B. When the body is polished, like the surface of a mirror, for instance, one portion of the light is reflected irregularly, as in the preceding case; at the same time another portion is regularly or specularly reflected, giving to the mirror the property of presenting to the eye, properly situated, the image of the body which sends its light to the reflector;-one consequence of this distinction is, that if we observe two plane surfaces reflecting white light differing from each other only in polish, it results that in those positions where the non-polished surface is visible, all its parts will be equally, or nearly equally, illuminated; while the eye, when it is in a position to receive only that which is reflected irregularly, will receive but little light from the polished surface; on the contrary, it will receive much more when it is in a position to receive light regularly reflected. (4.) If the light which falls upon a body is completely absorbed by it, as it would be in falling into a perfectly obscure cavity, then the body appears to us black, and it becomes visible only because it is contiguous to surfaces which transmit or reflect light. Among black substances, we know of none that are perfectly so, and it is because they reflect a small quantity of white light, that we conclude they have relief, like other material objects. Moreover, what proves this reflection of white light is, that the blackest bodies, when polished, reflect the images of illuminated objects placed before them. (5.) When light is reflected by an opaque coloured body, there is always A. reflection of white light,-and the latter is due to the fact that the body absorbs or extinguishes within its substance a certain number of coloured rays, and reflects others. It is evident that the coloured rays reflected are of a different colour from those absorbed; and also that, if they were combined with those absorbed, they would reproduce white light. We shall return to this subject in (6.). Further, it is also evident that opaque coloured bodies, when unpolished, reflect irregularly both white and coloured light, by which we are enabled to see their colours; and that those which are polished reflect only one portion of these two lights irregularly, while they reflect another portion in a regular manner. (6.) Let us now return to the relation which exists between the coloured light absorbed, and the coloured light reflected, by an opaque body, which makes it appear to us of the colour peculiar to this light. It is evident, from the manner in which we have considered the physical composition of solar light (2.), that if we reunited the total quantity of the coloured light absorbed by a coloured body, to the total quantity of coloured light reflected by it, we should reproduce white light: for it is this relation that two differently coloured lights, taken in given proportions, have of reproducing white light, that we express by the terms Coloured lights complementary to each other, or It is in this sense we say, That Red is complementary to Green, and vice versá; That Indigo is complementary to Orange-Yellow, and (7.) It must not be supposed that a red or yellow body reflects only red and yellow rays besides white light; they each reflect all kinds of coloured rays: only those rays which lead us to judge the bodies to be red or yellow, being more numerous than the other rays reflected, produce a greater effect. Nevertheless, those other rays have a certain influence in modifying the action of the red or yellow rays upon the organ of sight; and this will explain the innumerable varieties of hue which may be remarked among different red and yellow substances. It is also difficult not to admit that, among the differently coloured rays reflected by bodies, there is a certain number of them which, being complementary to each other, go to re-form white light upon reaching the eye. SECTION I. ON THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS, AND ITS DEMONSTRATION BY EXPERIMENT. CHAPTER I.-METHOD OF OBSERVING THE PHENOMENA OF SIMUL- Definition of Simultaneous Contrast (8). Experimental Experimental Demonstration of Contrast of Colour (13). CHAPTER II. THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS, AND THE FORMULA WHICH REPRESENTS IT (16). CHAPTER III. THE LAW OF SIMULTANEOUS CONTRAST DEMONSTRATED BY THE JUXTAPOSITION OF A GIVEN NUMBER OF COLOURED SUBSTANCES (20). CHAPTER IV. ON THE JUXTAPOSITION OF COLOURED SUBSTANCES WITH WHITE (44). CHAPTER CHAPTER V.-ON THE JUXTAPOSITION OF COLOURED SUBSTANCES WITH VI.-ON THE JUXTAPOSITION OF COLOURED SUBSTANCES WITH CHAPTER VII.-THE CHEMICAL COMPOSITION OF COLOURED SUBSTANCES CHAPTER VIII.-ON THE JUXTAPOSITION OF COLOURED SUBSTANCES BELONGING TO THE COLOURS OF THE SAME GROUP OF COLOURED RAYS (72). CHAPTER IX.-ON THE APPLICATION OF THE LAW OF CONTRAST TO THE CHAPTER I. METHOD OF OBSERVING THE PHENOMENA OF SIMULTANEOUS CONTRAST OF COLOURS. Definition of Simultaneous Contrast. (8.) If we look simultaneously upon two stripes of different tones of the same colour, or upon two stripes of the same tone of different colours placed side by side, if the stripes are not too wide, the eye perceives certain modifications which in the first place influence the intensity of colour, and in the second, the optical composition of the two juxtaposed colours respectively. Now as these modifications make the stripes appear dif ferent from what they really are, I give to them the name of simultaneous contrast of colours; and I call contrast of tone the modification in intensity of colour, and contrast of colour that which affects the optical composition of each juxtaposed colour. The following is a very simple method of convincing ourselves of the twofold phenomena of simultaneous contrast of colours. Experimental Demonstration of Contrast of Tone. (9.) Take two portions, o, o', fig. 1., of a sheet of unglazed paper of about twenty inches square, coloured light grey by means of a mixture of whiting and lamp-black; fix them in any manner upon a piece of brown-holland, placed opposite to a window, o being distant from o' about twelve inches. Then take two portions p and p' of another sheet of unglazed paper which differs from the first by being deeper, but still a grey composed of the same black and white. Fix p next to o, and p' at about twelves inches from p. If we now look at these four half-sheets, o, o', p, p′ for some seconds, we shall see that the piece o contiguous to p will be lighter than o', while p will, on the contrary, be deeper than p'. (10.) It is easy to prove that the modification is not equally intense over the whole extent of the surfaces of o and p, but that it becomes gradually weaker from the line of contact. It is sufficient to place a piece of card, fig. 2., cut out in the centre, upon op, in such manner that o and p each present three grey stripes; as is shown in fig. 3., the stripes 1, 1, are more modified than the stripes 2, 2; and these latter are more modified than the stripes 3, 3. For this modification to take place, it is not absolutely necessary for o and p to be contiguous; for if we cover the stripes 1, 1, we shall see the stripes 2, 2, 3, 3 modified. (11.) The following experiment, the natural sequence of the two preceding (9. and 10.), is very suitable for demonstrating the full extent of contrast of tone. Divide a piece of cardboard into ten stripes, each of about a quarter of an inch in width, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and cover it with a uniform wash of Indian ink, fig. 3. (bis.) When it is dry, spread a second wash over all the stripes except the first. When this second wash is dry, spread a third over all the stripes except 1 and 2; and proceed thus to cover all the stripes with a flat tint, each one becoming darker and darker, as it recedes from the first (1.). If we take ten stripes of paper of the same grey, but each of a different tone, and glue them upon a card so as to observe the preceding gradation, it will serve the same purpose. On now looking at the card, we shall perceive that instead of exhibiting flat tints, each stripe appears of a tone gradually shaded from the edge a a to the edge b b. In the band 1, the contrast is produced simply by the contiguity of the edge bb with the edge a a of the stripe 2; in the stripe 10 it is simply by the contact of the edge a a with the edge b b of the stripe 9. But in each of the intermediate stripes 2, 3, 4, 5, 6, 7, 8, and 9, the contrast is produced by a double cause: one, the contiguity of the edge a a with the edge b b of the stripe which precedes it; the other by the contiguity of the edge bb with the edge a a of the darker stripe which follows it. The first cause tends to raise the tone of the half of the intermediate stripe, while the second cause tends to lower the tone of the other half of this same stripe. |