This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
Under the full blaze of the sunlight, the earth throbs as with a million pulses Those substances which are most ethereal in their atomic structure, such as glass and crystals, vibrate most readily and most powerfully; but all things, even the most amorphous in structure, join more or less in the electrical pulsation - transmitting, reflecting, and modifying into colors, the limpid light which streams from the sunny skies.† When the sun sets, this vibratory motion of the earth's surface to a great degree ceases, is feebly kept up by the cold radiance of the moon, or fades into almost quiescence beneath the tremulous light of the stars. Put out the stars, and all seems absolute darkness. But is it so ? We trow not Draw the thickest curtain of cloud over the sky, - let neither moon nor star, nor feeblest glimmer of the violet-colored skies of night break the darkness; and yet, while men grope and stumble, and call to their aid the appliances of luciferous art, myriads of the lower creation - birds of the air, fish of the sea, and prowling and creeping things without number, ply their life as •easily as if with them it were not night but day.
What does this show, but that light and darkness are but relative terms, - that what is night for man, is day for other creatures; and that even in night-time the surface of the earth is vibrating, far too feebly, indeed, to excite vision in man, but sufficient for a vastly wide range of animal life, to whom eyes have been given extremely susceptible to the ethereal vibrations. The great Creator has furnished each class of his creatures with visual organs fitted for their peculiar sphere of action; and man, made for the day and the sunshine, has eyes whose range of discernment is limited to the diurnal phenomena. His organ of sight is adapted for a certain degree of light, more or less than which tends equally to blindness. He is not more baffled by the shadows of night than by a superabundance of the illuminating rays. Light itself may become darkness. The eagle gazes undaz-zled on the orb of day; but to us, the sun in its noontide splendor is an invisible spot in the sky; and " dark from excessive bright," is a phrase not more poetic than true.
Since, then, our range of vision is thus limited, let us beware of dogmatising as if light were a word of absolute instead of relative significance; and although we may not be able to see what Reichenbach's sensitives saw, still less to walk by the feeble rays which suffice for the lower creation, let us confess that the auroral and zodiacal lights, as well as all sound reasoning, show that Earth has a light of her own, by which it is as seemly that some orders of creatures should walk, as we, children of light and of the day, by the nobler radiance of the sun.
* The great HERSCHEL. expressly admits the correctness of this important and self-obvious, though little-thought-of truth, when speaking of the systems of Double Stars, and of the revolution of sun round sun, he says - "Each accompanied with its train of planets and their satellites, closely shrouded from our view by the splendor of their respective suns." - Outlines of Astronomy, Chap. xvi. §847.
† This vibratory action is indispensable in the process of vegetation; and, in regard to the prodigious effect of this vibratory influence of the solar rays, Prof. GREGORY says: " It has been calculated that the mechanical force exerted by The sun upon the amount of wood growing on one square foot of surface, in the course of a year, corresponds to what would be required to raise a weight of 486,000 pounds to the height of one foot; and this is only 1-llth of the whole effect of the sun's rays, of which only 14th reaches the plant, and half of that is lost." - Handbook of Organic Chem. p. 482.
It is known to men of science that every part of nature, even the hardest and most solid, is in a state of molecular motion, so subtle, as in most cases to defy occular scrutiny, yet indubitably revealing itself in its effects.* It is only when those vibrations grow strong and frequent that they become perceptible to our senses; and then they do so in the form of those ether-born twins, heat and light. Let us examine the spectrum, and see how this vibratory motion exhibits itself in the production of color. To the ordinary eye, the spectrum, produced by refracting or breaking up the symmetry of the solar beam, is merely a series of hues, beginning with red, brightening into yellow, and then fading away through violet into darkness. But if you examine it scientifically, you will find that those bright hues are produced by a series of tremors or vibrations of the broken ethereal ray; the strongest and slowest of which vibratory rays are least refracted, and form the red, and the feeblest and most rapid are most refracted and form the violet But the whole of the broken rays are not represented by the colors which meet the eye in the spectrum; for at either extremity, where the red and violet fade out of sight, a succession of rays spread out, invisible to our eyes, but which might be to some extent discernable had we the night-eyes of some of the lower animals.
The invisible rays at the red end are the strongest and rarest in the spectrum, only showing themselves by giving out heat, and an electricity which is positive; those at the violet end are the feeblest and densest, - only showing themselves by their chemical or actinic properties, and by an electricity which is negative. Thus the spectrum exhibits a complex phenomenon. Firstly, we have a series of rays steadily increasing in rapidity and weakening in force of vibration, from one end to the other: (similar in this respect to the atmospheric vibrations which produce sounds which emerging from silence as the spectral colors emerge from darkness, run through the scale of the musician, getting quicker and feebler in their vibrations, until they again become inaudible, - the ear hearing sounds as the eye sees colors, only so long as the vibrations continue within a certain range of velocity and force, which varies somewhat in different individuals and animals, - the savage Indian, for instance, hearing sounds and seeing objects where we can see or hear nothing; and dogs and the lower creation exhibiting the same powers to a still greater extent).* But superimposed upon this steadily ascending gamut of vibrations, we have another phenomenon, namely, that one-half of the rays of the spectrum are electrically positive, and give out heat, and that the other half are negative, and produce chemical action; † and that in the center, those opposite influences neutralise each other.
The varying phenomena of color, then, are not owing to a mere difference in the vibratory speed of the rays of the spectrum, but also to the electric difference of these rays, which, positive at the red end, and negative at the blue, flash up into yellow or white light in the center where they meet.
* "Nothing on be more certain," says Mrs. Somervills, " than that the minute particles of matter are constantly in motion, from the action of heat, mutual attraction and electricity. Prismatic crystals of salts of zinc are changed in a few seconds into crystals of a totally different form by the heat of the sun; - casts of shells are found in rocks, from which the animal matter has been removed, and its place supplied by mineral:- and the excavations made in rocks diminish sensibly in size, in a short time, if the rook be soft, and in a longer time when it is hard: circumstance* which shew an intestine motion of the particles, not only in their relative positions, but in space, which there is every reason to believe is owing to electricity, - a power which, if not the sole agent, must at least have co-operated essentially in the formation and filling of mineral veins." - Physical Geography, I, Chapt xv. p. 288-9.
In considering, then, the impression made on our eye by the colors of the spectrum, there are two points to be considered. In regard to illuminating power, the strongest point of the spectrum is the yellow, - in point of vibratory power, it is the red; and the color which makes the strongest impression on our visual sense is the red-orange, or scarlet, which, lying between the red and yellow, combines in fullest force the illuminating and vibrating powers. Hence it would appear that color is a vibratory phenomenon of the ethereal rays, - intermediate between heat on the one hand, and actinism on the other, and attended by an overlapping of the electro-positive and electro-negative rays, of which heat and actinism are the representatives. But whether heat and actinism are not themselves the necessary products of a certain rate of vibration in the ether, and so the whole phenomenon of color be practically reducible to one of ratio of vibration, we do not profess to say. Men will get at the root of all those things by-and-by. Meanwhile, it is instructive to observe, from the paper upon radiant heat, lately read before the British Association, by Professor Powell, that heat rays, or rays emanating from a hot body, when refracted, present identically the same phenomenon as those of light, namely: that the rays of the heat-spectrum which vibrate most slowly have a heating but not an illuminating power; those of greater velocity, a luminiferous property also; and those of the greatest velocity, little heating or luminiferous, but higher chemical power.
The reflected rays from the moon form a curious illustration of these and our preceding statements, - the strong electro-positive heat-rays of the solar beam being absorbed by the lunar orb, while the feebler and more rapidly-vibrating rays are reflected to our planet, and bring us a certain amount of illumination combined with a strong chemical influence; which latter shows itself, inter alia, (especially in tropical countries), by the well-established fact of the rapid decomposition of butcher-meat, etc, when exposed to the lunar beams.
* There it a Bosjesman tribe in South Africa, who exhibit in a remarkable manner the phenomenon call Nyctalo-phia, - deeping and resting during the day, when their eyes, either from natural or acquired organization, cannot bear the light of the sun, and carrying on their main pursuits during the night † It to this difference in the chemical action of the various rays which produce color that eonatitates the greatest stumbling block in the way of photography, - the colors at the blue end of the spectrum making an undue impression on the chemical surface compared with the others. This difficulty is being obviated, but much as photography has achieved, we believe the art is still in Its infancy.