20. Let us now apply the above formula to the seventeen observations of Chapter I., and we shall see that the modifications of contiguous colours are precisely those which would result from the addition to each of them of the complementary of the contiguous colour (18). The rank these colours occupy in Chapter I. may be readily noted, as I have attached to each the number applied to it in that chapter. And for the complementary of each colour see (6). (See Plate II.)

38. It is evident that, all other things being equal, the modifications of contiguous colours will be so much the more marked, as the complementary colour C or Cʹ, which is added to each of them, differs more from them; for the complementary Cʹ which is added to the colour O, is identical with it, as the complementary C is identical with the colour P to which it is added; and the modifications of O and P will simply augment the intensity of their colours. But are there yet any two bodies known which present to the observer two pure colours perfectly complementary to each other? Certainly not; all those which are coloured by reflection reflect ( 7), besides white light, a great number of rays of various colours. We cannot instance a red body and a green body, or an orange and a blue, or an orange-yellow and an indigo, or a greenish-yellow and a violet, which reflect pure or mixed colours, absolutely complementary to each other. So that the juxtaposition of these colours produces only a simple augmentation of their intensity. Hence, although it is less easy to verify the law of contrast with respect to red and green, or orange and blue, &c., than with respect to those which are the object of the seventeen experiments just described (15), yet, upon applying it to the former, it will be seen that their colours acquire a most remarkable brilliancy, strength, and purity. This result, perfectly conformable to the law, may be easily understood: for example, an orange-coloured object reflects blue rays, as a blue object reflects orange rays (7). Hence, when a blue stripe is put in contact with an orange stripe, although it is admitted that the first appears to the eye to receive blue from the orange of the second, as this appears to receive orange from the blue of the blue stripe; or what is the same thing, that the blue stripe appears to destroy the effect of the blue rays of the second stripe, as these appear to destroy the effect of the orange rays of the blue stripe—it is evident that the two colours so contrasted must purify each other, and become more intense. But the blue may incline to green or violet, and the orange to yellow or red; that is to say, the modification may not only affect the intensity of the colour, but also its physical composition. However, if this latter effect take place, it is always much more feeble than the first. Besides, if you look several times at the same coloured stripes, you will see that the blue which at first had appeared to you greenish will afterwards appear inclining to violet; and that the orange, which had appeared at first yellowish, will incline to red; so that the phenomena of modification, as it affects the physical composition of colour, will not have the constancy of those which are the subject of the preceding seventeen observations (15). I now proceed to state the observations I have made on colours which are most nearly complementary to each other.

Red and Green.

39. Red,[1] the complementary of Green, placed by
the side of Green, increases its intensity.

Green, the complementary of Red, placed by the side
of Red, increases its intensity.

Such is the theoretical result, the experimental result entirely agrees with it. When we place a green, inclining more to yellow than to blue, side by side with, 1st, a slightly orange-red, 2nd, a slightly crimson-red, and 3rd, an intermediate red, and repeat our observations several times on each of these assemblages of colour, we shall observe different results; that is to say, in one case the red will appear more orange and the green yellower, and in another the red will appear more violet and the green bluer. We shall find also that the change may be attributed as much to a difference in the intensity of the light upon the colours as to fatigue of the eye.

When we place a green, inclining rather to blue than to yellow, side by side with, 1st, a slightly orange-red, 2nd, a slightly crimson-red, and 3rd, an intermediate red, the results are the same as with the first green, but with this difference,—that in the assemblage of bluish-green and of slightly crimson-red, observed several times, the green and the red appear almost constantly yellower than they are separately. A result very easily understood.

Orange and Blue.

40. Blue, the complementary of Orange, placed by the
side of Orange, increases its intensity.

Orange, the complementary of Blue, placed by the
side of Blue, increases its intensity.

Upon repeating these observations with a deep blue and an orange which is not too red, the two colours appear commonly to become redder.

Orange-Yellow and Indigo.

41. Orange-yellow, the complementary of Indigo,
placed by the side of Indigo, increases its intensity.

Indigo, the complementary of Orange-yellow, placed
by the side of Orange-yellow, increases its intensity.

Greenish-Yellow and Violet.

42. Greenish-yellow, the complementary of Violet,
placed by the side of Violet, increases its intensity.

Violet, the complementary of Greenish-yellow, placed
by the side of Greenish-yellow, increases its intensity.

The result of observation is almost always in conformity with this law.

43. According to the law of the simultaneous contrast of colours, and the insensible gradation of modification, beginning at the contiguous edges of the colours in juxtaposition (11), we may show, by means of coloured circular spaces, the modifications which the principal colours induce in those which are contiguous to them.

Place wafers, circular pieces of paper, or any other convenient material about an inch and a half in diameter, coloured red, green, orange, blue, greenish-yellow, violet, indigo, and orange-yellow, each separately upon a sheet of white paper; then tint the white paper around the circle with its complementary colour, gradually softening it off from the coloured circle, when it will be found that

These figures are designed to exhibit the effects of contrast to those persons who, not having studied physical laws, are, notwithstanding, desirous of understanding these effects.