return to Introduction to Early Color Photography, August 2008
return to Evolution of Three Color Photography, April 2006
WHEN the compound nature of the solar light was first discovered
by the illustrious Newton, it was supposed to consist of seven
primary colours; but natural philosophers have long since arrived
at the conclusion that three only out of the seven are entitled to
be regarded as primaries, the remainder being produced by different
combinations of these three. The primary colours have been
considered as blue, yellow, and red; but certain experiments performed
by Professor Maxwell have led him to the conclusion that
although the number of the primary colours is really three, yellow
ought properly to be placed among the secondaries, being really a
combination of green and red-the former colour, green, being
entitled to rank as a primary.
The following is an authenticated abstract of a lecture, enunciating
these principles, recently delivered at the Royal Institution
by the learned Professor.]
The speaker commenced by showing that our power of vision depends
entirely on being able to distinguish the intensity and quality of
colours. The forms of visible objects are indicated to us only by differences in colour or brightness between them and surrounding objects, To
classify and arrange these colours, to ascel'tain the physical conditions
on which the differences of coloured rays depend, and to trace, as far as
we are able, tIle physiological process by which these different rays excite
in us various sensations of colour, we must avail ourselves of the
united experience of painters, opticians, and physiologists, The speaker
then proceeded to state the results obtained by these three classes of inquirers,
to explain their apparent inconsistency by means of Young's
Theory of Primary Colours, and to describe the tests to which he had
subjected that theory.
Painters have studied the relations of colours, in order to imitate them
by means of pigments. As there are only a limited number of coloured
substances adapted for painting, while the number of tints in nature is
infinite, painters are obliged to produce the tints they require by mixing
their pigments in proper proportions. This leads them to regard these
tints as actually compounded of other colours, corresponding to the pure
pigments in the mixture. It is found that by using three pigments only
we can produce all colours lying within certain limits of intensity and
purity. For instance, if we take carmine (red), chrome yellow, and
ultramarine (blue), we get by mixing the carmine and the chrome all
varieties of orange, passing through scarlet to crimson on the one side
and to yellow on the other; by mixing chrome and ultramarine we get
all hues of green; and by mixing ultramarine with carmine we get all
hues of purple, from "violet to mauve and crimson, Now, these are all
the strong colours that we ever see or can imagine: all others are like
these, only less pure in tint, Our three colours can be mixed so as to
form a neutral grey; and if this grey be mixed with any of the hues pro·
duced by mixing two colours only, all the tints of that hue will be exhibited,
from the pure colour to neutral grey. If we could assume that
the colour of a mixture of different kinds of paint is a true mixture of
the colours of the pigments, and in the same proportion, then an analysis
of colour might be made with the same ease as a chemical analysis of a
mixture of substances.
The colour of a mixture of pigments, however, is often very different
from a true mixture of the colours of the pure pigments. It is found to
depend on the size of the particles, a finely ground pigment producing
more effect than one coarsely ground. It has also been shown by Pro·
fessor Helmholtz that, when light falls on a mixture of pigments, part of
it is acted on by one pigment only and part of it by another, while a
third portion is acted on by both pigments in succession before it is sent
back to the eye, The two parts reflected directly from the pure pigments
enter the eye together, and form a true mixture of colours; but the third
portion, which has suffered absorption from both pigments, is often so
considerable as to give its own character to the resulting tint. This is
the explanation of the green tint produced by mixing most blue anel
yellow pigments.
In studying the mixture of colours, we must avoid these sources of
error, either by mixing the rays of light themselves, or by combining the
impressions of colours within the eye by the rotation of coloured papers
on a disc.
The speaker then stated what the opticians had discovered about
colour. White light, according to Newton, consists of a great number
of different kinds of coloured light, which can be separated by a prism,
Newton divided these into seven classes; but we now recognise many
thousand distinct kinds of light in the spectrum, none of which can be
shown to be a compound of more elementary rays. If we accept the theory
that light is an undulation, then, as there are undulations of every
different period from the one end of the spectrum to the other, there are
an infinite number of possible kinds of light, no one of which can be regarded
as compounded of any others.
Physical optics docs not lead us to any theory of three primary colours,
but leaves us in possession of an infinite number of pure rays, with an
infinitely more infinite number of compound beams of light, each containing
any proportions of any number of the pure rays.
These beams of light, passing through the transparent parts of the eye,
fall on a sensitive membrane, and we become aware of various colours.
We know that the colour we see depends on the nature of the light; but
the opticians say there are an infinite number of kinds of light, while the
painters, and all who pay attention to what they see, tell us that they
can account for all actual colours by supposing them mixtures of three
primary colours.
The speaker then next drew attention to the physiological difficulties in
accounting for the perception of colour. Some have supposed that the
different kinds of light are distinguished by the time of their vibration.
There are about 447 billions of vibrations of red light in a second, and
577 billions of vibrations of green light in the same time. It is certainly
not by any mental process of which we are conscious that we distinguish
between these infinitesimal portions of time, and it is difficult to conceive
any mechanism by which the vibrations could be counted so that we
should become conscious of the results, especially when many rays of
different periods of vibration act on the same part of the eye at
once.
Besides, all the evidence we have on the nature of nervous action goes
to prove that whatever be the nature of the agent which excites a nerve,
the sensation will differ only in being more or less acute. By acting on
a nerve in various ways we may produce the faintest sensation or the most violent pain; but if the intensity of the sensation be the same, its
quality must be the same.
Now, we perceive by our eyes a faint red light which may be made
stronger and stronger till our eyes are dazzled. We may then perform
the same experiment with a green light or a blue light. We shall thus
see that our sensation of colour may differ in other ways, besides in being
stronger or fainter. The sensation of colour, therefore, cannot be due to
one nerve only.
The speaker then proceeded to state the theory of Dr. Thomas Yonng,
as the only theory which completely reconciles these difficulties in
accounting for the perception of colour.
Young supposes that the eye is provided with three distinct sets of
nervous fibres, each set extending over the whole sensitive surface of the
eye. Each of these three systems of nerves, when excited, gives us a
different sensation. One of them, which gives us the sensation we call
red, is excited most by the red rays, but also by the orange and yellow, and slightly by the violet; another is acted on by the green rays, but
also by the orange and yellow, and part of the blue; while the third is
acted on by the blue and violet rays.
If we could excite one of these sets of nerves without acting on the
others, we should have the pure sensation corresponding to that set of
nerves. This would be truly a primary colour, whether the nerve were
excited by pure or by compound light, or even by the action of pressure
or disease.
If such experiments could be made, we should be able to see the
primary colours separately, and to deselibe their appearance by reference
to the scale of colours in the spectrum.
But we have no direct consciousness of the contrivances of our own
bodies, and we never feel any sensation which is not infinitely complex,
so that we can never know directly how many sensations are combined
when we see a colour. Still less can we isolate one or more sensations
by artificial means, so that in general when a ray enters the eye, though
it should be one of the pure rays of the spectrum, it may excite more than
one of the three sets of nerves, and thus produce a compound sensation.
The terms simple and compound, therefore, as applied to colour sensation,
have by no means the same meaning as they have when applied
to a ray of light.
The speaker then stated some of the consequences of Young's theory,
and described tbe tests to which he had subjected it:-
1st. There are three primary colours.
2nd. Every colour is either a primary colour or a mixture of primary
colours.
3rd. Four colours may always be arranged in one of two ways.
Either one of them is a mixture of the other three, or a mixture of two of
them can be found identical with a mixture of the other two.
4th. These results may be stated in the form of colour-equations, giving
the numerical value of the amount of each colour entering into any mixture.
By means of the colour top, such equations can be obtained for
coloured papers, and they may be obtained with a degree of accuracy
showing that the colour-judgment of the eye may be rendered very
perfect.
The speaker had tested in this way more than 100 different pigments
and mixtures, and had found the results agree with tho theory of three
primaries in every case. He had also examined all the colours of the
spectrum with the same result.
The experiments with pigments do not indicate what colours are to be
considered as primary; but experiments on the prismatic spectrum show
that all the colours of the spectrum, and therefore all the colours in
nature, are equivalent to mixtures of three colours of the spectrum itself,
namely, red, green (near the line E), and blue (near the line G). Yellow
was found to be a mixture of redl and green.
The speaker, assuming red, green, and blue as primary colours, then
exhibited them on a screen by means of three magic lanterns before
which were placed glass troughs containing respectively sulphocyanide
of iron, chloride of copper, and ammoniated copper.
A triangle was thus illuminated, so that the pure colours appeared at
its angles, while the rest of the triangle contained the various mixtures
of the colours as in Young's triangle of colour.
The graduated intensity of the primary colours in different parts of the
spectrum was exhibited by three coloured images, which, when superposed on the screen, gave an artificial representation of the spectrum.
Three photographs of a coloured ribbon taken through the three
coloured solutions respectively were introduced into the camera, giving
images representing the red, the green, and the blue parts separately, as
they would be seen by each of Young's three sets of nerves separately.
When these were superposed, a coloured image was seen, which, if the
red and green images had been as fully photographed as the hlue, would
have been a truly-coloured image of the ribbon. By finding photographic
materials more sensitive to the less refrangible rays, the representation of
the colours of objects migbt be greatly improved.
The speaker then proceeded to exhibit mixtures of the colours of the
pure spectrum. Light from the electric lamp was passed through a narrow
slit, a lens and a prism, so as to throw a pure spectrum on a screen
containing three moveable slits, through which three distinct portions of
the spectrum were suffered to pass. These portions were concentrated by
a lens on a screen at a distance, forming a large, uniformly coloured
image of the prism.
When the whole spectrum was allowed to pass, this image was white,
as in Newton 's experiment of combining the rays of the spectrum. When
portions of the spectrum were allowed to pass through the moveable slits,
tbe image was uniformly illuminated with a mixture of the corresponding
colours. In order to see these colours separately another lens was placed
between the moveable slits and the screen. A magnified image of the
slits was thus thrown on the screen, each slit showing, by its colour and
its breadth, the quality and quantity of the colour which it suffered to
pass. Several colours were thus exhibited, first separately, and then in
combination. Red and blue, for instance, produced purple; red and green
produced yellow; blue and yellow produced a pale pink; red, blue, and
green produced white; and red and a bluish green near the line F produced
a colour which appears very different to different eyes.
The speaker concluded by stating the peculiarities of colour blind vision,
and by showing that tbe investigation into the theory of colour is
truly a physiological inquiry, and that it requires the observations and
testimony of persons of every kind in order to discover and explain the
various peculiarities of vision.