Only about a third of the spectrum is visible to the eye. The other portion appears in the form of heat, inappreciable by the optic nerve as light. Color, therefore, is not a physical thing,--a quantity in Nature. Her beauty and glory, visible in her tints and hues, are in the brain of the observer,--a play of light reflected from the myriad objects upon which it breaks in infinite diversity of ethereal wavelet's. One may see colors which do not exist as undulations. For example, let one look fixedly at a brilliant red object for a while, and then close his eyes. He will behold an image of the same object of a green color. This green color, then, is a sensation in the optic nerve, which, being powerfully stimulated by the red, undergoes a reaction, resulting in a sensation similar to that which it would experience were it looking at the object in green. The color green, in this case, is certainly only nervous sensation. As light is now known to be the motion of matter, color, as the result of light, must inevitably be limited by it. The touch of the light-waves upon our nerves causes certain contractions which we call color, the contractions ceasing when the touch is withdrawn. A pane of green glass will cast upon a white marble a green light. Let us suppose that this play of light had always existed, so far as those two objects were concerned. The marble would appear to be permanently green, and not white; and if we had not a simple way of removing the light, we should certainly say it was green marble. Could we as effectually change the play of light which causes grass to appear green, we should at once demonstrate as readily, that its color was an appearance to the eye, not a part of the grass itself. It is very probable that we are extensively deceived in this way,--that many appearances in nature are only simulations which we have no means of detecting. Isomerism in minerals has been discovered,--a state in which quite different physical properties are coexistent with identity of component parts. What we always see, and what seems to be permanent, we naturally accept as a physical fact; and yet we can understand that our senses may, in many instances, be the sport of appearances which, because permanent, we conceive to be reality. Thus color is a cerebral sensation only, and grass is not green.