If the use of Maxwell disks had been known to Chevreul his deductions from his experiments would have been quite different in their details. For example, in accepting the proposition that there are three primaries, red, yellow and blue, which may be combined in pairs to make the secondaries, orange, green and violet, he states that owing to the impurities of the pigments the secondaries are not as pure as the primaries. Consequently he believes that this may account for many of the shortcomings which he was too observing to overlook; but notwithstanding such an error in theory this wonderful investigator made many practical experiments and established very valuable facts regarding color contrasts.
The term Simultaneous Contrast seems rather restricted for a title covering such a range of effects, and the author subdivides the subject into simultaneous contrasts, successive contrasts and mixed contrasts, which he defines as follows:—
Simultaneous Contrast.
"In the Simultaneous Contrast of Colors is included all the phenomena of modification which differently colored objects appear to undergo in their physical composition and in the height of tone of their respective colors, when seen simultaneously."
Successive Contrast.
"The Successive Contrast of Colors includes all the phenomena which are observed when the eyes, having looked at one or more colored objects for a certain length of time, perceive, upon turning them away, images of these objects having a color complementary to that which belongs to each of them."
Mixed Contrast.
"The distinction of Simultaneous and Successive Contrast renders it easy to comprehend a phenomenon which we may call the mixed contrast; because it results from the fact that the eye, having seen for a time a certain color, acquires an aptitude to see for another period the complementary of that color, and also a new color, presented to it by an exterior object; the sensation then perceived is that which results from this new color and the complementary of the first." These last two effects may be shown very clearly in simple experiments.
There are various phenomena which may be classed as successive contrasts sometimes called "after images." The phenomena which Chevreul groups under the term "Simultaneous Contrast of Colors" belong to a class of physio-psychological effects termed after images, and more definitely to the subdivision called negative images. The positive after images are not important in the consideration of the theories of color and therefore are not described here. The specific effect most directly involved in the subject of harmonies may be observed if the eyes are fixed upon a small disk of red paper on a white wall for a few seconds and then the paper is suddenly removed, as there will appear on the wall in place of the full red disk a faint tint of a blue green. This is called an after image, and is nearly or exactly a tint of the color complementary to red.
For making this experiment mount a circle of red paper, say three inches in diameter on a square white card, four or five inches across, and grasping the card by one corner hold it in front of a white wall or a sheet of white paper pinned on any support. Tell the observer to look intently at the red disk for a half minute, and then without giving any notice suddenly remove it and ask what color is seen in place of it. At the first trial the result may not be entirely successful, because the eyes of the observer may naturally follow the red spot when it is removed instead of remaining fixed in the original position, but a second trial will bring the expected result. To illustrate mixed contrast, fasten on the wall a piece of red tint No. 2 paper four or five inches square. This may be very conveniently done by using a bit of beeswax on each corner of the paper, which will not soil the wall. Then having the three-inch circle of standard red paper mounted on a white card somewhat larger than five inches square hold the card in front of the red tint on the wall and repeat the experiment as before. The effect now should be a three-inch disk of very light gray in the center of the pink square, which is a "mixed contrast" according to Chevreul. The reason is simple. The after-image or successive contrast of light blue-green is projected on the red tint and being complementary the resulting effect is a gray. If the red tint could be exactly graded to the complementary effect in the eye the resulting gray circle would be a true neutral gray. Another illustration of the same physical effect by which the complementary is induced may be shown by substituting for the tint of red a light tint of the blue-green paper retaining the full red disk as before. The same blue-green after image is now projected on to the light blue-green paper and hence a circle of more intense blue-green is produced. Thus it is seen that Chevreul's successive and mixed contrasts are both due to the same physiological effect, the only difference being in the ground on to which the after image is projected.