if we add a quantity of salt to the vessel of water, so as to form a dense solution, the point where the rays strike the bottom will move still nearer to the window. In like manner, if we draw off the salt water, and supply its place with alcohol, the beam of light will be still more highly refracted; and oil will refract yet more highly than alcohol.

The following simple experiment is well known: Take an empty basin, and place it on a table; then lay a silver dollar at the bottom of the basin, and let the spectator withdraw so far that the brim of the basin hides the dollar. Now, fill the basin with water, and the dollar, though lying unmoved, will come completely into sight. The explanation of this phenomenon is, that the ray of light producing vision in the eye is bent, as it emerges from the water, and has all the effect of conveying our sight round a corner. The refractive power of water is also observable when we thrust a straight stick into it; we see that the stick seems to be bent, and fails in reaching the point which we desired it should. On this account, the aim, by a person not directly over a fish, must be made at a point apparently below it, otherwise the weapon will miss, by striking too high. With regard to the refractive power of transparent substances, or media, the general rule, with certain limitations, is in proportion to the densities of the bodies; it increases, for instance, from the most perfect vacuum which can be formed, through air, fresh water, salt water, glass, and so on. But those substances which contain the most inflammable matter have the highest refractive power. It was from the great refractive powers of the diamond

and water, that Newton, with admirable sagacity, predicted that they contained inflammable principles.

The refraction of rays of light is observable in the case of common window-glass. The two sides of a pane not being perfectly parallel to each other, bodies seen through it appear as if distorted; and as the obliquities in the glass are very various, the distortions are equally grotesque and numerous. Some windows are

purposely ground on the surface, to produce universal and minute refraction; and thus so great a confusion is introduced among the rays, that objects are not distinguishable through the glass. When the obliquities on the surface of one side of a piece of glass stand distinct from each other, so as to admit of refraction in a clear and distinguishable manner, then each obliquity affords a separate view of an object on the opposite side, and thus an object seems to be multiplied as many times as there are obliquities. The refraction of light is also observable, on a great scale, in relation to our atmosphere. The rays of the sun, on reaching the confines of the atmospheric fluid which envelops the earth, enter a medium of greater density than that through which they have previously passed, and consequently are refracted, or bent. One obvious effect of this is, that we never see the sun in the actual position which he occupies. He always appears more or less raised in relation to our eyes, as was the case with the dollar in the abovedescribed experiment of the basin of water. This is peculiarly the fact in the morning, when his earliest rays meet our eyes. Entering a denser medium, these rays bend round to meet our vision, and we actually see the body of the sun a few minutes before he has

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