39. _Red,[1] the complementary of Green, placed by the side of Green, increases its intensity._ _Green, the complementary of Red, placed by the side of Red, increases its intensity._ [1] In the original French Edition there is evidently a confusion of terms in this and the two following articles (39, 40, 41, 42). The Red is there represented as placed in juxtaposition with _red_, and green with _green_, and so on with the other instances; orange and blue; orange-yellow and indigo; greenish-yellow and violet; instead of being made by contrast with their respective complementaries to intensify each other. Such is the theoretical result, the experimental result entirely agrees with it. When we place a green, inclining more to yellow than to blue, side by side with, 1st, a slightly orange-red, 2nd, a slightly crimson-red, and 3rd, an intermediate red, and repeat our observations several times on each of these assemblages of colour, we shall observe different results; that is to say, in one case the red will appear more orange and the green yellower, and in another the red will appear more violet and the green bluer. We shall find also that the change may be attributed as much to a difference in the intensity of the light upon the colours as to fatigue of the eye. When we place a green, inclining rather to blue than to yellow, side by side with, 1st, a slightly orange-red, 2nd, a slightly crimson-red, and 3rd, an intermediate red, the results are the same as with the first green, but with this difference,—that in the assemblage of bluish-green and of slightly crimson-red, observed several times, the green and the red appear almost constantly yellower than they are separately. A result very easily understood. _Orange and Blue._ 40. _Blue, the complementary of Orange, placed by the side of Orange, increases its intensity._ _Orange, the complementary of Blue, placed by the side of Blue, increases its intensity._ Upon repeating these observations with a deep blue and an orange which is not too red, the two colours appear commonly to become redder. _Orange-Yellow and Indigo._ 41. _Orange-yellow, the complementary of Indigo, placed by the side of Indigo, increases its intensity._ _Indigo, the complementary of Orange-yellow, placed by the side of Orange-yellow, increases its intensity._ [Illustration: PLATE III.] _Greenish-Yellow and Violet._ 42. _Greenish-yellow, the complementary of Violet, placed by the side of Violet, increases its intensity._ _Violet, the complementary of Greenish-yellow, placed by the side of Greenish-yellow, increases its intensity._ The result of observation is almost always in conformity with this law.