rendered visible by Light. 565 on black velvet, which returns hardly any light, the apartment will remain nearly dark. When the ray is received on a polished surface or mirror, which returns nearly the whole light, but only in one direction, and therefore throws it upon some other single object, the effect will be according to the nature of that object, and nearly as if the beam had fallen directly upon it. Now all bodies on earth, and among these the particles of the atmosphere, diffuse among themselves the light received directly from the sun, and by so doing, maintain everywhere that mild radiance or luminosity agreeable to the sight, which renders objects visible when the sun's direct rays do not fall upon them. It is this which constitutes the shade, or non-illumined part of the body, as distinguished from the shadow where the light is directly intercepted. But for this fact, indeed, all bodies shadowed from the sun, whether by intervening clouds or by any other more opaque masses, would be perfectly black or dark; that is, totally invisible. If the earth had no atmosphere, the sun would appear as a round intensely luminous mass in a perfectly black sky. On lofty mountain summits, where half the atmosphere is below the level, the direct rays of the sun are painfully intense, and the sky is by contrast dark. The moon is without an atmosphere, so that on the surface of this satellite, there is a sudden transition from absolute light to absolute darkness in all parts which do not receive the direct light of the sun. Under a good telescope, these deep black shadows of the mountains are easily seen, and the very sudden transition from light to darkness, is sharply marked by the irregular ridges on the illuminated side of the moon. 793. In the absence of any material substance to reflect it, light itself is, therefore, invisible. Professor Tyndall has applied this property of invisibility to the determination of the purity of atmospheric air. Smoke, or particles of dust, in any space traversed by the rays of light, reflect it and give an apparent body to the rays. When these motes, or particles, are removed from a confined quantity of air under a glass shade, by means of a small quantity of glycerine (which seems to absorb and fix them), the light traverses the shade without its presence being in any way indicated. There are, indeed, no material particles to reflect it. If, in a darkened room, a beam of light is thrown through three glass shades in succession,—the first and third containing the ordinary atmosphere of a crowded room, and the third purified by glycerine, the light is plainly seen traversung the first shade; it is entirely lost in the second, but re-appears 566 Colour of the Sea and Atmosphere. in the third. A more delicate test of the freedom of the atmosphere from imponderable particles of suspended matter, could not be desired. 794. The green and blue colours of sea-water are ascribed by Tyndall to the presence of finely suspended matters. When solar light meets the surface of the sea, the red rays are first extinguished, orange and yellow follow as the beam penetrates deeper into the sea, green foliows yellow, and the various shades of blue, where the water is deep enough, follow green. Absolute extinction of the solar beam would be the consequence, if the water were deep and uniform and contained no suspended matter. Such water would be as black as ink. A reflected glimmer of ordinary light would reach us from its surface, as it would from the surface of actual writing ink, but no light, hence no colour, would reach us from the body of the water. In very clear and very deep sea-water this condition is approximately fulfilled, and hence the extraordinary darkness of such water. The coloured rays reflected by minute suspended particles in a shallow sea, are either green or of a light blue. It also results from experiments performed by this physicist, that the blue light of the sky is entirely due to reflected light, and were there nothing in our atmosphere capable of reflecting the solar rays, we should see no blue firmament, but should look into the darkness of infinite space. The reflection of the blue is effected by minute colourless particles seen in the mass. Smallness of size alone is requisite to insure the selection and reflection of this colour. While the blue is owing to reflected, the crimson or orange glow of the sky in the evening and the morning, as well as the red colour of the sun and moon when on the horizon, is due, on the other hand, to transmitted light, that is to say, to light which in its passage through great atmospheric distances, has had its blue constituents sifted out of it by repeated reflection. "Light proceeds in straight lines, leaving shadows where it cannot fall." 795. A ray or straight line of light is seen on looking at the light of the sun entering into a dark room by a small aperture, in which fine dust is floating, and seen there as a line of "motes in the sunbeam." We cannot sce round a corner. We can see through a straight tube, but not through a crooked one. The vista through a long, straight tunnel is a striking illustration of this fact, as also of the Light proceeds in Straight Lines. 567 diminution of the apparent size of objects as they are more distant. If a person enter a straight canal-tunnel, two miles long, like that cut through the chalk hill between Gravesend and Rochester to join the Thames and Medway rivers (now converted into a railway), the opening at the distant end is clearly seen as a small luminous speck, having the exact form of the general arch. In taking aim with a gun or an arrow, one is merely preparing to make the projectile go to the desired object nearly by the path along which the light comes directly from the object to the eye. A carpenter looks along the edge of a plank to see whether it be straight. Because light moves in straight lines, if a number of similar objects are placed in a straight line with the eye, the nearest one hides the others. In the middle of a wood or a city, a person sees completely only a few of the trees or houses that are nearest to him. The forms of shadows prove that light moves in straight lines, for the outline of the shadow corresponds correctly with that of the object as seen from the luminous body. The shadow of a face on a wall is a correct profile or projection. A wheel presented edgeways to the eye appears only as a broad strip, but it seems oval or round as it is otherwise turned; so a wheel presented edgeways to the sun or other light, casts at first a straight shadow on the wall behind it, and the shadow becomes oval or round as the position is changed. A globe, a cylinder, a cone, and a flat circle, will all throw the same round shadow on a wall, if held with their axes pointing to the luminous body, and therefore by the shadow only, these objects could not be distinguished. A man under a vertical sun, as may be seen near the equator, stands at noon upon his own little round shadow; but as the sun declines in the afternoon, the shadow juts out on the opposite side, and at last may be so long as to extend across a field. A distant cloud, which appears to the eye of an observer on the ground only as a streak along the sky, may yet be broad enough to shadow a whole region; for clouds generally form in level strata, and when viewed by a spectator on the ground at a distance are scen nearly edgeways. 796. A body held between a source of light and a wall, not only darkens a portion of the wall, or casts its shadow there, but makes the whole space between it and the wall a shadowed space, so that any object introduced there, is as much shadowed as the portion of 568 Production of Shadows. the wall. A striking illustration of this is afforded when a flock of white pigeons in sunshine wheel round a tall steeple. At the moment when they enter the shadow of the steeple they seem to vanish, although there is nothing in the air between them and the eye of the spectator. Thus, also, all the planets, as they revolve about the sun, cast a long shadow beyond them or away from the sun, and when one of their satellites or moons passes where the shadow is, it suddenly disappears. The satellites or moons of Jupiter, when they suddenly disappear from the field of view of our telescopes, or are eclipsed as we term it, have generally only plunged into the shadow of the planet, and are not hidden from us by being then on the other side of its body, as many persons suppose. When our own moon is eclipsed,—a phenomenon so awful to men in the early ages of the world, she is only passing through the long shadow which the earth casts beyond itself. 797. In the case of a light-giving centre being broader than the object which casts a shadow, the shadow is less broad than the object, and terminates in a point. In the contrary case, the shadow is larger than the object, and still larger as the distance from the object is greater. When our moon passes between this earth and the broad sun, producing a total eclipse of the sun where its shadow falls, the band over which the shadow passes has to be computed, and is stated in the almanacs. Knowledge of the comparative sizes of the two globes gives the power of predicting what is to happen. The shadow of a hand held between a broad fire and the wall is small; while a small pasteboard figure of a man, placed near a narrow centre of light, like a candle, throws a shadow which is gigantic. When the surface which receives a shadow is oblique to the direction of the light, the shadow may be much longer or broader than the object, even when the sun is throwing the light. This is seen when a narrow projecting roof, or a veranda, shadows from the high sun of a summer noon, the whole front of a house; or, as is proved by the long morning and evening shadows of trees, houses, men, and other objects, on the ground in all countries. 798. The apparent darkness of a shadow is not proportioned to its real darkness, but to the comparative strength of the surrounding lights. A landscape may be clearly seen, even when the sun is veiled by a cloud, and then little or no shadow is anywhere per. ceived; but as soon as the cloud passes away, deep bright lights appear where direct sunshine falls, and shadows behind every object which the direct sunbeam does not reach. Yet the objects and places Photometry. Photometers. 569 the appearing dark, are in reality more illuminated than before the shadow existed, for they are receiving, and again scattering, new light from all the more intensely illuminated objects around them. A finger held between a candle and the wall, casts a shadow of a certain intensity; if another candle be then placed in the same line from the shadow, the shadow will appear much darker, although, in fact, more light will be reaching it, and reaching the eye from it, than before; it will be darker only by comparison. If the candles be separated a little sideways, so as to produce two shadows of the finger, but which coincide or overlap in one part, that part will be of double darkness, as compared with the remainders. A common mode of comparing the intensity of lights is to place them at such distances from a screen or wall, as to make them at the same time throw on the screen equally dark shadows of the same intervening object; and then, according to the law of decreasing intensity explained above, to calculate the intensities of the sources of light by the difference of their distances from the wall. The eye judges very correctly of the intensity of shadows so compared. 799. Bunsen's Photometer.*—In determining the relative illuminating power of coal-gas another method is adopted. A gas jet is burned at one end of a straight line, and a sperm candle of a certain standard quality is placed at the other end, and the space between the two is so graduated as to show how many times the one light exceeds the other in intensity. A disc of white blotting paper is rendered partly translucent by means of a solution of paraffine in benzole, the central portion of the disc remaining unchanged. This is mounted in a sliding frame, so that the light from each end can fall upon the corresponding side. So long as the lights are of unequal intensity, the opaque and translucent parts of the paper will be plainly distinguishable, but when they are equal, the difference will disappear, and the distance of the two lights from the movable screen will show their relative intensity marked off, as twelve, fifteen, twenty, or more candles. An Act of Parliament requires that a certain standard gas, tested on this principle, shall be supplied to the public. The action of the photometer depends on the fact that the paper disc presents a uniform appearance only when the light which passes through the translucent part is equal to that which is reflected by the opaque portion. Two gaslights or candles burning one at each end of the sliding scale, would show no difference in the paper when the disc was placed in the centre. * From pws, light, and μerpov, measure. |