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"The Theory of the Primary Colours." The British Journal of Photography, August 9, 1861

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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."