OPTICAL ILLUSION. An object ap pears large or small, near or distant, ac cording 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, the object must be small or dis tant. Experience alone enables us to de cide 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 by a comparison of the object in posi tion with other common objects, such as trees, houses, etc. The same is, of course, true of apparently small objects. But when all means for comparison are removed our judgment is at fault. Sim ilarly, we erroneously infer spherical solids at a distance to be flat disks, and a man in a white habit seems larger than he would if he wore a dark dress. Illu sions are also produced by external causes.
The persistence of impressions on the retina for about one-sixth of a second after the object which produced the im pression has been removed produces an other class of illusions. Common exam
ples of this are the illuminated circle formed by the rapid revolution of an ig nited carbon point, piece of red-hot iron, or other luminous body. Another form of illusion is produced to a person who is seated in a vehicle in motion. The illu sion is most complete when the attention is riveted on an object several yards off; this object then appears to be a center round which all the other objects re volve, those between the observer and the object moving backward, and those be yond the object moving forward. Other illusions arise from a disordered state of the organs of vision: e. g., the seeing of things double or movable, or of a color different from the true one.