On the physical side the influence of the red rays, although there is much evidence showing that it may be traced throughout the whole of organic nature, is certainly most strongly and convincingly exhibited on plants. The characteristic greenness of vegetation alone bears witness to this fact. The red rays are life to the chlorophyll-bearing plant, the violet rays are death. A meadow, it has been justly said, is a vast field of tongues of fire greedily licking up the red rays and vomiting forth the poisonous bile of blue and yellow. An experiment of Flammarion’s has beautifully shown the widely different reaction of plants to the red and violet rays. At the climatological station at Juvisy he constructed four greenhouses—one of ordinary transparent glass, another of red glass, another of green, the fourth of dark blue. The glass was monochromatic, as carefully tested by the spectroscope, and dark blue was used instead of violet because it was impossible to obtain a perfect violet glass. These were all placed under uniform meteorological and other conditions, and from certain plants such as the sensitive plant, previously sown on the same day in the same soil, eight of each kind were selected, all measuring 27 millimetres, and placed by two and two in the four greenhouses on the 4th of July. On the 15th of August there were notable differences in height, color and sensitiveness, and these differences continued to become marked; photographs of the plants on the 4th of October showed that while those under blue glass had made no progress, those under red glass had attained extraordinary development, red light acting like a manure. While those under blue glass became insensitive, under red glass the sensitive plants had become excessively sensitive to the least breath. They also flowered, those under transparent glass being vigorous and showing buds, but not flowering. The foliage under red glass was very light, under blue darkest. Similar but less marked effects were found in the case of geraniums, strawberries, etc. The strawberries under blue glass were no more advanced in October than in May; though not growing old their life was little more than a sleep. It appears, however, that the stimulating influence of red light fails to influence favorably the ripening of fruit. Zacharewiez, professor of agriculture at Vaucluse, has found that red, or rather orange, produces the greatest amount of vegetation, while as regards fruit, the finest and earliest was grown under clear glass, violet glass, indeed, causing the amount of fruit to increase but at the expense of the quality.

Moreover, the lowest as well as the highest plants participated in this response to the red rays, and in even a more marked degree, for they perish altogether under the influence of the violet rays. Marshall Ward and others have shown that the blue, violet and ultra-violet rays, but no others, are deleterious to bacteria. Finsen has successfully made use of this fact in the treatment of bacterial skin diseases. Reynolds Green has shown that while the ultra-violet rays have a destructive influence on diastase, the red rays have a powerfully stimulating effect, increasing diastase and converting zymogen into diastase.

While the influence of the red rays on the plant is thus so enormous and easily demonstrated, the physical effects of red on animals seem to be even opposite in character, although results of experiments are somewhat contradictory. Béclard found that the larvæ of the flesh fly raised under violet glass were three fourths larger than those raised under green glass; the order was violet, blue, red, yellow, white, green. In the case of tadpoles, Yung found that violet or blue was especially favorable to the growth of frogs; he also found that fish hatch most rapidly under violet light. Thus the influence that is practically death to plants is that most favorable to life in animals. Both effects, however, as Davenport truly remarks in his ‘Experimental Morphology,’ when summing up the results of investigations, are due to the same chemical metabolic changes, but while plants succumb to the influence of the violet rays, animals, being more highly organized, are able to take advantage of them and flourish.

At the same time the influence of violet rays on animal tissue is by no means invariably beneficial; they are often too powerful a stimulant. That the violet rays have an influence on the human skin which in the first place, at all events, is destructive and harmful in a high degree, is now clearly established by the observations and experiments of Charcot, Unna, Hammer, Bowles and others, while Finsen has made an important advance in the treatment of disease based on this fact. The conditions called ‘sun-burn,’ ‘snow-burn,’ ‘snow-blindness,’ for instance, which may affect even travelers on snow-fields and Arctic explorers, are now known to be wholly due to the violet and not to the red rays. Unna’s device of wearing a yellow veil, and Bowles’s plan of painting the skin brown, thus shutting off the violet rays, suffice to prevent sun-burn. The same effect is also obtained by nature, which under the stress of sunlight, and largely through the irritation of the violet rays themselves, weaves a pigmentary veil of yellow and brown on the skin, which thus protects from the further injurious influence of the violet rays and renders the sunlight a source of less alloyed joy and health.

That the presence of the red rays, or at all events the exclusion of the violet, is of great benefit in many skin diseases seems to be now beyond doubt. This has been shown by Finsen in his treatment of smallpox in red rooms; it appears that it was also known in the Middle Ages as well as in Japan, Tonquin and Roumania, red bed-covers, curtains or carpets being used to obtain the effect. Under the treatment by red light not only is the skin enabled to heal healthfully without scarring, but the whole course of the disease is beneficially affected and abbreviated, the fever is diminished and also the risk of complications. Another physician has discovered that a similar beneficial effect is produced by red light in measles. A child with a severe attack of measles was put into a room with red blinds and a photographic lamp. The rash speedily disappeared and the fever subsided, the child complaining only of the absence of light; the blinds were consequently removed, and the fever, rash and prostration returned, to disappear again when the blinds were resumed.

Whether red light, or the exclusion of violet, exerts a beneficial influence on the hæmoglobin of the blood and on metabolism generally has not been distinctly proved, but it seems to me to be indicated by such experiments as those of Marti published a few years ago in the Atti dei Lincei. This investigator found that while feeble irritation of the skin promotes the formation of blood corpuscles, strong irritation diminishes the blood corpuscles and also the hæmoglobin; at the same time he found that darkness also diminishes the number of red corpuscles, while continued exposure to intense light (even at night the electric light, which, however, is rich in violet rays) favors increased formation of red corpuscles, and in some degree of hæmoglobin. Finsen has shown that inflammation of the skin caused by chemical or violet light leads to contraction of the red corpuscles.

This brings us to the consideration of the influence of the red rays on the nervous system. From time to time experiments have been made as to the influence of various colored lights, chiefly on the insane, as first suggested by Father Secchi in 1895. Even yet, however, the specific mental influences of the various colors are not quite clear. It has been found by some that the red rays are far more soothing and comfortable, less irritating, than the total rays of uncolored light, and Garbini found that angry infants were soothed by the light through red glass, only slightly by that through green and not at all by other colored light. On the other hand, it is stated that a well-known dry plate manufacturer at Lyons was obliged to substitute green-colored glass in the windows of his large room for the usual red because the work people sang and gesticulated all day and the men made love to the women, while under the influence of green glass (which also allows yellow rays to pass) they became quiet and silent and seemed less fatigued when they left off work. We need not attach much value to these statements, but in this connection it is interesting to refer to the results obtained some years ago by Féré and recorded in his ‘Sensation et Mouvement.’ Experimenting on normal subjects as well as on nervous subjects, who were found more sensitive, with colored light passed through glass or sheets of gelatine, he found notable differences in muscular power, measured by the dynamometer, and in the circulation as measured by plethysmographic tracings of the forearm under the influence of different colors. He found in this manner with one subject whose normal muscular power was represented by 23 that blue light increased his power to 24, green to 28, yellow to 30, orange to 35 and red to 42. The dynamogenic powers of the different colors were thus found to rank in the spectral order, red representing the climax of energy, or, as Féré puts it, “the intensity of the visual sensation varies as the vibrations.” Féré found that colors need not be perceived in order to show their influence, thus proving the purely physical nature of that influence, for in a subject who was unable to see colors with one eye, the color stimulus had the same dynamogenic effect whether applied to the seeing or the defective eye. Increase of volume of blood in the limbs, measured by the plethysmograph, so far as we can rely on Féré’s experiments, ran parallel with the influence on muscular power, culminating with red, so that no metaphor is involved, Féré remarks, when we speak of red as a ‘warm’ color. On the insane the results attained by the use of colored glass do not seem to be quite coherent. Some of the earlier observers described the beneficial effects of blue glass in soothing maniacs. Pritchard Davies, however, was not able to find that red light had any beneficial effect, though on some cases blue had, while Roffegean found that, in the case of a somber and taciturn maniac who could rarely be persuaded to eat, three hours in a red-lighted room produced a markedly beneficial effect, and a man with delusions of persecution became quite rational and was even in a condition to be sent home after a few days in the same room. He also found that a violent maniac wearing a strait jacket, after a few hours in a room with blue glass windows became quite calm and gave no further trouble. Osburne has found, after many years’ experience, that in the absence of structural disease violet light (for from three to six hours) is most useful in the treatment of excitement, sleeplessness and acute mania; red he has found of some benefit, though to a much less degree in such cases (it must be remarked that violet light as usually applied is not free from red), while he has not found any color with which he has tried experiments (red, orange or violet) of benefit in melancholia. The significance of these facts is not altogether clear; the influence, as Pritchard Davies concluded, seems to be largely moral, though it may be that the colors of long wave-length are tonic and those of short wave-length sedative.

So far I have been chiefly concerned to point out that the immense emotional impressiveness of red has a basis in physical laws, being by no means altogether a matter of environmental associations. It is true that the two groups of influences overlap, and that we can not always distinguish them. We can not be sure that the greater sensitiveness to the red rays may not have been emphasized in the organism, not necessarily as the result of inherited acquirement, but probably as the perpetuation of a variation of sensibility, found beneficial in an environment where red was liable to be especially associated with objects that were to be avoided as terrible or sought as useful. In this way the physical and environmental factors would run in a circle.

We have to bear this consideration in mind when we take into account the susceptibilities of animals, especially of the higher animals, to red. The color sense, it is well known, is widely diffused among animals; indeed this fact has been brought forward, especially by Pouchet, to prove that there can have been no color evolution in man; this it can scarcely be said to show, since evolution does not run in a straight line, and it is quite conceivable and even probable that the ancestors of man were less dependent than many lower animals, for the means of living, on a highly developed color sense. Thus a color sense that among some creatures is so highly developed as to include even the ultra-violet rays, was among our own ape-like ancestors either never developed or partially lost.

Graber, in his important investigation into the color sense of animals, showed that of fifty animals studied by him forty showed strong color preferences in their places of abode. In general he found, without being able to explain the fact, that animals which prefer the dark are red lovers, those which prefer the light are blue lovers. The common worm, with head and tail cut off, still preferred red to blue nearly as much as when uninjured. (This would seem to indicate the same kind of susceptibility to unaccustomed violet rays which we have already encountered in the phenomenon of sun-burn.) The triton and cochineal, with eyes removed and heads covered with wax, still had delicate sense for color and brightness. The flea infesting the dog had a finer color sense than the bee, while nearly all the animals Graber investigated were more or less sensitive to the ultra-red rays.