DIPLOPIA, seeing a single object as double, occurs when ever one of the two eyes leaves the correct position of fixation, i.e., when one eye cannot focus on an object simultaneously with the other eye. An object clearly seen by the normal eye gives rise to an image in the deviated eye to one side of the macula, and is referred by the mind to a position different from that which it really occupies. Thus there seem to be two objects, one clearly seen with the normal eye, and one indistinctly seen by the deviated eye. Causes of deviation of one eye are (I) disturbances in the motor apparatus of the eye, (2) tumor or abscess in the orbit, (3) limitation of motion of the eyeball from without. Paralysis, or weakness of one or more of the eye muscles, is by far the commonest cause of ocular deviation, and it is in those cases which come on suddenly that diplopia is a striking feature. It occurs frequently in alcoholic intoxication and in encephalitis lethargica and less often in poisoning by lead or carbon monoxide, and in certain acute infections such as diphtheria and polioen cephalitis.