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.
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. 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 wavelets. 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 lightwaves 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.
Is sugar sweet ? That sugar has certain chemical constituents which go to make up a saccharine compound we know. But what evidence have we of its sweetness, except that the nerves of taste are peculiarly affected when brought in contact with it. Its sweetness is not measurable in the chemist’s scales. It can be analyzed, and its constituent elements accurately defined. But sweetness is not one of those elements. The test of that is the tongue. Pure sugar of milk has scarce any sweetness at all ; nevertheless, it is pure sugar. The influence which it has on the nerves of taste is only different from that of cane-sugar. Destroy the nice nervous connection between the tongue and the brain, and sweetness disappears. A severe cold will accomplish this, and while the touch of the sugar is felt, the delicate sympathy which is awakened by the sugar and is felt in the brain as sweetness is destroyed. The sweetness, like the color, is a nervous sensation. We can conceive of a development of the nerves of taste which might receive a host of new impressions from contact with objects now tasteless. The saccharine compound does exist as a chemical quantity, and has a special effect on the nerves of taste, exciting them peculiarly, the result of the excitement being the idea of sweetness.
Is the rose fragrant ? The sense of smell is indeed only a continuation of that of taste. In smelling, the nerves are touched by only infinitesimally small particles of the substances reaching them, and are only able to receive an impression from this excessive distribution. This is also true of taste, to a certain degree, as it is impossible to fully perceive a flavor until the substance is tolerably comminuted, as we smack our lips to obtain it. Indeed, it may be questioned whether the whole of taste may not lie in the capabilities of different substances for great subdivision of particles. If quartz could be made to dissolve into excessively minute particles as readily as sugar, it might have its own special flavor. Some odors are offensive in dense quantities which are highly agreeable when wafted to us in delicate atoms, — musk, for instance. The rose secretes a volatile oil, the wonderfully small atoms of which, on touching the nerves of smell, communicate a peculiar sensation. This odor, like the sweetness, exists only in the nerves affected ; and a trifling disaffection of the nerves suffices to destroy it entirely. The chemist can also analyze the oil, but he does not enumerate in its elements odor. In fact, we have no words to express the sensation of smell. We say sweet, sour, bitter; but have no terms to express the differing sensations produced on us by the rose, lily, violet, and pink. Their oily atoms awaken different sensations in the delicate nerves they touch. The sensation awakened may be due to chemical action induced by them in the system. But whether chemical or physical, the result of their touch is a motion of matter, an impulse communicated to the brain, the sensation of the organ being — the reception of this initiative force being — what we designate as odor. The fragrance of the rose lies, then, in the contractions of special nerves, which thus respond to the touch of the oily particles that are blown against them.
Does the trumpet sound ? A vibration of matter causes the surrounding air to vibrate in consonance with it; and the waves of air thus created, breaking against the auditory nerve, awaken a peculiar sensation which we call sound. The trumpet, vibrating variously, as the valves are moved and the air forced through it, initiates waves of air of different lengths ; and as they are communicated to the surrounding air with amazing rapidity, they successively strike the listener’s ear. As the waves of light touch the optic nerve, so do the grosser waves of air touch the auditory nerve. But sound is only a recognized sensation when the waves of air are within a certain measurement, a maximum and minimum of length. The rush of a whirlwind has no sound, except when arrested by some object, and smaller waves of the vast billows of rolling air are created. We say that the wind roars. But the tremendous currents above us, which sweep along the vast masses of vapor, are noiseless until they touch the earth, and some little trifling eddies are made in their lower sweep by hills and trees and houses. it is then only noise. The ear requires yet smaller waves of air to experience the sensation of tone. The lowest note of a piano has barely enough of it to give a definite idea. As the waves become shorter, the ear begins to be pleasantly affected, and the realm of music is reached. Within a certain restricted length of air-waves lies all of the pleasurable sensation which we call musical tone. But as we rise in the scale the tone begins to become uncertain, until the highest note of the instrument is again indefinite noise. The attenuated tone-waves of Nature are also inappreciable by the auditory nerves, and an obscure hum or buzz is all that can be perceived, until, finally, the eye detects motion which the ear utterly fails to perceive as sound. The results of the air-waves are appreciable by sight and feeling ; but the waves which are heard are not those which create the disturbance in nature we see and feel. The wild gust which seizes a tree and bows it to the earth is only heard when the branches it sways, or the leaves which it rustles, give out a secondary and far more attenuate series of waves. A locust, on a warm, sunny day, will agitate the air around him with a series of waves which affect the ear far more powerfully than the wind which sighs in the waving trees above him. Thus sound is the answering sensation of the auditory nerve to the touch of air-waves : and these waves must be within certain circumscribed limits of magnitude to awaken that sensation at all. The greater or less violence with which they strike the ear causes them to appear loud or soft. We can imagine a development of the nerves, or of the ear apparatus, which might allow them to be influenced by waves of greater volume and less rapid flow, and also by those of diminished size and accelerated movement. The trumpet then does not sound ; the ear sounds, and in the ear alone lies the music that it makes. The deaf man, whose auditory nerves are not sensitive to air-waves, sees the clouds move and the trees sway, the brook ripple and the trumpeter with his tube at his lips ; but the air-waves they all create pass by him, and sound is inconceivable. That sound is a mere nervous sensation is further proved by the fact that we have disturbances of the auditory nerve which we call singing in the ears. No waves of air create this disagreeable music. It arises from some affection of the nerve, which irritates it to a vibration similar to that which it undergoes when air-waves of a certain intensity reach it.
We say the sound rolled on, the odor was wafted, the color was printed, our language and our thoughts implying that the sound, the odor, the color, are things, when in reality they are all mere sensations, answering to the touch of physical agents. All sensation is nervemotion. Outer stimulus, applied to the nerves, causes contractions which, communicating with the brain, give the idea of color or taste or sound.
The sense of feeling is a recognition of the existence of objects by a duller perception than the others, though all of the senses attain their perceptions by feeling, in the strict meaning of the word. We say things feel hard or soft, the varying density of the objects being the cause of the varying sensations they awaken. Smoothness and roughness are varying outlines of surface, existing as physical conformation ; the pleasurable or disagreeable sensations awakened in us by contact being due to the greater or less irritation of the nerves of feeling that attrition with it occasions. Motion is absolutely necessary to give us an idea of the density or configuration of an object. The mere touch of that object is insufficient to possess us with its nature. Iron and down are indistinguishable, unless we, to a certain extent, manipulate them. Glass would be indistinguishable from sand-paper did we not to a certain extent pass our fingers over the different surfaces. Mere touch would not suffice. We have the evidence of all of our senses to prove to us the nature of an object. It tastes or smells or vibrates or is colored ; the varied sensations thus awakened combining to give us our totality of conception. The rose reflects light-waves which the eye feels red; it emits oil-particles which the nose feels fragrant; it touches our tongue, and feels pleasantly ; it touches our fingers, and feels soft and smooth. It exists in nature as a physical structure, and its existence is evident to us through the various sensations it creates in different nerves of our bodies, and through them alone.
One of the ancient philosophies maintained that all Nature is but the phantasm of our senses. Had it, after first granting that the senses themselves were evidences of matter and motion, maintained that Nature was only evident to us through them, it would have been simple truth. Our perceptions of Nature are limited to the capacity of our nervous structure. We frequently make the mistake of endowing matter with attributes which it does not possess, and which are resident only in the impression communicated to us by forces emanating from it, the forces being we know not what. And we can understand that there may be forces in nature as powerful as those which we perceive by our senses, but which are utterly unrecognized by them. We can understand that it were possible for organized beings to possess fifty instead of five senses, which might receive from nature other impressions and awaken other emotions as beautiful and as beneficent as those arising from sight and hearing.