By looking upon the blue figure for some minutes and then suddenly shifting the vision to the dot on the right, there will be seen a pure orange tint; and if we look upon the orange figure for some minutes and then suddenly shift the vision to the dot on the left, we will see a pure blue tint; and, finally, if we shift the vision at regular intervals of a second each, from one to the other of the two colors named, for a half minute, and then suddenly look at the dot between the two, we will see a clear white figure surrounded by gray. The white is produced by an equal mixture of the complements of blue and orange upon the retina of the eye; the fact that this experiment produces white, proves that the two colors are complementary.
Another very good method of finding the complement of any color is illustrated by the following experiment — see the orange figure on page 26. Take a slip of white paper in one hand, and while looking very intently at the dot upon the orange figure, suddenly move the slip up to the dot, hold it there a few seconds, and then withdraw it for a quarter minute; repeat these movements three or four times, but while doing so, keep looking intently at the dot. Bach time the slip is moved up to the dot, that part of it which covers the orange figure will show a most beautiful blue tint. This experiment also shows that blue is complementary to orange.
We will give another experiment as additional proof that orange (which is an equal mixture of red and yellow) is the complement of blue, by mixing the spectral complements of red and yellow upon the retina of the eye; see illustration on page 28.
After shifting the eyes at regular intervals of a second each from one to the other of the two colors named, for a half minute, then suddenly looking at the black dot between the two, we will see a pure tint of blue, which is produced by an equal mixture upon the retina, of a sea-green tint (the complement of red) with a violet
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