In the colors which we see in nature influences of a similar kind are constantly at play, for every object, besides being illuminated by the prevailing light, has thrown on to it colors which are reflected from near by objects. In analyzing these influences there are, as Rood has pointed out, at least three factors that must be borne in mind. These are: (1) the natural or "local color" of the object, the cause for which we have already explained; (2) the colored light which is reflected unaltered from its surface, just as we have seen white light to be; (3) the portion of this colored light which is not entirely reflected but which penetrates the surface and is then reflected. Let us suppose that we are regarding a red wall of glazed brick at the edge of a grass lawn: the local brick-red of the wall will be materially altered by surface reflection not only of the white light but also of blue-green which, being approximately its complementary, tends to lower its saturation and pull it towards neutrality; at the same time, the green rays which have penetrated will on reflection assume a yellowish orange hue. The total effect is therefore that the red is somewhat removed towards neutrality and at the same time made to assume an orange hue. But it is by no means always possible to analyze these color effects, so that we must depend rather on the accuracy of the impression which we receive, at the same time bearing in mind that even objects with which we usually associate the most positive of hues may under certain conditions become entirely altered in this regard. In their use of colors, the post-impressionists are most careful to allow for these influences, although they may employ hues to produce them which at first sight appear to be entirely out of place.
Finally, we must say a few words about the relative refractability of different colors, that is to say, the ease with which the different spectral hues are brought to a focus on the retina. The rays of slow vibration, as at the red end of the spectrum, are less readily focused than those which vibrate quickly, as at the violet end. Consequently, when red rays are in focus, violet rays are over-focused and vice versa. The application of these principles in art depends on the fact that our judgment of distance is partly associated with the amount of effort which we must make in order to accommodate our vision. At rest the optical apparatus of the e.ye is accommodated for distant objects so that when these come nearer than a certain point an effort is required to make the focusing stronger. From the amount of this effort we judge in part of the distance of the object. Now it takes more effort to focus red than green or blue rays so that we always tend to locate a red object as being nearer than one that is blue or green. These facts can be very beautifully demonstrated by looking at red and green lamps placed side by side; the green light appears to be behind the red. And in picture painting the same principles can be applied, and seem to be so in many of the post-impressionists' paintings; objects are brought forward by being colored in the reds and they are pushed back by the use of blues and violets.
These facts bring us to a discussion of the influence of the blue-
