608. Contrast, which augments the difference existing between two tones of the same colour, affords the means of judging with greater certainty than could otherwise be done, whether the numerous tones of a scale are at the same distance from each other. Thus, if the tone 2, placed between 3 and 4, appears equal to the tone 1, it follows, if the tones are equidistant, that 3 placed between 4 and 5 will appear equal to 2; that 4 put between 5 and 6 will appear equal to 3, and so with the others. If the tones are too near together to yield this result, we must move them successively, not one degree, but two or three. This means of judging of the equality of distance that separates the tones of the same scale, is based upon the fact, that _it is easier to establish an equality than to estimate a difference between patterns of the same colour._ OF THE BINARY ASSOCIATIONS OF COLOURS, CRITICALLY CONSIDERED. 609. In order to sum up in few words the generalities which must serve as the bases of our judgment, not on one colour compared with another of the same sort, but on the associations of two colours, which any object whatever presents to our eyes, we must consider combinations both of complementary and non-complementary colours. 1st. _Combination of Complementary Colours._ _This is the only association where the colours mutually improve, strengthen, and purify each other without leaving their respective scales._ This case is so advantageous to the associated colours, that the combination is also satisfactory when the colours are not absolutely complementary; also when they are made dull with grey. I therefore prescribe the complementary association when we have recourse to the harmonies of contrast in painting, in tapestry, in the arrangement of coloured glass windows, in the assortment of hangings with their borders, in that of stuffs for furniture and clothing; and, lastly, in the arrangement of flowers in our gardens. 2nd. _Combination of Non-complementary Colours._ _The product of this combination is distinguished from the preceding in this,—the complementary of the juxtaposed colours, differing from the other colour to which it is added, there must necessarily be a modification of hue in the two colours, as well as a modification of tone, if they are not taken at the same height._ 610. Juxtaposed non-complementary colours can _certainly_ give rise to three different results:— 1. They may improve each other. 2. The one may be improved, the other may lose some of its beauty. 3. They may injure each other. The greater the difference between the colours, the more favourable will the juxtaposition be to their mutual contrast; consequently, the more analogy they will have, and the more chances there are that the juxtaposition may injure their beauty. 1. _Two non-complementary colours improve each other by juxtaposition._