Although not an illusion but a physical reality, it is of interest in passing to note the colored halo or aureole surrounding the shadows of objects cast by the sun against a cloud, fog, or jet of steam. The most wonderful effects are seen by the aerial traveler over a bank of clouds when the upper sky is clear. For example, the shadow of the aircraft cast by the sun upon a dense layer of clouds is surrounded by a halo or aureole of the colors of the rainbow. The phenomenon is purely optical, involving diffraction of light. A well-known example of this is the "Spectre of the Brocken." XII PAINTING AND DECORATION In the arts where colors, brightnesses, contrasts, lines, forms, and perspectives mean so much, it is obvious that visual illusions are important. Sometimes they are evils which must be suppressed; in some cases they are boons to the artist if he is equal to the task of harnessing them. Ofttimes they appear unheralded and unexpected. The existence of visual illusions is sufficient to justify the artist's pride in his "eye" and his dependence upon his visual judgment rather than upon what he knows to be true. However true this may be, knowledge is as useful to the artist as to anyone else. The artist, if he is to produce art, is confronted with the tremendous task of perfecting an imperfect nature and he is handicapped with tools inferior to those which nature has at her disposal. He must deal with reflected lights from earthly materials. Nature has these besides the great primary light-sources--the sun, the moon, the stars, and, we might say, the sky. She also has the advantage of overwhelming magnitudes. These are only a few of the disadvantages under which the artist works, but they indicate that he must grasp any advantage here and there which he may. Knowledge cannot fail him; still, if he fears that it will take him out of his "dream world" and taint him with earthliness, let him ponder over da Vinci, Rembrandt, and such men. These men _knew_ many things. They possessed much knowledge and, after all, the latter is nothing more nor less than science when its facts are arranged in an orderly manner. If the arts are to speak "a noble and expressive language" despite the handicaps of the artist, knowledge cannot be drawn upon too deeply.