152. The complementary of a colour contiguous to another being once known, it is easy, according to the principles of combination, to determine the modification that the second must receive from the first; since this modification is the result of the mixture of the complementary with the contiguous colour. In fact, if there is no difficulty when the result is that of the non-complementary mixture with a simple colour, red, yellow, and blue, with a binary colour, orange, green, violet (using the language of painters, 76), there is no greater difficulty when the result is that of the mixture of two binary colours. For, the complementary being much less intense than the colour with which it is mixed, the result will be found by subtracting from the last binary colour the portion of its simple colour, which with the complementary forms white, or in other words, neutralises it. 153. _Examples._—1. Orange being added as a complementary to green, neutralises a portion of its blue, and consequently makes it appear less blue or more yellow. 2. Orange being added as complementary to violet, neutralises a portion of its blue, and consequently makes it appear less blue or more red. 3. Green being added as complementary to violet, neutralises a portion of its red, and consequently makes it appear less red or more blue. 154. These three examples are easily explained by subtracting from the binary colour a portion of its simple colour which is identical with that contiguous to it. Thus:— 1. Blue subtracted from Green, makes it appear more Yellow. 2. Blue ” ” Violet ” ” Red. 3. Red ” ” Violet ” ” Blue. 155. To put the diagram into practice we must adopt invariable types of colour, either in the solar spectrum, or in polarized light, or coloured rings, or colours developed in a constant manner, by any process whatever; then imitate them with the utmost fidelity, by means of colouring matters which should be applied to the circular plan of our chromatic diagram. These types must be sufficiently numerous to reproduce the principal colours, in order that a practised eye may without difficulty insert all the tones of the same scale and all the hues of which types are wanting. In fact the diagram thus established, should present terms so near that the various colours of the natural bodies might be referred to them. 156. 1. _That it represents all the Modifications resulting from the Mixture of Colours._—Thus any colour lowered by white and deepened with black may, retaining its place in the scale, give rise to an infinite variety of tones; infinite, inasmuch as an unlimited number may be inserted from tone 1 to tone 20. 157. 2. Pure colours, by their mutual modifications, may produce an infinite variety of hues; for between two adjacent hues we may insert as many as we desire.