We know that these varying conditions of light which affect us as color have an absolute being. The photographer carries on his nice operations behind a yellow screen undisturbed, when the substitution of a pink one would at once allow of the chemical action of the other rays of light on his plate, to the destruction of his image. Still, the pink and the yellow, as colors, are brain sensations. We feel them with our eyes, and the feeling they awaken we call color. The optic nerve receives the undulations of ether thrown back from grass, and the peculiar sensation thus awakened by their touch is called green. The color is not a part of the grass, not a quantitative constituent, like its carbon or silex. The grass has no color, because color is something existent in the eye of the beholder, not in the object awakening that something by its peculiar mode of reflecting light. A looking-glass does not possess, as a constituent part, the image of a human face; but that face, when put before it, appears to be a part of the glass; and if no looking-glass had ever existed except with a certain face before it, that face would be just as much a part of the glass as the color green is of grass. They both reflect. Some people are color-blind. They cannot perceive any difference between the rose and the leaves around it. Color is inconceivable to them. Let us suppose, then, that all men were color-blind. They would be fully cognizant of light, shadow, darkness; but the nicer sensations of the brain which we call colors would be utterly unknown to senses unable to feel their delicate touch. At the same time, the different undulations of the different colors might have been detected by other means than the sense of sight, as unseen gases have been discovered by the chemist. And we cannot say that Nature may not possess an inconceivable variety of influences inappreciable by our senses. We say grass is green; but is it always so? What varying colors does it possess under the varying light to which it is exposed. The same grass is light green in the sun, dark green in the shadow, almost black in the twilight, and at night what color is it? We may say that it is green, but that we cannot see it. By no means. If greenness were an inherent attribute, it would be persistent. The weight, density, chemical construction, and size of the plant do not change from midday to midnight. They are identical in the dark and the light. But the color depends entirely on the character of light poured upon it; as that color is only a peculiar reflection of that light, or part of it, and that reflection is only green when it stimulates an optic nerve to a sensation peculiar to its touch. The same grass becomes yellow or brown in autumn, possessing then new powers of absorption and reflection. The very limited capacity of the eye to receive sensation from light rays is proved by the discovery that the spectrum possesses other rays, called heat-rays, which the eye cannot perceive.