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PAINTERS' COLOURS, OILS,
COLOUR, COLOURS, PAINTS, AND VARNISHES.
Colour is a term used by persons in several senses; hence confusion sometimes arises, although, as a rule, the context leaves no doubt as to the particular sense intended. When a beam of white light is made to pass through an angle of a triangular prism in a certain manner, and the light which has passed through is received upon a screen, we find that it has undergone a wonderful change; instead of being one uniform colour, as it was originally, it is spread out into a band of many colours, of which seven can readily be distinguished—viz., red, orange, yellow, green, blue, indigo, and violet. We see these colours by the effect or sensation produced by their action on the retina of the
eye ; in a sense, therefore, these colours have an abstract existence only, we can see them by the eye, but we cannot handle them as we can a piece of cotton. When we speak of a red colour or a green colour, we use the term “colour" in an abstract sense to indicate the sensation which these colours create
On the other hand, we often speak of coloured bodies (that is bodies which give the sensation of being coloured when we look at them) as “colours,” especially when (as with vermilion, chrome yellow, emerald green, Prussian blue, and magenta) they can be used to impart colour to other bodies. colour” is used in a concrete sense to indicate
in our eyes.
In this way
tangible bodies which have the power of causing other bodies to which they may be applied to create, so to speak, the sensation of colour. Although the subject is one of some importance to users of colours, it is not intended to enter here into a long discussion of colour from an abstract point of view, inasmuch as space does not admit of doing so in any adequate manner, the reader must, therefore, be referred to other works specially devoted to the consideration of the subject.
The Spectrum-Colours.—When, then, a beam of white light is passed in a particular manner through the edge of a triangular prism, it undergoes two changes—(1) the direction of its course is altered, i.e., it becomes refracted ; and (2) the beam of white
light is separated into a divergent band of several differently-coloured light-rays. Fig. 1 represents the path of a beam of light through a triangular prism; a is a ray travelling in the direction of the arrow which strikes the prism at c. If the prism had not been there, it would have passed on and would have fallen
the screen, s, s, at b; but the prism, bending it out of this course, refracts it as
shown atc; it then passes Fig. 1.
through the prism in the
new direction until it emerges at d, where it is again refracted so as to take the new direction, d, f. As the amount of refraction differs for each ray according to its colour, the result is that the original white beam of light is differentiated into a long band of numerous distinct colours, known as the spectrum, which extends from e to fon the screen. The rainbow is a spectrum of this kind formed by the refraction of the sun's light during its passage through the drops of water in a shower of rain. In the latter case, however, the spectrum is seen in front of the drops, not behind them, as it is formed by the rays, which, falling on the drops, pass to the back, and are then reflected so as to emerge again on the side nearest the sun.
The colours of the spectrum are pure colours-i.e., they cannot be further split up; if, say, the red part of the spectrum be passed through a second prism, no new colours are produced; the light which passes through the second prism is still red, although it is distributed over a wider surface. It, therefore, follows that there are really a very large number of simple colours in the spectrum, although, owing to the limitations of language, it is impossible to separate and name every one of these in a popular manner; although scientists can do so in another manner which it is not necessary to describe here. It is, however, customary to follow the lead of Sir Isaac Newton, who discovered this property of white light, and to distinguish seven colours-viz., red, orange, yellow, green, blue, indigo, and violet; but it should be distinctly understood that in the spectrum there is no well-marked line of division between these seven so-called primary colours; the red passes insensibly into the orange, the orange into the yellow, and so on through the other colours in the order given above.
White from Coloured Light.-By passing the spectrum colours through a lens, or through another prism, in a particular manner, the seven colours can be recombined so as to form white light. It is not even necessary to use all the spectrum colours, as two or three will suffice if properly selected. Thus blue and yellow will when united form white light; as also red, green, and blue, and many other combinations, particulars of which will be found in special books on Colour, such as those of Professor Church and Mr. W. Benson. The consideration of this property of a few of the spectrum colours combining together to form white light led Young, and, later, Helmholtz, to consider that there are only three primary colours, red, green, and blue, from which all the other colours can be obtained ; thus, by combining red and green, yellow is produced; or by combining red and blue violet is the result.
Light from Coloured Bodies. When the light which is reflected from the surface of a coloured body like vermilion is passed through a prism, it is found to yield a spectrum; not, however, a complete one, such as is got from a ray of white light, but one more or less incomplete; thus, vermilion gives a spectrum containing some red, orange, and a little blue light; chrome yellow again gives a spectrum showing a few red, some yellow, and some green rays; in each case the eye distinguishes the effect due to the combined action of all these rays on the retina. No artificial colouring-matter is known which reflects rays of one colour only; in every case the rays of the dominant colour are mingled with those of other colours. The light from some bodies
is of a very complex character, while that from others is comparatively simple. It is this complexity in the composition of the light reflected that makes it so difficult to demonstrate the true laws and facts of colour with pigments or any artificial colouring-matters.
Cause of Colour in Coloured Bodies.—The actual reasons why bodies such as vermilion, magenta, or emerald green are coloured, it is almost impossible to investigate in the present state of knowledge, since the cause, whatever it may be, must be due to the molecular construction of the different compounds about which very little is known; still, we know something of some of the reasons why coloured bodies appear coloured. When light falls upon a substance, the light may be affected in one or two ways; it may be reflected, that is, it may be thrown back from the body; or it may be transmitted, that is, it may pass through, or, in some cases, be absorbed by the body on which it has fallen. As a rule, there is never either complete reflection or complete transmission of light, the most perfectly reflecting body allowing some rays to pass into it. It is by reflected light that we see bodies; when the reflection is complete, or as nearly so as is the case with mercury or a very highly polished plate of silver, the body is nearly invisible; it is only rendered visible because it does not reflect all the light which falls upon it in a regular manner; some is irregularly reflected and it is this light which enables us to see the body. Two kinds of reflection can, therefore, be distinguished-regular and irregular. Regular reflection is that where the light is thrown back in a straight line from the reflecting surface; if this is perfect, only the light that is reflected is seen, the reflector itself is invisible. Irregular reflection is that where the light is thrown back from the reflector in every direction; it is this light which makes the body visible, and it is due to the fact that no matter how apparently even the surface may appear to be, yet it is not even; it is sufficiently rough to cause the light which falls upon it to be irregularly reflected. Then bodies never reflect or absorb the whole of the light which falls upon them, some of it is absorbed; the most perfectly polished plate of silver (which is the most highly reflecting body known) does not reflect the whole of the light which falls upon it, while a piece of black cloth reflects only a little of the light that falls upon it. Upon the character of the reflected light thrown off from a body depends its colour, which is independent of the proportion of the light that falls upon and is reflected by the body. If all the rays of light falling upon it are reflected, then the body appears white; if all the light rays are absorbed, then the body
appears to be black. If, now, some of the spectral rays are absorbed and the rest reflected, then the body appears to be coloured, the colour depending upon the composition of the rays which are reflected; thus the rays from a red body, such as vermilion, are red, as are also those from Derby red and oxide of iron; similarly, the rays from a yellow body, such as chrome yellow or yellow ochre, are yellow, but it does not follow that the rays from all red bodies or from all yellow bodies are identical in composition. If the rays from, say, vermilion, oxide of iron, and crimson lake are passed through a prism, and the spectra of the coloured light which is reflected from each examined, they will be found to be different; that from the crimson lake will contain more blue rays than that from the vermilion, while that from the oxide of iron will contain more of the dark red and indigo rays than either of the others; and it is the same with the other classes of colours. There is no coloured body known which reflects what might be called a pure light, while the spectrum-colours are pure, as has been already stated. As stated above, it is this compound nature of the light which is reflected from coloured bodies that makes it extremely difficult to demonstrate the true laws of light and colour by the use of pigments.
In the same manner as the coloured light which is reflected from bodies is compound, so that which is transmitted is compound and, usually, similar to that which is reflected, but occasionally it is the complement to the reflected light. When it is the complement of that which is reflected, then the bodies which give rise to this phenomena are known as dichroic; in other cases both the reflected and transmitted rays are of the same general colour, although there is usually some difference in the actual tint of the two colours.
It is assumed that the coloured bodies have a selective action on the light which falls upon them, reflecting or transmitting, as the case may be, those coloured rays to which they owe their colour, while they absorb all the other rays. White bodies reflect all the rays which fall upon them, black bodies absorb all and are, in consequence, often nearly invisible. As to the character of the rays reflected from red, orange, yellow, green, or other coloured bodies, these will have been inferred from what has been said above.
Colour Theories.-Two theories of colour are in use to explain the coloured effects of light. The old theory, which is mostly due to Brewster, considers that there are three primary colours-viz., red, yellow, and blue; by the proper admixture of which in various proportions all the other colours can be