OPTICAL ILLUSION. Of all the senses none is more deceptive than the sense of sight; it often deceives us as to the distance, size, shape, and color of objects; it fre makes them appear as it In &tuations where their existence is impossible; and often makes us think them movable when they are not so, and rice versa. An object appears to us as large or small, near or distant, according as the rays from its opposite borders meeting at the eye form a large or a small angle: when the angle is large, the object is either large or near; when small, thet object must be small or distant. Practice alone enables us to decide whether an object of large apparent size is so on account of its real size, or of its proximity; and our decision.is arrived at bya comparison of the object in position, with other common objects, such as trees, houses, etc., which may chance to be near it, and of which we have by experience come to form a correct idea. The same is, of course, true of apparently small objects. But when all means for comparison are removed, as when we see a distant object floating on an extensive sheet of water, or erect in an boundless sandy plain, where no other object meets the eye, then our lodgment is vompletely at fault. Imperfection in the acquired perceptions of sight, as it is called, produces many other illusions; it leads us to consider spherical-solids at a distance as flat discs, and deceives us regarding the size of objects, by their color; the sun appears larger than lie would if illumined by a fainter light, and a man in a, white habit seems larger than lie would if he wore a dark dress. Illusions are also produced
by external causes; and instances of this sort are given under MIRAGE, REFLECTION, and REFRACTION.
The property which the eye possesses of retaining an impression for a very brief, though sensible period of time (about one-quarter of a second), after the object which produced the impression has been removed, produces a third class of illusions. Comnion examples of this are the illuminated circle formed by the rapid revolution of an ignited carbon point, piece of red-het iron, or other luminous body, and the fiery curve pro duced by a red-hot shot projected from a cannon.
Another form of illusion is produced to a person who is seated in avehicle in motion, and it is very deceptive when the motion is so equable as not to be felt by the person him self. The illusion is most complete when the attention is riveted on an object several yards off; this object then appears as a center round which all the other objects seem to revolve, those between the observer and the object moving backwards, and those beyond the object moving forwards. This illusion occurs on a large scale in the apparent motion of the heavenly bodies.
Other illusions arise from a disordered state of the organs of vision; such are the seeing of things doable or movable (if they are not so), or of a color different from the true one; the appearance as of insects crawling over a body at which the eye is directed, etc.