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!" Dr. Vince, an English philosopher, was once looking through a telescope at a ship which was so far off that 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.