75. Hence we see that the colours which painters term simple—red, yellow, and blue—pass insensibly by juxtaposition into the condition of compound colours, since the same red is purple or orange; the same yellow is orange or green; and the same blue is green or violet. CHAPTER VIII. _On the Application of the Law of Contrast to the Hypothesis that Red, Yellow, and Blue are the only Primary Colours; and that Orange, Green, Indigo, and Violet are Secondary or Composite Colours._ 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. [Illustration: PLATE VI.] 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.