"The Theory of the Primary Colours." The British Journal of Photography, August 9, 1861
From George Eastman House : Notes On Photographs
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AN interesting lecture on this subject was recently delivered at
the Royal Institution, by Professor Maxwell, in which he
mooted the novel theory that the three primary colours
instead of being, as is commonly, red, blue, and yellow,
are red, blue, and green. As our readers know, the
latter colour is generally held to be a secondary colour,
the product of a mixture of the two primaries yellow and
blue, in the same way that purple is the product of a
mixture of red and blue, and orange of the mixture of red
and yellow. Professor Maxwell holds, however, that green
is not it secondary mixed colour, but a simple and primary
colour, and that yellow, instead of being a primary colour.
is the result of a mixture of red and green; and he illustrated
his position by producing yellow by the refraction through
a prism of red and green light. The lecture was further
illustrated by photographic transparencies taken for the
purpose by Mr. Sutton, who in the Notes gives the following
account of their production :-
"A bow made of ribbon, striped with various colours, was
pinned upon a background of black velvet, and copied by photography
by means of it portrait lens of full aperture, having
various-coloured fluids placed immediately in front of it, and
through which the light from the object had to pass before it
reached the lens. The experiments were made out-of-doors, in
a good light, and the results were as follow :-
" 1st. A plate-glass bath, containing the ammoniacal sulphate
of copper which chemists use for the blue solution in the
bottles in their windows, was first placed immediately in front
of the lens. With an exposure of six seconds a perfect negative
was obtained. This exposure was about double that required
when the coloured solution was removed.
"2nd. A similar bath was used, containing a green solution
of chloride of copper. With an exposure of twelve minutes not
the slightest trace of a negative was obtained, although the
image was clearly visible upon the ground-glass. It was,
therefore, found advisable to dilute the solution considerably ;
and by doing this, and making the green tinge of the water
very much paler, a tolerable negative was eventually obtained
in twelve minutes .
" 3rd. A sheet of lemon-coloured glass was next placed in front
of the lens, and a good negative obtained with an exposure of
two minutes.
"4th. A plate-glass bath, similar to the others, and containing
a strong red solution of sulpho-cyanide of iron was next
used, and a good negative obtained with an exposure of eight
minutes.
" It is impossible to describe in words the exact shades of
colour, or intensity of these solutions. The thickness of the fluid
through which the light had to pass was about three-quarters of
an inch. The collodion was simply iodized, the bath neutral,
and the developer pyrogallic acid. The chemicals were in a
highly-sensitive state, and good working order, producing
clean and dense negatives, free from stains und streaks in all
cases.
" The negatives taken in the manner described were printed
by the tannin process upon glass, and exhibIted as transparencies. The picture taken through the red medium was at
the lecture illuminated by red light,-that through the blue
medium, by blue light,-that through the yellow medium, by
yellow light, and that through the green medium, by green
light ;-and when these different-coloured Images were superposed
upon the screen, a sort of photograph of the striped
ribbon was produced in the natural colours."
Mr. Sutton concludes from these experiments that yellow
glass is not so good for the windows of dark rooms, &c., as
red or green glass. This idea would scarcely accord with
the resuIts the spectroscopic analysis of various-coloured
glasses recorded in our "Scientific Gossip." Red glass, of
some kinds is found indeed to be quite impervious to actinic
light; but the colour is so irritating to a sensitive eye as to
be practically useless. The resistant power of green to the
passage of the actinic rays is strikingly shown in the experiment
narrated; but it can scarcely be assumed, as following,
that green glass would necessarily have a similar effect.
Green we know is, at least often, formed of a mixture of
blue and yellow, and in such proportion as blue may be
present, it may be expected to possess an actinic character.
It will be seen, from the spectroscopic examination of a piece
of green glass, that it is, in one instance at least, comparatively
speaking, worthless for the purposes of the dark room.
If we rightly understand the statement of the case, moreover,
it was lemon-coloured, and not a deep yellow, or orange-coloured
glass which was used in these experiments, and it
might be expected à priori to permit of the passage, to a
large extent of actinic light. The subject is very interesting,
and we commend it to the attention of our readers, as also
the following abstract of Professor Maxwell's lecture which
we extract from the Notes to which the Professor had contributed
it:-
"The speaker commenced by showing that our power of
vision depends entirely on our 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 ascertain the physical conditions on
which the differences of coloured rays depend, and to trace, as
far as we are able, the physiological process by which these
different rays excite in us various sensations of colour, we must
avail ourselves of the united experience of painters, opiticians,
and physiologists. The speaker then proceeded to state the
results obtained by these three classes of enquirers, 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 is mixed with any of the hues produced by
mixing two colours only, all the tints of that hue will be exhibited,
from the pure colour to the 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 Professor 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 and 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 does 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 they can account
for all actual colours by supposing them mixtures of three
primary colours.
" The speaker 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 infinitessimal 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 wiII 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 is the same, its quality
must be the same.
"Now we may 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 Young, 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.
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 produced a colour which appears Very different to
different eyes.
"If such experiments could be made, we should be able to
see the primary colours separately, and to describe their appearance
by reference to the scale of colours in the spectrum. But
we have no distinct 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.z
"The speaker then stated some of the consequences of
Young's theory, and described the tests to which he had subjected
it.
"lst. 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.
"4. 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 one hundred
different pigments and mixtures, and had found the results
agree with the 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,
and blue. Yellow was found to be a mixture of red 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 sulpho-cyanide 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 colours. 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 riband, taken through the
three coloured solutions respectively, were introduced into the
camera, giving images representing the reel, 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 of the riband was seen, which, if the red and
green images had been as fully photographed as the blue, would
have been a truly-coloured image of the riband. By finding
photograpbic materials more sensitive to the less refrangible
rays, the representation of the colours of objects might be
greatly improved. . .
"The spealter then proceeded to exhibit mixtures of the colours
of the pure spectrum. Light from the electric lamp was passed
first 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, the
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 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 the investigation into
the theory of colour is truly a physiological enquiry, and that it
requires the observations and testimony of persons of every
kind, in order to discover and explain the various peculiarities
of vision."