COLOUR, COLOURS, PAINTS, AND VARNISHES. 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. Is 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 upon the screen, s, s, at b; but the prism, bending it out of this course, refracts it as shown at c; it then passes through the prism in the new direction until it Fig. 1. 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 ƒ in 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 |