The eye is subject to deception, or illusion. Sometimes we mistake the shadow for the substance. The rays of light, passing through media of different densities, are refracted at different angles; hence the direction or nearness of different bodies becomes apparently altered, and experience alone corrects the error. On putting a pole or stick obliquely into clear water, the submerged portion seems to be bent at the surface of the water, and to rise higher in the water than it actually is. The bottom of a deep, clear river, or pond, seems to be nearer to the surface than it is in reality; in other instances, the water deceives by an apparent shallowness, the truth of which the inexperienced bather should always test before venturing in. If a gold or silver coin be put at the bottom of a basin, and the basin be then filled with water, and the eye be obliquely directed, so as not to see the coin, hidden by the rim of the basin, it will become apparent when the basin is filled. In spearing fish, allowance is made by the experienced for the difference of refraction of the rays of light in a rare and more dense medium. It is probable that birds which prey upon fish, by darting at them while at a certain distance under the water, as, for example, the fish-hawk, or osprey, and the gannet, are taught by _instinct_ to correct an apparent error of sense, or rather, an illusion, which man corrects by practice and experience. The eye is deluded by the mirage. The mirage (a French word, adopted into our language) is "the name given to a phenomenon of unusual attraction, for which we have no specific appellative, unless it be the sea-term _looming_. As a general definition, we may say, the mirage is an optical illusion, occasioned by the refraction of light through contiguous masses of air of different density, such refraction not unfrequently producing the same sensible effect as direct reflection. "The illusions of the mirage differ according to circumstances, but they may be all arranged under one or other of the three following classes: _vertical reflection_, _horizontal_ or _lateral reflection_, and _suspension_."