76. The experiments to which I have just applied the principle of the modification which colours undergo by juxtaposition, and the explanation consequent upon the manner in which white light has been considered, are also clearly explained in the language of painters and dyers, who admit of only three primary colours—red, yellow, and blue. As there are persons who, while they hold this opinion, desire, notwithstanding, to give a reason for the phenomena resulting from the juxtaposition of colours, I will explain them in accordance with this language; and, for greater clearness, I make five groups of juxtaposed colours, beginning with those which include the observations to which the preceding law is most easily applied. I shall presume that orange is composed of red and yellow, green of yellow and blue, indigo and violet of red and blue.

First Group.—Two Secondary Colours having a Simple Colour as a Common Element.

It is very easy to verify the law when we look at two colours which form part of the group: we see that by their reciprocal influence they lose more or less of the colour which is common to them. It is evident that they become more unlike each other in proportion to this loss.

1. Orange and Green.—These colours having the element yellow in common, lose it by juxtaposition. The orange appears redder, and the green bluer.

2. Orange and Indigo.—These colours having the element red in common, lose it by juxtaposition. The orange appears yellower, and the indigo greener.

3. Orange and Violet.—Like the preceding.

4. Green and Indigo.—These colours having the element blue in common, lose it by juxtaposition. The orange appears yellower, and the indigo bluer.

5. Green and Violet.—Like the preceding.

Second Group.—A Compound Colour, and a Simple Colour, which is found in the Compound.

Plate 6.

1. Orange and Red.—The orange loses red, and appears yellower; the red becomes bluer, to differ as much as possible from the orange.

2. Violet and Red.—The violet loses red, and appears bluer; the red becomes yellower, to differ as much as possible from the violet.

3. Indigo and Red.—Like the preceding.

4. Orange and Yellow.—The orange loses yellow, and appears redder; the yellow becomes bluer, to differ as much as possible from the orange.

5. Green and Yellow.—The green loses yellow, and appears bluer; the yellow becomes redder, to differ more from the green.

6. Green and Blue.—The green loses blue, and appears yellower; the blue becomes redder, to differ as much as possible from the green.

7 Violet and Blue.—The violet loses blue, and appears redder; the blue must become yellower to differ as much as possible from the violet.

8. Indigo and Blue.—Like the two preceding.

Third Group.—Two Simple Colours. (See  6,  7.)

1. Red and Yellow.—Red in losing yellow appears bluer; and the yellow losing red appears bluer; or, in other words, the red inclines to purple, and the yellow to green.

2. Red and Blue.—The red in losing blue appears yellower; and the blue losing red appears yellower; or, in other words, the red inclines to orange, and the blue to green.

3. Yellow and Blue.—The yellow in losing blue appears redder; and the blue losing yellow appears more violet; or, in other words, the yellow inclines to orange, and the blue to violet.

Fourth Group.—Two Compound Colours consisting of the same Simple Colours.

Indigo and Violet.—As indigo differs from violet only in containing more blue in proportion to red, it follows that the difference will be greatest when the indigo loses red and inclines to greenish blue, while the violet, by gaining red, inclines toward that colour. It is clear that, if the violet lost red, or if the indigo acquired it, the two colours would approximate; but, as they differ from each other, the first effect ensues.

We may further explain the preceding phenomena by considering indigo relatively to violet, as blue; then it will lose its blue, which is common to both colours, and incline to green; while the violet, also losing its blue, will appear redder.

Fifth Group.—A Compound Colour, and a Simple Colour, which is not found in the Compound.

1. Orange and Blue. 2. Green and Orange. 3. Violet and Greenish Yellow.

Upon the hypothesis that orange, green, and violet are compound colours, and that red, blue, and yellow are simple, it follows that in opposing them in the order in which they are reciprocally complementary, and supposing also that the colours so juxtaposed are entirely free from any foreign colour, there appears no reason for the compound colour losing one of its elements rather than another, or for the simple colour being unlike one of the elementary colours rather than another. For instance, in the juxtaposition of green and red, we see no reason why green should pass into blue rather than into yellow, or why the red should incline to blue rather than to yellow.