risen above the horizon; like the dollar in the basin, we see him round a corner. In proportion as the sun approaches the zenith, the refraction diminishes; and as he recedes toward setting, it increases. So considerable is it, in the hazy atmosphere of evening, that we retain a sight of the sun's disk after it has sunk. The same phenomena occur in relation to the other heavenly luminaries.

From these explanations, it will appear that the directness of our vision is at all times liable to be disturbed by atmospheric conditions. So long as the atmosphere between our person and the object we are looking at is of the same density, we may be said to see in a straight line to the object. But if, by any cause, a portion of that atmosphere is rendered less or more dense, the line of vision is bent, or refracted, from its course. A thorough comprehension of this truth in science has banished a mass of superstition. It has been found that, by means of powerful refraction, objects at great distances, and round the back of a hill, or considerably beneath the horizon, are brought into sight. In some countries this phenomenon is called mirage. The following is one of the most interesting and best-authenticated cases of the kind. In a voyage performed by Captain Scoresby, in 1822, he was able to recognize his father's ship, when below the horizon, from the inverted image of it which appeared in the air. "It was," says he, "so well defined, that I could distinguish, by a telescope, every sail, the general rig of the ship, and its particular character, insomuch that I confidently pronounced it to be my father's ship, the Fame, which it afterwards proved to be, though,

on comparing notes with my father, I found that our relative position, at the time, gave our distance from one another very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues beyond the limit of direct vision!"

he could only see the upper part of the masts. The hull was entirely hidden by the bending of the water; but, between himself and the ship, he saw two perfect images of it in the air. These were of the same form and color as the real ship; but one of them was turned completely upside down.

In the sandy plains of Egypt, the mirage is seen to great advantage. These plains are often interrupted by small eminences, upon which the inhabitants have built their villages in order to escape the inundations of the Nile. In the morning and evening, objects are seen in their natural form and position; but when the surface of the sandy ground is heated by the sun, the land seems terminated, at a particular distance, by a general inundation; the villages which are beyond it appear like so many islands in a great lake; and an inverted image of a village appears between the hills.

The Swedish sailors long searched for a supposed magic island, which, from time to time, could be descried between the Island of Aland and the coast of Upland. It proved to be a rock, the image of which was presented in the air by mirage. At one time, the English saw with terror the coast of Calais and Boulogne, in France, rising up on the opposite side of the Channel, and apparently approaching their island. But the most celebrated example of mirage is exhibited in the Straits of Messina. The inhabitants of the Calabrian shore behold images of palaces, embattled ramparts, houses, and ships, and all the varied objects of towns and landscapes, in the air-being refracted images from the Sicilian coast. This wonderful phenomenon is

regarded by the common people as the work of fairies, and is known by the name of the fata morgana.

COLOR BY REFRACTION. One of the most remarkable phenomena attending refraction, is, that the rays of light, which seem to us to be white, may be separated into rays of various colors. It will be obvious that light has the effect of representing colors when no color substantially exists, by noticing the glancing and varied hues on irregular surfaces of glass, ice, or other crystallized substances. The proper method of analyzing the rays of light, and discovering into what colors they may be resolved, is by the use of a prism, or three-sided rod of glass. The experiment may be made in the following manner: Into a darkened room admit a beam of sunlight through a hole in the shutter; let this fall upon the prism, and, instead of passing in a direct line through it, and forming a circular white spot upon the wall opposite, the rays will be refracted upwards, and form an oblong image upon the wall, divided into seven colors-red, orange, yellow, green, blue, indigo, and violet. This lengthened image of the sun is called the solar or prismatic spectrum. No lines are seen across the divisions between the different colors, and it is extremely difficult for the sharpest eye to point out their boundaries. This experiment shows that common white light is compounded of seven different colors, and that they all differ in their powers of refraction; that is, the glass, or whatever medium through which they pass, attracts no two of them with the same degree of force. As they differ in refraction, so also they differ in their powers of reflection; and hence arise all the various colors of bodies. Those bodies

which reflect only the red rays, for instance, and absorb all the others, appear red; and so of the other colors. Those which reflect all the rays appear white, and those which absorb all the rays, or nearly so, appear black. Hence it is that black clothes are warmer than any other color, as they absorb more light, and light is never unaccompanied by heat. On the other hand, white is the coolest dress that can be worn.

The rainbow is formed by a combined process of reflection and refraction. It is never seen, except when rain is falling between the spectator and the sky opposite the sun. If we look into a globe of glass, or water, held above the head, and opposite to the sun, we shall see a prismatic spectrum reflected from the farther side of the globe. In this spectrum, the violet rays will be innermost, and the spectrum vertical. If we hold the globe on a level with the eye, so as to see the sun's light reflected in a horizontal plane, we shall see a horizontal spectrum with the violet rays innermost; and a corresponding variation will be observed in other positions. Now, since, in a shower of rain, there will be drops in all positions relative to the eye, the eye will receive spectra inclined at all angles to the horizon; so that, when combined, they will form the large, curved spectrum called the rainbow. In a very strong sunlight, a secondary bow is seen outside of the primary one: the colors are fainter, because the bow consists of rays that have suffered two reflections instead of one. Red rainbows, distorted rainbows, and inverted rainbows on the grass, have been observed. The latter are formed by the drops of rain suspended on the spiders' webs in the fields.