(From duplus, and visus).

A depravity of sight, by which the same objects appear double. The symptom is almost always of short dura-tion, and we bear it freely. So long as the object is not within the distance of distinct vision, two images opposite to one faramen, or aperture, having fallen upon the eye, are not united in the retina, but in distinct places; and,therefore, they have not the optic point as a centre: whence the image appears double. The optic portion is a circular point in the bottom of the eye, whose centre the optic axis occupies: but as often as we look at any object with both eyes, so often, unless there should be some defect in the organs, we turn the eyes, that each axis may concur in the same point of the object; and we learn by long habit, that a double image answers to one object, and consequently we judge that object single: but if a double image should fall upon the same eye, and not concur in the optic point, then the same object appears to be seen in two different places, and therefore double. It frequently arises from weakness, when we lose the power over the muscles of the eye, so as not to be able to direct them with accuracy. The diplopia, then, is the forerunner of death, or in fevers, of delirium. At times, the defect seems to be in the brain beyond the eye; and it has been sometimes an early symptom of hydrocephalus, or of an abscess in the basis of the brain near the thalami nervorum opticorum. Dr. Cullen makes it a variety of the second species of pseudoblepsis, which he calls mutans, in which objects appear changed from what they really are: and the disease varies according to the variety of the remote cause, of which he enumerates, from Sau-vages, ten species. See Nosologia Methodica Sauva-gesii, et Culleni. Wallis's Nosologia Methodica Ocu-lorum, with notes.