Popular Science Monthly/Volume 27/September 1885/The Physiology of Colors



ALIGHT is defined by two qualities, brightness and color. The comparison of two lights of the same color can be made without the assistance of our eyes, and by physical means alone, but it is impossible to compare different colors without bringing in the intervention of the physiological impression. It has been known since Newton's experiments that white light, or, to be more precise, the light of the sun, is formed of a large number of different colors, and that the union of all these in equal proportion, acting upon the eye, either simultaneously or at very close intervals, produces the impression of white. Starting from a preconceived analogy with the notes of the gamut, Newton divided the solar spectrum, or the image obtained by decomposing white light with a refracting prism, into seven different colors. This division is really arbitrary, for the colors pass from one to another by insensible transitions, and each of them may be characterized either by the degree of its refraction in the prism, or by the length of the undulations to which it corresponds.

When we collect a part of the spectral rays in one point, we obtain either one of the primitive colors in a greater or less state of purity, or a new tint. If we divide the spectrum arbitrarily into two parts and collect the rays of these parts separately, we obtain two distinct colors, the superposition of one of which on the other gives white light. The experiment can be performed with an ordinary spectrum divided arbitrarily into two parts; and it is effected, we might say naturally, in the phenomena of rotatory polarization in which the most brilliant hues are shown.

We mention these properties to deduce two conclusions from them. We remark, first, that the mixture of simple or homogeneous lights in any proportion always produces upon the eye a single impression, that of one color. While the ear can distinguish all the notes that go to make up a harmony, the eye can grasp only one color, without being able to distinguish whether it is really simple, or is formed of different lights.

In the second place, the mixture of colors provokes only one new impression, that of purple, for example, which we may obtain by mixing red and violet, while the varieties of rose are nothing but mixtures of purple and white. White may be produced by two simple colors alone, as by red and green; more generally, if we isolate three suitably chosen colors in the spectrum, such as particular shades of red, green, and violet, we may, by mixing them in different proportions, imitate the impressions produced by all the colors. Artificial colors, formed, for example, of rays selected from the spectrum, may be simple or compound, without the eye being informed of the difference, except, perhaps, when they have a shade of purple or of rose, for we know then that those colors do not exist in a simple state.

The same is the case with the colors of nature, or of industry. An object appears colored to us because it sends us only a part of the light it borrows from the general illumination. The sorting out is made either by transmission, as in colored glasses, or by reflection, as in the case of the metals, or by diffraction, as in the wings of some butterflies, or in the coronas which we sometimes perceive around the moon; the portion of the light that does not reach the eye having been absorbed or sent off in a different direction. Leaving out of the account the effects of fluorescence, we perceive that objects do not have colors of themselves, but simply borrow from the general lighting the tint that is suited to them, and present very different aspects according to the mode in which they are lighted. A red ribbon, for example, placed successively in the different colors of the spectrum, appears black, except in the red region; it therefore returns by reflection an almost homogeneous light. A rose ribbon appears very unequally luminous in different parts of the spectrum. The light which it reflects is, therefore, complex.

We may ask, then, What would be the condition of nature if the light that shines upon us were absolutely homogeneous? Some bodies would absorb it completely, and would appear dark like black velvet; others would reflect it more or less actively, and would have a corresponding degree of brilliancy. As there would be no criterion for comparison, the eye would have only the sensations of white, black, and the intermediate rays.

Pascal said that nothing better enabled him to comprehend the properties of the air than what took place where there was none. So, nothing better enables us to comprehend the properties of colors than the appearance of the world under an illumination of homogeneous light. The volatilization of a salt of soda in the flame of a Bunsen burner almost perfectly fulfills this condition. With such a light, cloths dyed in the richest colors show only white, black, and gray, and the art of painting has no place.

The estimation of color being connected with the impression produced upon the retina, it is readily to be seen that the human eye will not always equally well perform that function. The different points of the retina are not alike ready to appreciate colors. To distinguish the details of au object, it is necessary to direct the look toward it, or, in other words, to produce an image upon the central region of the retina, where the acuteness of physiological perception is much the greatest. The same is the case for colors. When we keep the look in a determined direction, and put a colored body in the visual field in such a way that its image is produced laterally, we remark that the notion of color is more and more weakened as we remove from central vision, and disappears at the limits of the field. But the most important fact is, that in the different views the colors are not distinguished from one another with equal facility, and that we sometimes come to the point of confounding colors which really seem to be most discordant, as green and red. The discovery of this particular form of infirmity is due to Dalton, who was very strongly affected by it, and who carefully analyzed the errors of his judgment. This fault, which remained unperceived for so long a time, is in reality quite frequent. About ten persons in a hundred make mistakes in the comparison of colors marked enough to be detected by an attentive examination. Generally the imperfection is not accompanied with grave inconveniences, and is corrected unconsciously by the operation of habit, the recollection of objects, and the judgments of others. But the annoyance becomes extreme when one can not distinguish, for example, red from green, a cherry or a ripe strawberry amid the foliage, or a green from a red light in railway or ship signals. Artists sometimes have marked predilections for certain colors. Lesueur put a profusion of blue into all his paintings, and Turner seems to have sought and found red everywhere. It might be worth while to investigate whether the choice of their favorite colors by some painters is wholly intentional, or is a consequence of a physiological state. Color-blind persons are generally so by birth, but the affection may sometimes be the result of an accident. In some nervous affections it is occasionally manifested temporarily and under the strangest forms. The sight may thus, more than the other senses, be the victim of numerous errors and illusions. To speak only of those which have relation to colors, I notice the effects of contrast of two neighboring colors, or those which follow the impression of an image, or the subjective colors we see with our eyes shut, the result of a mechanical action on the eye, and I shall limit myself to describing some experiments relative to the apparent relief of colors. When we examine on a screen the image of a spectrum produced by a direct-vision prism, the successive colors appear as if situated on the same plane; but, if we slowly turn the slit or the prism, we shall have the illusion of a colored blade in relief with the red extremity forward. The effect is more sensible when the slit is V-shaped, in which case the spectrum resembles a groove. If we substitute for the slit the word DAVY in transparent letters, there will appear to be produced on the screen an exaggerated form of letters in relief like those we see on some shop-signs.

Outside of the colors we are accustomed to see, the solar spectrum includes other rays, some less refrangible than the red, which make themselves manifest by their calorific properties, and others more refrangible than the violet, which are remarked by their photographic effects, and by the action they exercise on fluorescent substances. The solar ultra-violet spectrum produced in the prism occupies an extent nearly equal to that of the luminous spectrum; while Mr. Stokes has shown that the electric arc gives an ultra-violet spectrum five or six times more extended.

We may be surprised that the sight of man is restricted to so small a part of the rays emitted by a luminous source. We have to remark on this point that the case is the same with the other senses. The touch can give an idea of the temperature of bodies only within very narrow limits; the ear can perceive neither extremely grave nor extremely acute tones, and the highest sounds it can hear produce a painful impression. On the side of the infra-red, the visible spectrum stops very abruptly, and the efforts of Brewster extended the range of the rays that the eye can perceive only slightly. Visibility, on the other side of the spectrum, persists in a remarkable manner. Helmholtz had already discovered that with certain precautions he could perceive the whole ultra-violet spectrum as it is revealed by photography. Having had occasion to study the light emitted by metallic vapors, I have ascertained that, with a prism of Iceland spar, an ordinary sight can distinguish an ultra-violet spectrum three or four times as extended as the luminous spectrum; one of my co-laborers saw much farther still, and pointed out in advance all the rays which it was possible for me to photograph. If instead of regarding the refraction of these rays, which varies with the nature of the substances, we define them by their wave-lengths or by the duration of their undulations, we may say that the ordinary luminous spectrum comprises the interval of an octave, and that it is possible to perceive a second higher or more acute octave.

Sir William Thomson has expressed surprise that Nature has forgotten to give us a special sense for perceiving the magnetic phenomena amid which we are living. In the case of light, we are in the presence of rays that are not luminous in sunlight, or at least are not seen by us, which are energetically absorbed by most transparent media and especially by the humors of the eye, for which we give ourselves no concern whatever in current life, and which nevertheless act upon the retina. Does it not seem as if we possessed in this respect a superfluous sensibility, and as if there were a lack of harmony between the structure of the organ and the wants to which it should respond? A question has been raised on this subject that presents a very great interest in the philosophical point of view, as to whether man is susceptible of an organic development, and if it is possible to detect a trace of any progress that may have been accomplished in the vision of colors, and consequently in the structure of the eye. An eminent Englishman has not disdained to engage himself with this question. Mr. Gladstone has summed up all the expressions used by Homer to designate the color of objects, from which it appears that the great poet was accustomed to apply the terms in a very uncertain manner, and confounded green with yellow and blue with black. Before concluding, from this curious observation, that the sense of color was but little developed in Homer's age, we should, perhaps, remark that the interval that separates us from him is but a short time in the history of mankind; that the Greeks afterward made much use of colors in their pictures and in the painted statuettes of which we possess numerous specimens; that the frescoes of Pompeii exhibit the most various colors; and that a careful examination of modern authors might lead us to draw the same conclusions with respect to their time as would be drawn from the Homeric writings. Is it not singular that in the middle of the seventeenth century, when Lesueur was using blue extensively in painting, that emphatically naturalistic poet, La Fontaine, did not once employ the term blue to designate any colored object or the color of the sky?

Even if mankind were capable of a rapid progress toward perfection, it might be supposed that peoples who have continued in the conditions of the Stone age, like the natives of Cape Horn, should not have participated in the general progress. The French expedition, that recently spent a year at Terra del Fuego, made a special study of the natives in respect to this point. The Fuegian language has terms for only two colors, one for red and analogous tints, the other for blue and green. But it is thus poor only because colors do not play an important part in Fuegian life, for it was found that with a little practice the people learned to distinguish and classify colors and their different shades with all the exactness of the most civilized European. The organic development of their visual apparatus, therefore, leaves nothing to be desired. The question whether the vision of animals is the same as that of man, or whether some of them may not have the faculty of perceiving rays to which we are insensible, has been taken up by M. Paul Bert. He placed in a glass vessel a number of freshwater crustaceans of the family of Daphneæ. When light was cast upon a point in the vessel, the Daphnias precipitated themselves upon it and arranged themselves along the beam. Most animals show a similar disposition, and seek the light when it is not too glaring. When a spectrum was thrown upon the vessel, the Daphnias still spread themselves over the illuminated region, but with some quite remarkable peculiarities of arrangement. The smallest ones were scattered through the whole spectrum, being rare in the red, abundant in the yellow and green, and more numerous in the blue and violet, while some of them fixed themselves in the ultra-violet. The largest ones, however, were almost exclusively localized upon a narrow band situated between the green and the blue. These animals, then, see the same rays as we, notwithstanding the distance that separates them from us in the zoölogical scale, and even seem to share our infirmities, for some of them behaved as if they were affected by color-blindness. Sir John Lubbock has made a series of brilliant researches, in the laboratory of the Royal Institution, on the vision of ants, bees, and wasps, from which the curious result has been deduced that the ultra-violet rays appear brighter to ants than the ordinary luminous spectrum. The history of animals regarding this point would therefore be of the highest interest.

We have, so far, considered colors only as one of Nature's decorations, but their influence on the development of living beings is exercised under the most various conditions. Without doubt, light and colors act upon the condition of our mind, and the moral impression thus produced can be nothing but the translation of a physiological action. In some sanitary establishments, where mental disorders are treated, patients are sometimes kept in a yellow light, which seems to exercise a happy influence on their disposition, and to promote calmer feelings. It. is not the yellow light of soda that produces this result, but a kind of white light, in which the extreme blue and red rays have been softened so as to cause a predominance of the rose and yellow tones.

The predilection of animals for particular colors is not the result of an artistic preference. If the Daphnias seek the green light and the ants the ultra-violet, it is, doubtless, because they find better conditions of existence in them. Plants yield themselves more conveniently to studies of this kind. A common plant, like those we have habitually under our eyes, increases, develops itself in every quality, adds to its weight, produces leaves, flowers, and fruits, and respires, or keeps up a constant exchange between the elements it contains and the gases of the atmosphere. These different acts of vegetable life are very unequally affected by the various luminous or calorific radiations. The growth of plants, by the elongation and multiplication of cells, takes place mostly under the influence of the calorific rays, and there is for each plant a preferred temperature. If a plant receives heat only from one side, it is more developed on that side, and forms a curve in the opposite direction. This is the phenomenon of thermotropism. A plant grows less rapidly in the light than in the darkness, but with good effect on its general nutrition and transverse development. In this case the different colors have a very marked specific action. With a good light, the retarding action, insensible in the duller rays, exhibits a first maximum toward the red end, a minimum in the yellow, where the light is most intense, and a grand maximum in the violet. The rays of greater wave-length are, therefore, the more active ones. Hence results a very simple explanation of heliotropism, or of the marked tendency of plants to bend toward the light. When a plant is exposed to a lateral light, the illuminated parts lengthen less rapidly than those which remain in the shade, and the plant bends its head toward the light. We are able to go still further into the mechanism of nutrition. Besides the loss of water by evaporation, plants have two kinds of respiration—one which is continuous day and night, disengaging carbonic acid, a kind of combustion correlative with life and quite analogous to the respiration of animals; and the other intermittent, and taking place only in the light, the result of which is to borrow from the carbonic acid of the atmosphere the carbon from which the plant makes sugar and wood, and to disengage oxygen. The coloring-matter of the leaves, chlorophyl, plays the principal part in this nutritive respiration. Now, chlorophyl has to be made first, and then to perform its respiratory functions; and in this, again, the different colors act very unequally.

If we examine the formation of chlorophyl in the plant with a moderate light, we shall find that it takes place through the whole extent of the solar spectrum, very weakly in the infra-red, reaching a maximum in the deep yellow, and undergoing a regular diminution to the ultra-violet. The curve of this action takes a direction analogous to that which Fraunhofer has given for the distribution of lumnious intensity in the spectrum, but is more prolonged toward the more refrangible rays. Here, again, is a preferred intensity, beyond which chlorophyl is formed less easily. Experiment has also shown that the production of oxygen takes place only where there are grains of chlorophyl. If we resolve with a prism a ray of white light that we have passed through a solution of chlorophyl, we will remark a strong absorption-band in the red, and two others in the blue and violet. These represent the rays that have been absorbed by the green substance, and which effect the reduction of carbonic acid.

In his experiments on fermentations, M. Pasteur distinguished the minute microscopic beings into aërobes and anaërobes; the former respire and are developed in the presence of the oxygen of the air, while the others are killed by oxygen. If we examine the aërobes of a liquid with the microscope, we will find them collected around bubbles of an*, where they can find oxygen. If the liquid is deprived of air-bubbles and incloses a filament of a green alga, the bacteria will distribute themselves indifferently in the medium so long as it is illuminated with a very weak light, or, which is better, with a light that has been filtered through a solution of chlorophyl. In white light, the bacteria will be seen to precipitate themselves upon the grains of chlorophyl, to get the oxygen disengaged from them. They thus constitute a very delicate reagent. To witness the effect of the different colors, we let the microscopic spectrum fall upon a filament of conferva, or a transverse leaf-section. The bacteria will collect upon the plant in the red, at the point of maximum absorption, next in the blue, and the density of the population will nearly follow the absorption-curve of the coloring-matter.

I will not insist too long upon these facts in natural history; but I must add that great specific differences exist in the proper color of different plants, resulting from unequal absorption by their coloring matters. One example of this kind will be enough. The color of seawater varies according to the thickness through which we observe it, on account of the unequal absorption of the different rays; hence a marine plant will find itself in a condition more or less favorable, and be better or less equipped for the struggle for existence, according to the depth of the soil on which it rests. If we examine a bottom which the tide has just left, we will find blue sea-weeds on the edges of the deepest waters, farther down green sea-weeds, beyond these brown ones, and, lastly, red plants in the places which are least frequently uncovered. From the top of a bank we may thus perceive a series of concentric bands of different colors defining the limits within which each species, better fitted to the physical conditions, has overcome and eliminated the neighboring species. This is not a question of depth, because we find red sea-weeds at the water-level in sheltered places, the hollows of rocks, and deep caves, like the one at Capri, where the light comes in weakened.

Light, then, is an inexhaustible source from which living beings obtain energy under all forms and in the most unforeseen conditions; or, as Lavoisier has said, we might believe prophetically, considering the time when he spoke: "Organization, feeling, spontaneous movement, and life exist only at the surface of the earth, and in places exposed to the light. We might say that the fable of the torch of Prometheus was the expression of a philosophical truth that did not escape the ancients. Without light, nature would be bereft of life, dead, inanimate. A beneficent God in giving light has spread organization, feeling, and thought over the surface of the earth."

  1. An address before the Royal Institution of Great Britain. Translated from the French for "The Popular Science Monthly."