NOTE.—_The chemical nature of coloured substances has no influence upon the phenomena of simultaneous contrast._ 71. The chemical nature of coloured bodies in juxtaposition has no influence upon the modifications of their colours. Whatever may happen to be the chemical composition of the coloured bodies, provided they be identical to the sight, they yield the same results. I need only cite the following examples:—Indigo, Prussian blue, cobalt, ultramarine, as nearly alike as possible, gave the same sort of modification; orange prepared from red lead, annotto, or a mixture of woad and madder, caused the same modification of the colours to which they were adjacent. CHAPTER VII. _On the Juxtaposition of Coloured Bodies belonging to the Colours of the same Group of Coloured Rays._ 72. Whenever there is a great difference between two contiguous colours, the difference becomes still more appreciable by putting the same colour successively in juxtaposition with the various colours belonging to the same group. For example, _orange and red_. Orange being placed beside scarlet-red, pure red and crimson-red, it will be seen that the red acquires a purple and the orange a yellow tint. Violet being placed beside scarlet-red, pure red and crimson-red, gives analogous results: the violet always appears bluer, and the red yellower or less purple. 73. These observations explain why we obtain results in accordance with the formula, even when such coloured substances are used as are far from exhibiting pure colours, namely, stained papers or stuffs. 74. The juxtaposition of coloured stripes is a means of demonstrating the difficulty of determining the types of pure colours by common pigments; at least, if we do not take into consideration the law of simultaneous contrast. For instance— 1. Place red in contact with orange-red; the first will appear purple and the second yellower, as above; but if the first red be placed beside purplish-red, the latter will appear bluish, and the former more yellow or orange; so that the same red will be purple in one case and orange in the other 2. Place yellow beside orange-yellow, the former will appear greenish and the latter redder; but if you put the first yellow beside a greenish-yellow, the last will appear greener, and the yellow more orange; so that the same yellow will incline to green in one case, and to orange in the other. 3. Place blue beside greenish-blue, the former will incline to violet, and the second will appear yellower. Put the same blue beside a violet-blue, the former will incline to green, and the second will appear redder; so that the same blue will be violet in one case and greenish in the other.