Color, one of those simple and obvious qualities of physical objects, as perceived by us, which can only be defined by its syno-nymes, hue, dye, etc, or by some theory respecting the nature of light, of bodies, or of vision, but the mode of manifestation or production of which is far from being equally obvious. The color of healthy arterial blood in the higher animals is always florid red; that of pure sky or air, blue; that of most growing leaves, some shade of green. Many chemical compounds are known by certain colors, but by heat or other agencies these colors are often changed. The colors of certain paints and dyes are to a certain degree permanent, but by exposure to light and air they undergo a gradual change. Probably no colors are absolutely permanent, but those thus named are each so during a certain condition of the substance to which it belongs. Other colors, as those shown by a diamond cut in certain forms, or by a prism, those of mother-of-pearl, of the plumage of birds, and of soap bubbles, depend on some accident of form or size of bodies, or of the structure of their surfaces, and these change with the position of the observer; hence these are known as variable colors.

When white or solar light is transmitted through triangular prisms of glass, or other media differing in dispersive power from the air, the beam or ray of white is analyzed, being separated into the seven primary colors, red, orange, yellow, green, blue, indigo, and violet. The proof that these are the elements of white light was first furnished by the experiments of Sir Isaac Newton, in 1672. It must be added, however, that between any two of the simple colors of the spectrum there is a gradual interchange of hue, so that in fact the different colored rays existing in and obtainable from the white ray are not seven, but indefinite in number. The same sensible hue of certain colors which, when found in the spectrum, cannot be further decomposed, can always be separated by the prism when found in natural objects, giving two or more, but not all of the spectral colors; that is, the colors of natural objects are always compound. Light from white paper gives a complete spectrum; from yellow, a fragment of a spectrum, showing the yellow space bright, with some green and orange, sometimes a faint tinge additionally of blue and red; blue paper gives a whole spectrum, less the orange and red; and so of others. The light of the purest yellow flower, analyzed, shows also some green and orange.

On account of the impossibility of obtaining pure pigments, a colored solution or glass transmits not only the rays that make up its own color, but other colors that in the spectrum are neighboring to these. Light coming through a blue glass is found to contain a strong blue element, feebler green and indigo, and still feebler yellow and violet. Looking through the blue glass at a spectrum, the sarofe colors are seen, and in corresponding degrees of vividness. The blue medium passes through itself also the closely related rays, but it extinguishes or cuts off the more distant. Bodies vary in color with their mechanical division. Light when reflected from the surface of gold is yellow, and when transmitted through thin leaves of this metal is green; but when gold is reduced to a fine powder and suspended in water, the mixture will transmit, according to the state of division, blue, purple, or ruby light, as was shown by Faraday. Finely divided sulphur ordinarily transmits red light, but on precipitating it from a solution of sul-phydric acid, and adding sesquichloride of iron, the liquid will for a short time transmit purple light. By decomposing sulphydric acid in solution with heat, the water holding the suspended sulphur appears blue.

The theory now commonly received is that proposed by Sir David Brewster, namely, that when a body appears permanently of any given color, simple or compound, it is because it absorbs the remaining colors of the spectrum, i. e., of the complete or white ray, and reflects those which constitute the color of which it appears. Thus a scarlet flower is a body having such relations to light that it absorbs violet, blue, etc, reflecting only certain proportions of red, orange, and yellow. In this view, bodies, as snow and milk, appear white when they reflect or transmit entire or white light; others, as certain inks and coal, appear black, because they absorb all the colored rays in the proportions which form white light. But as all bodies both absorb and reflect in some degree, the proportion of light which white bodies absorb and black bodies reflect is also a mixture of the various colors in the proportions forming white. To prove that the most intensely black body still reflects some light, look through a long narrow tube into a room completely darkened; this may be considered as presenting before the eye an absolutely black spot.

Let a piece of coal or black velvet be brought against the inner end of the tube; the body is at once visible, as something differing clearly from the black space. Oersted separated the light coming from bodies into two parts. First, he found that all bodies have in some degree the property of mirrors, but that in those not polished the light is reflected from so many small surfaces in various directions that no image is produced. Secondly, he found that illuminated bodies gave out another portion of light not reflected, which had the proper color of the bodies themselves. A beam of light, in passing through certain media, or in being reflected from a surface at a certain angle, has all its vibrations reduced to a common plane, and is then said to be polarized. (See LIGHT.) Arago proved that from illuminated unpolished bodies a small and usually imperceptible amount of light having the color of the luminary is returned to the eye, and that this light is polarized by reflection; while much the larger portion of their light, having the color of which the body is seen, is polarized by refraction, and hence must have come from beneath the surface of the bodies emitting it.

A theory long since proposed in substance by Euler is explained at length in his " Letters to a German Princess." Two viols or other stringed instruments being attuned in unison, though many feet apart, if a string of one be made to give out a musical sound, the corresponding string of the other, being fitted to vibrate in the same times, takes up the vibrations from the air, and actually emits the same sound, or some chord of that sound. The first induces vibrations in the second; the second reciprocates the vibrations of the first; but the conditions of the second must first be suitable. In this view, when sunlight falls upon various bodies, it induces (according to the undulatory theory, by the communication of agitation from the ethereal medium to the molecules of the bodies) a luminous condition or power in the illuminated body, but which in most bodies remains only so long as the action of the original luminary is exerted upon them. But though the body receives the whole impulsion of white light, it will in turn reproduce the whole, or such part as the condition of its molecules fits it for reciprocating.

If it reciprocate all the colored rays in due proportion, and in considerable amount, it will be white; if in like proportion, but in almost imperceptible amount, it will be called black. If a body reciprocate mainly the blue and allied rays, it will be some shade of blue; and so of all colors. Moreover, whatever rays the molecular condition of the body forbids it to reciprocate, it will of necessity extinguish; but as force can no more be destroyed than matter, this extinguished light is not lost, but becomes manifest again as heat in the body receiving the rays, or perhaps in certain circumstances as affinity or electricity. According to Brewster's theory, a body is exactly of the color which it rejects, or does not possess; according to Euler's, the body is of the color it shows, and color is not a quality of light merely, but a secondary physical quality of bodies, due to a primary mechanical one. Thus a white body, as a screen, can respond to any color thrown upon it; but a colored body can only respond to rays within a certain limit, and if it receive rays only of colors beyond this limit, it must appear black.

In the undulatory theory, as now received by the highest authorities in physics, there is assumed to be a fixed and invariable connection between the color of a given ray and its re-frangibility; and the latter again depends on its rate and time of vibration. Thus, as calculated by Dr. Young, the middle red ray has a wave length of 1/39,180 of an inch, and corresponds to 477 millions of millions of pulsations per second of the ether, or of the retina; while the middle violet has a wave length of 1/57,490 of an inch, and corresponds to 699 millions of millions of pulsations per second; the other colors having wave lengths intermediate to these. Sir David Brewster, however, who set out with a partiality for the now exploded corpuscular theory, examined the spectrum with variously colored glasses, and declared that he detected some red, yellow, and blue in all parts of it. He accordingly proposed a theory of white light as composed of three elementary colors only, those just named; each color having all the various degrees of refrangibility, and the other colors being mixtures of these three.

Helmholtz has shown that many of Brewster's results were due to using imperfect prisms, and that when these sources of error are in a high degree removed, the mixture of rays through the spectrum fails to appear. - Complementary Colors. Solar or white light being regarded as a whole produced by the union of many elements (colors), it is plain that any portion of these elements being blended to form a composite hue, the remainder will form by mixture some other hue; and either of these being added to the other, the result will be the reproducing of complete or white light. In mathematical language, that which by addition to any part completes it, or makes a whole, is termed a complement. Transferring the mathematical conception to light, any hue, simple or compound, so related to another that by blending it produces with the latter white light, is termed its complementary color, and vice versa. Suppose the four higher prismatic colors blended; their dominant hue is blue. If now the three lower colors, red, orange, and yellow, be mingled in a separate hue, this will be an orange-red. But this combined with the compound blue just named must produce complete or white light.

Hence, in the particular hues of each thus obtained, blue and orange-red are complementary; but there may be many slight variations of these hues which shall go by the same names, and yet not be exact complements, the result of their mixture being then a white tinged with whatever color happens to be in excess; i. e., a light tint of that color. All the colors but yellow and green being blended give a hue of violet-red; and of this the yellow-green is complementary. If one of the compound hues be varied by the incorporation of more of a certain element, the other must lose an equivalent of the same element. If the yellow-green be moved toward green by adding blue, the violet-red, in order still to be complementary, must be moved toward red, by losing the same amount of blue as the other gains. If one of the hues be, irrespective of light or shade, increased in intensity, the other must also be made more intense; since, if the relative quantities of each color existing in the normal spectrum be not maintained, an uncompensated portion of one or more colors must remain.

The following pairs of colors are complementary to each other: Red - bluish-green; orange - blue (azure); yellow - indigo; green - reddish-violet; blue - orange-red; indigo - orange-yellow; violet - yellow-green. M. Chevreul, superintendent of the dyeing department of the Gobelins manufactory, constructed a chart of 24 hues, each shown in 24 different tones, and so arranged that not only the composition and relations, but also the complementary color of each of these, are obvious at a glance. - Subjective or Accidental Colors. Place on a sheet of white paper a red wafer, and look on it intently for some seconds; move the wafer suddenly away, and in its place is seen a bluish-green spot of sensibly the same size. After looking at an orange wafer an azure image will appear; indigo will follow yellow, and violet succeed yellowish green, and the reverse. The color of the image is in all cases complementary to that of the original object. If small objects of any colors be well illuminated and viewed on a black ground, the complementary image is seen upon closing the eyes. Under the same circumstances, a white object will give a black image.

The color thus following upon the contemplation of its complement has been termed accidental, and also, from the fact that it arises from some state produced within the visual organ itself, subjective. A complete account of the ordinary phenomenon may be summed up thus: The color of the object itself grows gradually more faint, as it is viewed; after the cessation of the direct action of the object on the retina, there is, 1st, the persistence for about 1/10 of a second of the primitive impression; 2d, the appearance of the accidental image; 3d, the permanence for a longer or shorter period of the latter, its intensity and duration depending on the intensity and time of the direct impression, while the color gradually fades and then gives place to others; or if the eyes be successively opened and shut, or directed to light and dark surfaces in turn, a succession of appearances and disappearances of the image, usually attended with changes of its color. Any one may witness these curious results by taking a momentary glance at the sun or a candle flame, or looking steadfastly on a strongly lighted colored object, and then proceeding as already indicated.

The subjective color obtained from the red sun of morning or evening will be a hue of green, passing, if the eyes be kept closed, gradually to darker hues, as blue and violet, or restored to red by looking momentarily on a white surface, and then followed by green on closing the eyes; that obtained from the yellowish-white sun of midday will be indigo or violet. In either case, if the eyes are turned at once to a white surface, the image at first appears of its darkest color, passing successively through the lighter colors to white; and whatever color appears when the eyes are closed, its complementary is seen when they are fixed on a white surface. Place, again, any small colored object upon a white surface, and look on it for some time: colors will be seen to develop themselves about the edges of the object, the color being in each case the complement of that of the object, bluish green surrounding red, and so on. The general explanation of these phenomena is, that the retina, having been once impressed with any color, gradually loses its sensibility to that color, and acquires a disposition to be affected by its complementary; and that this tendency is manifested both successively, or in time, and simultaneously, or in space.

M. Scherffer considers that the continued or powerful action of certain colored rays enfeebles or fatigues the sensibility of the retina to those rays; so that when the eye afterward receives white light, it is affected for a time by the other or complementary rays only. M. Plateau explains both the persistence of the original, and the appearance and changes of the successive accidental colors, by supposing these changes to constitute the transition in time of a portion of the retina from an excited to its normal state; while irradiation and the simultaneous accidental color constitute a similar transition in space, or from the actually excited portion of the retina to that which is in repose. When we look continuously on any color, as red, this color loses its vividness and beauty, because a color the opposite of red is excited in the eye, and blends with it; but its complementary, or any color near to this, as green, being now presented, the latter is at once improved, rendered more pure and vivid, by the acquired tendency of the eye to see that color. This is successive contrast; and it is thus shown that colors which will harmonize, or affect the eye agreeably, and be mutually improved, by being viewed in succession, are opposites or complements of each other.

Colors nearly allied will be injured when thus beheld, and will affect the eye unfavorably. A purchaser who is shown in succession several pieces of bright red cloth will pronounce those last seen to be much inferior in brightness and beauty of color to the first; but if his attention be now directed to green stuffs, he will declare these extremely bright; and after them will see red stuffs quite as favorably. Again, two hues of red or blue seen side by side are not improved, because the impression made by either tends to excite an impression of green or orange in its neighborhood, which impression blends with the actual color of the other piece, and impairs it. But if blue and orange strips be viewed side by side, the blue throws orange upon the orange, and vice versa, so that the brilliancy and purity of both colors are improved. This effect constitutes simultaneous contrast; and it shows that harmony in colors viewed at once and near to each other also requires that these should be, or approach to, complementaries of each other. If different tones be associated, the effect is always to make the tinted appear lighter, and the shaded darker, than it really is.

This is easily shown by placing side by side several gray strips, passing gradually from quite light to dark; although the shade of each strip is homogeneous, yet its side toward a darker strip will appear to be the lighter, and that toward a lighter strip the darker. Chevreul's law, both for hues and tones, deduced from facts such as those now stated, may be thus expressed: "When the eye sees at the same time two contiguous colors, they will appear as dissimilar as possible, both in optical composition and in height of tone. Guided by this principle, the juxtaposition of colors in painting, in dress, in furniture, in the planning of gardens, in bouquets, and indeed wherever colors are employed with a view to artistic effect, ceases to be a matter of accident or an ill-understood experience, and becomes a subject for the rules and predictions of science. In all chromatic arrangements, harmonies of contrast are first to be sought. But as these are limited, harmonies of analogy are also called into requisition, with less striking, but often with very pleasing results.

These may be secured in three ways: by arranging different tones in a series; by associating nearly related hues of a like tone, except where these, as blue and violet, distinctly injure each other; and by viewing appropriate groupings of colors by colored light, as that from a stained window, which modifies them all in a particular direction. The effect of the contiguity of white is to deepen all hues in whatever tone, unless it may be a light yellow; but with the deeper hues and tones, the contrast with white is generally too violent. Black accords well with almost any hue or tone, except that the deeper, as indigo or violet, render it apparently gray and faded. - The reader is referred to M. Chevreul's work on this subject, or to Prof. Youmans's "Household Science," for full details in relation to the arrangement or matching of colors. A few examples may be given. Thus, to trim orange articles of dress with yellow, to bring violet and deep blue flowers into juxtaposition, to upholster mahogany chairs with crimson or dark orange stuffs, without an intervening band of black or green, or to place heavy gilt frames near to strong red or orange in a picture, is a violation of chromatic harmony. Black and dark colors diminish, white and lighter tones enlarge, the apparent size of the wearer.

Large figures or horizontal stripes shorten, while narrow vertical stripes heighten, the apparent stature. All colors in the vicinity of the face influence the complexion, as already explained. Hues and tones of green improve a pale or blonde complexion, by throwing on it their complement of rose; while orange throws blue on the too abundant orange of the brunette complexion, and blending with the latter produces a whitening effect. Light or tinted colors agree best with light, deep or shaded colors with dark complexions. Car-pets, paper hangings, curtains, and furniture for rooms should be of colors chosen with reference both to their effects upon each other, and upon the complexions of the inmates. The beauty of red flowers is heightened by the neighborhood of green foliage; and in the hues of flowers it is easily observed that chromatic discords are seldom met with. The association of a yellow or orange pollen with a violet, purple, or blue corolla is familiar. An eye delicately susceptible to colors will note also the frequent examples of modification of color by contrast, that come under daily observation.

Thus an orange-red sunset appears heightened to bright .scarlet when seen through openings in green foliage; narrow bands of gray clouds moving over such a sky appear of a rich light or olive green; and at a later hour, when the distant forest is simply black, it appears bordered at the top with a vivid green from the same cause. One who looks intently at a bright horizon will see after some seconds a dark curtain appear to drop down to near the horizon, while between this and the earth a brightened band of sky of changing width will be visible. The colors of thin plates and films, as mica, mother-of-pearl, and soap bubbles, and similar phenomena, are treated of in the article Light.