Produced by Chris Curnow, Hazel Batey and the Online
Distributed Proofreading Team at https://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
Illustration: COLOUR-PATCH APPARATUS.
_THE ROMANCE OF SCIENCE._
COLOUR MEASUREMENT AND MIXTURE.
=With Numerous Illustrations.=
BY CAPTAIN W. de W. ABNEY, C.B., R.E., D.C.L., F.R.S.
PUBLISHED UNDER THE DIRECTION OF THE COMMITTEE
OF GENERAL LITERATURE AND EDUCATION APPOINTED BY THE
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE.
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE.
LONDON: NORTHUMBERLAND AVENUE, W.C.;
43, QUEEN VICTORIA STREET, E.C.
BRIGHTON: 135, NORTH STREET.
NEW YORK: E. & J. B. YOUNG & CO.
1891.
PREFACE.
Some ten years ago there were three measurements of the spectrum which I
set myself to carry out; the last two, at all events, involving new
methods of experimenting. The three measurements were: (1st) The heating
effect; (2nd) the luminosity; and (3rd) the chemical effect on various
salts, of the different rays of the spectrum. The task is now completed,
and it was in carrying out the second part of it that General Festing,
who joined me in the research, and myself were led into a wider study of
colour than at first intended, as the apparatus we devised enabled us to
carry out experiments which, whilst difficult under ordinary
circumstances, became easy to make. On two occasions, at the invitation
of the Society of Arts, I have delivered a short course of lectures on
the subject of Colour, and naturally I chose to treat it from the point
of view of our own methods of experimenting; and these lectures,
expanded and modified, form the basis of the present volume.
As a treatise it must necessarily be incomplete, as it scarcely touches
on the history of the subject--a part which must always be of deep
interest. The solely physiological aspect of colour has also been
scarcely dealt with; that part which the physicist can submit to
measurement being that which alone was practicable under the
circumstances.
W. De W. Abney.
_South Kensington,
1st May, 1891._
CONTENTS.
CHAPTER I.
Sources of Light--Reflected Light--Reflection from Roughened
Surfaces--Colour Constants p. 11
CHAPTER II.
A Standard of Light--Formation of the Spectrum by Prisms and by the
Diffraction Grating--Wave-lengths of the principal Fraunhofer
Line--Position of Colours in the Spectrum p. 17
CHAPTER III.
The Visible and Invisible Parts of the Spectrum--Methods for showing the
Existence of the Invisible Portions--Phosphorescence--Photography of the
Dark Rays--Thermo-Electric Currents p. 30
CHAPTER IV.
Description of Colour Patch Apparatus--Rotating Sectors--Method of
making a Scale for the Spectrum p. 41
CHAPTER V.
Absorption of the Spectrum--Analysis of Colour--Vibrations of
Rays--Absorption by Pigments--Phosphorescence--Interference p. 51
CHAPTER VI.
Scattered Light--Sunset Colours--Law of the Scattering by Fine
Particles--Sunset Clouds--Luminosities of Sunlight at different
Altitudes of the Sun p. 62
CHAPTER VII.
Luminosity of the Spectrum to Normal-eyed and Colour-blind
Persons--Method of determining the Luminosity of Pigments--Addition of
one Luminosity to another p. 76
CHAPTER VIII.
Methods of Measuring the Intensity of the Different Colours of the
Spectrum, reflected from Pigmented Surfaces--Templates for the Spectrum
p. 88
CHAPTER IX.
Colour Mixtures--Yellow Spot in the Eye--Comparison of Different
Lights--Simple Colours by Mixing Simple Colours--Yellow and Blue from
White p. 112
CHAPTER X.
Extinction of Colour by White Light--Extinction of White Light by Colour
p. 126
CHAPTER XI.
Primary Colours--Molecular Swings--Colour Sensations--Sensations absent
in the Colour-blind p. 133
CHAPTER XII.
Formation of Colour Equations--K[oe]nig's Curves--Maxwell's Apparatus and
Curves p. 147
CHAPTER XIII.
Match of Compound Colours with Simple Colours--All Colours reduced to
Numbers--Method of Matching a Colour with a Spectrum Colour and White
Light p. 156
CHAPTER XIV.
Complementary Colours--Complementary Pigment Colours--Measurement of
Complementary Colours p. 167
CHAPTER XV.
Persistence of Images on the Retina--The Use of Coloured Discs p. 179
CHAPTER XVI.
Contrast Colours--Measurement of Contrast Colours--Fatigue of the
Eye--After-Images p. 196
LIST OF ILLUSTRATIONS.
FIG. PAGE
Colour-patch apparatus _Frontispiece_
1. Spectrum of sunlight 18
2. Carbon poles of an electric light 20
3. Curve for converting prismatic spectrum into wave-lengths 28
4. The thermopile 35
5. Heating effect of different sources of radiation 38
6. Colour-patch apparatus 42
7. Rotating sectors 45
8. Spectrum of Carbon Sodium and Lithium 48
9. Interference bands 60
10. Absorption of rays by the atmosphere 68
11. Luminosity curve of spectrum of the positive pole of the
electric light 79
12. Rectangles of white and vermilion 82
13. Arrangement for measuring the luminosities of pigments 83
14. Measurement of the intensity of rays reflected from white
and coloured surfaces 88
15. Intensity of rays reflected from vermilion, emerald green,
and French ultramarine 92
16. Method of obtaining two patches of identical colour 95
17. Absorption by red, blue, and green glasses 99
18. Light reflected from metallic surfaces 100
19. Intensities of vermilion, carmine, mercuric iodide, and
Indian red 101
20. Intensities of gamboge, Indian yellow, cadmium yellow,
and yellow ochre 101
21. Intensities of emerald green, chromous oxide, and terre
verte 103
22. Intensities of indigo, Antwerp blue, cobalt, and French
ultramarine 104
23. Method of obtaining a colour template 104
24. Template of carmine 106
25. Template of luminosity of white light 108
26. Absorption of transmitted and reflected light by
Prussian blue and carmine 107
27. Collimator for comparing the intensity of two sources
of light 109
28. Spectrum intensities of sunlight, gaslight, and blue
sky 109
29. Comparison of sun and sky lights 111
30. Slide with slits to be used in the spectrum 113
31. Screen on which to match gamboge 116
32. Diaphragm in front of prism 128
33. Curve of sensitiveness of silver bromo-iodide 136
34. Curves of colour sensations 139
35. K[oe]nig's curves of colour sensations 151
36. Maxwell's colour-box 152
37. Maxwell's curves of colour sensations 154
38. Chromatic circle 168
39. Disc to cause alternate opening and closing of two
slits 179
40. Disc painted blue and red 181
41. Electro-motor with discs attached 183
42. Method of cutting disc to allow an overlap of a second
disc 184
43. Arrangement to find value of gamboge in terms of emerald
green and vermilion 188
44. Disc arranged to give approximately all the spectrum
colours 192
45. Method of showing contrast colours 196
COLOUR MEASUREMENT AND MIXTURE.
CHAPTER I.
Sources of Light--Reflected Light--Reflection from Roughened
Surfaces--Colour Constants.
There is nothing, perhaps, in our everyday life which appeals more to
the mind than colour, yet so accustomed are the generality of mankind to
its influence that but few stop to inquire the "why and wherefore" of
its existence, or its cause. To those few, however, there is a source of
endless and boundless enjoyment in its study; for in the realms of
physical and physiological science there is perhaps no other subject in
which experiments give results so fascinating and often so beautiful.
Although its serious study must be undertaken with a clear mind, a good
eye, and a fair supply of patience, yet a general idea of the subject
may be grasped by those who are possessed of but ordinary intelligence.
Colour phenomena are encountered nearly every day of one's life, and the
fact that they are so frequently met with, prevents that attention to
them, or even their remark. Who amongst us, for instance, has noticed
the existence of what are called positive and negative after images,
after looking at some strongly illuminated object, or would have gauged
the fact that a certain portion of the nervous system can be fatigued by
a colour, and give rise to images of its complementary, had not an
enterprising advertiser, who manufactures a household necessary, drawn
attention to it in a manner that could not be misunderstood.
If on an autumn afternoon we pass through a garden whilst it is still
perfectly light, we can notice the gorgeous colouring of the flowers,
and appreciate with the eyes the beauty of each tint. As evening comes
on the tints darken, the darkest-coloured flowers begin to lose their
colour, and only the brightest strike the eye. When night still further
closes in every colour goes, though the outlines of the flowers may
still be distinguished; and it would not be impossible, in some parts,
to see a tiny speck of pale light upon the ground amongst them. This
speck of light we should know from experience to be the light from a
glow-worm. Why is it that we lose the colour of the flowers and
recognize the tiny light from this small worm? The reason for the one is
that in order for objects which are not self-luminous to be seen at all,
light must fall on them and illuminate them, and the light which they
reflect may be coloured if they possess the qualities to reflect
coloured light. The glow-worm's light is seen, not because it does not
emit light in the day-time, but because the eye, being limited in
sensitiveness, is unable to distinguish it when it is flooded with the
light of day. The glow-worm, however, is self-luminous, as is shown by
the fact that it emits light in the dark, the light itself being
slightly coloured if compared with that of day. That a candle-flame or
the sun is self-luminous is an axiom, and need not be philosophised
upon; but what must be impressed on the reader is, that though an object
which requires to be illuminated to be seen, is not self-luminous, yet
when illuminated it does in fact become a source of illumination to the
eye, although the light is only light reflected from its surface. It is
a point worth remembering that the rougher the surface of an object, the
brighter to the eye it will be. That is, a coloured object when polished
will be a bad secondary source of illumination, as the light incident
upon it will be very nearly reflected from the surface, according to
the ordinary laws of reflection; but if it be roughened it will become a
much better source, as the roughnesses, though obeying the laws of
reflection, will reflect light in every direction. A good example of
this is an ordinary sheet of glass. Light from a source falling on its
surface is scarcely reflected in any direction except in that determined
by the ordinary laws of reflection, and it will be scarcely visible to
the eye. Grind its surface, however, and the innumerable facets caused
by the grinding will reflect light back to the eye in whatever position
it be placed, and will thus be distinctly seen.
We may here premise that even the roughest surface will reflect a
greater percentage--varying greatly according to the nature of the
surface--of light in the direction which it would do if it were a smooth
surface than in any other; and in taking measurements of the light
irregularly reflected from a rough surface, this fact must be borne in
mind.
Not only must we know how colour is produced, but we must also be able
to refer it to some standard which shall be readily reproduced, and
which shall be unalterable. There are two variable factors which have to
be taken into account in colour experiments: the first is the quality of
light which illuminates the object, and the second is the sensitiveness
of the eye which perceives it, as light is only a sensation which is
recognized by the brain through the medium of the eye. We shall, as we
go on, see that different qualities of light may cause objects to appear
of different hues, and further that eyes may vary in perceptive power,
to an extent of which the large majority of people are not aware. Hence
it becomes necessary as far as possible to eliminate these variables.
The task which we have set ourselves to perform then, is first to find a
suitable light for experimental work, and next to endeavour to refer
colour to an eye which has no abnormal defects. This being accomplished,
we have then to find means to measure the different constants which are
involved in colour, and to refer the measurements to some standard.
Colour constants are three, viz. hue, luminosity, and purity; and it
will be seen that if these three are determined, the measurement of the
colour is complete.
Perhaps the meaning of these terms may require to be explained. The hue
of a colour is what in common parlance is often called the colour. Thus
we talk of rose, violet, magenta, emerald green, and so on, but for
measuring purposes the hue had best be referred to the spectrum colours
as a standard (the means of doing so will be shortly explained), for
they are simple colours, which can be expressed by numbers. Compound
colours, which it may be said are invariably to be found in nature,
being mixtures of simple colours, can be just as readily referred to the
spectrum. By the luminosity of a colour we mean its brightness, the
standard of reference being the brightness of a white surface when
illuminated by the same white light. By the purity of a colour we mean
its freedom from admixture with white light. An example of different
degrees of purity will be found in washes of water-colours of different
tenuity. Thus if we wash a sheet of paper with a light tint of carmine,
the whiteness of the paper is not obliterated; if we pass another wash
over it the whiteness of the paper is lessened, and so on. The lightest
tint is that which is most lacking in purity.