7. It is by light reflected from opake bodies that most of the objects around us are rendered visible. When a lighted candle is brought into a dark room, not only the candle but all other bodies in the room become visible. Rays of the sun passing into a dark room render luminous a sheet of paper on which they fall, and this sheet in its turn enlightens, to a certain extent, the whole apartment, and renders objects in it visible, so long as it receives the rays of the sun. In like manner, the moon and the planets are opake bodies, but the light of the sun falling upon them, and being reflected from their surfaces, renders them visible. Were no light to fall on them from the sun, or were they not endued with a power of reflecting it, they would be altogether invisible to our sight. When the moon comes between us and the sun, as in a total eclipse of that luminary, as no solar light is reflected from the surface next the earth, she is invisible—only the curve or outline of her figure being distinguished by her shadow. In this case, however, there is a certain portion of reflected light on the lunar hemisphere next the earth, though not distinguishable during a solar eclipse. The earth is enlightened by the sun, and a portion of the rays which fall upon it is reflected upon the dark hemisphere of the moon which is then towards the earth. This reflected light from the earth is distinctly perceptible, when the moon appears as a slender crescent, two or three days after new moon—when the earth reflects its light back on the moon, in the same manner as the full moon reflects her light on the earth. Hence, even at this period of the moon, her whole face becomes visible to us, but its light is not uniform or of equal intensity. The thin crescent on which the full blaze of the solar light falls, is very brilliant and distinctly seen, while the other part, on which falls only a comparatively feeble light from the earth, appears very faint, and is little more than visible to the naked eye, but with a telescope of moderate power,—if the atmosphere be very clear—it appears beautifully distinct, so that the relative positions of many of the lunar spots may be distinguished.
The intensity of reflected light is very small, when compared with that which proceeds directly from luminous bodies. M. Bouguer, a French philosopher, who made a variety of experiments to ascertain the proportion of light emitted by the heavenly bodies, concluded from these experiments, that the light transmitted from the sun to the earth is at least 300,000 times as great as that which descends to us from the full moon—and that, of 300,000 rays which the moon receives, from 170,000 to 200,000 are absorbed. Hence we find that, however brilliant the moon may appear at night—in the day time she appears as obscure as a small portion of dusky cloud to which she happens to be adjacent, and reflects no more light than a portion of whitish cloud of the same size. And as the full moon fills only the ninety thousandth part of the sky, it would require at least ninety thousand moons to produce as much light as we enjoy in the day-time under a cloudy sky.
As the moon and the planets are rendered visible to us only by light reflected from their surfaces, so it is in the same way that the images of most of the objects around us are conveyed to our organs of vision. We behold all the objects which compose an extensive landscape,—the hills and vales, the woods and lawns, the lakes and rivers, and the habitations of man—in consequence of the capacity with which they are endued of sending forth reflected rays to the eye, from every point of their surfaces and in all directions. In connection with the reflection of light, the following curious observation may be stated. Baron Funk, visiting some silver mines in Sweden, observed, that, ‘in a clear day, it was as dark as pitch underground in the eye of a pit, at sixty or seventy fathoms deep; whereas, in a cloudy or rainy day, he could see to read even at 106 fathoms deep. Enquiring of the miners, he was informed that this is always the case; and reflecting upon it, he imagined it arose from this circumstance, that when the atmosphere is full of clouds, light is reflected from them into the pit in all directions, and that thereby a considerable proportion of the rays are reflected perpendicularly upon the earth: whereas when the atmosphere is clear, there are no opaque bodies to reflect the light in this manner, at least in a sufficient quantity; and rays from the sun himself can never fall perpendicularly in that country.’—The reason here assigned is, in all probability, the true cause of the phenomenon now described.
8. It is supposed by some philosophers that light is subject to the same laws of attraction that govern all other material substances—and that it is imbibed and forms a constituent part of certain bodies. This has been inferred from the phenomena of the Bolognian stone, and what are generally called the solar phosphori. The Bolognian stone was first discovered about the year 1680, by Leascariolo, a shoe-maker of Bologna. Having collected together some stones of a shining appearance at the bottom of Monte Paterno, and being in quest of some alchemical secret, he put them into a crucible to calcine them—that is, to reduce them to the state of cinders. Having taken them out of the crucible, and exposed them to the light of the sun, he afterwards happened to carry them into a dark place, when to his surprise, he observed that they possessed a self-illuminating power, and continued to emit faint rays of light for some hours afterwards. In consequence of this discovery, the Bolognian spar came into considerable demand among natural philosophers and the curious in general; and the best way of preparing it seems to have been hit upon by the family of Zagoni, who supplied all Europe with Bolognian phosphorus, till the discovery of more powerful phosphoric substances put an end to their monopoly.—In the year 1677, Baldwin, a native of Misnia, observed that chalk dissolved in aqua-fortis exactly resembled the Bolognian stone in its property of imbibing light, and emitting it after it was brought into the dark ; and hence it has obtained the name of Baldwin’s phosphorus.
In 1730 M. du Fay directed his attention to this subject, and observed that all earthy substances susceptible of calcination, either by mere fire, or when assisted by the previous action of nitrous acid, possessed the property of becoming more or less luminous, when calcined and exposed for a short time in the light—that the most perfect of these phosphori were limestones, and other kinds of carbonated lime, gypsum, and particularly the topaz, and that some diamonds were also observed to be luminous by simple exposure to the sun’s rays. Sometime afterwards, Beccaria discovered that a great variety of other bodies were convertible into phosphori by exposure to the mere light of the sun, such as, organic animal remains, most compound salts, nitre and borax—all the farinaceous and oily seeds of vegetable substances, all the gums and several of the resins—the white woods and vegetable fibre, either in the form of paper or linen; also starch and loaf-sugar proved to be good phosphori, after being made thoroughly dry, and exposed to the direct rays of the sun. Certain animal substances by a similar treatment were also converted into phosphori; particularly bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The same property was communicated to rock crystal and some other of the gems, by rubbing them against each other so as to roughen their surfaces, and then placing them for some minutes in the focus of a lens, by which the rays of light were concentrated upon them, at the same time that they were also moderately heated.
In the year 1768 Mr. Canton contributed some important facts in relation to solar phosphori, and communicated a method of preparing a very powerful one, which, after the inventor, is usually called Canton’s phosphorus. He affirms that his phosphorus, enclosed in a glass flask, and hermetically sealed, retains its property of becoming luminous for at least four years, without any apparent decrease of activity. It has also been found that, if a common box smoothing-iron, heated in the usual manner, be placed for half a minute on a sheet of dry, white paper, and the paper be then exposed to the light, and