red. But all these mixtures, when observed through a prism, are resolved again into their elements, they being separated by refraction; whereas the colours produced by homogeneous rays are not changed by the prism, and this forms their essential characteristic. 168. We may also imitate the simple colours more or less perfectly, by mixing together powders of different colours. This is the way in which painters compose their colours; but these can never equal, in vividness, the colours obtained by the analysis of light, and the prism immediately separates them, and nakes known their composition. upon We may even imitate white by such mixtures, but yet in a ery imperfect manner, for the coloured powders, made use of or this purpose, always absorb a great part of the light which lls them; and it is because they thus absorb a part, that ey appear of the colour supplementary to this part. In genal, therefore, we can compose, with such mixtures, only a grayh and dirty white, such as results from the mixture of a powr perfectly white with a black powder. But if we increase e intensity of the light by which they are illuminated, we are abled to add to their lustre and to render it equal to that of the itest bodies, as paper, for example, but paper faintly illumited and placed in an unfavourable light. 169. These mixtures, varied continually, would present an inite variety of shades, which make no part of the simple colrs of the spectrum; but there is no one of these shades which ght not be produced by a proper mixture of simple colours, h the advantage of a far greater lustre. By considering the ferent portions of simple light which compose the spectrum, so many forces acting upon the organ, and measuring the uence of these forces, in the various mixtures which may be med of the prismatic colours, collected at the focus of a lens, to the process indicated above, Newton arrived at an rule, by means of which we are enabled to calculate nd the kind of colour that will result from a mixture of ays, when the kinds and proportions are given. the centre C, and a radius equal to unity, describe a Fig. 112. and divide its circumference into seven parts proportional e numbers, 18, 10, 1, 10, T; so that when these parts it by uniting the different colours obtained from different prisms; and this explains the progressive decomposition which the same beam, if it be of sensible magnitude, experiences in its different parts, according to its distance from the prism where it has been refracted.* Compound Colours produced by the Mixture of Simple ones. 167. By the preceding experiments we have proved, in the most rigorous manner, that each ray carries in itself its colorific. property, which, like its refrangibility, cannot be destroyed or changed. But what is very remarkable, this property is not peculiar to them; for we can compose artificial mixtures of colours, which produce on our organs the same sensation. This may be done in several ways; one of the most simple, consists in employing the apparatus represented in figure 109, where the rays, after being dispersed by a prism, are concentrated by a lens in one uncoloured focus. Suppose a piece of black pasteFig. 111. board TT, placed before the lens. In the middle of this pasteboard, let several rectangular openings VV, II, be made, and let slides be adjusted in such a manner as to admit of their being covered or uncovered at pleasure. If the pasteboard be so placed as to cause the spectrum to fall upon these openings, we can allow such rays as we choose to pass, and in such propor-tions as we choose; and these, uniting at the focus of the lens, will paint there, on the white screen, the particular colour which results from their mixture. We find in this way, that each colour may be imitated by the mixture of the two colours which border upon it. Thus the mixture of orange and greenish yellow, produces yellow; the mixture of greenish yellow and bluish green, produces deep green; which may be imitated also, but not so well, by blue and yellow. Blue is imitated by the mixture of bluish green and indigo; and, what is rather remarkable, violet itself may be imitated by the mixture of blue and * For further particulars on this subject see Biot's Traité de Physique. red. But all these mixtures, when observed through a prism, are resolved again into their elements, they being separated by refraction; whereas the colours produced by homogeneous rays are not changed by the prism, and this forms their essential characteristic. 168. We may also imitate the simple colours more or less perfectly, by mixing together powders of different colours. This is the way in which painters compose their colours; but these can never equal, in vividness, the colours obtained by the analysis of light, and the prism immediately separates them, and makes known their composition. We may even imitate white by such mixtures, but yet in a very imperfect manner, for the coloured powders, made use of for this purpose, always absorb a great part of the light which falls upon them; and it is because they thus absorb a part, that they appear of the colour supplementary to this part. In general, therefore, we can compose, with such mixtures, only a grayish and dirty white, such as results from the mixture of a powder perfectly white with a black powder. But if we increase the intensity of the light by which they are illuminated, we are enabled to add to their lustre and to render it equal to that of the whitest bodies, as paper, for example, but paper faintly illuminated and placed in an unfavourable light. 169. These mixtures, varied continually, would present an infinite variety of shades, which make no part of the simple colours of the spectrum; but there is no one of these shades which might not be produced by a proper mixture of simple colours, with the advantage of a far greater lustre. By considering the different portions of simple light which compose the spectrum, as so many forces acting upon the organ, and measuring the influence of these forces, in the various mixtures which may be formed of the prismatic colours, collected at the focus of a lens, according to the process indicated above, Newton arrived at an empirical rule, by means of which we are enabled to calculate beforehand the kind of colour that will result from a mixture of simple rays, when the kinds and proportions are given. With the centre C, and a radius equal to unity, describe a Fig. 112. circle, and divide its circumference into seven parts proportional to the numbers, to, to, t, To, T, ; so that when these parts Let us now consider these different arcs in the order in which they follow each other, as representing the seven principal colours of simple light, which compose the spectrum; so that the entire circumference will represent the whole series of shades through which this light passes, from the first rays of red to the last of violet. Then, having determined the centres of gravity r, o, y, g, b, i, v, of all these successive arcs, suppose in each of them a weight proportional to the corresponding arc; and conceive these weights to be so many forces tending to draw to themselves the centre C, and the eye supposed to be placed there. According to this supposition, it is evident that the eye will remain at rest, being placed at the centre of gravity of all the weights; and this rest will correspond to the perfect white produced by the simultaneous sensation of all the shades of simple light, when they are mixed together according to the proportions in which they exist naturally in the spectrum. But suppose these proportions changed, as they always are in a coloured mixture which differs from white; then it will be necessary to place in each partial centre of gravity, not the total weight of the corresponding arc, but the half or third, or generally the n part of this weight, according as the given mixture contains the half, the third, or generally the nth part of all the light which composes this colour in the spectrum. This being done, if we find the common centre of gravity of all the partial *This division requires only the simple rule of fellowship. As the sum of all the fractions is to 360°, so is one of the fractions to the arc which corresponds to it. The sum of the fractions is +&+ % or 7. If, for example, we would find the arc cor 79 120 weights, it will, in general, no longer coincide with the centre of the circle. While it does coincide, the colour of the mixture will still be white; but when, on the contrary, it approaches considerably one of the partial centres, the mixture will present, as the predominant colour, that which belongs to this centre. Finally, wherever it falls, as in G', for example, we have only to draw from the centre to this point the line CG'; then the direction of this line will indicate the predominant colour of the mixture, and its length, or the distance from the point Gʻ to the centre C, will indicate its vividness. If, for example, CG' is situated exactly midway between CY and CG, the compound colour will be the most perfect yellow; but if CG' approaches more to CG or CY, this yellow will incline more to the orange or the green. On the first supposition, if the point G' falls very near the circumference, the colour will be strong and vivid in the highest degree; if it falls half way between the circumference and the centre, the intensity of the colour will be diminished one half, and it will be that of a mixture of the liveliest yellow with an equal quantity of white. In general, if we represent the distance CG by A, the radius of the circle being 1, 1 — A will express very nearly the proportion of white which it will be necessary to employ, in order to imitate to the eye the colour in question; this white being mixed with the proportion ▲, of the single colour towards which CG is directed. In this manner, then, the nature and intensity of the colour are ascertained. It is proper, however, to remark, that if the point G' falls upon the line CR, or very near this line, the red and violet being then the principal elements of the compound colour, it will no longer correspond to any of the prismatic colours, but will be a purple, inclining to the red or violet, according as the point G' varies from the line CR, towards the one or the other of these colours; and in these two cases, the purple or mixed violet, will be more dazzling and brilliant than the simple violet. In general, we observe the greatest analogy between the effects of the violet and the red; so much so that we can form with blue and red, mixtures which produce on the eye the sensation of a fine violet; and it is probably this kind of return of tints upon themselves, perfectly analogous to the consonance of octaves, which led Newton to compare, as he has frequently done, the impressions produced by the different colours to those produced by the musical intervals. |