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namely, that where the retina branches out from the optic nerve. This may easily be observed by placing three white patches of paper on a dark wall, at equal distances on the same horizontal line, and standing at about four or five times as far from the wall as the papers are from each other. If then one eye be closed, and the other be directed to one of the outside patches, the middle one will be quite invisible, though the other one, which is farther from the axis of the eye, is clearly perceptible.
Dr. Young makes this experiment rather differently. He says: "To find the place of the entrance of the optic nerve, I fix two candles at ten inches distance, retire sixteen feet, and direct my eye to a point four or five feet to the left of the middle of the space between them: they are then lost in a confused spot of light; but any inclination of the eye brings one or the other of them into the field of view."
147. It is undoubtedly experience alone that enables us to judge of the magnitudes and distances of objects by the sight, though the precise manner in which this takes place has never yet been satisfactorily determined.
All that the eye furnishes, in the first instance, is the angle, plane or solid, subtended by an external object: if this were the only criterion afforded, we should of course often imagine a small object to be larger than another greatly exceeding it, if the former were placed so much nearer the eye that it would mask the other, if they were in the same line from the eye. Now we know perfectly well, not only that children make constant mistakes of this nature, but that men are likewise very often deceived, when placed in situations in which they have not had previous experience to modify their observations. For instance, a native of this island placed for the first time of his life among the Alps, forms the most absurdly incorrect notions of the distances and magnitudes of the parts of those scenes, which are so much more extended than any to which he has been accustomed. An easier illustration of this may be had by ascending any eminence much greater than those which one is accustomed to look from. It is almost impossible to imagine that the human beings one sees below are of the same size as one's self, till a few minutes' consideration dispels the delusion.
