Contrast Colours--Measurement of Contrast Colours--Fatigue of the
Eye--After-Images.
Fig. 45.--Method of showing Contrast Colours.
There is a phenomenon in colour which must be alluded to, and which
possesses more than a passing interest to the art world, and that is
colour contrast. Perhaps one of the best methods of showing this is by
our colour patch apparatus. If we throw the reflected beam and the
colour patch on a square as before, and place a rather thinner rod in
front, so that the two shadows lie on a background of the combined white
light and spectral colours, on passing a slit through the spectrum, the
shadow which is illuminated by white light will appear anything but
white. Thus if we allow yellow spectral light to illuminate one shadow,
the other will appear decidedly of a blue hue; if a green ray it will
be of a ruddy hue; if a blue ray of a yellow hue; that is, all the
contrast hues will appear to the eye to tend towards a complementary
tone to the spectral light. The kind of white light illuminating the
shadow has a marked effect on the tone, as might be expected. The
following table shows the contrast colour of the white illuminated
shadow when the white light used was that of a candle.
+---------------+-------------------+---------------+------------------+
| | Contrast | | Contrast |
| Spectrum | Colours in | Spectrum | Colours in |
| Colour. | Electric light. | Colour. | Gaslight. |
+---------------+-------------------+---------------+------------------+
| Cherry red | Green gray | Cherry red | Green gray |
| Scarlet | Bluish green gray | Scarlet | Sap green |
| Terra-cotta | Blue gray | Light red | Green gray |
| Raw sienna | Light blue gray | Olive green | Pink gray |
| Olive green | Umber | Apple green | Mauve & black |
| Emerald green | Pinkish lavender | Emerald green | Pink terra-cotta |
| Grass green | Light pink | Emerald green | Pink terra-cotta |
| Bluish green | Dark pink | Bluish green |Pinker terra-cotta|
| Signal green | Salmon | Peacock blue | Salmon |
| Cyanine blue | Yellow ochre | Prussian blue | Reddish yellow |
| Ultramarine | Raw sienna | Ultramarine | Raw sienna |
| Violet blue | Brownish yellow | Violet blue | Brownish Orange |
| Blue violet | Green yellow brown| Blue violet | Brownish yellow |
| Violet | Burnt sienna | Violet | Yellow ochre |
+---------------+-------------------+---------------+------------------+
The contrasts here shown are not so visible when the two shadows of the
rod occupy the whole of the white square, but are decidedly increased
by the shadows occupying only a part of the field, the margins being
illuminated with a mixture of the two lights. Not only are there
contrasts with coloured light and white, but the relative position of
one colour to another may alter the hue of each to the eye. The
following experiments indicate what change can be expected in contrasted
colours. The double colour apparatus was used as described at page 122,
and a slit was placed in four different positions in the spectrum, viz.
in the red, orange, green, and violet, to form patches, and another slit
was placed in the same four positions in the other spectrum, and the
contrasts noted.
+-----------------+----------------------------------------------+
|Original Colours.| Change due to Contrast. |
+--------+--------+----------------------+-----------------------+
| Red | Orange | Red became yellower | Orange became green |
| | | | grey |
| " | Green | " unaltered, but | Green unaltered, but |
| | | brighter | brighter |
| " | Blue | " became more | Blue became greener |
| | | orange | |
| " | Violet | " became orange | Violet, no marked |
| | | | change |
| Green | Orange | Green became bluer | Orange became yellower|
| " | Blue | " became olive | Blue became more |
| | | | violet |
| " | Violet | " became yellower| Violet became bluer |
| Orange | Blue | Orange became redder | Blue became bluer |
| " | Violet | " became greener | Violet became bluer |
| Violet | Blue | Hardly altered | Hardly altered |
+--------+--------+----------------------+-----------------------+
These contrasts were in most cases very marked, as would be seen by
causing the same colours to fall on a different part of the screen,
outside that on which the comparisons were made.
This phenomenon of contrast is one which is most valuable for artistic
purposes, for it gives a power of increasing the value of the colour of
pigments which is used by the artist almost intuitively. Thus he can
heighten the tone of his orange pigment, with which he makes a sunset
sky, by placing in juxtaposition with it some bit of blue coloured
space. The blue becomes bluer, and the orange more orange, by this
artifice. All these artifices--or rather we should say intuitive
applications of science--are most necessary when the small range of
luminosity of colours with which he has to deal is taken into account.
For instance, in a picture of a sun-lighted snow mountain and deep pine
forests, the utmost luminosity he can give to the former is that of
white paper when seen in the shade, which, in comparison with what he
sees, is really a mixture of 90% of black with the light from the snow,
so that his range of luminosity is only nine-tenths of that which occurs
in nature. It is in adapting this low scale to his picture that true
genius of the artist is seen.
It might seem that these contrast colours being only a physiological
effect, could not be accurately measured, but such is not the case, if
a little artifice be employed. If we use the second colour patch
apparatus side by side with the first, we can very readily and with very
close approximation determine the contrast colours we see. Suppose by
the second apparatus we form a colour patch of say red, and place a thin
rod in the beam of this ray and of the reflected beam, and about six
inches from it form another patch with the first apparatus, using the
three slits to make colour mixtures; by first noting the contrast
colour, and then approximating in the second patch to what the eye
perceives, we can little by little get a fairly exact match to the
contrast colour, and can definitely note it. We now give the results of
three measures made for the contrast colours which presented themselves
to the eye when they were caused by a red ray near the lithium line,
another near the E line in the green, and the third near the G line in
the violet.
To make white light and the contrast colours, the slits had to be of the
following apertures--
+-----------------+-------+--------+---------+
| Colour. | Red. | Green. | Violet. |
+-----------------+-------+--------+---------+
| White light | 15·7 | 6·5 | 9·8 |
| Contrast to Red | 13·5 | 11·8 | 22·5 |
| " Green | 15·8 | 5·1 | 4·8 |
| " Violet | 15·9 | 7·2 | 4·2 |
+-----------------+-------+--------+---------+
Thus to form the contrast to red took 13·5 of red, 11·8 of green, and
22·5 of violet. Now from each of these there can be deducted the amount
of white light, which will leave only two colours mixed. Calculating
this out we find that the contrasts are--
+-----------------+-------+--------+---------+
| Contrast Colour | Red. | Green. | Violet. |
| to | | | |
+-----------------+-------+--------+---------|
|Red | -- | 3·5 | 16·7 |
|Green | 15·7 | 3·2 | -- |
|Violet | 19·4 | 9·5 | -- |
+-----------------+-------+--------+---------+
If the contrasts were exactly complementary colours, the proportions of
the two colours left should be the same as those of the same colours as
given, which with the original colour make white light. It will be seen
that such is not the case. A very simple way of testing this is to form
a patch of white light with the three slits in the first apparatus, and
then to obtain the contrasts by the other apparatus, with the same
colours one after the other that pass through the three slits. If now we
cover up the slit in the first apparatus through which the colour whose
contrast in the second apparatus is sought passes, we may dilute it with
white light as we will, but in no case has the writer found that an
exact match to the contrast colour can be obtained in this way. Thus,
supposing we wanted to try the experiment with the same red light as
that which comes through the red slit, we should use that same light in
the second apparatus, and form the contrast colour with the white beam,
and then in the first apparatus cover up the red slit, leaving the
violet and green to form a patch on the screen. We should then dilute
the colour of this patch with white light, and note if it appeared the
same as the contrast colour.
Another phenomenon which presents itself is the fatigue of the
colour-sensation apparatus of the eye, induced by looking at a bright
object. For instance, if we look at a crimson wafer or spot for some
time, and then turn the eye so that it rests on a grey surface, an image
of the spot will still be seen, but as of a greenish-blue colour. This
is due to the fact that the red-seeing apparatus is fatigued and
exhausted, whilst the green and violet-seeing machinery has not been
largely exercised. Consequently when looking at grey paper the grey of
the paper is seen in the retina at all parts as grey, except in the
small part of the retina which has got diminished power of perceiving a
red sensation; hence a sea-green image will be seen until the fatigue
has passed away. This colour can be reproduced with very fair accuracy
by allowing only one eye to be fatigued, and then using the other to
obtain a colour mixture corresponding to it. It will then be found that
the colour is the same as the complementary colour, much diluted with
white light.
To the same cause may be traced positive and negative after-images, as
they are called. If we look at a strongly-illuminated coloured form,
such as a church window, and close the eyes, the window will still be
seen, at first of its original colour (a positive after-image), and it
will then fade and be seen in its complementary colours (a negative
after-image). The positive image is due to the persistence of what we
may call nerve irritation, whilst the negative image is due to the
physiological excitation of all the nerve fibrils, which ordinarily
speaking give the sensation of a very dull white light. The previous
fatigue of one set of fibrils, however, prevents them being excited to
the same degree as the others, hence we get a complementary image. It
would be out of place to pursue this subject further, as we have only
dealt with the physical measurement of colour-sensations, and these are
beyond it.
INDEX.
Absorption by red, blue, and green glasses, 53
Absorption of light in the earth's atmosphere, 67
Absorption, reference to law of, 53
After-glow, 74
Arc light, 20
Artists and colours, 195
Balmain's paint, 33
Black body, 18
Blindness to green, 142
Blindness to red, 79-142
Bromo-iodide of silver, 136
Carbon poles, 20
Carmine, light reflected from, 107
" template, 106
Chlorophyll, green solution of, 51
Collimating lens, focal length of, 22
Colour, analysis of, 52
Colour-blind, red and green, 79, 80
Colour-blindness, 142-146, 157, 159
Colour constants, 15
Colour equations, formation of, 147, 148
Colour, extinction of, by white light, 126
Colour mixtures, 113
Colour patch apparatus, 41-52
Colour sensation of the eye, 202
Coloured discs, use of, 189
Coloured glasses, measurement of, 162
Colours, complementary of pigments, 170-172
Colours, complementary of spectrum, 167
Colours, how matched, 156, 157
Complementary colours, measurement of, 173-178
Compound colours, definition of, 16
Continuous spectrum, 17
Contrast colours, 196-200
Diffraction gratings, 23
" spectra, 24
Dimness and brightness of spectrum, 29
Discs, spinning, 182
Dust, particles of, 62
Electric light, contrast colours in, 197
Electric light, crater of positive pole of, 20
Emerald green, light reflected from, 94
Equations, colour, 147
Essentials of spectrum, 22
Extraction of colour by white light, 126
Extraction of white light by colour, 131
Eye, sensitiveness of, 15
Fatigue of the retina, 202
Fluorescence, 31
Fundamental sensations, 140
Gamboge, matching, 189
Glass, light from sheet of, 14
Glass prisms, 21, 22
Glow-worm, 13
Green colour-blindness, 142
Heating effect of radiation, 38
Hue, 15
Images, after, 202
Images, persistence of, on retina, 179
Impurity of simple colours, 124
Indicator of sectors, 48
Infra-red rays, 32
" photography with, 34
Insensitiveness of the yellow spot to green, 118
Intensities of limelight, gaslight, and blue sky
compared, 110, 121
Interference, 58, 59
Interference bands on soap film, 60
Invisible spectrum, methods for showing existence of, 32, 33
K[oe]nig's curves, 151
K[oe]nig's experiments, 140
Law of the scattering by fine particles, 66
Light from sun, imitation of, 63
Light, quality of, illumining object, 14
Light scattered, 62
Limelight, 19
Lines in solar spectrum, 26
Luminosity, 13
Luminosity, addition of one to another, 85-87
Luminosity of colour, 16
Luminosity of pigments, methods of determining, 81, 82
Luminosity of spectrum to normal-eyed and colour-blind
persons, 76-78
Luminosity of sun at different altitudes, 69-71
Maxwell's colour-box, 152, 153
Maxwell's discs, 184-186
Measurement of amount of light reflected by different
pigments, 88-92
Metals, light reflected from, 100
Mock suns, cause of change of colour in, 64
Molecular physics, 54
Molecular swings, 136, 137
Monochromatic light, 47
Negative images, 203
Normal vision, 77
Orange, finding luminosity of, 190
Percentages of skylight, sunlight, and gaslight, 110, 111
Phosphorescence, 32, 56
Pigments, absorption by, 57, 58
Plan of forming spectrum, 21
Polished and uneven surfaces, 13
Primary colours, definition of, 133-135
Prism, Iceland spar, 96
Prismatic spectrum into wave-lengths, conversion of, 28
Prisms, drawback to use of, 23
Prussian blue template, 107
" " light reflected from, 107
Purity of colour, 16
Rays, infra-red, 34
Rays, photography of dark, 34
Rays, ultra-violet, 34
Registering tint of pigments, 116
" " colours, 156
Retina, persistence of images on, 179
Ritter's rays, 32
Rood's colour scale, 26
Rotating sectors, 46
Scaling of spectrum, 49
Sectors, rotating, 46
Simple colours, how obtained, 112, 113
Slits placed in spectrum, 113
Soap-bubbles, 58, 59
Soap-films, 59
Spectrum, absorption of, 51, 52
Spectrum of sunlight, 18
Sun, mock, 64
Sunset clouds, 68, 69, 72, 73
Sunset sky, 72, 73
Thermopile, heating effects of, 36
Thermopile, principle of, 35
Ultramarine, light reflected from, 95
Ultra-violet rays, 31
Vermilion, light reflected from, 93
Vibrations of rays per second, 55
Violet bands, brightness of, 21
Visible and invisible parts of spectrum, 30
Water, particles of, 62
Wave-length of lines in solar spectrum, 26
White light and contrast colours, 200-202
White light, extinction of by colour, 131
White light, formation of by mixture of yellow and blue, 125
White light, how made, 114, 115, 119-123
White light, impression of, 81
Yellow and blue make white, 125
Yellow, chrome, luminosity of, 191
Yellow spot, 117
Young-Helmholtz theory, 138
THE END.
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Transcribers Note:
Page 162 The following equation:
Therefore _Z_ + _x´X_´ + [Mu]´_W_ = [Alpha]_wW_
_Z_ = ([Alpha]_w_ - [Mu]´)_W_ - _x´X´_
Is printed as
Therefore _Z_ + _x1X_´ + [Mu]´_W_ = [Alpha]_wW_
_Z_ = ([Alpha]_w_ - [Mu]´)_W_ - _x´X´_
in the original.
End of Project Gutenberg's Colour Measurement and Mixture, by W. de W. Abney