually
done is to compare the sensations produced by given amounts of light
of different colours for each observer. Thus we take some one kind
of light as a standard and compare the relative stimulation of the
components produced by other kinds of light with that produced by this
standard light. In determining a luminosity curve the standard is the
white light of the recombined spectrum. In examining the colour-
defective the complication arises that this standard white is not the
same for the normal and colour-blind observers. The defect influences
the sensation derived from stimulation with white light as well as the
sensations derived from colours. Watson has so lucidly explained the
bearing of this fact upon the deductions that I cannot do better than
quote his remarks^.

" As long as we confine our attention to the part of the spectrum
between the extreme red and the blue, the effect of the blue sensation
on the luminosity may in general be neglected as being too small to

1 Proc. Boy. Soc. Lond. A. Lxxxvin., p. 410, 1913,

RESEARCHES BASED UPON THE THEORY 237

produce any appreciable effect ; hence, in what follows, the effect of the
red and green sensations on the luminosity will alone be considered.

" The easiest way to show the manner in which the luminosity
curve of a colour-deficient person is obtained will be to consider a
particular case, say, one where the red sensation is deficient to such
an extent that all the ordinates of the red sensation curve are only
half the normal. Such a person may be said to possess half-normal
red sensation and will be indicated by the symbol 0-5 RS. Since each
of the ordinates of the red sensation curve is half the normal, the total
area of that curve will also be half the normal. As in the light from
the crater of the electric arc the areas of the red and green sensation
curves are as 579 to 248, it follows that the areas for the 0*5 RS are as
290 to 248.

" Now suppose such a person determines the luminosity of a colour
of which the wave-length is A, and that A;, and A,^ are the ordinates of the
normal red and green sensation curves for this colour, the corresponding
ordinates for the observer will be hK, and A^,. The total sensation
produced by the colour will be the sum of the two sensations, that is,
for the normal it will be A,, -f A^, and for the observer (0-5 RS) -|A,. -1- A^.

" The sensation produced by the comparison white in the luminosity
measurement will be proportional to the sum of the areas of the red
and green sensation curves. Hence, if we represent the areas of these
curves for the normal by Ur and Zg respectively, the sensations produced
by the white for the normal will be Ur + Eg, and for the colour-deficient
will be \Er -\- Eg.

" Thus the brightness of the coloured hght is for the 0-5 RS observer

reduced in the ratio (|A,. + \g)l{K + A^,), while that of the white is

reduced in the ratio {\Er + Eg)l{Er + Eg). Let w be the intensity of

the white when the normal observer makes the luminosity setting and

w' the intensity of the white when the colour-deficient observer makes

the setting. Then we have

w = a (A,. + A,,) (1)

and w' ^-^-^^ -^ a (|A, + A,) (2),

where a is a constant which depends on the unit used to measure the
intensity of the white light.

" Thus the ratio of the colour-deficient observer's luminosity to that
of the normal is given by

^(/ _ ^Xr + A„ Er + Eg

w A, + A, "^lEr+Eg ^'^'-

238 COLOUR VISION

Thus, knowing iv, we can calculate what is the ordinate w of the
luminosity curve for the 0-5 BS observer corresponding to the
colour A.

" The following statement may make the above argument clearer.
When a colour-deficient observer makes a luminosity setting he matches
the brightness of the colour as it appears to him against the brightness
of the white as this appears to him. Owing to his deficiency both the
colour and the white appear less bright. If there were no reduction
in the brightness of the white to him, his luminosity setting would be
{\X,. + A,^)/(A,. + A^) of the normal setting. Owing, however, to the
comparison white being also reduced in brightness the size of his white
unit is reduced in the ratio of \Er + Eg to Er -\- Eg ; and hence, as the
size of the unit has decreased, he requires more of these units to match
the colour, that is, we must multiply the number given above by
{Er + Eg)l{\Er + Eg). This decrease in the value of the white unit
is the reason why the luminosity curve for a red-blind observer is
higher than the normal in the green, and that for a green-blind observer
it is higher than the normal in the red. These high values do not
indicate that a red-blind person receives a greater stimulus from a given
green light, or a green-blind receives a greater stimulus from a red
light, than does a normal, but simply that relatively to the stimulus
received from a given white light the stimulus received is greater.

" Using equation (3) and the luminosity curves for a normal eye
given by Abney, the luminosity curves for persons having the following
red sensations in terms of the normal, 0 RS, 0-33 RS, 0-7 RS, and
the following green sensations 0 GS, 0-33 GS, and 0-6 GS, have been
calculated. The results together with the normal luminosity curve are
shown in Fig. 68.

" It will be observed that all the luminositv curves intersect at one
point, P, which corresponds to SSN 48-8 or a wave-length 5770 A.U.
The condition that the luminosity should be the same for the normal as
for, say, the 0-5 RS observer is that [equations (1) and (2)]

... . Er + Eg . .
(2 A. + A,) prr^^^ = A, + A„

XrEg X Er A,. Er

that is at a wave-length such that for both observers the ratio of the
ordinates of their red and green sensation curves is the same as the ratio

RESEARCHES BASED UPON THE THEORY

239

of the total areas of these curves^. Equation (4) shows that the wave-
length of the light corresponding to thepointof intersection is independent
of the amount or kind of the deficiency in colour sensation of the observer.

" The curves given in Fig. 68 depend on :

" (1) The accuracy of Abney's sensation curves.

" (2) The correctness of Abney's theory that in the case of the
ordinary types of total or partial red or green colour blindness the

120

100
90
80
70
60
50
40
30
20
10

tc-^^^^ -^^

Jit-^m.^ ^^

tt^'^'^A%Z^^ ^v

uW^ /r%^^^ ^v

H®z yt ^1^^^ ^^

IS7 t.^ §§^^-5 ^

it mt-L 7 SSv^Si^^

WL t t ^W\r% 5

Mil _r tSSi^ ^

Mht '1 o|S^^^^

m-t^ t ^SSS^ ^

Wt'^ ^ ^%i\\

Mr ± c^55^ ^

MMl^ i S5S5S\

Miii_v ^mt\\

"fc 7 ^t%^^\

wt ^ ^^^5\

Mi i_ Nx^Nv,^

Mu / / ^i$§^^^

2^ ^ "^^^^

-^^ .^'^ ii

64 62

7000'

60 58

56 54 52

6000

50 48

I I

46
I

55|oO

40 38

32 30 28
I I

Fig. 68. Luminosity curves of persons having normal and reduced red and green sensa-
tions. Abscissae, wave-lengths of the prismatic spectrum of the arc light ; ordinates,
arbitrary scale. (Watson.)

ordinates of one of the sensation curves are all reduced in the same
proportion, and

" (3) The additive property which has been assumed and which
involves the corollary that the areas of the luminosity curves obtained
by normal and colour-deficient persons are the same. Thus, if it can
be shown that the observed luminosity curves of persons who are colour-
deficient agree with the calculated curves it is strong evidence in favour
of the correctness of the above three assumptions."

1 The wave-length at which all the curves will intersect depends on the distribution
of lit^ht in the spectrum employed in the experiments, i.e. on the source of light.

240

COLOUR VISION

Watson has adduced conclusive evidence in this direction, partly
from his own observations by the flicker method, and partly by calcula-
tion from Abney's published cases. Since the observations in Abney's
cases were all published before the calculated curves were obtained,
and some were indeed published before the sensation curves used in
the calculations had been obtained, they comprise an overwhelming
volume of evidence.

If therefore we have the normal luminosity curve we can calculate
the luminosity curves for various grades of approximate dichromatism
(Fig. 68). If these are plotted on a large scale they will afford a means

120
110
100

64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28

— J.. -

^'~tL"'-A^ '^v

/ >'4~'^^M^^—\>^ ^

/v^55||^In ^

24^5^7 li&^ S:

Jtll^jj ^^^\ \

IWIZTI S^^X^^

JM^-tt Sss^ S

ImriJ- ^^t%\K

timJ^2^ twi\-^

jfm / // 9'o;i^.\\ \

^jfjkk k |\\^.\-!:A

jkmi^J^ ^mxlx

Mtt^ 7 ^l^vX

-Mt-J t ^kW\^

IjWrr 1 ^M^

-iWT t m%S

m^ J- S|^\

^// -/ ^§ii

^i£^ y ^§1 ^

2 ^ ^--?

)i I I lecjoo' I I I

I I I I

SOpOAU

Fig. 69. Luminosity curves of a normal trichromat with excessive macular pigmentation.
+ + + foveal ; O O O Q parafoveal. (Watson.)

of measuring directly the amount of deficiency of red or green sensation
in any protanope or deuteranope, whether partial or complete. All that
is necessary is to obtain the examinee's flicker luminosity curve with the
same source of light and under as nearly identical conditions as possible.
The fact that all the curves intersect at a particular wave-length affords
one means of deciding that the case belongs to the group of approximate
dichromats. His average readings at this point should agree with the
average readings of the normal trichromat, and in every case the normal
readings should be taken at the same time. For purposes of diagnosis

RESEARCHES BASED UPON THE THEORY

241

it is not necessary to take readings at all points of the curve. Readings
at SSN 48-8 (5770 A.U.), the point of intersection, and at SSN 42*8
(5410 A.U.), in the green, and SSN 53-2 (6090 A.U.), in the red,
usually suffice.

We have now to consider some divergences from the calculated
curves. In most cases these are slight and are limited to the green-
blue, where differences may be expected, since the effect of the blue
sensation has been neglected in the calculated curves ; and further,
slight differences in macular pigmentation begin to be important in
this region. These divergences do not vitiate the method for purposes

izu —

/ . .^Y \

-/Zc^m^Z\ "^

qn LCZa%Z%^Z\ K

-^J^it '"^^^^tK ^

80 ttlLt%l 1^^^"^

-StA-Tt SgS^^^

70 IWAriJ- ^%.\^z^

tiW--ri- xiSv^^^-

60 -M-'T- t ±-Sg^\ ^

®° mU q^o ^5^S^S^^^

50 mW-I: t^ twi^x

^° j!LL^tri$i ^ty^^

40 'iiM'J-^ J"^ _s^\mv\v

^° mt-r 7 ^^.^^^

in '/LT^ 1 o\k\^\

^° Mtj' 4 5Slx^^

20 Jm-t t §SSS

'° M^ 4 T Si§^^

,0 Mv -y^ §11^^^

10 'My'/ y ■ ■ *$^ *

^^ ^^

64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28

Fig. 70. Luminosity curves of a normal trichromat with subnormal macular
pigmentation. + + + foveal ; O O O parafoveal. (Watson.)

of diagnosis of protanopes and deuteranopes, since they occur in a
part of the spectrum where their characteristic colour defects are least
important from the practical point of view.

Cases, however, occur of marked excess or deficiency of macular
pigmentation, and in these the luminosity curves show marked differ-
ences from the normal (Figs. 69, 70). Fig. 69 shows excess of pigmenta-
tion. Fig. 70 deficiency. By using a central fixation dot and a white
ring, the inner and outer edges of which subtend visual angles of 3° 22'
and 5°, Watson was able to obtain luminosity curves of a parafoveal

p. c. V. 16

242

COLOUE VISION

area. Fig. 71 shows his foveal and parafoveal curves. Unusual macular
pigmentation may be combined with approximate dichromatism, as in
Fig. 72, where partial deuteranopia coexists with excessive macular
pigmentation.

Other methods can be employed for diagnosing approximate dichro-
matism. In addition to taking the ordinary equation, i.e., matching
the white of the source of light with a mixture of homogeneous red,
green and violet, a pure colour may be matched with a mixture of two
pure colours, as in the Rayleigh equation ; or a mixed colour, such as
that obtained by passing the white light of the re-combined spectrum

120
110
100

64 62 60 56 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28

70|00' ' I ' 65[nol ' I ' ..o|on I t 1 t ..joo till jojooAU ' '

Fig. 71. Luminosity curves of a normal trichromat with average macular
pigmentation. + + + foveal ; 0 O O parafoveal. (Watson.)

through a saturated solution of potassium chromate, may be matched
with a spectral colour.

A saturated solution of this salt, | inch thick, will filter off nearly
all the blue in the white light, so that the light appears yellow. Now,
from about SSN 50 to the red end of the spectrum there is no measurable
amount of blue sensation present. If a slit is caused to traverse the
spectrum a position will be found where the light exactly matches
that transmitted through the chromate. For the normal eye it is at
SSN 49-6 (583)Lt/i). If a partial deuteranope makes the match the

/— »^_r)y'^ "N

/ ' ^i— ■ ^

/7lc<^n%%Z\ ^

rrztvK%%Z\ ^

vsz^s^" SSS^^ V

wJirn W^%^t-

JWAtt W^- V

rJn& ^^^^ ^ °

^23^^ k ^^^ %

VM-J ^ ^S&s^^^ it

Mau- It- ^s^§$vS

dLt t P °^^\ N

MW1-^J~ ^ix\

mti- t ^SSo^

-M-M- t ^%Wk

wr/7 / ^^\K

M^^ 7" ^iyis>-

M/ /- i|3^-.GL

'©-' y^ ■^

RESEARCHES BASED UPON THE THEORY

243

slit must be moved towards the red, if a partial protanope towards
the green.

There is a considerable band of the spectrum over which the slit
may be moved without vitiating the approximate dichromat's match,
but Watson^ has pointed out that the match which satisfies the
normal person is always one limit of the band, though the band lies
sometimes on one side and sometimes on the other of this position.
There is at present no explanation of this fact. We shall find that the

izu —

■i ^ r\

—

■^

1 r\r\

•^

fS-

'/,

"7ri

i .

r*''

S-f.

p.n

/ '

"if

I'd

^o^

FLA

Ijk

I'l

l<r

?fh

■/

■^

^in

■^

^\^

+"■

' -,

P-

N-

64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28

I r 1 I I I I I I I I I I I I „,

7000 65 00 6000 55

|oo

I I

50 00 AD

Fig. 72. Foveal luminosity curve of a deuteranope with excessive macular

pigmentation. (Watson.)

anomalous trichromat whose peculiarity is due to a shift of one of the
sensation curves makes a much more accurate setting by this method.

IV. Anomalous Trichromatic Vision

Shift of a Sensation Curve

Abney and Watson^ have pubHshed an exhaustive account of a case
of anomalous trichromatic vision in which the green sensation curve is
of normal shape and size but is displaced towards the red end of the

1 Proc. Boy. Soc. Lond. May 28, 1914. « Ibid. A. LXXXIX, 232, 1913.

16—2

244

COLOUR VISION

spectrum. There is good reason to think that cases of the same nature
are not uncommon, but in the cases hitherto examined the shift has
always affected the green sensation curve and in the same manner.

Figs. 73 and 74 show the normal sensation curves in terms of
luminosity, Fig. 73 being the equal areas curve.

If an observer is an " approximate dichromat," e.g., a partial

^ 22 24 26 ze 50 32 M 36 58 40 42 44464650525456566062 ^
4^0 5/000 5500 6,000 6i500 zoooA.U

Fig. 73. Normal sensation curves in terms of luminosity. (Abney and Watson.)

;0I2I4.I6ie2O222426 28 50 52>t3636 4042444<i48| :i052

4J00

I I / laic I 1^1 I I I I
5500 6/900 6^500

54 56 58 ^62

r.oooAU.

I I I I I I I I
4600 5,000

Fig. 74. Normal sensation curves on equal area scale, (Abney and Watson.)

deuteranope with half the normal green sensation, on the luminosity
scale the ordinates of his green sensation curve will be half those of the
normal curve. On the equal area scale, however, his green sensation
curve will be the same as the normal, for since the area of the green
sensation curve on the luminosity scale is now 124, to obtain the equal
area scale we must multiply by 579/124 = 4-42 (cf. p. 228). Hence,

RESEARCHES BASED UPON THE THEORY 245

since the multiplier is twice as great as for the normal, the resulting
curve will be the same as for the normal.

" Now, if we take a yellow at SSN 48-4 (5760 A.U.), i.e. at a (Fig. 74),
and mix it with a violet at SSN 9-5 (4235 A.U.), i.e. at b (Fig. 74),
the width of the violet slit being 2-5 times that of the yellow, and read
of¥ the three sensations at these places from the full line curves given in
Fig. 74, the values in the violet being multiplied by 2-5, we get the
following numbers :

Position

Sensations

of slit

Red

Green

Blue

67-9

69-3

1-6

1-4

67-7

Sums . .

. . 69-3

69-3

Since the sums for the three sensations are the same it follows that
the mixture will look white to the normal eve. Further, since the
curves for the person who has half the green sensation are precisely
similar, the sums will be equal for him also and hence he will match
the mixed colour with his own white. Similarly for any other case of
colour defect, where the defect is due to a deficiency of one of the
sensations, the curves of the sensations on the equal area scale will be
the same as those of the normal eye.

" Although the person who has the defect of one of the sensations
will agree with the normal match, it will be found that when making
the match the position of the yellow slit can be moved some little
distance from the correct position for the normal without the match
becoming defective to him." Watson has measured the range over
which the match is valid by making himself artificially colour-blind by
fatiguing his eye with a colour {v. p. 106).

" If the sensation curves are the same for a given observer as for
the normal, except that one of them is shifted along the spectrum,
quite a different result will be obtained. Thus suppose that the green
sensation curve is shifted towards the red end of the spectrum by an
amount equal to 2 SSN and occupies the position shown by the dotted
curves in Figs. 73 and 74. The sensations at the points a and b for
such a person are as follows :

osition

Sensations

of sHt

Red

Green

Blue

67-9

79-2

1-6

1-4

67-7

Sums

69-3

79-2

09-3

246 COLOUR VISION

"The sums of the blue and red sensations are still equal but the
sum of the green sensation ordinates is greater, and hence the mixed
colour will not match the white but will appear to such a person too
green. If, however, the yellow slit is moved towards the red to c,
Fig. 74 (SSN 49-8, 5860 A.U.) and the width of the violet slit is made
2-72 times that of the yellow slit we get

Position

Sensations

of slit

Red

Green

Blue

71-8

73-5

0-5

1-7

73-0

Sums. . .

. 73-5

73-5

" The three sums are now equal and hence the mixed colour will
match the white to such an observer, although it will appear orange to
normal vision.

" Hence if an observer does not agree with the normal when violet
and yellow are matched to form white, but requires that the yellow
slit be moved towards the red to form a match, we conclude that his
green sensation curve is displaced towards the red and vice versa."

This explains the accuracy of the match which is made with the
potassium chromate method when there is a shift as compared with
cases where there is a reduction of one of the sensation curves {v.
p. 243).

" In the case of a shift of the green sensation towards the red amount-
ing to 2 SSN as indicated by the dotted curves in Figs. 73 and 74 we
should expect the following effects to be produced :

"1. The part of the spectrum which to the normal appears yellow
will appear greenish, for owing to the displacement the green sensation
excited will be greater than in the normal. In the same way what
appears orange to the normal will appear yellow, and so on.

" 2. If we place three slits in the spectrum, one at the place of the
red lithium line, d, Fig. 74, another at the b magnesium line e, Fig. 74,
and the third in the violet at b, Fig. 74, and by varying the width of the
three slits produce a mixture which to the normal appears to match
the white, this match will not appear correct to the observer with the
green shift (whom, for short, we may designate by R., the normal being
mdicated by N.). To R. the match will be imperfect, for the green
sensation he receives from the light passing through the green slit at
e will not be as great as it is to N. By opening the green slit, we can,
however, obtain a match which is correct for R., but his match will

RESEARCHES BASED UPON THE THEORY 247

appear green to N., and he will never agree that the normal match
is correct.

" If now we move the green slit to/. Fig. 74, where the normal green
sensation curve cuts the displaced curve we shall find that a match
which is correct for N. is also correct for R., and that either can detect
a small departure from this setting.

" 3. The above is one arrangement of the three slits such that
R. and N. make the same match. Another such position is obtained
if the red slit is moved to g, Fig. 74 {SSN 52-4, 6000 A.U.). The light
which now conies through the red slit excites green sensation in the case
of both A^. and R., but to a greater extent in the case of the latter.
Since the red and blue sensations are the same for both it will be sufficient
to consider the equality of the red and green in the two cases. If the
width of the red slit is 0*62 times that of the green we get the following
values of the sensations on the equal area scale :

Sensations

Position

of slit

Red

Green

Red

Green

21-1

48-5

21-1

34-5

43-8

16-4

43-8

30-4

Sums. .

64-9

where the sums are the same in the two cases and hence the mixture
appears white to both N. and R. Thus if the green slit is kept in a
constant position and the red slit is gradually moved up towards the
green the matches made by R. appear to N. at first too green, but the
excess of green gradually decreases till the red slit is at SSN 52-4.
If the red slit is moved further towards the green the mixture which
appears correct to R. will then appear too red to N.

" 4. Owing to the displacement of the green sensation curve i^.'s
luminosity curve will be higher than the normal on the red side of the
point where the normal and displaced green sensation luminosity curves
cut and lower on the green side of this point, for the ordinates of the
luminosity curve are the sums of the ordinates of the three luminosity
sensation curves. The resulting luminosity curve for a displacement
of 2 SSN towards the red is shown in Fig. 73 by the dotted curve, the
corresponding normal curve being given by the thick continuous line."

The observer (/?.), whom Abney and Watson examined, made a series
of matches throughout the spectrum, and from these his sensation
curves were deduced in the manner described on pp. 224, sqq.

" His red and practically his blue sensation curves are identical with

248 COLOUR VISION

those of the normal, but his green sensation curve is markedly different.
It is similar in shape to the normal, but is displaced by about 2 SSN
towards the red end of the spectrum. Thus to him the maximum
for the electric arc light occurs at wave-length 5690 A.U. in place of
at 5575 A.U., which is that of the normal.

" When white is matched by mixing light which passes through
three slits placed at the points d, e, and h, Fig. 74 (6705, 5190,
4235 A.U.), R. requires very much more green than the normal, as is
shown by the first line in the next table. If, however, the red slit is
moved towards the yellow, the green and violet slits remaining fixed in
position, the excess of green required by B. got less and less, till finally

Position of

Slit Widths

" red " slit

W.W.

SSN

AU.

Red

C4reen

Violet

Red

Green

Violet

59-8

6710

28-3

17-2

26-0

62-5

17-2

26-0

571

6450

8-5

17-2

260

16-5

17-2

26-0

54-5

6200

7-2

17-2

260

8-6

17-2

26-0

530

6080

0-2

17-2

33-0

9-2

17-2

33-0

52-5

6040

10-4

17-2

33-0

9-5

17-2

33 0

520

6000

14-8

17-2

450

10-0

17-2

45-0

Changes in slit- widths required to match white by R. and a normal trichromat (W.W.).
The positions of the green and the violet slits were kept constant, as was also the
width of the green slit, the match being obtained by varying the widths of the red
and violet slits and the brightness of the comparison white. With the red slit at
52'5 the match made by either was correct for the other.

a position for the red slit was found where the mixture matched white
both to R. and to the normal. If the red slit is moved further towards
the yellow R. required less green than the normal, so that his mixture
looked slightly red to the normal. The above changes indicated that
in the case of R. we had to deal with a shift of the green sensation
curve."

" When matching D fight with a mixture of red and green fight,
if the red is at the lithium line, R. required considerably more green
than did the normal. If, however, the red sfit were moved towards
the yellow, just as in the case of the white matches, the excess of green
gradually decreased, though, owing to the fact that the D light in the
case of R. excites the green sensation more strongly than the normal,
we did not get the marked change in the appearance of i^.'s match to
the normal which has been referred to in the case of the white match "
{v. p. 247). " For this reason the white match is preferable to the
Rayleigh match for bringing out the characteristic changes when the

RESEARCHES BASED UPON THE THEORY 249

position of the red slit is altered. Another advantage of the white
match is that the yellow produced by mixing green and red to match
the D light is less saturated than the D light itself, and this causes
considerable difficulty with some observers when making the match.

" When matching white by a mixture of violet light {SSN 9-5,
4235 A.U.) and yellow light the following results were obtained :

W.W.
Position of yellow slit . . . . SSN 48-9 or 5780 A.U

R.
SSN 50-0 or 5860 A.U.

showing that the complementary to the violet is in the case of R. dis-
placed towards the red, as has been shown on p. 246 we should obtain
if the green sensation curve was shifted towards the red end of the
spectrum.

" Again it was found that if the red and violet slits were in the
standard positions d and b, Fig. 74, a position for the green slit, /,
Fig. 74, could be found such that the mixed light matched white both
for R. and for TV.

" The luminosity curve obtained hy R., using the equality of bright-
ness method, agrees with what it ought to be if his green sensation curve
is displaced towards the red by about 2 SSN, such a calculated curve
being given in Fig. 73. Attention may be drawn to the fact that -R.'s
luminosity curve does not agree with the normal at SSN 48-6, as it
would if i^.'s abnormality were due to a deficiency of either the red or
green sensations " {v. p. 238).

" It is of practical importance to consider what effect a shift of the
green sensation curve such as that exhibited by R. will have on the
power of discriminating colours, particularly those colours which are
used as signals at sea and on railways. As has been mentioned, one
effect of the displacement of the green sensation curve is that the part
of the spectrum which to the normal appears yellow, to such persons
appears green or greenish. Thus R. places the change from green to
yellow in the spectrum at SSN 49-9 or 5810 A.U. while to a normal
(W.W.) this point appeared to be at SSN 48-8 or 5780 A.U. One effect
of this difference is that a light, such as that given by a paraffin lamp,
which to the normal appears decidedly yellow appears to a person with
the shift of a greenish hue and in fact R. often calls such a light green.

" Another effect of the displacement is that the perception of a green
light when diluted with white light is very much more difficult than for

250 COLOUR VISION

the normal. The reason for this effect is at once apparent from a con-
sideration of the sensation curves. Consider a green at SSN 36 (5090
A.U.). At this point in the spectrum the red and blue sensation curves
for N. on the equal area scale intersect. Hence we may regard the effect
produced by light of this wave-length as an amount of green sensa-
tion (represented by the difference between the ordinate of the green
sensation curve and the ordinate of either the red or the blue sensation
curve) diluted by white light (this white light corresponding to the
equal amounts of red, green, and blue sensation excited). It will be
observed that the amount of the diluting tvhite is the same for the
normal and the person with the displaced green curve ; but to R., the
amount of residual green sensation is less than half that of the normal.
In other words the green perceived by R. when light of this wave-length
enters his eye is very much more diluted than it is to a person having
normal colour vision. As a certain amount of dilution with white
light will obliterate the perception of green in the coloured ray, it
follows that the amount of white light which will obliterate it is
considerably less for R. than it is for a normal vision."

"Further, the want of saturation of the green light makes the
chromatic threshold for green much higher than normal, so that a green
light must be made brighter for a person with a green shift before it
can be distinguished from a white light than for the normal.

" The above results both as to the effect of dilution with white and
as to the chromatic threshold are of great practical importance, for
they both affect the power of an observer to identify green lights such
as those used at sea. These lights are never pure spectral lights, though
they are equivalent to spectral colours diluted with white. Thus the
Board of Trade standard light-green light can be matched by a mixture
of spectral green at SSN 37-4 or 5115 A.U. with an equal amount of
white (arc light). Further, if the size of the image of the coloured patch
on the retina is diminished, it must be remembered that the amount of
white required to extinguish a spectral colour is very much reduced."

SECTION IV

THE OPPONENT COLOURS THEORY (HERING)

CHAPTER I

STATEMENT OF THE THEORY

The three-components theory approaches the subject from what I
have called the synthetic point of view. It provides a theory of colour
sensations in terms of stimulus intensities. The opponent colours
theory approaches the subject from the analytic point of view and
provides a theory in terms of visual sensations.

Mach^ had already pointed out in 1865 that the black-white series
of sensations differed fundamentally from the chromatic series. On
the principle of psychophysical parallelism the two series should have
different physiological bases. Hering^ adopted the psychological
analysis of Goethe, Mach and others, that red, yellow, green, and blue
were the only simple and unmixed colour-sensations, but advanced
novel views about black. He adopted the view generally accepted by
psychologists^ that black is a sensation and is not the expression of the
mere absence of stimulation^. The completely dark-adapted eye when
sheltered from all external stimuli gives a sensation which is variously
described as the hght chaos, the intrinsic light of the retina, and so on.
Hering called this sensation " mean grey." According to him " black "
occurs only as the result of external stimulation, i.e., under the influence
of simultaneous or successive contrast. The " black '" of a black patch
seen on a white background, or of the after-image of a white patch, is
blacker than the intrinsic light of the eye and is regarded by Hering as
the true black sensation.

1 Sitz. d. Wiener AJcad. Ln. 2. 320, 1865. * Zur Lehre vom Lichtsinne, 1876.

3 Cf., however, Ward, Brit. .11. of Psychol, i. 407, 1905.

* Contrast Leonardo da Vinci — " L' ombra e diminuzione di luce, tenebre fe privazione
di luce." Trattato delta Pittura, ed. 1817, Rome, p. 274.

252 COLOUR VISION

Hering thus arrived at six primary sensations, white, black, red,
green, yellow and blue, from which all other visual sensations are derived.
His theory consists essentially in the arrangement of these primaries
into three pairs, the members of each pair being antagonistic to each
other and dependent upon antagonistic physiological processes. The
physiological processes are assumed to affect three different hypothetical
visual substances, white-black, red-green and yellow-blue substances.
These substances exist somewhere in the sub-cortical visual paths :
their exact position is not defined, and it is indeed unnecessary to
predicate three separate substances, except for the sake of clearness^.
The opponent physiological processes are expressed in terms of an-
tagonistic directions of metabolic change.

Hering- supposes that when a living substance is protected from
external stimuli it undergoes spontaneous autonomous metabolic
changes. Some molecules break down or undergo dissimilation (or
katabolism), fresh ones are built up or undergo assimilation (or anabol-
ism). When the two processes balance each other the substance is in
a state of autonomous equilibrium. It is to be noted that autonomous
equilibrium does not necessarily mean physiological inactivity. Fresh
formative matter (Beale) may be brought from the blood in the exact
quantity necessary to replace the formed matter which is poured out
into the blood. If the substance is acted upon by an external stimulus
allonomous metabolic changes are set up. They may be either anabolic
or katabolic, but they induce a spontaneous tendency in the opposite
direction so as to re-produce autonomous equilibrium, i.e., allonomous
katabohsm, for example, induces autonomous anabohsm. With con-
stant stimulation the autonomous anabolism becomes equal to the
allonomous katabolism, and a new condition of equilibrium at a lower
potential is set up, which is called allonomous equilibriutn. Upon
removal of the stimulation autonomous anabolism will prevail for a
time until autonomous equilibrium is again set up.

Hering's metabolic theory of the activities of living substances has
been elaborated and put into more concrete form by Verworn^. It has
had a considerable influence upon modern physiology. The funda-
mental principles have been adversely criticised by McDougalH.

Applying Hering's hypotheses to the visual sensations^, black, green

1 Hering, Sitz. d. Wiener Akad. xcviii. 3, 73, 1889.

- Vorgdmje der lehenden Materie, Prag, 1888 : translated in Brain, xx. 232, 1897-
3 AUgemeine Physiologie, Jena, 1901. * Brain, xxvi, 153, 1903.

s Rivers, Schdfefs Physiology, p. 1112, 1900.

STATEMENT OF THE THEORY 253

and blue excite assimilation, white, red and yellow dissimilation in the
respective substances. Autonomous equilibrium in the black-white
substance corresponds to " mean grey " ; a descending change, or
excess of dissimilation, causes a whiter sensation, an ascending change
a blacker sensation. It is clearly a difficulty of the theory that
autonomous equilibrium of the black-white substance causes a definite
sensation.

In most kinds of stimulation of the retina all three substances
are excited and the character of the sensation depends upon the relative
amounts of action of each and their direction. All coloured lights,
except the four primary colours, have three values or valencies, corre-
sponding to their action on the three substances. The physiological
value or " moment " of a light depends upon its physical value and also
upon the condition of excitability of the visual mechanism.

Hering's explanation of the facts of adaptation and induction is
lucidly summarised by Rivers^ thus :

" The conditions of allonomous equilibrium are the basis of adapta-
tion. When the black-white substance is completely adapted to the
dark, it is in the condition of mean potential, corresponding to autono-
mous equilibrium. During the day the eye is always adapted to the
surrounding illumination. The exact condition of adaptation must vary
to some extent with the continual changes of external illumination to
which one is normally exposed, but for practical purposes the condition
of adaptation may be regarded as changing very slowly, and in any
definite interval of time as corresponding to the mean illumination
during that period. The white-black substance is then in a condition
of allonomous equilibrium at a low potential. The brighter the
illumination to which the eye is adapted, the lower is the potential at
which equilibrium occurs. During the daytime the white-black sub-
stance is always to be regarded as in a condition of relatively low potential.
The condition of allonomous equilibrium at a high potential is exceptional
in the case of the black-white substance, owing to the fact that there is
no external stimulus to anabolism. Black proper, according to Hering,
only occurs under the influence of simultaneous or successive contrast ;
and the best example of allonomous equilibrium at relatively high
potential, in the case of the black-white substance, occurs in Hering's
simultaneous induction. If a black square on a white ground is fixed,
the whole surface after a time is seen as a uniform grey. The black-
white substance corresponding to the black square will have undergone

^ Loc. cit. p. 1114.

254 COLOUR VISION

an ascending change, that corresponding to the white a descending
change, and the condition of uniform grey of the whole surface corre-
sponds to equihbrium, which in the former case is at a high potential,
and in the latter at a low potential. The potential of the former will
only, however, be higher than that of the eye completely adapted to
the dark, if the experiment is carried out with the eye adapted to the
dark. In the case of the white-black substance it would seem as if
allonomous equilibrium might in ordinary life take place at very different
levels of potential, less than that of the autonomous condition ; and
that allonomous equilibrium, at a potential higher than that of the
dark-adapted eye, only occurs under very exceptional conditions.
This might be urged as an objection to the terminology adopted by
Hering. It is due, however, to the absence of proper external stimuli
to anabolism, and this difficulty does not occur in Hering's treatment
of the chromatic substances.

" When the red-green and yellow-blue substances undergo the descend-
ing change, the corresponding sensations are red and yellow respectively.
When they undergo the ascending change, they are green and blue
respectively. The rapidity of the change (or the predominance of one
process over the other) partly determines the weight of the particular
element in question in the sensational complex, or, in other words, the
purity or saturation of the colour ; the other factor determining the
purity being the degree of simultaneous stimulation of the other sub-
stances.

" The conditions of adaptation to coloured light may be readily
referred to different conditions of allonomous equilibrium. When the
chromatic substances are in a condition of equilibrium, they do not
contribute to the quality of the sensational complex. They are equally
in a condition of autonomous equilibrium, whether the eye has been
wholly unstimulated or whether the stimulation has been exclusively
by mixed colourless Kght. When the eye, after exposure to red light,
no longer sees objects red, the red-green substance has become adapted
to the light, and is in a condition of allonomous equilibrium, but at a
low potential ; on the other hand, in adaptation to green light,
this substance is in allonomous equilibrium at high potential. With
removal of the light to which the eye has become adapted, the comple-
mentary after-image colour is due to the autonomous change back to a
condition of mean potential. After-images occupying limited portions
of the visual field are due to local adaptation, and are explained on
the same lines as general adaptation. The after-images seen with

STATEMENT OF THE THEORY 255

complete exclusion of light from the eye, which formed the great
difficulty of the fatigue theory, are here referred to autonomous changes.
Hering has formulated the nature of the active opposed change, which
Plateau had previously suggested as the basis of after-image in
distinction from the passive condition advocated by Fechner and
Helmholtz.

" The absence of colour in all kinds of light at low intensity is,
according to Hering, a function of the dark adaptation of the eye rather
than of the low intensity of the stimulus, and is due to the fact that
in the dark the white-black substance rises greatly in potential, while
the chromatic substances remain in the condition of autonomous
equilibrium. Consequently, the weight of the chromatic is very small
compared with that of the colourless elements, and the former remain
under the threshold."

Hering ascribes the positive after-image produced by closing the
eyes after gazing at a bright obj ect to exhaustion of assimilation during
fixation. As a consequence all that remains is a feeble process of
dissimilation due to internal stimulation. The fact that a positive
after-image becomes negative if a surface, the brightness of which is
greater than that of the positive after-image, is fixated, negatives this
theory.

Simultaneous contrast is explained by Hering in the same manner
as successive contrast. Allonomous katabohsm in a retinal area acts
as a stimulus to autonomous anabolism in that area, but the effect
is not Hmited to the area. It extends to adjoining areas, being most
marked at the junction of the reacting areas.

Hering attributes the bright halo, which under certain conditions
surrounds complementary after-images {v. p. 101), to a process of succes-
sive induction. When, for example, a white square upon a black back-
ground has been for some time fixated, the assimilation process, which
according to Hering has been especially active just beyond the contrast-
ing edges of the white square, reaches such a height that finally a
process of simultaneous induction is set up ; assimilation gives way to
dissimilation. When now the eyes are turned away from the white
square, this dissimilation process continues. It is owing to this succes-
sive induction that a bright halo appears around the dark after-image
of the white surface. And the dark after-image results from the con-
trasting assimilation process, which is evoked by the dissimilation
process that produces the halo.

The dark halo, which under other conditions surrounds an

256 COLOUR VISION

after-image, is attributed by Hering to the effect of simultaneous
contrast. Thus, when a black square has been fixated upon a white
background, the bright after-image of the former is produced by a
subsequent process of assimilation which evokes a simultaneous dis-
similation process in the adjoining retinal region.

Hering's theory thus gives a satisfactory explanation for the most
important facts of temporal and spatial induction, and a less compre-
hensive explanation for the facts of adaptation. Hitherto the question
of luminosity has been left out of account. In the earliest exposition
of his theory {Zur Lehre vo?n Lichtsinne) Hering considered the luminosity
of a visual sensation as wholly due to the black-white component.
If this were true the four primary colours seen in a state of purity would
be of equal brightness. This explanation of luminosity is untenable,
and the " specific brightness of colours " has been substituted for it^.
According to it the warm colours, red and yellow, which are due to
dissimilation, increase the total brightness, whilst the cold colours,
green and blue, which are due to assimilation, diminish it. Red and
yellow therefore possess an inherent brightness {Eigenhelligkeit), green
and blue an inherent darkness {Eigendunkelheit). " A toned colour
may generally be regarded as made up of four fundamental components,
two toned and two tone-free (white and black). It is only in colours
of the tone of a primary that a single toned component is present.
In any red-yellow colour, e.g. orange, we have therefore to distinguish
three bright fundamental components (red, yellow, white) and one dark
(black) ; in any green-blue on the other hand three dark (green, blue,
black) and one bright (white). The red-blue and green-yellow colours
would contain two bright and two dark fundamental components^."
Hering deduces the following rules :

" If two colours of equal tone and equal purity differ in brightness
it is due to the difference in their black-white components."

" Two colours of different tone may dift'er in brightness in spite of
equal purity and equality of their black- white components."

" When the black- white components are equal, a yellow, red, or
yellowish red colour is so much brighter, and a blue, green, or bluish
green so much darker the more distinct the tone of the colour in compari-
son with the black-white components^."

The specific brightness of colours is held to explain the achromatic

1 Hillebrand, SUz. d. Wiener Akud. xcvm. ?,, 70, 1889.

2 Hering, in Graeje-Saemisch Handb. d. ge.i. AugenhlJcde, i. iii, xii, p. 61, 1905.
^ Loc. cit.

STATEMENT OF THE THEORY 257

scotopic luminosity of the spectrum. In dark adaptation the black-
white substance is raised to a condition of high potential, while the
chromatic substances are not appreciably affected. Hence with slight
stimulation the coloured components remain below the threshold, and
the curve of luminosity of the spectrum is that of the black-white sub-
stance. With increase of physical intensity the red and yellow add to
the brightness, the green and blue detract from it, and consequently
the point of maximum brightness shifts towards the red end of the
spectrum.

This identification of the achromatic scotopic luminosity curve with
the activity of the black-white substance has led many to the erroneous
opinion that Hering regards the visual purple as being the black-white
substance.

As Tschermak^, a pupil of Hering, says : " The theory of the specific
brightness of colours was propounded by Hering and Hillebrand to
explain the unequal changes in brightness of different coloured lights
in indirect vision and in dark adaptation (Purkinje's phenomenon), and
therewith also to explain the different distribution of subjective bright-
ness in the spectrum for the colour-seeing light-adapted eye as compared
with the non-colour-seeing dark-adapted or totally colour-blind eye.
The colour sensations, in general terms, may be said, according to
Hering's theory, to be compounded of (zusammengesetzt) or analysable
into (zerlegbar) a chromatic part and a non-chromatic grey part of
different nature (brightness) determining the nuance, and of different
relative magnitudes determining the saturation, so that so-called
coloured lights bring about a double excitatory effect in the light-adapted
eye, combining in addition to a colour valency also a white valency.
The latter — in conjunction with the continuous processes which produce
the intrinsic grey of the eye and with the contrast effects (increase of
blackness) which are due to reciprocal action from the surrounding
areas— determines the nature and relative magnitude of the non-
chromatic sensation constituent, the grey components. The bright-
ness of a colour sensation cannot be referred only to the brightness
of its grey components, but must also depend upon the colour-
components. Colour impressions, therefore, even with the greatest
possible saturation, i.e., with the greatest possible abstraction of the
grey components, possess also ' brightness ' — a quahty which cannot
be more closely defined and is only absent from absolute black."

The tone-free sensation derived from the mixture of two

1 Ergeb. d. Physiol, i. 792, 1902.
P. c.v. 17

258 COLOUR VISION

complementary colours is due to the combined action on the black-
white substance. With the correct intensities the anabolic effect of
one colour neutralises the katabolic effect of the complementary on the
chromatic substances, whilst their combined black and white valencies
manifest themselves as white or grey.

The regional effects of stimulation of the retina are attributed to
absence of response of one or of two of the substances. In the outermost
zone only the black- white substance is stimulated ; in the middle zone
only the black- white and the blue-yellow substances are stimulated.
Hess's experiments on the whole confirm this hypothesis {v. p. 70).

The fact, however, that the colour values diminish unequally with
increasing excentricity, those red- wards of green (495 jjcfji) most rapidly,
those violet-wards most slowly, necessitates that the sensibility of the
red-green substance diminishes more quickly than that of the yellow-blue.
The total colour blindness of the extreme periphery affords another
method of determining white valencies, but it gives totally different
results from the values obtained with low intensities and dark adaptation.
The latter can be explained as due to a different mechanism, the rods.
If the normal peripheral values give the true white valencies then they
must be different in the protanopic eye. But Hering considers both
protanopia and deuteranopia as due to absence of the red-green substance
{vide infra). This would be insufficient alone to account for the alteration
in white valencies, which must depend upon change in the white-black
substance.

As shown by Hess's experiments, confirmed by Baird and others,
Hering's fundamental colours are those which, when passed from the
periphery to the centre of the retina, develop sensations which undergo
no change in hue. The red is a spectral red mixed with a small amount
of blue. According to Hering spectral red has a not inconsiderable
action on the yellow-blue substance. A mixture of spectral red and green
produces the sensation of yellow because, although the red and green
processes neutralise each other, the effect of the spectral red stimulus
on the yellow-blue substance remains. It is noteworthy that the
fundamental red and blue agree with those adopted by adherents of the
Young-Helmholtz theory (p. 220).

Colour blindness is explained on the same principle. Thus, in total
colour blindness only the black- white substance responds to stimulation
by all lights. Hence the luminosity curve of the totally colour-blind
should be the same as that of the achromatic scotopic eye, which is
the case. On the other hand, the lummpgjty values of coloured lights

STATEMENT OF THE THEORY 259

should be the same for the totally colour-blind and for the extreme
periphery of the retina, which is not the case (v. Kries) {v. p. 71).
Dichromatic vision is attributed to insensitiveness or absence of the
red-green substance, or of the yellow-blue substance (tritanopia).

The two classes of red-green blindness, protanopia and deuteranopia,
offer serious objections to Hering's hypothesis, and these have not yet
been overcome satisfactorily. His original explanation was that the
differences were due to physical causes, especially macular pigmentation,
and the same was applied by him to the variations in the Rayleigh
match (anomalous trichromatic vision). Hering^ very carefully ex-
amined Prof. Biedermann and Dr Singer, well-marked examples of indi-
vidual variation in pigmentation. He regarded the former as relatively
yellow-sighted, with little macular pigmentation, the latter as relatively
blue-sighted, with greater pigmentation. He found Biedermann's
matches agreed with deuteranopic matches, Singer's with protanopic.
Experiments in which the subjects looked through the macular region
of dried human retinae were held to give confirmatory results. His
own extrafoveal matches compared with his fovea] matches were of
the same nature as the deuteranopic to the protanopic foveal matches.
On the other hand Biedermann and Singer's peripheral matches did not
agree, as should have been the case, but the differences were diminished
in amount, and were attributed to absorption by the crystalline lenses.

V. Kries {v. p. 168), Abney and others have shown conclusively that
the differences between protanopic and deuteranopic vision cannot be
explained by any such physical causes.

Some of Hering's assumptions are difficult to accept. The absence
of any direct evidence of anabolic processes acting as physiological
stimuli has already been mentioned, but it is not a serious objection.
The balance between assimilation and dissimilation is obviously com-
parable to the balance between excitation and inhibition^, and analogies
can be readily found in Sherrington's researches.

It is, however, a distinct objection that the theory demands twice
as many variables as the results of colour-mixtures demand. Brunner^
has attempted to eliminate this difficulty by ascribing to each substance
a reversible photochemical process*.

1 Lotos, VI 142, 1885.

2 Cf. Bernstein, Naturwissenschaftliche Rundschau, xxi. 497, 1906.

3 Arch. J. d. ges. Physiol, cxxi. .370, 1908.

* See also Pauli, Der kolloidale Zustand u. die Vorgdnge in der lebendigen Substanz, 1902.
For examples of reversible photochemical processes, see Stobbe, Ann. d. Chemie, cccux. 1,
1908.

17—2

260 COLOUR VISION

Further, whilst red-green and yellow-blue show a null-point or
absence of sensation, black-white shows no such null-point. Moreover
the autonomous and allonomous equilibrium of the black- white are very
different greys. In the light of the duplicity theory Hering's mean
grey must be referred to a different mechanisn from the chromatic
mechanism. Whilst long fixation of a toned light certainly tends to
produce a tone-less sensation, the behaviour of black and white is quite
different. Yet on no theory can the chromatic mechanism be deprived
of its capacity to produce a tone-less sensation.

We have already stated that a given colour can be defined by its
hue, its luminosity and its degree of saturation. Hering hypothecates
three ratios : white to black, red to green, and yellow to blue. The
question arises how the hue, the luminosity and the saturation are
determined by these three ratios. Hering's early hypothesis that the
luminosity was determined by the white to black ratio proved untenable
and gave place to the specific brightness of colours. Similarly, the hue
cannot be determined simply by the two chromatic ratios, since we have
seen that the hue changes for most colours on admixture with white.
These are serious difficulties attending acceptance of the theory, but it
must be admitted that the relationship of luminosity to hue and saturation
is not satisfactorily explained either by the three components or any
other theory.

Hering's theory starts from the choice of psychologically pure colour
sensations. If, however, spectral red and green are chosen and mixed
they prove not to be complementary colours^. The colour adopted
by Hering and his followers as fundamental red is a distinctly carmine
or bluish-red {v. p. 258). Psychological analysis, however, does not
afford a very secure foundation for the determination of the purity or
simplicity of a colour sensation 2. Most people would say that green
is not a simple sensation, doubtless owing to experience derived from
the mixture of pigments. On the other hand, most people would agree
that yellow is a simple sensation, and this assumption is a necessary part
of Hering's theory. Yet there are many arguments, apart from those
derived from colour mixtures and fatigue already discussed, against
such a view. McDougalP brings forward the following. (1) If patches
of red and green light be thrown upon one area of one retina, they may
under suitable conditions show the phenomenon of struggle as well as
of fusion, i.e., at one moment they may appear as yellow, at another

^ Ladd- Franklin, Psychol. Rev, vtn. ^ See Section I. Chap. ni.

3 Mind, X. N.S. 380. 1901.

STATEMENT OF THE THEORY 261

the red or the green may alone affect consciousness, thus proving that
the actions of the red and green rays on the retina are not mentally
antagonistic or destructive, but that they proceed side by side in the one
area of the retina. (2) On fixation of a bright yellow light it becomes
after a few seconds a bright pure red or a bright pure green or shows
struggle of red and green. (3) In the after-image of very bright light,
whether white, or yellow, or of other colour, yellow never appears
except as an incident in the struggle between red and green, just as purple
occurs as an incident in the struggle of red and blue : it never appears
as do red, green, and blue, forming one of the phases of constant colour
of the recurring cycle, red, green, blue, red, green, blue {v. p. 111).
(4) On diminishing the illumination of a patch of yellow after fixating
it for 30 sees, or more, it usually becomes red unless the period of fixation
has been considerably prolonged, when it usually becomes blue. (5) The
after-image of bright yellow light may be yellow in the first phase, but
this yellow always resolves itself into green struggling upon red as in
the case of an initial yellow phase following bright white light. In the
after-image of less bright yellow light blue usually appears, but red and
green usually predominate. In fact "it is impossible to determine
from the character of a sensation that the physiological process under-
lying it is simple and that the sensation is not the result of psychical
fusion of the effects on consciousness of two or more separate physio-
logical processes."

Although the theory of opponent colours offers an easily com-
prehensible explanation of the most prominent facts of induction,
both successive and simultaneous, it is incapable of accounting for
many other facts of the same nature. McDougall^ brings forward the
following formidable list.

(1) The occurrence of the sense of absolute darkness or blackness
in the absence of any stimulation of white light which could produce
it by simultaneous or successive contrast. If a dull or moderately
bright patch of white or coloured light on a dark ground is fixated very
steadily it will suddenly disappear. If the patch be upon a background
not quite dark, but feebly illuminated, it happens not infrequently
that the whole field of vision disappears, leaving a sense of complete
and extreme darkness which is more complete than is experienced on
excluding all light from the eyes. This " complete fading " occurs more
readily if accommodation is relaxed.

^ Loc. cit.

262 COLOUR VISION

(2) Simultaneously induced light appearing :

(a) on a ground that has shown no previous contrast effect ;
{h) almost or quite as bright as the inducing light ;
(c) failing to appear on a ground on which a marked contrast is
produced.

(3) Binocular contrast, whether of white or coloured light.

(4) Binocular fusion of complementary colours to give a white
sensation.

(3) and (4) are only to be reconciled to the theory of opponent
colours on the assumption, in the face of strong evidence to the contrary,
that the cerebral areas for the two eyes are identical.

(5) Certain features of uniocular contrast :

(a) the total inhibition of a patch of colour by bright white light
falling on an area of the retina at some distance from that affected by
the coloured light ;

(b) the inhibition of colour of low saturation by a white ground ;

(6) Positive after-images in general :

{a) the phenomena of fading of after-images, especially of white
light, with its recurring cycle of phases of pure and highly saturated
red, green, and blue {v. p. Ill) ;

(b) the fact that fixation of white light is invariably followed
by an after-image brighter than the ground, if the conditions that lead
to the production of a relatively bright halo are avoided ;

(d) the frequent appearance of a same- coloured phase in the after-
image of less bright coloured lights {v. p. 102).

(7) The variety of colours that may appear in the after-image
of any light save the dullest -that will give any after-image, e.g., pure
blue phases in the after-image of red light.

(8) The array of facts indicating that the yellow sensation is due to
psychical fusion of red and green {v. p. 260).

STATEMENT OF THE THEORY 263

(9) The reversal of the colour of a fixated patch by addition of
white light.

(10). The reversal of the colour of a patch of hght during simple
prolonged fixation.

(11) The appearance of colours of fair saturation during simple
fixation of bright white light.

(12) The uniocular struggle of complementary colours during which
they fuse at moments to give white and at other moments appear
separately in consciousness.

(13) The fact that a white image, produced by uniocular fusion
of two complementary colours, may be followed by an after-image, the
character of which proves that the rays of each colour have produced
throughout the appropriate retino-cerebral colour-systems their specific
effects, and that therefore there has been no mutual destruction or
interference of their physiological effects. (See McDougall's Theory
in Section V.)

CHAPTER II

RESEARCHES BASED UPON THE THEORY

We have already, in Part I, discussed many of the researches which
were inspired by Hering and his theory. Of these the most noteworthy
are those referred to under the names of Hering, Hess, and Tschermak,
They deal chiefly with the equivalence of colour equations under
different conditions, — adaptation, area and region of retina stimulated,
etc. — and with the facts of temporal and spatial induction. Those
devoted to colour mixtures have been largely polemical and directed
to controvert the statements of adherents of the duplicity theory or of
the Young-Helmholtz theory,

Hering's theory affords an explanation of the general facts of succes-
sive contrast. If for example, the eye is stimulated with blue light,
allonomous anabolism is set up in the yellow-blue substance, and a
large amount of the substance is formed. If now yellow light stimulates
the retina katabolism is set up in this much increased material and the
resulting sensation is much greater than without previous stimulation.
The abnormal saturation of the complementary to the stimulating
colour is thus explained. A positive after-image is explained by a

264 COLOUR VISION

continuation for a certain period of the anabolic or katabolic process
after the stimulus has been withdrawn.

V. Kries, however, pointed out that the theory fails to explain
certain details adequately. Thus, since stimulation with colourless
light does not act upon the chromatic substances it was to be expected
that after previous stimulation with white light the same quantities of
coloured light would be required on both the stimulated and resting
areas to produce a colour-match. He found that it was not the case.
Heringi made many objections to v. Kries's experiments and explained
away his results {v. p. 108). He appears to hold that whilst previous
stimulation with white does not affect colour valency, chromatic stimula-
tion markedly affects white valency. As Greenwood points out^, Bering's
hypothesis becomes unsatisfactory by multiplying its detailed sub-
hypotheses.

On simultaneous contrast Hering himself has done much work and
has been the means of initiating more. The researches of Hess and
Pretori have already been discussed. Those of Pretori and Sachs^ on
colour contrast are scarcely comprehensible except in terms of the
theory and may now be mentioned. These authors endeavoured to
discover the quantitative relations underlying colour contrast. Accord-
ing to Hering toned lights possess both colour and white valencies.
In addition to these exogenous valencies there is an endogenous black
valency dependent upon autonomous processes and manifesting itself
in the intrinsic grey of the resting eye. There exists therefore between
a coloured " contrast-exciting " (kontrasferregend) and a colourless
" contrast-responding " (kotitrasterleidend) field besides the colour con-
trast also a colourless brightness contrast so long as the white valency
of the contrast-responding field is not exactly equal to the black induction
due to the white valency of the contrast-exciting field. The saturation
of the contrast colour depends therefore upon the relationship of the
colour contrast to the simultaneous colourless contrast. Hering* has
demonstrated the disturbing influence of simultaneous brightness con-
trast on colour contrast by special experiments.

Pretori and Sachs used a rotating disc with variable sectors. Three
sets of circular papers were mounted concentrically on the disc. The
diameters of the three sets were : upper 8 cm., middle 11-4 cm., lower
19-6 cm. ; so that the middle set formed a ring 17 mm. in breadth and

1 Arch.f. d. ges. Physiol, xciv. 533, 1903.

2 Physiology of the Special Senses, p. 200.

•■' Arch. f. d. ges. Physiol. LX. 71, 1895. * Loc. cif. xli. 27, 1887.

RESEARCHES BASED UPON THE THEORY 265

constituted the contrast-responding field. The inner circle (upper set)
and outer ring (lower set) always had the same sectors and together
constituted the contrast-exciting field. If the exciting field was red,
for example, and the responding ring black, the black appeared reddish^.
If a white sector was introduced into the responding field the red dis-
appeared, and if the white sector was increased in size the grey ring
became tinged with the contrast colour, green. By further adding a
suitable red sector to the responding field the contrast green could be
counteracted. The size of the red sector afforded a measure of the
contrast effect. If the red sector in the responding field was increased
by 10° each time from zero to 120°, the white and black sectors being
altered suitably to eliminate the contrast colour, it was found that with
a constant coloured exciting field the grey responding field remained grey
when its red and white valencies increased proportionately.

The contrast-exciting field was then varied in three ways : (1) the
colour valency was changed while the white valency was kept constant ;
(2) the white valency was changed while the colour valency was kept
constant ; (3) both colour and white valencies were changed, but the
ratio between them was kept constant.

(1) In the contrast-exciting field there were a coloured sector and
sectors of white and black such that the grey produced by rotation looked
exactly like the colour when seen by achromatic scotopic vision. In
the contrast-responding field a definite grey made up of black and white
sectors was used and a coloured sector was introduced until the initial
contrast colour was eliminated. In the experiments the coloured sector
in the exciting field was gradually increased ; that in the responding field
was kept constant ; the contrast colour was eliminated by gradually
increasing the white sector from zero. It was found that with constant
white valency of the contrast-exciting field (and therefore constant
black induction due to it), combined with increase of colour valency,
the same amount of colour contrast is caused by a simply proportional
diminution of white valency in the contrast-responding field. If there-
fore the white valency of the responding field is kept constant the con-
trast effect increases proportionally to the increase in colour valency.
If the exciting light is kept constant and the white valency of the respond-
ing field increased from zero the saturation of the contrast colour in-
creases up to a certain optimum value.

(2) In the contrast-exciting field the coloured sector was kept
constant and the black-white sectors varied. In the contrast-responding

1 Cf. V. Helmholtz, 1st ed. p. 400.

266 COLOUR VISION

field the coloured sector for eliminating initial contrast was kept con-
stant and the white gradually increased from zero until the contrast
colour was eliminated. It was found that with constant colour valency
of the exciting field, combined with increase in the white valency (and
therefore increase of black induction due to it), the same amount of
colour contrast is caused by a simply proportional increase of white
valency in the responding field. Increase of colourless contrast there-
fore proportionally diminishes the colour contrast.

(3) In the contrast-exciting field the coloured sector and the white
sector were increased in such a manner that the colour valency and the
white valency increased in the same proportions. In the colour-
responding field the coloured sector for eliminating initial contrast
was kept constant and the white sector gradually increased from zero
until the contrast colour was eliminated. It was found that with
proportional increase of colour valency and white valency in the exciting
field there was generally no increase in contrast, but in some experiments
the contrast effect increased nearly proportionally to the intensity of
the contrast-exciting mixture up to a certain optimum value.

With regard to colour blindness Heringi in 1885 examined a series
of cases of anomalous trichromats as to their colour mixtures with rota-
ting discs. He found that they could be divided into two groups, one
group making the luminosity match of spectral red (660 /xju.) to spectral
blue (447 fifx) as M5 : 1, the other as 7 : 1. From his observations he
called the first group, i.e. partial deuteranopes, relatively yellow-sighted ;
the second group, i.e. partial protanopes, relatively blue-sighted. Since
colour equations are purely relative no exception can be taken to this
nomenclature. He found the position of his fundamental green in the
spectrum to differ in the two classes, for the first a relatively longer
wave-length, for the seond a relatively shorter. For colourless mixtures
of spectral red and blue-green, or yellow-green and violet, or to a less
extent yellow and blue, the relatively yellow-sighted required a larger
amount of the short wave-length component. Similarly they required
more green for a match of spectral yellow with a red and yellow-green
mixture, or of greenish-yellow with an orange and yellow-green mixture.
He found the results inconclusive with the blue-sighted owing to too
great differences of saturation. There was no typical difference in the
matches of blue with a green and violet mixture. Similar, though less
marked, differences of the same nature were found in peripheral matches
by Biedermann (yellow-sighted) and Singer (blue-sighted). The

1 Lotos, N. F. VI. 1885.

RESEARCHES BASED UPON THE THEORY 267

relatively yellow-sighted saw farther to the red end of the spectrum than
the blue-sighted, and the violet end was less saturated to them. The
yellow-sighted appeared to have more sensitive chromatic substances
than the blue-sighted.

Hering compared the sensations of the two types of " red-green "
blindness, i.e., protanopes and deuteranopes, with those of the anomalous
cases. He showed that the red-green blind with unshortened spectrum
(deuteranopes) were analogous to the yellow-sighted in their grey matches.
Similarly the red-green blind with shortened spectrum (protanopes)
were analogous to the blue-sighted^. The colourless green of the
neutral point, matched with a red-violet mixture, required a greater
intensity of the green in the so-called green-blind than in the so-called
red-blind. Hering specially recommends this match for statistical
purposes, but unfortunately it is much vitiated by pigmentary absorp-
tion. Another typical difference was the greater luminosity of the red
end of the spectrum for the so-called green-blind, the red-blind requiring
a much darker grey luminosity-match. These and other effects were
attributed to a weaker blue sensation in comparison with the white
sensation in the red-blind as compared with the green-blind. Similar
differences were found in peripheral vision. It will be noticed that
Bering's researches confirmed those of Konig and other observers, but
his interpretations are different.

Hering's researches on the totally colour-blind^ first proved the
identity of their luminosity curve with that of the normal dark-adapted
eye for low intensities. He regarded this fact as strong confirmation
of his estimates of the white valencies of lights for the dark-adapted
eye. He also found that the sensitiveness of the totally colour-blind
eye for slight differences of luminosity was the same as that of the normal
achromatic scotopic eye. Hess and Hering^ and their followers have
strenuously opposed the view that the totally colour-blind have a central
scotoma, but as we have seen, too much stress has been laid upon the
negative evidence.

We have seen {v. p. 256) that Hering has attempted to explain the
differences of the photopic and achromatic scotopic luminosity curves
by the theory of the specific brightness of colours. Tschermak* does

1 Cf. Rose, Arch f. Ophth. vn. 2, 72. 1861.

2 Hering and Hillcbrand, Sitz. d. Wiener Akad. xcvm. 70, 1889 ; Hering, Arch. /. d.
ges. Physiol, xltx. 563, 1891.

3 Arch.f. d. ges. Physiol. Lxxi. 105, 1898.

« Ibid. Lxx. 297, 1898 ; lxxxii. 559, 1900.

268 COLOUR VISION

not think that this theory applies to the differences of luminosity
observed in various colourless mixtures in passing from parafoveal to
peripheral vision or on changing the condition of adaptation of the eye.
He regards these changes as functions of the physical stimuli or wave-
lengths of the light, not of their physiological valencies. He found that
in the normal photopic eye the white valencies of different lights change
unequally in passing from the centre to the periphery, as well as at the
same site with dark adaptation. He therefore concludes that the white
valencies of all lights change under different conditions. They increase
for the normal and the colour-blind eye in passing from light to dark
adaptation. In the normal this change is unequal, in that the white
valencies of the long wave-length lights increase relatively less than those
of the short wave-length lights. With relative yellow-sightedness this
adaptative change of the white valencies is more pronounced than with
relative blue-sightedness. The two types of red-green blind persons
show analogous changes. In typical totally colour-blind people there
is a change in the absolute white valencies, but none in their relative
values.

These and the regional changes Tschermak attributes to changes
" in the photo-chemical stimulus-intermediaries in the absorption
apparatus of the visual organ." One of the photo-chemical white
substances may be the visual purple and its accumulation during
protection of the eye from light may underlie the increased excitability
of peripheral parts of the retina. Thus, change in concentration of
an elective absorbing substance alters the absorption equivalents, and
might be expected to act in the opposite sense to the Purkinje pheno-
menon, a view which has been adopted by v. Kries^ {v. p. 206).

Tschermak^ indeed considers the theory of the specific brightness
of colours insecurely founded {nicht hinldnglich begrundet), chiefly on
the grounds of the supposed unequal summation of luminosities in
mixtures. Thus Hering^ mixed the lights from a red and a green glass
of the same subjective brightness, and found that half the quantity of
the mixed light was darker than either of the components alone. Similar
results were obtained with complementary colours, thus agreeing with
Ewald and Kiihne's results^. Bonders^ obtained similar results with

1 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxix. 81, 1902.

2 Ergeb. d. Physiol, i. 2, 797, 1902.

3 Lotos, N. F. n. 31, 1882 ; vi. 57, 1885.

* Untersuch. a. d. physiol. Inst, zu Heidelberg, i. 153, 208, 1878.
5 Arch f. Ophth. xxx. 1, 15, 1884.

RESEAKCHES BASED UPON THE THEORY 269

the Rayleigh mixture. These experiments, however, require confirma-
tion by more trustworthy methods of heterochromatic photometry.

Whilst the researches of Hering and his school have contributed
many facts of value to our knowledge of colour vision, especially in
the domain of induction, one cannot help being struck by the fact that
they have led to far-reaching modifications and increased complexity
in the theory itself.

SECTION V

OTHER THEORIES

I. DoNDERs' Theory

Donders^ accepted the principle of three " fundamental " colours,
derived from Young and v. Helmholtz, and later elaborated by Konig
and others. He also accepted four " simple " colour-sensations on
psychological grounds, — red, yellow, green and blue. He further
accepted the absolute correspondence of psychical and physical elements
in Fechner's sense, that they may be compared to the concave and
convex sides of the same curve. He was thus led, with Mach and
Hering, to postulate four corresponding specific processes, the sensations
of white and black excluded.

Donders placed the seat of these processes in the brain. The
peripheral or retinal processes are trichromatic, the stimuli acting upon
the three mechanisms of the " fundamental " sensations. Here each
process acts upon a single form-element. In the cerebral centres more
than one process can act upon the same element. From the combined
action of two colours a third, containing none of the characteristics of
either, can arise, e.g., yellow from red and green, so that from two pro-
cesses a third, sui generis, is evolved.

The processes may be regarded as chemical dissociations. Complete
dissociation of the molecules gives rise to the sensation of white. It
must be complete because the sensation continues unchanged and does
not dispose to secondary sensations ; this is the case with white only.
Restoration and dissociation occur somewhat as described in Hering's
theory.

The sensations of the " simple " colours are attributed to partial
dissociation of the same molecules. In opposition to white they call

1 Arch. f. Ophth. xxvn. 1, 155, 1881.

OTHER THEORIES 271

forth the complementaries, which process once begun increases in
strength ; hence the loss of saturation of colours on prolonged fixation.
The primary, partial dissociation leaves a residuum of molecules which
undergo secondary dissociation, giving rise to the complementary
sensation. The residual molecules gradually dissociate spontaneously,
without adequate stimulation, and this process increases more and more
until there is equilibrium of direct and indirect dissociation, giving rise
to the neutral sensation. If the eye is then stimulated with white light,
the complementaries become more pronounced until, with the destruc-
tion of the residual molecules, equilibrium is re-established.

Bonders thus introduced the ideas of chemical dissociation and
separation of the peripheral and central processes, both of which have
been further elaborated by later theorists.

II. Ladd-Franklin's Theory

Mrs Ladd-Franklin^ propounded the following theory.

In the earliest stage of its development the visual sense con-
sisted only in the sensation of grey. The term " grey " is used to
express the whole series of black-white sensations. This sensation of
grey was brought about by the action upon the retinal nerve-endings
of a chemical substance set free by means of the decomposition of a
certain kind of molecule, the grey molecule. This molecule is composed
of an outer range of atoms, somewhat loosely attached to a firmer inner
core, and having various different periods of vibration. The decomposi-
tion of this molecule consists in the tearing off of its outer portion, which
then becomes the exciter of the nerve-endings and the immediate cause
of the sensation of grey. The tearing off is brought about by the ether
vibrations of the entire visible part of the spectrum, but in the greatest
amount by those near the middle part, as is shown by the sensations
of the totally colour-blind.

In the course of the development of the colour-sense some of the
grey molecules become differentiated into colour-molecules in the
following manner. The atoms of the outer range segregate themselves
into three groups or pairs of groups at right angles to each other and
having three different average velocities. The adaptation between the
present structure of the retina (as regards colour) and the constitution
of physical light consists in the fact that the mean vibration periods of

1 Ztsch.f. Psychol, u Physiol, d Sinnesorg. iv. 211, 1893; Mind, N. S. u. 473, 1893.

272 COLOUR VISION

the atoms oi each group are synchronous with, and probably sub-
multiples of certain vibration periods of the ether, i.e., the vibration
periods of the three fundamental colour-tones. Hence, when light of
a fundamental colour-tone, e.g., green, falls on the retina it will have the
effect of tearing off from a large number of molecules those atom-groups
whose periodicity is such as to render them particularly exposed to its
shocks, and hence that special substance will be set free which is the
exciter of the sensation of green.

When the wave-length of the light which falls onthe retina is any-
where between the wave-lengths of two fundamental colour-tones — for
example, blue and green — then a certain number of molecules lose their
blue constituents and a certain number their green constituents, and
the resulting sensation is a mixture of green and blue. This fact explains
why we are unable to distinguish between a single intermediate wave-
length and the appropriate mixture of two out- lying wave-lengths.

There will be certain mixtures of objective light which will set free
all three kinds of nerve-exciting substances in equal amounts. These
three substances, however, are the chemical constituents of the exciter
of the grey sensation. Hence, when they are present in the right amount
they recombine to form that substance and the sensation produced
is exactly the same as that caused by the decomposition of the grey
molecules.

There are five instances in which we are incapable of receiving any
sensation but that of grey. In all, the grey molecule is alone decomposed.
Thus in the peripheral totally colour-blind zone of the retina the diffe-
rentiation of the grey molecule into colour-molecules has not taken
place. Similarly, total colour blindness is an example of atavism, the
grey molecules remaining undifferentiated. When such small areas of
the retina are stimulated, or areas are stimulated with light of such weak
intensity, that no colour-sensation is aroused it is to be concluded that
the colour-molecules are not decomposed in sufficient quantity. When
the stimulus is so intense that the colour-sensation is blotted out, it is
to be supposed that the colour-molecules have become exhausted sooner
than the grey, or that a strong energy of ether vibrations affects all
the colour constituents equally without reference to their periodicity.
In all these cases the important feature is the capacity of the substance
which excites the sensation of grey for independent existence.

Mrs Ladd-Franklin holds that the theory explains not only the facts
of colour-mixture, but also after-images and simultaneous contrast.
Thus, in her own words : " When a red light, say, has fallen for some

OTHEE THEORIES 273

time upon the retina a large number of molecules have lost their red
constituents — they have become partly mutilated molecules. But in
this condition they are extremely unstable ; they gradually go to pieces
completely, and the setting free of their remaining constituents, the blue
and the green producing parts of the molecules, causes a sensation
of blue-green. The red sensation, therefore, in the case of careful
fixation, becomes paler and paler ; if the objective illumination is
weakened, it may even be overpowered by the blue-green sensation ;
and if the eyes are closed, the blue-green sensation alone remains after
a few seconds and continues until the injured molecules have all become
completely destroyed. Since, as is well-known, the circulation of the
retina is extremely rapid, the half mutilated molecules are in large
numbers dragged across the border of the original image and there their
complete destruction causes the phenomenon of simultaneous contrast."

The facts of scotopic vision are explained by the theory on the view
that the grey substance is present in the rods, which subserve vision
at low intensities, whilst the differentiated material is present in the
cones, which subserve colour-vision and vision at higher intensities.
A novel suggestion is that the grey substance in the rods is responsible
for the change in the luminosity curve at higher intensities, its decom-
position undergoing change as the intensity increases.

The theory is held to offer a satisfactory explanation of the relative
saturation of colours in the spectrum. The number of molecules
decomposed by a given light depends on the closeness of the coincidence
of the vibration periods. For wave-lengths half-way between those of
two fundamental colour-tones this coincidence will be very slight, and
hence the number of molecules decomposed will be small, so that the
resulting colour-tone is very little saturated. Green is less saturated
than red and blue because the grey substance is most decomposed by
lights of this region of the spectrum.

It also explains the relative sensitiveness of the eye to change of
colour per change of wave-length, which is greater in the yellow and
blue-green than anywhere else (Part I, Section II, Chap. ii). Where
the number of colour-molecules decomposed by a light of given wave-
length is relatively small a given amount of change of wave-length
is necessarily more effective in changing the quality of the sensation.

Mrs Ladd-Franklin severely criticises both the Young-Helmholtz
and Hering theories, yet it must be admitted that her theory is a
legitimate offspring of the three-components theory. It perhaps owes
more than she would readily admit to Bonders' theory.

p. c. V. 18

274 COLOUR VISION

V. Kries^ raises the fundamental objection that the complete decom-
position of the differentiated molecule gives rise to the same sensation
as the undifferentiated. This objection may be surmounted by suppos-
ing that the differentiated molecule still retains a core or substratum of
undifferentiated atoms, which are in a more stable condition than the
rest of the differentiated molecule.

Whilst the explanation of after-images may pass muster, that of
simultaneous contrast is unsatisfactory. We know of no " retinal
circulation " Avhich is so rapid as to account for the phenomena. It
would have to be almost instantaneous and it is impossible on physical
grounds to conceive of a mechanism so potent as to transfer molecules
so instantaneously through a finite distance. The reciprocal action of
retinal areas must be explained on some more plausible grounds.

The suggestion that both the scotopic and photopic luminosities
are to be attributed to the rods and their contained grey substance is
attractive, but the fact that the luminosity curve of the totally colour-
blind remains the same as the normal achromatic scotopic curve even
when the intensities of the incident light are raised far above the
scotopic level is against the conjecture.

III. McDougall's Theory

McDougall^ strongly supports the three-components theory of colour
vision. He prefers to term it Young's theory rather than the Young-
Helmholtz theory, because he considers that v. Helmholtz did it scant
service by his far-fetched psychological explanations of the facts of
induction.

McDougall's views of colour vision are part of a general theory of
psycho-physical processes^. He defines a psychological process as that
part of the total process of physiological excitation within the nervous
system which stands in a relation of immediate interaction with psychical
process or consciousness. He thus rejects the hypothesis of psycho-
physical parallelism held by Fechner, G. E. Miiller and others. He
adduces strong evidence derived from anatomy, physiology and psycho-
logy in favour of the view that the seat of the psycho-physical process
is in the synapses or arborisations at the sites of contact of nerve cells
or neurones. Many of his arguments on this part of his theory receive

^ In Nagel's Hayidb. d. Physiol, d. Menschen, p. 277.

2 Mind, X. N.S. 52, 210, 347. 1901.

3 Brain, xxiv. 577, 1901 ; xx\^. 153, 1903.

OTHER THEORIES 275

considerable support from Sherrington's researches^. Reflex action
as compared with the conduction of an impulse along a nerve, is charac-
terised by " lost time " or appreciable delay. Sherrington has brought
forward abundant evidence to show that the delay is due to resistance
to the passage of the impulses and that the resistance probably has its
seat in the synapses. It is further probable that in the lower levels of
the nervous system, e.y., in the spinal cord, where the reflex paths are
paths of high degree of constancy of function, the synapses are very
thoroughly organised, i.e., their degree of resistance has been reduced
to a minimum by frequent repetition of the particular reflex action in
the individual and in the race, while in the higher parts of the nervous
system the resistance, and therefore the loss of time, occasioned by the
synapses is greater in the inverse order of their degree of organisation.

Repeated stimulation causes fatigue, which also probably is due to
changes occurring in the synapses. This is the explanation of the
" complete fading " of retinal images under certain conditions {v. p. 261).
Complete fading may be brought about with greater ease by the simul-
taneous effect of a second stimulus. Thus, if a small white patch a is fixated
for 15 sees, or more, and then a second similar patch h is suddenly exposed,
so that the image of h falls on another part of the same retina or on a
non-corresponding part of the other retina, a will usually disappear from
consciousness at once, remain absent several seconds, and then return
suddenly, appearing equally bright with h. McDougall has described
numerous instances of these " mutual inhibitions " of visual images,
amongst which those of binocular and uniocular struggle are the most
striking. Thus, if a bright white surface is fixated with a red glass
before one eye and a blue glass before the other, the surface usually
appears alternately red and blue. If the white surface is less bright
there is greater tendency for the colour impressions to fuse^. This
alternation is most easily explained by supposing that one excitation
ceases to pass through the neurones of the visual cortex, and that the
factor which determines the regular alternation of the tAvo images is
the rapidly oncoming and rapidly disappearing fatigue of the synapses
of the cortex.

In support of this view we have the following facts : (1) A very
similar alternation in consciousness, i.e., alternate predominance of an
image and inhibition of it by another image, may occur in the case of
two after-images formed on adjacent areas of one retina; (2) if, when a red

^ See The Integrative Action of the Nervous System, London, 1900.
^ Cf. Rivers, Camb. Phil. Soc. viii.

18—2

276 COLOUR VISION

field is presented to one eye and a blue field to the corresponding area
of the other eye, one eye be closed or covered for a brief period — one
second will suffice — the colour presented to that eye always predominates
over and inhibits the colour presented to the other eye as soon as the
eye is uncovered, i.e., the rested tract predominates over the relatively
fatigued tract, even if the period of rest be not more than one second ;
(3) if by any one of several devices, as, for example, slight movements
of objects in the field (the drawing of a hair across the field a little before
the eye will suffice), the predominance of one field, say the red field,
be prolonged, the blue field of the other eye tends more and more
strongly to assert itself until, no matter how vigorous the movements
in the red field, the blue predominates and the red field disappears from
consciousness ; all such devices merely direct attention to the one field,
i.e., they cause the excitation of the one cortical conduction-path to be
reinforced by the activity of higher levels, and we see that as the
excitation continues to pass through this path the resistance of the path
increases until, in spite of such reinforcement, it yields to the inhibitory
influence of the other rested tract ; (4) if, accommodation being relaxed,
a white field be presented to the right eye and a much less bright white
or grey field to the corresponding area of the left eye, and if the left eye
be covered over for some ten to fifteen seconds, while the right eye
continues to fixate the brighter field, then on uncovering the left eye
the bright image of the right eye yields to the much less bright image of
the left eye, and disappears from consciousness for some seconds.
McDougall gives many examples of uniocular struggle^.

So far as colour vision is concerned McDougall supplements the
original Young theory by the addition of an independent white mechan-
ism, the end organ of which is the rods. In other words he adopts the
duphcity theory. It is a difficulty, even of these combined theories,
that there is no place in them for a special black-exciting process.
McDougall boldly accepts the view that such a process is unnecessary
to explain the facts, and adduces cogent arguments in its favour^.
The sensation of black, according to him, is experienced when " com-
plete fading " occurs. The visual cortex is then at complete rest, as
opposed to the condition of normal tone which manifests itself as the
intrinsic light (Eigenlicht).

This combined theory therefore assumes a separate retino-cerebral
apparatus or system for each of the primary or simple colour-affections,
red, green, blue, and (scotopic) white. The next point of importance
Mind, X. N.S. Sec. V. ^ cf. also Ward, Brit. Jl. of Psychol, i. 407, 1905.

OTHER THEORIES 277

is the necessity for assuming that the cortical areas of the two eyes are
separate, i.e., that each retina contains the peripheral endings of the
retino-cerebral nerve-fibres of its own set of four colour systems, and
these are distinct from the four systems of the other retina. The
view that the central connections of corresponding points of the
two retinae are anatomically identical is untenable. It was rejected
by V. Helmholtz^ chiefly from a consideration of the influence of
voluntary attention in effecting the predominance of one or other of
two struggling fields. As already mentioned McDougall has shown that
momentary stimulation of the retina by dim equally diffused light will
bring back to consciousness after-images which have completely faded.
This must be due to the second stimulus raising the excitability of the
visual cortex, so that the feeble impulses coming from the retina are
enabled to break down the resistance in these highest levels. Now,
if the cortical areas of the two retinae were identical, when the after-
image is formed on one retina only, the admission of dim light to either
eye should be equally effective in reviving the faded after-image.
But it is found that though it has a perceptible effect of this sort in both
cases, that effect is very much slighter in degree, and ceases to be eft'ective
much more rapidly in the case of the eye in which there is no after-image.
The slight effect of stimulating this eye shows that there is some close
and sympathetic connection between the cortical areas of the tw^o
retinae, although they are not identical. McDougall represents the
retino-cerebral systems demanded by the theory diagrammatically in
Fig. 75.

It is to be noted that Hering's theory requires the same explanation
of the cortical areas of the two eyes, with the greater complication of
three pairs of colour-systems for each eye. If, however, the opponent
metabolic processes are peripheral the theory then fails to explain the
facts of binocular struggle with red and green lights and so on ; and
wherever the processes occur requires much modification to meet the
case'^.

McDougall's experiments on the fading of after-images, already
mentioned {v. p. Ill), demonstrate the recurrence of colour-sensations
attributable to four primary colour-systems. They further exhibit
instances of uniocular struggle between the primary or simple colours.
Per contra, experiments devised to exhibit uniocular struggle afford
confirmatory examples of the spontaneous development of pure primary

1 § 771, 3rd Ed. iii. p. 407.

2 Cf. G. E. Miiller, Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xiv. § 32, 1897

Fig. 75. Diagram representing the two retinae and the cortical centres connected with
them. The upper four circles represent the cortical levels of the red, green, blue,
and white systems connected with the left retina, the lower four those connected with
the right retina. The broken line bisecting all the circles represents the purely
anatomical separation of each centre into two halves lying in opposite cerebral hemi-
spheres. The continuous lines on the left side represent the sympathetic relation
between corresponding jjoints of the two systems of similar function connected with
the right and left retinae respectively. The dotted lines represent the antagonism
between any point of any one colour-system of the one eye and the corresponding
points of the other two colour-systems of the other eye. The broken lines represent
the feebler antagonism between any point of any one colour-system of one eye and the
corresponding points of the other two colour-systems of the same eye. The arrows
radiating from the centre of each circle represent the antagonism between any point of
the cortical level of any one of the systems and every other point of the same.
(McDougall.)

OTHER THEORIES 279

colour sensations. These facts are strikingly in favour of the three-
components theory.

Perhaps the greatest service which McDougall has done to the theory
is to show that it can be made to give a reasonable explanation of the
facts of induction. His theory of contrast was adumbrated by Rollet
in 18671.

The darkening contrast effect exerted by a white area on adjoining
grey areas may be attributed, as shown above, to the inhibitory effect
of the more vigorous cortical processes excited by the white area on the
feebler cortical processes excited by the grey areas. If the explanation
be true, then by the theory it must also hold for colour-contrast. For
(photopic) white being the resultant of simultaneous activity of the red,
green, and blue colour-systems, inhibition of white by white must be
expected to involve inhibition of red by red, green by green, and blue
by blue. In the cortical level of each colour system the activity of any
one part tends to inhibit the activity of all other parts, and when any
one part is more intensely excited than the rest its activity does partially
or completely inhibit that of all other parts of the cortical area of the
same colour system (see Fig. 75). Suppose then that all parts of all
three colour-systems are equally excited to a moderate degree, except
that one small part of one system, e.g., the red, is more highly excited.
Then the activity of this more highly excited part of the red cortical
area depresses that of the rest of the red cortical area, more especially
that of the immediately surrounding parts, with the result that in the
parts of the field surrounding the red patch the activity of the blue and
green systems predominates over that of the red, and the grey ground
appears blue-green. When the balance of the activities of the three
cortical levels has once been turned in this way in favour of the blue and
green systems, the predominance of the blue and green systems must
be still further increased by the antagonism between the corresponding
parts of the cortical areas of the three colour-systems, i.e., the activity
of the red cortical area must be still further depressed or inhibited by
that of the blue and green areas. In favour of these views are the facts
that coloured light on one retinal area can be inhibited by white light
on another area and one of the constituents of the sensation of white
can be inhibited by a patch of bright coloured light.

Simultaneous induction, in Hering's sense of the term {v. p. 125),
is the appearance of the same colour around a patch of any colour
when it has been steadily fixated for some time, generally 10 sees, or
1 Berichit d. Wiener Akad. lv. 344, 424, 741, 18G7.

280 COLOUR VISION

more. It may appear on a surrounding grey ground in the absence of
or as a sudden reversal of the contrast- colour. It is most readily
induced on an intensely black background, a fact which militates
strongly against Hering's explanation of it {v. p. 255). The longer
fixation is continued the brighter the induced light becomes and the
further it spreads away from the inducing patch of light over the ground.
McDougall attributes simultaneous induction to retinal changes. The
light rays decompose certain retinal mother substances. Thus red
light sets free more of the red substance than of the blue and green
substances. The substances diffuse into adjoining areas of the retina,
and as the red substance is in excess its excitatory effect upon the nerve-
endings of the red system manifests itself beyond that of the green and
blue substances upon their respective systems. It is to be noted that
in the phenomenon of simultaneous induction (in Hering's meaning of
the term) there is time for such diffusion to occur^. The effects of these
peripheral stimuli, which are regarded as the basis of simultaneous
induction, may of course be abolished or altered by inhibitory effects in
accordance with the principles suggested above as the basis of contrast.

The explanation of after-images, founded on the same principles,
is as might be expected much more complicated. All after-images are
primarily due to the persistence in the retina of substances set free in
it by the action of the light rays on stored-up mother substances.
These specific substances continue to act upon the retinal nerve-endings
and thereby to be used up gradually. In the dark the relative intensity
of the action of any one of the different specific substances is chiefly
a function of the quantity of that substance present in unit area of the
retina. The frequently recurring changes in the brightness and colour
effects of after-images are, with the exception of the gradual diminution
of intensity due to the using up of the substances, all determined by
changes in the cortex and not by changes in the retina.

There are two stages in the chemical changes, (a) the setting free of
the specific substances (red, green, blue, and (scotopic) white substances),
(b) the excitation of the nerve-endings by the substances. Red light,
for example, may be conceived to cause the activity of the red system
to predominate in three ways : (1) by setting free the red substance in
larger quantity than the blue and green substances while reinforcing
the exciting action of all three equally ; (2) by setting free the three
substances in equal amounts while reinforcing the action of the red more
than that of the blue and green substances ; (3) by exerting a more
1 Contrast Ladd-Franklin, p. 274 ; Edridge-Green, p. 297.

OTHER THEORIES 281

vigorous effect upon the red substance in both stages. Reasons are
given for deciding in favour of the third possibihty. That the red
substance is set free in excess in the first stage is shown bv the pheno-
menon of simultaneous induction. That red light exerts a greater
action on the red substance in the second stage is shown by the pre-
dominance of red sensation over green and blue in spite of facts which
show that the free green and blue substances become at least equal in
quantity to the free red substance. These facts are : (1) the induced
light, at first red, on prolonged fixation loses in saturation as it gains
in brightness until it becomes white or slightly tinged with green ;
(2) the phenomenon of reversal of red light to green-blue or green during
continued fixation, either on diminishing its brightness or on adding
white light to it, or on simple prolonged fixation ; (3) the predominance,
sometimes complete, of the green and blue systems in the after-image
of red light of medium brightness.

This gradual increase in the green and blue substance cannot be
attributed to fatigue of the process of setting free the red substance,
but if the red light is more selective on the retinal processes in the second
stage, that of excitation of the nerve-fibres, it follows as a corollarv.
For, if, for example, the red, green, and blue substances are set free in
the proportion 3:2:2 and are used up in the proportion 4:2:2, the
green and blue substances must accumulate more rapidly. This purely
hypothetical assumption explains complementary after-images and the
various cases of the reversal of colour.

Though there is no fatigue in the retinal processes of the first stage,
it occurs in the second stage. It accounts for the rapid fall in intensity
of the sensation during the first few seconds when a patch of bright
white light is fixated, and also for the fact that the after-image of
white fight is more vivid the shorter the period of fixation, but declines
in brightness more rapidly. The more intense the fight the more the
first retinal process exceeds the second in activity. The preponderance
of the retinal excitement of the red system over the excitement of the
blue and green systems is probably slight, even when a sensation of
red of good saturation is experienced. For owing to the mutual anta-
gonism between corresponding areas of the cortex any predominance of
the excitement of one system in the retina is greatly exaggerated in the
cortex.

McDougall has shown that these principles can be made to explain
all the phenomena of after-images without undue straining. He
emphasises the importance of the part played by the cortex by the

282 COLOUR VISION

following facts : (1) the tendency for well-defined and homogeneous
parts of a complex image, whether a direct image or an after-image,
to undergo " complete fading " and to be revived in consciousness as
independent wholes ; (2) the fact that when two images are fixated
successively and so as to fall partially upon the same area of one
retina each forms an after-image distinct and separate from that
of the other, so that the two after-images may appear simultaneously
or alternately in consciousness ; (3) when an after-image is projected
upon a not too bright surface it is not always modified at once by
the effects of the light from this surface, but may maintain itself in
consciousness unaltered for a certain length of time. Fixation of a
patch of light therefore throws the cortical areas affected into a state
of preparedness to function again or to continue to function in the same
way as during the action of the light. This tendency may override and
obscure changes initiated in the retina.

Hence, it is not proper to say that an after-image has its seat either
in the retina or in the brain ; both retain an impress from the original
stimulation, and though the persistence of the exciting substances in
the retina is perhaps the more important element, yet the cortical
impress plays a large part in determining the exact form in which the
after-image shall appear.

McDougall's views on the evolution of the colour-sense are shown
in the following extract^.

" It will be generally admitted that if we try to form a conception
of the course of development of the colour-processes we must begin by
assuming the vision of the lower animals, in which vision is but little
developed, to be monochromatic, i.e., we must assume the visual
sensations to be of one kind only, varying only in intensity or bright-
ness ; and we must suppose one kind of light sensation to be similar
to our white or grey sensation or at least to stand to it in the
relation of a direct ancestor. If we then seek the probable first step
in the development of the colour-processes from this stage of simple
grey-vision, we must, I think, assume that it consisted in a differentia-
tion of the effects of the light of the warm and the cold ends of the
spectrum ; the rays of the cold end would begin to set free, in addition
to a white- exciting substance, a substance that by the excitement of a
concurrently differentiated retino-cerebral apparatus would add the
sensation of blue to that of white ; and in just the same way the rays
of the warm end would begin to set free an additional substance that

1 Mind, N.S. X. pp. 212-214, 1901.

OTHER THEORIES 283

by the excitement of a second concurrently differentiated retino-
cerebral apparatus would add the sensation of yellow to that of white.
If we then consider the state of a species in which the visual apparatus
has achieved this degree of development we shall see that it would
obviously be an advantageous arrangement that, when the retina was
stimulated by white light, i.e., by light containing rays of all wave-
lengths, the two new colour-systems, the yellow and the blue, both
being excited in addition to the white system, should have the sensa-
tion-elements determined by them fused in consciousness to white.
This compound white sensation-element would then add itself to and
so reinforce the sensation of white due to the excitement of the older
white apparatus. For suppose that the yellow and blue sensations
neutralised each other when the yellow and blue systems were excited
together, this would leave a sensation of white but would involve a
waste of the energy that, under the other arrangement, would go to
reinforce the white sensation. Or suppose the third possibility,
namely, that yellow and blue when excited together fused to give a
new kind of sensation. Then stimulation by mixed light would result
in a sensation compounded of white and this new yellow-blue sensation ;
the ancient and primitive sensation of pure white or grey would have
been lost, it could never again be experienced, and in place of the three
perfectly distinct kinds of sensation yellow and blue and white yielded
by the first arrangement, there would be possible only two, yellow and
blue, and a mixture, bluish-yellow. To illustrate this by an example —
it is obvious that a species or a variety endowed with the sensations
of red and blue and white, would in this respect have an advantage
over one endowed with red and blue and purple only.

" Further, it is obvious that the original white apparatus would
not be likely to undergo much further development if the yellow and blue
systems developed in importance and in the intensity of the impression
produced by them in consciousness, for they would yield when excited
together a white sensation of correspondingly developed intensity.

" It seems not unnatural to suppose that the developing differentia-
tion of the colour-sensibility of the retina should have proceeded out-
wards from the centre, the region of acutest vision, and the one that
is most used. In the peripheral zone of the human retina we have then
the perpetuation of the primitive monochromatic stage of development
of the eye, while the very rarely occurring monochromatic eyes are
cases of reversion to, or arrested development in, this remote ancestral
condition. In the same way the zone of tlie human retina, stimulation

284 COLOUR VISION

of which causes the sensations of yellow and blue and white only,
remains in that stage of development in which only the first step of
differentiation has been effected and the frequent cases of bichromatic
vision, in which yellow and blue and white seem to be the only sensa-
tions that can be aroused by stimulation of the retina, are cases of
reversion to or arrested development in this more recent ancestral
condition.

" If we try to picture the further evolution of the colour-sense,
the process that would seem to be likely to give the best results, and
therefore the one most likely to be effected by the factors that have
controlled the origin and development of species, is a repetition of the
process of differentiation such as gave rise to the blue and yellow
systems, but occurring within either the blue or the yellow system.
For reasons, which we can hardly hope to determine, this differentiation
has proceeded in the yellow system. The light of the two ends of
the warm half of the spectrum must be supposed to have begun to set
free, within the retinal elements of the yellow apparatus, two different
substances in addition to the yellow substance, and with these two
new substances, the red and the green, we must assume the concurrent
differentiation of the red and green retino-cerebral systems. Then just
as it was obviously advantageous that yellow and blue sensation-
elements, when excited together, should fuse to give white, so obviously
it would have been advantageous that red and green sensations when
excited together should fuse to give yellow, else the primitive white and
the original yellow sensation would again be lost. As these two new
colour-systems became developed in the retina from the fovea centralis
outwards, the primitive yellow apparatus would lose its importance
and would probably undergo atrophy in this central region of most
highly developed colour sensibility, just as the primitive white apparatus
has become lost in the very centre, the fovea centralis itself. That the
primitive white apparatus remains functional throughout the rest of
these parts of the retina in which the colour-systems are developed is
probably due to its having assumed the special function of vision under
dim illumination, while no analogous functioning has justified the
continued existence of the primitive yellow system in the area of red and
green sensibility."

OTHER THEORIES 285

IV. Schenck's Theory

Schenck's theory^ is founded upon the Young-Helmholtz theory.
From the psychological point of view each simple or pure light-sensation
is held to depend upon an independent physiological process. The
simple light-sensations are white, red, green, yellow and blue. Since
all sensations can be synthesised out of red, green and blue, but not out
of white or yellow and two other simple sensations, red, green and blue
are the three fundamental sensations. Their physiological counterparts
are three " visual substances."

The difficulty that white and yellow have no simple physiological
counterparts is explained developmentally. In an earlier phase of
development the photopic apparatus, the cones, contained only one
visual substance, the stimulation of which gave the sensation white,
and which was nearly allied to the scotopic visual substance of the rods.

The white substance first underwent a " panchromatising " change
whereby it became more sensitive to lights of long wave-length. It
next became differentiated into two substances, a yellow substance,
specially sensitive to rays of long wave-length, and a blue substance,
specially sensitive to rays of short wave-length. These substances
retain the peculiarity of the mother-substance that when they are
simultaneously stimulated with strengths which correspond to the
effect of ordinary daylight they give rise to the sensation white.

Analogously the yellow substance became differentiated into a
red and a green substance, which when equally and simultaneously
stimulated arouse the sensation yellow.

These changes may be represented diagrammatically thus :

Regional Effects. I. In the photopic normal eye : (a) the centre
of the retina is trichromatic, (b) the middle peripheral zone is red-green-
blind, (c) the outer peripheral zone is totally colour-blind.

II. The limit of the red-green-blind zone is only relative : it
extends further peripherally with increase of size, intensity, and satura-
tion of the observed object.

III. Colour matches valid for the centre are valid for the periphery.

IV. Luminosity matches valid for the centre are valid for the
periphery.

' Arch.f. d. ges. Physiol, cxvm. 129, 1907.

286 COLOUR VISION

Arguments from Regional Effects. I. For the completely developed
colour sense only three visual substances are necessary, the red, the
green, and the blue. For the red-green-blindness of the middle zone
only two are necessary, the blue and the yellow. For the total colour
blindness of the outermost zone only one is necessary, the white.

II. The three developmental phases of the visual substances
increase from without towards the centre. W is maximal at the
periphery and diminishes steadily towards the centre ; B and Y are
slight at the periphery, maximal in the middle zone and slight at the
centre ; R and G are minimal at the periphery and increase to a maximum
at the centre.

III. The validity of colour matches for centre and periphery is
associated with a reduction of sensation. Hence visual substances of
the same nature are present throughout, though perhaps in smaller
number in the periphery, i.e., either (a) one or two of the visual sub-
stances are absent, or (6) all the visual substances are present but initiate
a smaller number of sensations.

IV. The validity of luminosity matches for centre and periphery
necessitates the presence of all the visual substances associated with a
smaller number of sensations. If one visual substance were absent the
spectral colour which specially stimulates it would appear darker in
peripheral vision, which is not the case. In the middle zone, of the
fundamental colour sensations only blue is present ; red and green are
replaced by yellow. Hence the red and green substances here give
rise to a single sensation, yellow, and must be represented by the yellow
substance. In the outer zone all three fundamental sensations are
absent, and all three must be represented by the white substance.
These deductions are legitimate because each pair of derivative sub-
stances together combine the peculiarities of the respective mother
substances.

On this basis the theory may be extended. Each visual substance
must be compounded of two functionally distinct parts, one for lumino-
sity, the other for hue. The first part, which is first affected by the
incident light, may be called the stimulus-receptor {Reizempf anger).
The second part, which determines the nature and intensity of the aroused
sensation, may be called the sensation-stimulator (Empfindungserreger).
The amount of energy set free by the receptor determines the excitation
of the stimulator and thus the luminosity of the resultant sensation.

In the fully developed colour-sense the receptor for long waves in

OTHER THEORIES 287

the red substance is associated physiologically with the red stimulator
only, the receptor for medium waves in the green substance with the
green stimulator, and the receptor for short waves in the blue substance
with the blue stimulator. That is, each receptor transfers its whole
energy to the corresponding stimulator.

In the early developmental stages other associations are possible.
In the totally colour-blind zone each receptor is associated with each
stimulator, so that each receptor divides its stimulating energy between
the three stimulators. Hence no matter what the stimulating light the
resultant sensation is white. In the red-green-blind zone the short
wave receptor is limited to the blue stimulator, whilst the two others
are associated indiscriminately with the other two stimulators. Hence
in this case the red and green stimulators always initiate the sensation
of yellow.

It was suggested by Schaum that the receptors act as optical sensi-
tisers, and by Richarz that they may be regarded as optical resonators.

Deutermiopia. This type of colour blindness shows the following
characteristics :

I. It is a reduction-system from normal colour vision.

II. It agrees thus far with the red-green-bhndness of the middle
retinal zone that (1) affected people describe three sensations, white,
yellow and blue, and (2) their sensations are the same in quality and
nearly so in luminosity as the sensations of white, yellow, and blue in
regional red-green-blindness.

III. It differs from regional red-green-blindness in that (1) it is
also present on direct fixation, and (2) it is not relative, since increase
of area, intensity, and saturation of the object never arouses the fully
developed colour-sense.

Hence it is to be regarded as an arrested development, in which the

second phase [Y\ , ) has not taken place.

The strongest arguments in favour of this view are the equivalence
of colour matches for the normal and deuteranope {v. p. 170) and the
almost complete identity of the normal and deuteranopic luminosity
curves.

Protanopia is a reduction-system which differs from deuteranopia
and normal colour vision in the difference of the luminosity curves.
Lights of long wave-length have strikingly low stimulus values for the

288 COLOUR VISION

protanopes, whereas beyond 580 /x/x violetwards the stimulus values are
relatively higher than for deuteranopes and normals.

Hence protanopia is to be regarded as an arrested development in
which a receptor is absent, and that receptor must be the one which is
set in action by lights of long wave-length. Hence the receptor or
resonator of the red substances is absent, whilst the stimulator is
present.

Tritanopia offers difficulties to the theory, but is explained as due to
the second developmental phase having reached a certain stage without
the first having taken place.

Monochromatic Vision also offers special difficulties and is probably
complex. Schenck distinguishes three types : (1) the peripheral total
colour blindness of normals and deuteranopes ; (2) the peripheral total
colour blindness of protanopes ; (3) scotopic total colour blindness.
He concludes that (1) the white visual substance of the ro4s develops
in such a manner as to become relatively more sensitive to rays of
medium wave-length, thus producing the white substance of the
protanopic cones ; (2) from the protanopic cone white substance the
normal peripheral cone white substance is developed by further differen-
tiation, so that it becomes more sensitive to rays of long wave-length.

Complete monochromatic vision may be due to (1) pure rod- vision ;
(2) arrest of cone development at the rod stage (total colour blindness
without central scotoma) ; (3) arrest of cone development at the
protanopic peripheral cone stage (pathological cases) ; (4) arrest of cone
development at the normal peripheral cone stage (the cases of Becker^,
Piper^, Scholer and Uhthoff^, Siemerling and K5nig^, and Pergens^).

Ationialous Trichromatic Systems. Partial deuteranopia is caused by
the receptor for light of medium wave-length possessing an absolutely
diminished sensitiveness, with relatively diminished sensitiveness for
about 560 y^t/x, and relatively increased sensitiveness for about 580 fifj..
Partial protanopia is caused by absence of the receptor for light of
long wave-length and modification of the receptor for light of medium
wave-length.

It will be noticed that this theory is essentially the Young-Helmholtz
theory modified and elaborated so as to overcome certain difficulties.

1 Arch./. Ophlh xxv. 2, 205, 1879.

^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvni. 155, 1905.

^ Beitrdge z. Path. d. Sehnerven, Berlin, 1884.

* Arch, f Psychint. und NervenJcr. xxi. 284, 1889 ; in Konig, p. 206.

* Klin. Monaishl. f. Augenhlk. XL. 42, 46, 1902.

OTHER THEORIES 289

Its weakest feature is the failure common to the three-components
theory and most of its variants to account satisfactorily for the facts
of induction. These are presumably relegated to the central nervous
organs as in the theory of zones. Schenck elaborates Ad. Fick's^ views
as to the peculiar position of yellow. He adopts Tschermak's division^
of each visual substance into two parts, a stimulus-receptor or inter-
mediary, and a sensation-stimulator.

The rod visual substance is the primitive visual substance : it
possesses a receptor or resonator for light of short wave-length, and a
slightly damped resonator for light of medium wave-length.

The development of the cone visual substance from the primitive
rod substance is accompanied by increased damping of the resonator
for light of medium wave-length, and also panchromatisation, i.e.,
development of a resonator for light of long wave-length.

As regards the sensation stimulators there is a white stimulator in
the rods, and there are red, green, and blue stimulators in the cones.

In the lowest developmental phase of the cones all the resonator
and all the stimulator molecules are intermingled, so that they act as
a physiological unit and are equivalent to a white substance.

In the next phase the resonator for short waves becomes limited to
the blue stimulator and thus forms a blue substance, whilst the others
remain intermingled and are equivalent to a yellow substance.

In the final phase each resonator is limited to its corresponding
stimulator.

As has been seen the theory has to be strained to account for all the
facts of the different varieties of colour blindness, and it may well be
doubted whether the complications thus introduced make it more
acceptable than the ordinary three-components theory.

V. Wundt's Photochemical Theory

Wundt^, from the data available at the time, concluded that the
facts of colour-mixtures and so on did not necessitate a multiplicity of
specifically different stimulation elements or substances. His theory is
as follows.

Two different stimulation processes are set in action by every
retinal excitation, a chromatic and an achromatic. The chromatic
excitation is a function of the wave-length of the light ; the achromatic,

1 Arch, f d. ges. Physiol. XLvn. 275, 1890. * m^^ lxxxii. 589, 1900

^ Orundziige d. physiol. Psychologie, i. 535. 1893.

p. c. V. 19

290 COLOUR VISION

so far as its relative intensity is concerned, is also dependent upon the
wave-length, and reaches its maximum in the yellow. Both stimula-
tions follow different laws with increasing stimulus intensities. At low
intensities the achromatic surpasses the chromatic ; at moderate
intensities the chromatic is the relatively stronger ; at the highest
intensities the achromatic regains superiority.

The achromatic stimulation consists in a uniform photochemical
process, the intensity of which is partly dependent upon the objective
Hght intensity and partly upon the wave-length, since it reaches its
maximum in the yellow and falls off towards both ends of the spectrum.

The chromatic stimulation consists in a poly form photochemical
process, which changes continuously with the wave-length. Thus
extreme differences of wave-length produce effects which are of almost
the same nature, whilst the effects of certain different intervening
wave-lengths are related in such a manner that they completely com-
pensate each other like opposed phases of one and the same movement.

Every photochemical process of stimulation outlasts the excitation
for a certain period and exhausts the sensibility of the nerve-substance
for the particular form of excitation. Positive and homochromatic
after-images are to be explained by the persistence, negative and com-
plementary after-images by the exhaustion of the sensibility. The
phenomena of contrast are to be explained by the general principle
that all impressions of light and colour are experienced in relation to
each other, i.e., by the general law of relativity.

VI. G. E. MiJLLER's Theory

G. E. Miiller's theory^ is a modification of Hering's theory in the
light of psychophysical principles. It is set forth at great length and
would already require further qualification to account for the facts
which have been discovered since its promulgation. It is based on the
fundamental psychophysical hypothesis, which he wrongly terms an
axiom, that " an equality, similarity, or difference in the condition of
sensations corresponds to an equahty, similarity, or difference in the
condition of the psychophysical processes."

Miiller's essential disagreement with Hering is that the antagonistic
action of white and black, red and green, yellow and blue, would be
different from what it is if Hering's assumption were true that the

1 Ztsch. f. Psychol, u. Physiol, d. Sinvcxorg. x. 1 and .321, 1896 ; xrv. 1 and 161,
1897

OTHER THEORIES 291

sensations depend solely upon the relationship of the three underlying
metabolic processes. He considers that the light does not act directly
upon the achromatic and chromatic substances. There are materials
present in the retina — white, red, green, etc. materials — which are
chemically altered by the action of the light. The chemical action is
reversible under different conditions, thus resembling Hering's A and
D changes. It is the modifications in these peripheral substances that
are transmitted to the cerebral substrata of the actual sensations.

In addition to four chromatic retinal processes, red, yellow, green,
blue, at the periphery, there are six central " values," the red process
exciting the red, yellow, and white values, the green exciting the green,
blue, and black values, and the blue exciting the blue, red and black
values. A yellow stimulus excites the red and yellow processes, thereby
, exciting the red, yellow, green, and white central values, of which the
red and green neutralise one another. By adding another link in the
psychophysical chain many more possible explanations are available
for the phenomena of colour vision. Thus, the red material has a yellow
valency, and the total yellow valency is made up of two parts, a direct
and an indirect, and so on.

The reasoning throughout is very involved, but one feature stands
out clearly, viz., the division of the psychophysical processes into two
parts, a peripheral and a central.

VII. Edridge-Green's Theory

Edridge-Green's theory is thus summarised in his own words.

" A ray of light impinging on the retina liberates the visual purple
from the rods and a photograph is formed. The rods are concerned
only with the formation and distribution of the visual purple, not with
the conveyance of light impulses to the brain. There are cases in which
the visual purple is differently constituted and is not sensitive to certain
rays at one or both ends of the spectrum. The decomposition of the
visual purple by light chemically stimulates the ends of the cones (very
probably through the electricity which is produced), and a visual
impulse is set up which is conveyed through the optic nerve fibres to
the brain. If it were possible, in a case in which the spectrum appeared
of similar length and brightness to both, for a normal-sighted person and
a colour-blind one to exchange eyes, the normal-sighted would still see
colours properly and the colour-blind would still be colour-blind. The
character of the impulse set up differs according to the wave-length of

19—2

292 COLOUR VISION

the light causing it. Therefore in the impulse itself we have the physio-
logical basis of light, and in the quality of the impulse the physiological
basis of colour. The impulse being conveyed along the optic nerve
to the brain, stimulates the visual centre, causing a sensation of hght,
and then passing on to the colour-perceiving centre, causes a sensation
of colour. But though the impulses vary in character according to
the wave-length of the light causing them, the colour-perceiving centre
is not able to discriminate between the character of adjacent impulses,
the nerve cells not being sufficiently developed for the purpose. At
most seven distinct colours are seen, whilst others see in proportion to
the development of their colour-perceiving centres, only six, five, four,
three, two, or one. This causes colour blindness, the person seeing
only two or three colours instead of the normal six, putting colours
together as alike which are seen by the normal-sighted to be different.
In the degree of colour blindness just preceding total, only the colours
at the extremes of the spectrum are recognised as different, the re-
mainder of the spectrum appearing grey^."

" It will be noticed that the theory really consists of two parts,
one concerned with the retina and the other with the whole retino-
cerebral apparatus^."

Edridge-Green holds that the visual purple is the sole visual sub-
stance and the essential feature in the retina which enables it to trans-
form light into visual impulses. He admits that visual purple is found
only in the rods and not in the cones, but he believes that it is liberated
from the rods and stimulates the cones. From entoptic observations he
considers that he has proved the inflow of visual purple to the fovea,
chiefly by way of four canals which radiate from the fovea and branch^.
He claims to have proved microscopically the presence of visual purple
between the cones of the fovea in the dark-adapted eyes of two
monkeys*. Edridge-Green thus supports the theory of the transference
of the visual purple which was first suggested by Mrs Ladd-Franklin
{v. p. 273).

It follows from this theory that " the Purkinje phenomenon, the
variation in optical white equations by a state of light and dark adapta-
tion, the colourless interval for spectral lights of increasing intensity,
and the varying phases of the after-image" are present in the fovea.

" It is reasonable to suppose that the visual purple which is formed

^ Colour- Blindness and Colour-Perception, p. 318, 1909.

- Hunterian Lecture^ p. 11, 1911. 3 ./. oj Physiol. XLI. 274, 1910.

* Trans. Opkth. Sac. xxii. 300, 1902.

OTHER THEORIES 293

by the pigment cells under the influence of a bright light would be
somewhat different in character from that which is formed in darkness."
This supposition affords the basis of an explanation of the photopic and
scotopic luminosity curves, of the Purkinje phenomenon, of erythropsia
or red- vision, and of green- vision.

The disappearance of lights falling upon the fovea is attributed
to temporary absence of visual purple from the fovea.

" It is very probable that light acting upon the visual purple is,
according to its wave-length, absorbed by particular atoms or molecules,
the amplitude of their vibrations being increased. These vibrations
may cause corresponding vibrations in certain discs of the outer segments
of the cones, which seem especially constructed to take up vibrations.
We know that when lisfht falls on the retina it causes an electric current.
We know how the telephone is able through electricity to convey waves
of sound, and something similar may be present in the eye, the apparatus
being especially constructed for vibrations of small wave-length. The
current of electricity set up by hght may cause the sensation of light,
and the vibrations of the atoms or molecules the sensation of colour.

" In all vital processes there is a condition of katabolism or chemical
change in the protoplasm, and an anabolic or building-up process, in
which the protoplasm is restored to its normal state. We have there-
fore to consider two definite processes in the visual purple — namely,
a breaking down of the visual purple photochemically by light and its
restoration by the pigment cells and rods. Under ordinary conditions
of light, and during the whole of the daytime, the visual purple is con-
tinually being bleached and re-formed. It is obvious, therefore, that
when the eye has been kept in the dark and is then exposed to light, an
observation taken immediately will not be comparable with one taken
a few seconds afterwards, because in the first observation we have only
to consider the katabolic change ; whilst in the second observation the
anabolic change has to be considered as well, as the visual purple has
to be re-formed for subsequent seeing. There appears to be very little
evidence in ordinary circumstances of this anabohc process ; for instance,
if we fatigue the eye with sodium light in a dark room, and then immedi-
ately examine a spectrum, we find that though all the yellow has dis-
appeared there is no increase in the blue ; in fact, the blue seems rather
diminished than otherwise. Again, there is not the slightest diminution
in either the red or green, showing conclusively that yellow cannot be
a compound sensation made up by a combination of red and green^."

^ Hunierian, Lectures, p. 21, 1911.

294 COLOUR VISION

The observations on fatigue are at variance with those of Burch,
Abney and others.

The " cerebral " part of Edridge-Green's theory is founded on the
principle of psychophysical units and series which dates from the time
of Fechner. The reasoning upon which it is based will be found in the
early chapters of his book on Colour-blindness and Colour-perception.
According to the theory the colour-sense has been evolved in stages.
First there was only a sensation of light. When colours were first
recognised only red and violet — the two ends of the spectrum — were
seen, and a spectrum seen by such an eye was red at one end, violet at
the other, the colours merging in the centre. Such eyes are called
" dichromic." In the next stage of evolution a third colour appeared
between the other two, viz., green : these persons were " trichromic."
In the next stage a fourth colour, yellow, appeared between the red and
green (" tetrachromic "). In the next stage a fifth colour, blue, was
seen between the green and violet (" pentachromic "). In the next
stage a sixth colour, orange, was seen between the red and yellow.
Persons having this type of vision are " hexachromic " and the majority
of people to-day belong to this class. The highest development which
has yet been reached is that of the " heptachromic," who in addition to
the other colours distinguish a seventh colour between the blue and
violet, viz. indigo.

Colour blindness is atavistic, and all stages are represented amongst
the colour-blind. The " dichromics " correspond most clearly to what
we have called dichromats for the sake of distinction from Edridge-
Green's nomenclature. The anomalous trichromats, on this theory,
include tri-, tetra-, and pentachromics.

In addition to these classes of the colour-blind there are others
distinguished by shortening either of the red or the violet end of the
spectrum. Such cases show a shift in the position of their psycho-
physical units towards the unshortened end.

Light perception and colour perception are therefore according to
this theory quite distinct^, and cases of colour blindness can be divided
into two classes, according as the defect is one of light perception, or
one of colour perception or differentiation without any defect in light
perception : both defects may be present in the same individual.

Edridge-Green finds that the normal hexachromic, who describes
six colours in the spectrum — red, orange, yellow, green, blue, violet, — •
will map out about 18 monochromatic patches. The supernormal

1 Proc. Roy. Soc. Lond. B, lxxxii. 458, 1910.

OTHER THEORIES 295

heptachromics, who interpose indigo between blue and violet, usually
map out 22 — 29 such patches. Konig, Lord Rayleigh, and all previous
observers found a much greater discrimination sensibility {v. p. 30),
and the very careful researches of Steindler confirm their results.
There is a fundamental fallacy here which requires explanation. The
discrepancy is borne out by the protocols of the author's investigations
of 14 dichromics^, in whom the number of monochromatic patches varies
from 11 to 2. " There may be shortening of the red or the violet end
of the spectrum ; there may be defective perception for some of the
other spectral rays ; the luminosity curve may have its maximum at
a different place from the normal ; there may be defective perception
when the image on the retina is diminished in size ; and the size of the
neutral region is very variable^." If so many varieties of one type of
colour blindness occur it is difficult to imagine how they can be explained
by any theory which is not a mere generalisation of facts, of little value
for purposes of classification. Edridge-Green summarises his views of
dichromics as follows : "(1) There are many degrees and varieties of
dichromic vision. (2) There are not two well-defined varieties of
dichromic vision, there are innumerable gradations connecting the two.
(3) In many cases precisely the same errors are made both by those
with and those without defective perception of red, when the rays for
which there is defective perception are not involved. (4) All dichromics
are not equally colour-blind, that is, one may have a much better hue
perception than another. (5) Dichromic vision may be associated with
defects of light perception which are also found in cases in which the
vision is not dichromic. (6) Dichromics may have a perception of
shade and a luminosity curve similar to the normal. (7) Many dichro-
mics match very accurately, their colour perception being sufficient for
the purpose when the colours are not too close in the spectrum. (8) The
degree of colour blindness varies with the state of health. (9) Colour
discrimination is diminished as a whole in dichromic vision. (10) Dichro-
mic vision appears to be due to a defective power of colour differentiation
probably corresponding to an earlier state in evolution of the colour sense.
(11) The two colours seen are red and violet."

Edridge-Green's conclusions with regard to anomalous trichromatic
vision are thus summarised by him^ :

" (1) Trichromic vision is not synonymous with anomalous trichro-
matism. (2) Many persons with otherwise normal colour perception

^ Ophthalw.oscopc, xii. 1914. ^ Loc. ell. p. 77.

» Proc. Roy. Soc. Loud. B, lxxxvi. 164 191.3.

296 COLOUR VISION

make an anomalous equation. (3) Many colour-blind persons (dichro-
mics and trichromics) make an absolutely normal match with no greater
mean deviation than the normal. (4) Colour weakness is not charac-
teristic of anomalous trichromatism but of trichromic vision. (5) Ano-
malous trichromatism and colour weakness are not synonymous.
(6) A large mean deviation indicates colour weakness. (7) Anomalous
trichromatism appears to be due to an alteration in the normal relations
of the response to the three colours (lights) used in the equation. If the
eye be more or less sensitive to one of the components of the mixed
colour whilst the other has its normal effect, an anomalous equation will
result. An anomalous equation will also result when the yellow is more
allied to green or red than is normal."

It is difficult to understand how any concordant classification of
the colour-blind can be arrived at by Edridge-Green's methods. His
classification is based upon the number of different colours distinguished
in the spectrum, yet these may be subdivided into a variable number of
monochromatic patches. He says : " They (the physicists) appear to
take for granted that the perceptions of others are similar to those
experienced by themselves^." Yet there can be no doubt that in
practice he is much influenced by the names which the examinee applies
to the various parts of the spectrum. " When it (the colour perception
spectrometer) 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^." Indeed, he constantly
lays stress upon the importance of colour names in testing for colour
blindness. As has already been pointed out, the colour-blind subject
uses a terminology for colours which is suited to describe the perceptions
of normal-sighted people, and which is unsuited to describe his own,
yet these colour names are apparently regarded by Edridge-Green as
an accurate and reliable criterion of what the colour-blind person sees^.

So far as the " retinal " part of Edridge-Green's theory is concerned
I think that the evidence is strongly against it. It appears to me to
be very improbable that the rods have no direct visual function.
Anatomically and embryologically they must be regarded as the primitive
visual neuroepithelium, the cones being a more highly differentiated

^ Colour- Blindness, p. 14, 1909 ^ Hunterian Lectures, p. 75, 1911.

^ To avoid any misconception on this important point I wish to emphasise the fact
that these remarks apply to the determination of the type of colour blindness of the
examinee. In practical testing the object aimed at is the determination of whether the
examinee recognises red, green, and white lights as red, green, and white lights respectively,
and it is obvious that the names which he applies are of great importance.

OTHER THEORIES 297

type. The arguments in favour of the duplicity theory are destructive
of this part of Edridge-Green's theory. The presence of Purkinje's
phenomenon, a photochromatic interval, and so on, at the fovea is
admittedly disputed, and good observers are ranged on each side.
The arguments in favour of the presence of visual purple in the fovea
are of two kinds, direct observation and entoptic phenomena. So far
as the former is concerned the observation has not been confirmed ;
if it should be it is most probable that the substance will be found there
only in traces and will represent a vestige of the early ancestry of the
cones {v. p. 204). In any case the evidence appears to be against the
view that visual purple is the sole and indispensable factor in fovea!
vision. So far as entoptic phenomena, other than those due to opacities
in the dioptric media, are concerned, they are open to the most various
explanations. The " intrinsic light of the retina " is often described
as a " light chaos," full of waves and currents ; it is no safe foundation
for any fundamental theory of vision. The rod-free area of retina subtends
at least 1° — 2° and therefore occupies a very definite finite space. That
visual purple in solution should be transmitted to the centre of this area
with the velocity necessary to account for the facts on the theory is
very improbable, if not impossible on physical grounds. We are
familiar with the theory of " sensitisers " and much may be. said in
support of them, but they are either bodies which travel in the circula-
tion and show no such rapidity of action^, or they are bodies produced
by molecular change and act in situ. Those referred to in Schenck's
theory, for example, belong to the latter category.

Much of the evidence in support of the " retinal " part of Edridge-
Green's theory is founded upon after-images. As has been pointed out in
Part I, Section VI, these phenomena belong to some of the most vari-
able in the whole range of colour vision. They vary with the previous
stimulation of the eye— and Burch has brought forward evidence to
show that " fatigue " effects last much longer than has been thought —
with a multiplicity of physical and physiological factors which may be
easily overlooked or under-estimated, and above all with psychological
factors which are almost uncontrollable. They can only be considered
valid if they are repeatedly confirmed by different observers, and even
then are usually open to a variety of explanations. This fact is well
exemplified by McDougall's experiments. Such observations can be
appraised only at the value which an uncertain method merits.
The " cerebral "• part of Edridge-Green's theory offers a possible

1 Cf. McT^oucrall, p. 280.

19—5

298 COLOUR VISION

explanation of the ps?/chological evolution of colour perceptions. It
must, however, be regarded as a pure hypothesis and must stand or
fall bv the accumulation of evidence for or against it. So far as the
physiological processes underlying colour sensations are concerned it
fails to afford any satisfactory explanation. In particular it does not
account for the trichromatism of normal colour vision as revealed by
the mixture of pure-colour stimuli. Edridge-Green says : "We must
therefore explain in another way the apparent trichromatism of normal
colour vision, which is so well known to every photographer, especially
those who are concerned with colour photography. If my theory of
the evolution of the colour sense be the correct one, and we have cases
of colour blindness corresponding to every degree of the evolutionary
process, we have an explanation of the facts. In past ages all saw the
rainbow made up of only three colours — red, green, and violet. When
a new colour (yellow) appeared between the red and green, it is obvious
that a mixture of red and green would give rise, not to red-green, but
to the colour which had replaced it— namely, yellow^." This is obviously
at most a very partial explanation of the trichromatism of normal
colour vision, which is a fact and not a theory.

It must be admitted that the evidence which has been collected from
various sources on the evolution of the colour sense is of an uncertain
character, but such as it is it affords no support to the theory. The dis-
cordant results and conclusions arrived at by different observers on the
colour perceptions of lower animals are recorded in Part I, Section VII,
Chap. II. If we consider only those arrived at by v. Hess, as being the
most recent, most exhaustive, and in many respects the most accurate,
we find that mammals have the same spectral limits as men ; in birds
and reptiles the spectrum is shortened at the violet end only ; amphibia
resemble mammals ; and fishes are totally colour-blind. With regard
to primitive races I agree with Myers that we have not sufficient evidence
to show that the colour sense materially differs in different peoples,
save that, as shown by Rivers's careful observations, their colour
sense is defective for light of the violet end of the spectrum. On the
recapitulation theory that the developing child passes rapidly through
the stages of evolution of the race one might expect to obtain useful
information from the development of colour perceptions in the infant.
I do not think that the experiments recorded afford any evidence on
this point, but only on the affective values of different colours for the

^ Hunferian Lectures, [>. 23 1911.

OTHER THEORIES 299

infant. If, however, this view is erroneous, the evidence is in favour
of delayed development of perception of blue.

Further, if congenital colour blindness is atavistic, then such
evidence, again uncertain and little trustworthy, as we have of the
nature of the sensations experienced by gross cases of dichromatism
— and most of these presumably belong to Edridge-Green's class of
dichromics — is that the two colours they perceive resemble the blue
and yellow of normal vision. " Anomalous trichromats " certainly
perceive more than two colours, but we have adduced reasons for be-
lieving that this group includes different types, and it is by no means
certain that any of these are due to atavism. If they are, the problem
of discovering the nature of their colour sensations as compared with the
normal is even more difficult than in the case of the dichromats, and
certainly no dogmatic statements can be rightly made in the present
state of knowledge.

INDEX OF SUBJECTS

{The numbers in heavy type indicate the principal references to the subjects.)

Absorption, 6

by visual purple, 13, 205
Achromatic acotopia, 14, 53
Achromatopia, 186
Action-time, 90

chromatic, 91, 235
Adaptation, 17, 49, 203, 254

in dichromats, 174
Adequate stimuli, 19
After-images, 101, 218, 254, 272, 280

fading of, 110
Akyanoblepsia, 180
Allonomous equilibrium, 252
Amphibia, anatomy, 11, 15

colour vision of, 142
Amphioxus, 145
Anatomy, 7

comparative, 10, 142, 196

Angstrom unit, 4

Animals, vision of, 131, 196, 203, 298

Annulus, 5

Anomalous trichromatic vision, 159, 182,

216, 235, 243, 266, 288
Apperception, 24
Approximate dichromatism, 235
Area, light, 119

rod-free, 10, 203

sensibility, 119
Areal effects, 51, 79, 95, 97, 117, 180, 250

in anomalous trichromats, 185

in dichromats, 180, 250
Autonomous equilibrium, 252.

Bidwell's ghost, 86, 88
Binocular contrast, 129, 262, 276
Binocular struggle, 276
Birds, anatomy, 11, 15

colour vision of, 140
Black, 25, 36, 103, 201, 202, 251, 276
Bleaching of visual purple 13, 55, 206, 212
Blind spot, 8, 129
Border contrast, 127
Brightness, 28, 42, 96, 100, 223, 236, 294

specific, 256, 267
Brown, 25, 36

Cerebral centres, 277, 292
Charpentier's bands, 87
Chemical changes due to light, 12
Choroid, 8
Chromate test, 242

Chromatic scotopia, 53
Chromatic threshold, 19, 60
Coefficients, law of, 107
Coefficient of vai'iation, 149
Colour blindness, acquired, 160

facts, 158

in primitive races, 147, 148, 150

partial, 158 ,

theories, 193

total, 159

unilateral congenital, 180
Colour diagram, 37, 213, 216, 229
Colour equations, 38, 213, 226
Colour, extinction of, 61, 121
Colour ignorance, 161
Colour mixtures, 33, 162
Colour names, 160, 296
Colour sensations of dichromats, 161, 180
Colour triangle, >7, 213, 216, 229
Colour vision, evolution of, 130. 282. 285,
294, 298

of primitive races, 145
Colours, fundamental, 214, 220, 230, 258

invariable, 70

memory, 22

pigment, 6, 78

simple, 25, 27, 104, 220, 224, 251, 276
Comparative anatomy, 10, 142, 196
Comparative psychology, 131, 211, 298
Complementary after-image, 101
Complementary colours, 35, 101, 128, 226,

254
Complete fading, 261, 275
Conditional reflexes, 134
Cones, 9, 196, 203
Contrast, binocular, 129, 262

border, 127

luminosity, 127

simultaneous, 17, 125, 219, 255, 264,
279.

successive, 101, 219, 254, 279

surface, 127
Critical frequency method, 96
Curve, equal-area, 228

luminosity, 42, 172, 189, 223, 240

minimal field, 72

minimal tinu;, 74

peripheral luminosity, 71, 173

persistency, 62

sensation, 232

valency, 215

302

INDEX OF SUBJECTS

Ddmmertingssehen {see Scotopia), 17
Deuteranopic vision, 159, 232, 287
Deviation, standard, 149
Dichromatic vision, 158, 162, 216, 232, 267

adaptation in, 174
Dichromatism, approximate, 235
Dichromic, 294

Differential threshold or Kminal discrimina-
tion value, 19, 30, 230, 233, 294
Diffraction spectrum, 3
Discrimination of hue, 30, 230, 233, 294
Dispersion spectrum, 3
Double-room, Hering's, 58, 74, 126
Duplicity theory, 198, 203

Electrical changes due to light, 15, 139, 206,

219
Energy of spcctrunij 28
Episcotister, 5, 92
Epithelium, pigment. 11, 145, 197, 211.

212
Equal-area curves, 228
Equality of brightness method. 44, 98, 107
Evolution of colour vision, 130, 282, 288,

294, 298
Extinction of colour, 61, 121
of Hght, 62. 121

Fading, complete, 261, 275

of after-images, 110
Fatigue, 112, 219, 231, 281
Feclmer-Helmholtz law, 107
Fechner's law, 20
Field of vision, 66
Fishes, anatomy, 11, 13

colour vision of, 143
Flicker method, 45, 55, 173, 191
Form sense, 204
Fovea centrafis, 9, 16. 81, 105, 117, 208

night blindness of, 50, 83, 208
Fraunhofer lines, 3, 29, 47
Fundamental colours, 214, 220, 230, 258

Gauging the spectrum, 39, 164, 215
General threshold or liminal value, 19, 60,

80, 117
Grassmann's laws, 33
Grey, 271, 282
mean, 251, 260

"Hemeralopia" (night-blindness), 49, 197.

209
Heptachromic. 294
Heredity, 159, 186
Hering's "double-room," 58. 74, 126
Hering's theory, 200, 251
Heterochromatic photometry, 43. 96
Hexachromic. 294
Hue, 27

discrimination of, 30, 230, 233, 294

Inadequate stimuli, 19
Induction. 17, 49, 101, 125, 218, 254, 279
temporal, 17, 49, 203, 255

Induction, simultaneous (Hering), 125, 127.
219, 255, 279
spatial, 17, 125, 219, 255, 264
successive, 101, 219, 254, 279

Infant vision, 131, 152

Inhibition. 93, 259, 275

Intensity, 28, 57, 78, 91, 95

Interference spectrum, 3

Intrinsic light, 104, 251, 276 ^

Invariable colours, 70

Law, Fechner-Helmholtz, 107

Fechner's, 20

Grassmann's, 33

MiiUer's, 18

Porter's, 96, 208

Talbot-Plateau, 92

Weber's, 19

of Coefficients, 107

of Specific energies of the senses, 18
Least perceptible differences, method of,

19
Lenticular pigmentation, 8, 41. 181
Light area, 119

chaos, 104, 251

extinction of, 62, 121

intrinsic. 104, 251, 276

sense, 204
Luminosity, 28, 42. 96, 100, 223. 236,

294
Luminosity contrast, 127
Luminosity curves of dichromats, 172, 240

of monochromats, 189

of trichromats, 42, 223

Macula lutea, 9, 81

Macular pigmentation, 41, 169, 181, 183.

241, 259
Mammals, anatomy, 11. 14

colour vision of, 132
Mean error, method of, 19
Mean grey, 251, 260
Mean variation, 149
Memory colours, 22
Method, constant, 19

critical frequency, 96

equality of brightness. 44, 98, 107

flicker, 45, 55, 173, 191

of least perceptible differences, 19

limiting, 19

of near error, 19

preference, 132, 137

of production, 19

of right and wTong answers, 19

visual acuity, 44, 99
Minimal field luminosity curve, 72
Minimal time luminosity curve, 74
Mixture of pure-colour stimuli. 33, 162
Monochromatic vision, 159, 186, 209. 217,

267. 288
MiiUer's law, 18

Negative after-image, 101
Neutral point, 163, 169, 207, 233

INDEX OF SUBJECTS

303

Night blindness, 49, 197, 209

of fovea, 50, 83, 208
Null-point, 216
Nystagmus, 187

Olive-green, 25, 36

Opponent colours theory, 200, 251

Optogram, 11, 13

Pentachromic. 294

Peripheral luminosity curve, 71, 173
Peripheral vision, 17, 50, 66, 123, 204, 208,
218 283
in diciiromats, 173, 176, 179
Persistency curve, 62
Phenomenon, flicker, 93

Purkinje, 57, 84, 96, 175, 177, 206, 210,
257
PhUology, 145
Photerythrous, 168
Photochemical theory, 289, 291
Photochromatic interval, 60. 80, 81, 120
Photometry, heterochromatic, 43, 96
Photopia, l7, 27, 203
Phototrophism, 130, 142
Pigmentation, lenticular, 8, 41, 181

macular, 41, 169, 181, 183, 241, 259
Pigment epithelium, 11, 145. 197, 211,

212
Pigments. 6, 78
Porter's law, 96, 208
Preference method, 132, 137
Primitive races, colour blindness in. 147,
148, 150

colour vision of, 131, 145
Protanopic vision, 159, 232, 287
Psychological process, 274
Psychologv, comparative, of colour vision,

131,*^ 211, 298
Psychology of colour vision, 17
Psycho-physical methods. 19

paraUeHsm, 21, 274, 290
Pupil reactions, 139, 141, 206
Purkinje's phenomenon, 57, 84 96, 206,
210, 257

in dichromats, 175, 177

reversed, 97
Purple, 27

visual, 12, 196, 205, 212, 220, 292

Rayleigh mixture. 182, 242
Reactions, pupil, 139, 141, 206
Recurrent vision, 85

Regional effects, 66, 95, 98, 117, 123, 258,
285

in dichromats, 177
Reptiles, anatomy, 11, 15

colour vision of, 142
Retina, anatomy of, 8
Retinitis pigmentosa, 209, 211
Rhodopsin, 12 (see Visual purple)
Right and wrong answers, method of, 19
Rod -free area, 10, 203
Rods and cones, 9, 196, 203

Santonin, 211

"SateUite," 86

Saturation, 29. 36, 39, 59, 229. 250

Scoterythrous. 168

Scotoma, 103

central, 188, 267
Scotopia, 17, 49, 52, 203

achromatic, 14, 53

chromatic, .53
Sectors, rotating, 5,. 92
Sensation curves, 222
Sensation stimulator, 286
Sensibility area, 119
Shift of sensation curve, 243
Shortening of the spectrum, 168
Simple colours, 25, 27, 104, 220, 224, 251,

■ 276
Simultaneous contrast, 17, 125, 219, 255,
264, 279

induction (Hermg,) 125, 127, 219. 255,
279
Spatial induction, 17, 125, 219, 255, 264
Specific brightness of colours, 256, 267
"Specific energies'' of the senses, law of the,

18
Specific threshold or liminal value, 19, 60
Spectrophotometer, 5, 222
Spectrum, 1, 27, 49

diffraction, 3

dispersing, 3

energy of, 28

interference, 3

prismatic, 3
Spectrum, gauging the, 39, 164, 215
Standard deviation, 149
Stimuli, adequate and inadequate, 19
Stimulus receptor, 286
Structural changes due to light, 11
Successive contrast, 101, 219. 254, 279

induction, 101, 219, 254, 279
Surface contrast, 127
Synapses, 274

Talbot-Plateau law 92

Temporal effects, 85, 208

Temporal induction, 17, 49, 203, 255

Tetrachromic, 294

Theories, 193

Theory, 193

Bonders', 270

duplicity, 198, 203

Edridge-Green's, 291

Hermg' s, 200, 251

Ladd-Franklin's, 271

McDougaU's, 274

G. E. Mailer's, 290

opponent colours, 200, 251.

physical, 198

Schenck's, 285

three-components, 199, 213

of two retinas, 197

Wundt's, 289

Young- Helmholtz, 199, 213

of zones, 200

304

INDEX OF SUBJECTS

Threshold, chromatic, 19, 60

differential, 19, 30, 230, 233, 294

general, 19, 60, 80, 117

specific, 19, 60
Tone, 28
Total colour-blindness, 159, 186, 209, 217,

267, 288
Trichromatic vision, 38, 158, 213, 274

anomalous, 159, 182, 216, 235. 243, 266,
288
Trichromic, 294
Tritanopic vision, 159, 180, 233, 288

Uniocular struggle, 275, 277
Unit of sensation, 20, 61

Valency, 202, 253

curves, 215

white, 44, 202, 257, 264, 268
Variation, mean, 149
Vision of animals, 131, 196, 203, 298

of primitive races, 131, 145

anomalous trichromatic, 159, 182, 216,
235, 243, 266, 288

deuteranopic, 159, 232, 287

dichromatic, 158, 162, 216, 232, 267

Vision, monochromatic, 159, 186, 217
288

protanopic, 159, 232, 287

recurrent, 85

tritanopic, 159 180, 233, 288

trichromatic, 38, 158, 213, 274
Visual acuity. 204

method, 44, 99
Visual purple, 12, 196, 205, 212, 220, 292

absorption by. 13, 205

bleaching of, 13, 55, 206, 212
Visual substances. 12, 201, 219, 253, 270,

271, 280, 285, 291
Visual yellow, 13, 197, 220

Weber's law, 19

White, 4, 25, 201, 202, 207, 211, 223, 226,

283, 285
White valency, 44, 202, 257, 264, 268

Yellow, 115, 198, 260, 283, 285, 298
Yellow spot, 9, 81 {see Fovea centralis)
Yellow, visual. 13, 197. 220
Young-Helmholtz theory, 199, 213

Zones, theory of, 200

INDEX OF AUTHORS

Abady, 94

Abbott, 134

Abclsdorff, 12, 13, 140, 205

Abney, Sir WUliam, 2, 4, 5, 39, 40, 42, 43,
44, 45, 46, 47, 48, 53, 54, 56. 60, 61, 62, 63,
64, 65, 66, 72, 74. 75, 76, 77, 78, 79, 80,
81, 92, 94, 96, 106, 107, 116, 121, 122, 123,
126, 163, 173, 175, 180, 181, 182, 183, 190,
192, 219, 221, 222, 223, 224, 225, 227, 228,
229. 230, 231. 232, 236, 238, 239, 240, 243,
244, 247, 259, 294

Aircy, 94

Albert, 59

Allen, 94, 96, 98

Allen, Grant, 146

Angelucci. 11, 12, 15

Angier, 35, 36, 179

Angstrom, 55

Arago, 50

Asher, 118, 119

Aubert, 4, 5, 29, 49, 60, 68, 118, 126, 127,

129, 200
Axenfeld, D., 129

Baird, 68, 70, 258

Baldwin, 152, 153

Bateson, 143

Basler, 204

Bauer, 145, 212

Bcale, 252

Becker, 180, 191, 288

Behr, 210

Bell, 98

Bellarminoff, 95

Bentley 143, 144

Benzaky, 146

Bernstein, 259

Berry, 197

Bidwell, Shelford, 86, 87, 88, 89, 94

Birch-Hirschfeld, 11

Bjerrura, 187, 189, 192

Bloom, 51, 208

Bois-Reymond, du, 15

Boll, 12, 13, 15, 196

Boltunow, 50

Bonnet, 18

Bosscha, 86, 89

Boswell, 66

Brandes, 127

Brauneck, 207

Breuer, 51, 183

Brewster, 129

Brodhun, 30, 59, 92, 173, 175, 207, 220, 233

Briicke, 43, 93, 112, 125, 126, 128

Briickner, 43, 45, 91, 235

Brunner, 259

Buffon, 44

Buhler, 105

Bull, Ole, 70

Burch, 3, 94, 102, 112, 113, 114, 115, 129,

219, 232, 294, 297
Burford, 133, 135

Cajal, Ramon y, 12, 212
Celsius, 44
ChaUis, 198

Charpentier, 45, 50, 51. 6]. (iS. 69. 79, 81,
86, 87, 89, 90, 95, 117, 120, 129, 197, 198
Chauveau, 129
Chevreul, 38, 125, 126
Clerk-MaxweU, 4, 21, 29, 40, 115, 126, 200,

220, 221
Cohn, 181, 191
Colbum, 192
Cole, 134, 142
Collin, 181
Colvin, 133, 135
Crova, 45

Dalton, John, 158, 202, 232

Davis, 134

Davis, A. S., 86

Dewar, 16

Dieterici, 30, 31, 35, 39, 182. 190, 191, 222,

223
Dimmer, 9, 10
Dittler, 12, 84, 90, 105, 108
Dobrowolski, 30
Donders, 50, 69, 120, 159, 163, 168, 181, 182,

183, 185, 191, 200, 221, 232, 268, 270, 271,

273
Dor, 191
Dove, 58, 126
Dow, 94, 98, 208
Draper, 42, 60
Dreher, 70
Drobisch, 198

Ebbinghaus, 36, 60, 127, 128, 129, 207
Edridge-Green, 12, 13, 29, 30, 109. 115, 116,

280, 291, 292, 294, 295, 296, 297, 29S, 299
Eisenmeier, 90
Engelmann, 11, 12, 16
Ewald, 268
Exner, F., 30. 86, 90. 94, 95, 212, 221, 223,

230

306

INDEX OF AUTHORS

Fechner, 20, 21, 42, 52, 94, 103, 107, 109,
110, 112, 126, 127, 128, 129, 255, 270, 274,
294

Feilchenfeld, 59, 120

Ferry, 94, 96

Festing, 42. 45, 53, 54, 61, 62, 64

Fick, Ad., 51, 81, 92, 110, 127, 200. 202,
218, 289

Filehne, 95

Fleming, 45

Franz, 145, 203

Frey, v., 35, 41, 124

Frisch, v., 145

Fritsch, 10

Fuchs, 14, 16

Fujita. 123, 124, 208

Fukala, 188, 192

Galezowski, 191

Garbini, 152

Gardner, E. A., 146

Garten, 11, 12, 13, 51, 208

Gates, Elmer, 132

Geiger, 145, 146, 147

Gertz, 187, 188, 189, 192

Gladstone, 145, 146

Glan, 35, 40

Goethe, 21, 126, 128, 158, 180, 200, 251

Golant, 4

Gorham, 45

Gotch. 15, 16, 82, 219

Graber, 132. 143

Grailich, 58, 198

Grassmann, 29. 33, 43, 200

Gray, 60

Greeff, 9, 10, 11. 203

Greenwood, 36, 38, 182, 216. 264

Griinbaum, 0., 92, 94, 95

Grunert, 187, 188, 189, 192

Guillery, 50, 80

GuUstrand, 10

Giirber, 110

Guttmann, 183, 184, 185

Haab, 13, 197

Haas, de, 16

Halben, 183, 184

Hamaker, 86, 90

Hartley, 198

Hartridge. 198

Haycraft, 45, 55, 56. 94, 96, 97

Hayes, 180

Hegg, 70

Heine, 10

Heinrichsdorff, 210

Helmholtz, v., 1, 4, 5. 16, 20, 28. 29, 34.
35, 36, 37, 42, 43, 44, 52, 58, 92, 93, 103,
104, 107, 109, 110, 111. 112. 113. 120. 124,
126, 127. 128, 129, 158, 159, 165, 198, 200,
202, 206, 209, 212, 214, 215, 216, 218, 219,
220, 222, 232, 255, 274, 277

Henius, 123, 124, 208

Hering, H, 21, 22, 23, 24, 25, 41. 44, 53. 57.
58, 61, 68, 74, 84. 90, 101, 106, 107, 108,

110, 118, 124, 125, 126, 127, 128, 129, 159,
181. 182, 183, 188, 189. 190, 191, 192, 193,
197, 200, 202, 207, 251, 252, 253, 254, 255,
256, 257. 258, 259, 260, 263, 264, 266, 267,
269, 270, 273, 277, 279, 280, 290, 291

Hermann, 127, 129, 181

Hersehel, Sir Wm., 44

Hess. 11. 12, 15. 68, 70, 72. 74, 80, 86, 89,
110, 126, 127, 128, 129, 132, 134, 139. 140,
142, 143, 144, 145, 181, 187, 188, 189, 192,
202, 210, 211. 212, 258, 263, 264, 267, 298

Hessberg, 187, 189, 192

Hilbert. 102

HiUebrand, 53, 172, 189, 256, 257, 267

Himstedt, 16, 132, 206, 219

Hippel, V, 180, 187, 188, 191

Hirschberg, 120, 123

Hofler, 38

Holmgren, 16, 147, 148, 180, 181

d'Hombres-Firmas, 191

Homer. 145, 146

Huddart, 158, 191

Hyde, 92

Ives, 48. 94, 96, 97. 98, 99, 100, 208,
223

Javal, 146

Juler, 187, 188, 192

Jurin, 127

KaLischer, 135, 136

Katz, 22

Kinnaman, 133

Kinoshita, 90

Kirsch, 91, 235

Kirschmann, 128

KofEka, 185

Koike, 84

Ivollikcr 0

K6Uner,'l60, 181, 187, 192

Konig, 13, 30, 31, 32, 34, 35, 39. 41, 42, 45,
46, 47, 55, 57, 58 59, 60, 61, 64, 72, 81,
84, 100, 163, 164, 167, 172, 173, 174, 175,
180, 181. 182, 183, 187. 188. 189, 190, 191,
197, 202. 207, 208. 209. 220, 221, 222, 223,
224, 230, 232, 233, 234, 236, 267, 270, 280,
288, 295

Koranyi. 142

Koster, 9, 10, 61, 81, 84

Kottgen, 13, 205

Krarup, 35, 36, 40, 47, 55

Krause, 11, 129

Kressig, 191

Krienes, 181

Kries, v . 20. 21, 31, 34, 36, 37, 38, 39, 41,
43, 50, 51, 53, 58, 61, 70. 71, 81, 83, 84,
86, 89, 93, 94, 105, 106, 107, 108, 109, 124,
125, 159, 163, 164, 166, 167, 168, 169, 170,
171, 172, 173, 174, 176, 177 178, 179, 182,
183. 188, 189, 190, 191, 192, 198, 200, 202,
203. 207, 208, 209, 216, 217, 21 8, 219 259,
264, 268, 274

Kruss, 94

INDEX OF AUTHORS

307

Kiihne, 11. 12. 13. 14, 16, 196, 197, 205, 212,

268
Kuhnt, 9, 129
Kunkel, 90

Ladd-Fraiiklin, Christine, 50, 60, 188, 207,

260, 271, 272, 273, 280, 292
Lambert, 38

Landolt. 66, 68, 69, 187, 191
Leber, 202
Lehmann, 127
Leroy, 118
Levy, 179, 181, 183
Liebermann, 234
Liesegang, 197, 202
Lipps. 20
LleweUyn, 188
Lodato, 12
Loeb, 142

Loeser, 52, 120, 123, 208
Lohmann, 210
Lotze, 182, 183, 184, 194
Lubbock, 132
Luckiest, 98
Lummer, 60, 84, 92

MacDougaU, 21, 24. 85, 86, 87, 88, 89, 90,
91, 93^101, 102, 104, 111, 150, 151, 152,
153, 156, 193, 200, 208, 252, 260, 261, 263,
274, 275, 276, 277, 278, 279. 280, 281, 282,
297

Mach, 21, 119, 127, 200, 251, 270

Maclver, Randall-, 148

McKendrick, 16

Magnus, 146, 191

Maltzew, Fraulein v., 179, 184

Mandelstamm, 30

Marsden, 162, 153

Marshall, Devereux, 12, 115

Martius, 45, 90

Marx, 234

Mast, 130

MaxweU, Clerk-, 4, 21, 29, 40, 115, 126,
200, 220, 221

May, 188, 190

Mayer, 129

Merkel, 9

Messmer, 210

Meyer, H., 126, 127, 129

Millar, 94

Morgan, Lloyd, 140

Miiller, G. E. 16, 20, 129, 274. 277, 290

MiiUer, H., 9, 12, 196

Miiller, Johannes, 18, 126, 200, 221

Miiller, P., 90

Munk, 86

Myers, 19, 151, 152, 154, 156, 157, 195, 298

Nagel, 16, 31. 43, 53, 54, 60. 61, 66, 71, 83,
84, 106, 107, 132, 148, 163, 164, 166, 167,
173, 174, 176, 177, 178, 180, 181, 182, 183,
185, 187, 188, 189, 190, 191, 192, 206, 209,
212. 216, 219

Nahmacher, 12

Nettleship, 159, 186, 187, 191. 192, 209
Newton, Sir Isaac, 3. 33, 34. 42, 43, 58, 84,

198, 199, 200, 207
Nichols, 28
Nicolai, 133
Nutting, 100

Orbeli, 105, 134

Parinaud, 61, 81, 197, 198, 210

Parker, G. H., 142

Parsons, 8, 44, 52, 66, 117, 159. 187, 208,

209. 211
Pauli, 259
Pawlow, 134
Pearse, 142
Peirce, 30
Percival, 94
Pergens, 192, 288
Pertz, 51
Pfluger, 188, 192
Pheophilaktowa, 132
Piper, 11, 14, 52, 123, 124, 139, 181, 192,

206, 208, 288
Plateau, 91, 92, 93, 95, 103, 110, 255
Pole, 180

Polimanti. 45, 48, 72, 94. 173, 176, 177, 183
Porter, A. W., 109, 115, 116
Porter, J. P., 140

Porter, T. C, 94, 95, 96, 97, 191, 208
Pretori, 127, 128, 264
Preyer, 152, 175, 202. 221
Purkinje, 9, 43, 57. 58, 59, 67, 74, 84, 86,

96, 97, 98. 112, 129, 145, 175, 206, 207,

208, 210, 211, 257, 268, 293, 297
Putter, 203
Pyle, 103

Querenghi, 191

Raehlmann, 53, 187, 191

Rayleigh, Lord, 3, 159, 182, 186, 235, 242,

249, 259, 269, 295
Reighard, 144
Ricco, 52, 117, 118, 120
Richarz, 287
Richter, 108

Ritter, 42, 45, 172, 175, 190
Rivers, 45, 95, 146. 147, 148, 149, 150, 151,

168, 252, 253. 275, 298
Rollet, 127, 128, 279
Ronne. 187, 189, 192
Rood, 45, 94
Rose, 191, 267
Rosier, 191
Rouse, 140
Runge, 38
Ruppert, 204

Sachs, 40, 55. 128, 140, 169, 205, 264

Samojloff, 86, 90, 132

Schafhautl 45

Schatcrnikoff, 53, 54, 55. 191, 208

Schaum, 287

308

INDEX OF AUTHORS

Schenck, 95, 179, 181, 218, 285, 288, 289,

297
Scholer, 191, 288
Schon, 109
Schopenhauer. 191
Schoute, 119,' 120

Schultze, Max, 9, 10, 11, 19(3, 197, 198, 203
Scina, Ragona, 12(3
Seebeck, 163, 170
Seguin, 110
Sewall, 205
Sherman, 84

Sherrington, 93, 94, 95, 218, 259, 275
Shinn, 152
Siebeck, 72, 73
SiemerUng, 288
Simmance, 94
Simon, 50, 51
Siven, 211

Smith, Miss E. M., 136, 138, 139
SneUen, 89
Spicer, Holmes, 192
Stargardt, 210
Stegmann, 60, 206

Steindler, 30, 31, 32, 230, 233, 234, 235, 295
Steiner, 16
Stilling, 180, 181
Stobbe, 259

Stort, van Genderen, 12, 15
Stout, 24
Swan, 90

Tait, 12, 13

Talbot, 16, 45, 91, 92, 93, 95

Thiirmel, 100

Tigerstedt, 4, 6

Titchener, 103

Tonn, 175, 207

Torelle, 142

Treitel, 51

Trendelenburg, 13, 14, 35, 36, 53, 55, 206

Trotter, 45

Tschermak, 49, 51, 53, 58, 61, 74, 81, 83,

84. 103. 106, 108, 125, 126, 128, 129,

174. 257. 263, 267, 268, 289
Tufts, 94. 99, 223
Turberville, 158
Tyndall, 60

Uhthoff, 30, 31. 32, 187, 188, 189, 191, 192.
209, 288

Valentine, 152, 155, 156, 157

Vaughan, 50

Verworn, 252

Vierordt, 42

Vinci, Leonardo da, 126, 251

Vintschgau, 181

Virchow, 146

Voeste, 107

Volkmann, 118

Wadsworth, 9

Waller, 16

Wanach, 116

Ward, 251, 276

Washburn, 111, 131, 134, 143, 144

Watson, J. B., 133

Watson. Prof. W., 4, 39, 40, 48, 94, 106,

173. 182, 183, 184, 190, 199, 222, 228,

236, 239, 240, 241, 242, 243, 244, 245,

247
Waugh, 134

Weber, E. H.. 19, 20, 42, 91
Weber, H. F., 60
Wehrli, 192
Weiss, 197
Welcker, 9
Welponer. 14
Wendt, 211

Weyde, van der, 163, 175, 179
Whisson. 158
Whiting, 94
Whitman, 94, 99
Wild, 94
Williams, 60
Wii-th, 107
Woinow, 202
Wolliflin, 84, 210
Wundt, 84, 107, 126, 127, 128, 181,

289

Yerkes, 133, 134, 142

Young, C. A., 85

Young, Thos., 158, 198, 199, 200, 202, 219,

220, 232, 271. 274, 276
Young- Helmholtz, 163, 199, 200, 213, 215,

216, 217, 220, 224. 230, 258, 263, 270,

273, 274, 285, 288

Zahn, 74
Zehender, 183
Zolotnitzki, 143, 144

Cambridge: printed by john clay, m.a. at the university press

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