Write with French chalk on a looking-glass; wipe it with a handkerchief, and the lines will disappear; breathe on it, and they will re-appear. This alteration will take place for a great number of times, and after the lapse of a considerable period. [Illustration] [Illustration: SIGHT & SOUND.] [Illustration] SIGHT AND SOUND. ARTIFICIAL MIRAGE. THE mirage is an optical phenomenon, produced by the refractive power of the atmosphere. The appearance presented is that of the double image of an object in the air; one of the images being in the natural position, and the other inverted, so as to resemble a natural object and its image in the water. The mirage is commonly vertical, or upright, that is, presenting the appearance, above described, of one object over another, like a ship above its shadow in the water. Sometimes, however, the image is horizontal, or upon the water, and at other times, it is seen on the right or left hand of the real object, or on both sides. All the effects of the mirage may be represented artificially to the eye. For this purpose, provide a glass tumbler two-thirds full of water, and pour spirit of wine upon it; or pour into a tumbler some syrup, and fill it up with water: as the water and spirit, or the syrup and water incorporate, they will produce a refractive power; then, by looking through the mixed or intermediate liquids at any object held behind the tumblers, its inverted image may be seen. The same effect, Dr. Walloston has shown, may be produced, by looking along the side of a red-hot poker at a word or object ten or twelve feet distant. At a distance less than three-eighths of an inch from the line of the poker, an inverted image was seen; and within and without that, an erect image. The above phenomena may likewise be illustrated, by holding a heated iron above a tumbler of water, until the whole becomes changed; then withdraw the iron, and, through the water, the phenomena of the mirage may be seen in the finest manner. Or, look directly above the flame of a candle, or over the glass of a lighted lamp, and a tremulous motion may be observed; because the warm air rises, and its refracting power being less than that of the colder air, the currents are rendered visible by the distortion of objects viewed through them. The same effect is observable over chimney pots, and slated roofs which have been heated by the sun. MOTION OF THE EYE. On entering a room, we imagine that we see the whole side of it at once, as the cornice, the pattern of the paper-hanging, pictures, chairs, &c., but we are deceived; for each object is rapidly, but singly presented to the eye, by its constant motion. If the eye were steady, vision would be lost. For example, fix the eye on one point, and you will find the whole scene become more and more obscure, till it vanishes. Then, if you change the direction of the eye ever so little, at once the whole scene will be again perfect before you.