which he was standing, a wind-mill, his own figure and that of a friend, depicted immediately opposite to him on the sea. This appearance lasted about ten minutes, till the sun had risen nearly his own diameter above the surface of the waves. The whole then seemed to be elevated into the air and successively vanished. The rays of the sun fell upon the cliff at an incidence of 73° from the perpendicular, and the sea was covered with a dense fog many yards in height, which gradually receded before the rising sun. When extraordinary refraction takes place laterally, the strata of variable density are perpendicular to the horizon, and when it is combined with vertical refraction, the objects are magnified as if seen through a telescope. From this cause, on the 26th of July, 1798, the cliffs of France, fifty miles off, were seen as distinctly from Hastings as if they had been close at hand, and even Dieppe was said to have been visible in the afternoon.

The stratum of air in the horizon is so much thicker and more dense than the stratum in the vertical, that the sun's light is diminished 1300 times in passing through it, which enables us to look at him when setting without being dazzled. The loss of light, and consequently of heat, by the absorbing power of the atmosphere, increases with the obliquity of incidence. Of ten thousand rays. falling on its surface, 8123 arrive at a given point of the earth if they fall perpendicularly; 7024 arrive if the angle of direction be fifty degrees; 2831 if it be seven degrees; and only five rays will arrive through a hori zontal stratum. Since so great a quantity of light is lost in passing through the atmosphere, many celestial objects may be altogether invisible from the plain, which may be seen from elevated situations. Diminished splendor and

the false estimate we make of distance from the number of intervening objects, lead us to suppose the sun and moon to be much larger when in the horizon than at any other altitude, though their apparent diameters are then somewhat less. Instead of the sudden transitions of light and darkness, the reflective power of the air adorns nature with the rosy and golden hues of the Aurora, and twilight. Even when the sun is eighteen degrees below the horizon, a sufficient portion of light remains to show that, at the height of thirty miles, it is still dense enough to reflect light. The atmosphere scatters the sun's rays, and gives all the beautiful tints and cheerfulness of day. It transmits the blue light in greatest abundance; the higher we ascend, the sky assumes a deeper hue, but in the expanse of space, the sun and stars must appear like brilliant specks in profound blackness.

SECTION XX.

It is impossible thus to trace the path of a sunbeam through our atmosphere without feeling a desire to know its nature, by what power it traverses the immensity of space, and the various modifications it undergoes at the surfaces and at the interior of terrestrial substances.

Sir Isaac Newton proved the compound nature of white light, as emitted from the sun, by passing a sunbeam through a glass prism, which, separating the rays by refraction, formed a spectrum or oblong image of the sun, consisting of seven colors, red, orange, yellow, green, blue, indigo and violet; of which the red is the least refrangible, and the violet the most; but when he reunited these

seven rays by means of a lens, the compound beam became pure white as before. He insulated each colored ray, and finding that it was no longer capable of decomposition by refraction, concluded that white light consists of seven kinds of homogeneous light, and that to the same color the same refrangibility ever belongs, and to the same refrangibility the same color. Since the discovery of absorbient media, however, it appears that this is not the constitution of the solar spectrum.

We know of no substance that is either perfectly opaque or perfectly transparent; for even gold may be beaten so thin as to be pervious to light; and, on the contrary, the clearest crystal, the purest air or water, stop or absorb its rays when transmitted, and gradually extinguish them as they penetrate to greater depths. On this account, objects cannot be seen at the bottom of very deep water, and many more stars are visible to the naked eye from the tops of mountains than from the valleys. The quantity of light that is incident on any transparent substance is always greater than the sum of the reflected and refracted rays. A small quantity is irregularly reflected in all directions by the imperfections of the polish by which we are enabled to see the surface; but a much greater portion is absorbed by the body. Bodies that reflect all the rays appear white; those that absorb them all seem black; but most substances, after decomposing the white light which falls upon them, reflect some colors and absorb the rest. A violet reflects the violet rays alone, and absorbs the others; scarlet cloth absorbs almost all the colors except red; yellow cloth reflects the yellow rays most abundantly, and blue cloth those that are blue; consequently color is not a property of matter, but arises from the action of matter

but

upon light. Thus a white ribbon reflects all the rays, when dyed red, the particles of the silk acquire the property of reflecting the red rays most abundantly and of absorbing the others. Upon this property of unequal absorption, the colors of transparent media depend; for they also receive their color from their power of stopping or absorbing some of the colors of white light and transmitting others; as, for example, black and red ink, though equally homogenous, absorb different kinds of rays; and when exposed to the sun, they become heated in different degrees, while pure water seems to transmit all rays equally, and is not sensibly heated by the passing light of the sun. The rich dark light transmitted by a smalt blue finger-glass is not a homogenous color, like the blue or indigo of the spectrum, but is a mixture of all the colors of white light which the glass has not absorbed; and the colors absorbed are such as mixed with the blue tint, would form white light. When the spectrum of seven colors is viewed through a thin plate of this glass, they are all visible; and when the plate is very thick, every color is absorbed between the extreme red and the extreme violet, the interval being perfectly black. But if the spectrum be viewed through a certain thickness of the glass intermediate between the two, it will be found that the middle of the red space, the whole of the orange, a great part of the green, a considerable part of the blue, a little of the indigo, and a very little of the violet, vanish, being absorbed by the blue glass; and that the yellow rays occupy a larger space, covering part of that formerly occupied by the orange on one side, and by the green on the other; so that the blue glass absorbs the red light, which, when mixed with the yellow, constitutes orange; and also ab

sorbs the blue light, which when mixed with the yellow forms the part of the green space next to the yellow. Hence, by absorption, green light is decomposed into yellow and blue, and orange light into yellow and red. Consequently the orange and green rays, though incapable of decomposition by refraction, can be resolved by absorption, and actually consist of two different colors possessing the same degree of refrangibility. Difference of color, therefore, is not a test of difference of refrangibility, and the conclusion deduced by Newton is no longer admissible as a general truth. By this analysis of the spectrum, not only with blue glass but with a variety of colored media, Sir David Brewster, so justly celebrated for his optical discoveries, has proved, that the solar spectrum consists of three primary colors, red, yellow, and blue, each of which exists throughout its whole extent, but with different degrees of intensity in different parts; and that the superposition of these three produces all the seven hues according as each primary color is in excess or defect. Since a certain portion of red, yellow, and blue rays constitute white light, the color of any point of the spectrum may be considered as consisting of the predominating color at that point mixed with white light; consequently, by absorbing the excess of any color at any point of the spectrum above what is necessary to form white light, such white light will appear at that point as never mortal eye looked upon before this experiment, since it possesses the remarkable property of remaining the same after any number of refractions, and of being capable of decomposition by absorption alone.

When the prism is very perfect and the sun-beam small so that the spectrum may be received on a sheet of white