It was a beautiful idea that real impressions of external objects are made upon the seat of vision, and that they are viewed, as in a picture, by something behind the screen,—that these pictures become dormant, but are capable of being revived by the operations of the mind in peculiar conditions; but we can only regard it as a philosophical speculation of a poetic character, the truth or falsehood of which we are never likely to be enabled to establish.[108]
That which sees will never itself be visible. The secret principle of sensation,—the mystery of the life that is in us,—will never be unfolded to finite minds.
Numerous experiments have been made from time to time on the influence of light upon animal life. It has been proved that the excitement of the solar rays is too great for the healthful growth of young animals; but, at the same time, it appears probable that the development of the functional organs of animals requires, in some way, the influence of the solar rays. This might, indeed, have been inferred from the discovery that animal life ceases in situations from which light is absolutely excluded. The instance of the Proteus of the Illyrian lakes may appear against this conclusion. This remarkable creature is found in the deep and dark recesses of the calcareous rocks of Adelsburg, at Sittich; and it is stated, also in Sicily, and in the Mammoth caves of Kentucky. Sir Humphry Davy describes the Proteus anguinus as “an animal to whom the presence of light is not essential, and who can live indifferently in air and in water, on the surface of the rock, or in the depths of the mud.” The geological character of rocks, however, renders it extremely probable that these animals may have descended with the water, percolating through fissures from very near the surface of the ground. All the facts with which science has made us acquainted—and both natural and physical science has been labouring with most untiring industry in the pursuit of truth—go to prove that light is absolutely necessary to organization. It is possible the influence of the solar radiations may extend beyond the powers of the human senses to detect luminous or thermic action, and that consequently a development of animal and vegetable forms may occur where the human eye can detect no light; and under such conditions the Proteus may be produced in its cavernous abodes, and also those creatures which live buried deep in mud. Some further consideration of the probable agency of light will occupy us, when we come to examine the phenomena of vital forces.
Light is essentially necessary to vegetable life; and to it science refers the powers which the plant possesses of separating carbon from the air breathed by the leaves, and secreting it within its tissues for the purpose of adding to its woody structure. As, however, we have, in the growing plant, the action of several physical powers exerted to different ends at the same time, the remarkable facts which connect themselves with vegetable chemistry and physiology are deferred for a separate examination.
The power of the solar rays to produce in bodies that peculiar gleaming light which we call phosphorescence, and the curious conditions under which this phenomenon is sometimes apparent, independent of the sun’s direct influence, present a very remarkable chapter in the science of luminous powers.
The phosphorescence of animals is amongst the most surprising of nature’s phenomena, and it is not the less so from our almost entire ignorance of the cause of it. Many very poetical fancies have been applied in description of these luminous creations; and imagination has found reason why they should be gifted with these extraordinary powers. The glow-worm lights her lamp to lure her lover to her bower, and the luminous animalcules of the ocean are employed in lighting up the fathomless depths where the sun’s rays cannot penetrate, to aid its monsters in their search for prey. “The lamp of love—the pharos—the telegraph of the night,—which scintillates and marks, in the silence of darkness, the spot appointed for the lover’s rendezvous,”[109] is but a pretty fiction; for the glow-worm shines in its infant state, in that of the larva, and when in its aurelian condition. Of the dark depths of the ocean it may be safely affirmed that no organized creation lives or moves in its grave-like silence to require this fairy aid. Fiction has frequently borrowed her creations from science. In these cases science appears to have made free with the rights of fiction.
The glow-worms (lampyris noctiluca), it is well known, have the power of emitting from their bodies a beautiful pale bluish-white light, shining during the hours of night in the hedge-row, like crystal spheres. It appears, from the observations of naturalists, that these insects never exhibit their light without some motion of the body or legs;—from this it would seem that the phosphorescence was dependent upon nervous action, regulated at pleasure by the insect; for they certainly have the power of obscuring it entirely. If the glow-worm is crushed, and the hands or face are rubbed with it, luminous streaks, similar to those produced by phosphorus, appear. They shine with greatly increased brilliancy in oxygen gas and in nitrous oxide. From these facts may we not infer that the process by which this luminosity is produced, whatever it may be, has a strong resemblance to that of respiration?
There are several varieties of flies, and three species of beetles of the genus Elater, which have the power of emitting luminous rays. The great lantern-fly of South America is one of the most brilliant, a single insect giving sufficient light to enable a person to read. In Surinam a very numerous class of these insects are found, which often illuminate the air in a remarkable manner. In some of the bogs of Ireland a worm exists which gives out a bright green light; and there are many other kinds of creatures which, under certain circumstances, become luminous in the dark. This is always dependent upon vitality; for all these animals, when deprived of life, cease to shine.
At the same time we have many very curious instances of phosphorescence in dead animal and vegetable matter; the lobster among the Crustacea, and the whiting among fishes, are striking examples; decayed wood also emits much light under certain conditions of the atmosphere. This development of light does not appear to be at all dependent upon putrefaction; indeed, as this process progresses, the luminosity diminishes. We cannot but imagine that this light is owing, in the first place, to direct absorption by, and fixation within, the corpuscular structure of those bodies, and that it is developed by the decomposition of the particles under the influence of our oxygenous atmosphere.