It would seem to be folly for any one to maintain that grass is not green, that sugar is not sweet, that the rose has no odor and the trumpet no tone. A man would seem to be out of his senses deliberately to doubt what the world thinks to be simple truths. Yet this paper will deliberately question these truths. It will endeavor to demonstrate that the greenness, the sweetness, the fragrance, the music, are not inherent qualities of the objects themselves, but are cerebral sensations, whose existence is limited to the senses of organized beings. Is grass green? First let us inquire what green is,--what color is. Light is now understood to be an undulation of the interstellar ether, that inconceivably rare, elastic expanse of matter which occupies all space,--an undulation communicated by the incandescent envelope of suns. It moves with such wondrous rapidity as to traverse hundreds of thousands of miles in a second. Such is the generally received explanation of the phenomenon of light; but there is much yet to be explained for which this simple undulation of matter seems to be an insufficient cause. These waves of motion have different lengths and rates of velocity; but the union of them all gives to the human eye the impression of white light. When a prism intercepts their flow, it, so to speak, assorts these differing waves; and, being separated, they then impress the eye with the color of the spectrum, the retina being differently affected by the differing velocities with which it is touched by the ethereal waves. Color, then, is the sensation of the brain, responsive to the touch of the motion of ether; and the brain is only thus affected when these waves are thrown back from some object to the eye. The multiplicity of tints and hues are reflections from the objects which appear to possess them as structural characters. Some of the waves pass into the objects and through them, others are arrested by them and absorbed, others rebound from them like a ball from a wall; and these last, breaking upon the optic nerve, give to it certain sensations which we designate as colors. A wave of a certain velocity and length gives us a certain sensation which we call blue; another awakens the sensation we call yellow. The two series of waves, mingling, produce a new sensation which we call green. The necessity of reflection for the production of these sensations is evident. The mingled waves have no color in their incident flow; but, striking some object, these waves become separated, some being absorbed, and the reflected ones produce the peculiar sensation we call color.