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A celebrated experiment of the Abbe Mariotte, which consists in placing two small round spots on a wall at some distance from each other, standing opposite the left-hand object, and looking at it with the right eye, the left being closed. By walking backwards, until the distance from the object is about five times as great as the distance between the two objects, the latter will be found to disappear. Mariotte and, after him, many ophthalmologists, inferred that the optic nerve, on which the ray doubtless falls in this experiment, is insensible; and hence that the choroid may be the seat of vision,-not the retina. The inference is illogical; for it doubtless falls on the part of the optic nerve where the central artery enters, and the central vein leaves the eyeball, and where there is necessarily no neurine.