Optical Illusions are produced when, owing to some peculiarity of the human eye, objects appear to be either in some place where they are not or to be different from what they really are. The phenomenon of persistence of vision, owing to which the impression on the retina of an object presented to the eye for a very short time remains for about one-tenth of a second, enables a number of illusions to be produced, as when a bright object rapidly swung round at the end of n string produces the effect of a continuous ring. If a number of different pictures, representing a moving object in different positions, are presented to the eye in rapid succession, and if a screen is interposed during the moment of change, the transition from one picture to the next is not noticed, and the figure itself seems to move; this is exemplified in the zoetrope and many similar devices. If a disc having differently-coloured sectors is rapidly rotated, it will appear to be uniformly of a colour which is a mixture of the component colours. Two objects seen separately with the two eyes may appear to occupy the same space; if one looks down a tube with one eye and holds a solid object such as a. brick near the other, it is not difficult to make it appear that there is a.hole through the brick. If a bird and a cage are drawn near each other on a sheet of paper which is held close to the eyes, the cage will be really seen by one eye and the bird with the other, but the bird will appear to walk into the cage. Many illusions may be produced by reflections. When an object is placed near a concave mirror and screened from direct view, an image or ghost may be seen at a corresponding conjugate focus (q.v.). When a sheet of clear glass is arranged across the stage of a theatre at an angle of 45° to the audience, an image of a figure behind the scenes will be seen on the stage; this is the well-known "Pepper's Ghost." If a coloured surface be viewed for a time sufficient to fatigue the retina, and if the eyes be quickly turned to a white surface, the latter will appear tinged with the complementary colour. The term optical illusion is commonly used todesignate tricks rather than more important optical phenomena, which, of course, do really produce an illusory effect. Optic Atrophy. This condition is detected by the altered appearance presented by the optic disc on ophthalmoscopic examination. The main symptom of the disease is failure of vision, and thqre is usually some alteration of the colour sense. Atrophy'may be a sequel of optic neuritis (q.v.), or may occur apart from previous inflammatory change. The latter form of atrophy is met with in locomotor ataxia, in syphilis, and in some other conditions. The treatment of optic atrophy is, as a rule, unsatisfactory, cases tending to proceed from bad to worse.