When the different rays of the solar spectrum strike the eye separately they each produce a particular characteristic and subjective impression, which is called color. Ingenious theories have been set forth by physiologists, like Young, Helmholtz, Hering, and others, to explain the perception of colors by our eye, but the problem still awaits solution, and is not likely to be explained from that side, because it is rather psychical. The laws regulating the perception of colors are not physiological; we perceive only relations. We know that the sense of color may be modified independently of that of light and of space. Two phases may be distinguished in its evolution. Every light, whether chromatic or not, produces a simple luminous impression on the retina--a simple excitation of the optic nerve, without being analyzed by it. In the second phase the brain, the psychic center of color, intervenes. There may obviously be considerable differences between persons in the interpretation of what we call colors, and we may judge that there is an education of this psychical center, and that it is an important matter. Different as the ways of interpreting a sensation of color may be, there are still some fundamental ideas in the matter which painters, for example, do not all observe. Some, like the impressionists, exaggerate them, and others neglect them. Which of these are wrong? and which right? are questions we are not concerned with, our purpose being to show that many of the phenomena of color, shade, sources of light, etc., escape a large proportion of persons unless they are attentive observers. If we visit the exhibitions of the impressionists we shall be entertained at the criticisms we hear over the canvases of such painters as Renoir and Monet; youths who have just come out of the drawing school declaring that their master never taught them to put blue on a face, and that in Nature all shadows are gray or black, and none red or violet; and we should astonish a great many people if we should say that a white robe should never be painted in a portrait picture with white lead alone. "All skies are blue, all trees are green, all pantaloons are red," said a celebrated painter who was trying to show how the habit of seeing a colored object in a certain way prevented one from perceiving the different colors that might be applied to it. We recollect the trouble of a brave youth who, having sat for his portrait to a celebrated painter, was distracted at perceiving green in the reflections of the hair of his likeness. Yet there are in Nature shadows that are blue and reflections that are green, and if we do not see them habitually it is because we do not give sufficient attention to them.