It has been calculated by Bouguer, that if our atmosphere, in its purest state, could be extended rather more than 700 miles from the earth’s surface instead of nearly 40, as it is at present, the sun’s rays could not penetrate it, and this globe would roll on in darkness and silence, without a vestige of vegetable form or of animal life. In the Hebrew version of the Mosaic History, the reading is, “Let light appear:” may not this really mean that the earth’s atmosphere was so cleared of obstructing vapours, that the solar rays were enabled to reach the earth? The same calculation supposes that sea-water loses all its transparency at the depth of 730 feet; but a dim twilight must prevail much deeper in the ocean.
The researches of Professor Edward Forbes have proved, that at the depth of 230 fathoms in the Ægean sea, the few shelled animals that exist are colourless: no plants are found within that zone; and that industrious naturalist fixes the zero of animal life of those waters at about 300 fathoms.[91] Since these zones mark the rapidly diminishing light, it is evident that where life ceases to be must be beyond the limits to which life can penetrate.
Our atmosphere, charged with aqueous vapour, serves to shield us from the intense action of the solar powers. By it we are protected from the destructive influences of the sun’s light and heat; enjoy those modified conditions which are most conducive to the healthful being of organic forms; to it we owe “the blue sky bending over all,” and those beauties of morning and evening twilight of which
—— Sound and motion own the potent sway.
Responding to the charm with its own mystery.
To defective transparency, or rather to the different degrees of it, we must attribute, in part, the colours of permeable media. Thus, a glass or fluid appears yellow to the eye, because it has the property of admitting the permeation of a larger quantity of the yellow rays than of any others;—red, because the red rays pass it with the greatest freedom; and so on for every other colour. In most cases the powers of transmission and of reflection are similar; but it is not so in all; a variety of fluor spar, which, while it transmits green light, reflects blue, and the precious opal, are striking instances to the contrary. Some glasses, which transmit yellow light have the singular power of dispersing blue rays from one surface; and a solution of quinine in water acidulated with sulphuric acid, although perfectly transparent and colourless when held between the eye and the light, exhibits, if viewed in a particular direction, a lively cerulean tint. These effects being supposed to be due to the conditions of the surface, have been called epipolic phenomena.[92]
The careful investigation of these phenomena has made us acquainted with some very interesting facts, and indeed discovered to us a set of luminous rays which were previously unknown. The dispersion of blue light from the surface of some yellow glasses—such as have been coloured by the oxide of silver—is of a different order from that which takes place with the solution of sulphate of quinine, or with the fluor spar. The first depends upon a peculiar condition of the surface, while the latter phenomena are due to a dispersion which takes place within the solid or fluid. In addition to the sulphate of quinine, and the fluor spar, we obtain the same results in a very marked manner by a canary yellow glass, coloured with the oxide of uranium, and by a decoction of the inner bark of the horse-chesnut tree. Mr. Stokes, who has investigated this class of phenomena, and proposes to call it Fluorescence, from its being naturally seen in fluor-spar, has shown that the peculiar internal dispersion, and the consequent alteration of the colour of the ray, is due to an alteration in its refrangibility. Whether this hypothesis prove to be the correct one or not, it is certain that there exists a set of rays of far higher refrangibility than those seen in the ordinary Newtonian spectrum. This may be shown in the following manner: taking either of the solutions named, or a block of uranium glass, throw upon one face, by means of a prism, a very pure spectrum. On looking into the glass or fluid there will be seen, commencing amidst the most refrangible rays, a new set of spectral rays, struggling to make their way through the absorbent medium. These are of a blue colour in the quinine or chesnut solution, and green in the uranium glass, and are seen extending themselves far beyond the most refrangible rays of the ordinary Newtonian spectrum. This is the space over which those rays which have the power of producing chemical changes, such as are rendered familiar by the practice of Photography, are detected in their greatest activity. It has, therefore, been supposed that these fluorescent rays are the chemical rays rendered luminous by the alteration of their refrangibility. This view has received much support from the fact that the extra spectral rays are crossed with numerous dark lines, and that in the chemical impressions these lines are marked by unchanged spaces which exactly coincide with them. There is, however, much doubt of the correctness of this, since, in the uranium glass of such a thickness that these visible rays are quite absorbed, the chemical rays still pass.
However, the whole question requires, and is receiving, the most searching investigation. The discovery of these phenomena, which are included under the term of Fluorescence, is of that interesting and important character, that it must be ranked as the most decided advance which has been made in physical optics since the days of Newton.
It is not improbable that those rays of such high refrangibility may, although they are under ordinary circumstances invisible to the human eye, be adapted to produce the necessary degree of excitement upon which vision depends in the optic nerves of the night-roaming animals. The bat, the owl, and the cat, may see in the gloom of night by the aid of rays which are invisible to, or inactive on the eyes of man, or of those animals which require the light of day for perfect vision.