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of color is a purely physical stimulus capable of measurement and reproduction as accurately as may be required. The effect on the observer of this stimulus, and so the color he sees, depends not only upon this stimulus but also upon the way in which the eye transmits it to the brain and even upon the brain's condition. That the interpretation of the physical stimulus is effected to a great degree by previous and existing conditions is a matter of common observation. . Ordinarily the moon looks pale yellow. If, however, we look steadily at a red fire for a few minutes and then glance at the moon, it will seem quite green. This is but an example of a well-known characteristic of vision.
To one emerging suddenly from a dark room ordinary daylight is dazzling to such a degree as to deprive one temporarily of the entire use of his eyes. After a few minutes the eye "adapts" itself to the new condition — that is, becomes less sensitive. On the contrary, when entering a darkened room from daylight many objects can at first not be seen which after a time become easily visible. Thus we see that the apparent brightness of an object depends not only upon the energy coming from it, but on the ''adaptation level" of the eye determined by the light which has been falling upon it.
The Young-Helmholtz theory of color vision involved the supposition in the retina of three sets of cells, each responding to light of all the visible waves lengths, but one set responding most strongly to approximately red light, one to green and one to blue. This means that when the first stimulates the brain strongly and the other two but weakly, we get a sensation we call "red." If now for some reason, such as prolonged observation of "red" light, the "red" set of cells becomes less sensitive than the others, a light that under normal conditions would cause an equal stimulus to be sent to the brain from each set of cells, and would so produce the sensation of white, will now cause a stimulus that is weaker in red than in blue or green and we get a sensation of blue-green. Nor, of course, does this weakening of the red, effect the judgment of neutral tints alone, but every object, whatever its color, will be weaker in red than it would be to the normally adapted eye and will accordingly be bluer and greener. In fact, if we look at even a monochromatic light for a long time those cells most strongly affected will become less sensitive than the others and accordingly the color becomes more and more neutral. Of course the point of total neutrality is never reached. The color is simply not as saturated.
This adaptability of the eye has some interesting consequences. If a bright object be viewed for an instant and then the eyes be closed, a bright "after-image" of the object will be seen against the dark background of the red. The brightness of this "positive" image rapidly decays until it is even darker (i. e., "negative" afterimage) than the field, if this be slightly illuminated by light falling on the outside of the eyelid. If the original object was white, the positive images in their decay pass through blue-green, indigo, violetpink, dark orange, etc. If the object was not white, the positive
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