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. SECTION II. ON THE DISTINCTION BETWEEN SIMULTANEOUS, SUCCESSIVE, AND MIXED CONTRAST OF COLOURS, AND ON THE CONNEXION BETWEEN THE EXPERIMENTS MADE BY THE AUTHOR, AND THOSE PREVIOUSLY MADE BY OTHER OBSERVERS. DISTINCTION BETWEEN SIMULTANEOUS, SUCCESSIVE, AND MIXED CONTRAST OF COLOURS.