CHAPTER XIV IMAGINATION AS AN AID TO VISION
W E see very largely with the mind, and only partly with the eyes. The phenomena of vision depend upon the mind's interpreta tion of the impression upon the retina. What we see is not that impression, but our own interpretation of it. Our impressions of size, color, form and location can be demonstrated to depend upon the interpretation by the mind of the retinal picture. The moon looks smaller at the zenith than it does at the horizon, though the optical angle is the same and the impression on the retina may be the same, because at the horizon the mind uncon sciously compares the picture with the pictures of sur rounding objects, while at the zenith there is nothing to compare it with. The figure of a man on a high build ing, or on the topmast of a vessel, looks small to the landsman; but to the sailor it appears to be of ordinary size, because he is accustomed to seeing the human figure in such positions.
Persons with normal vision use their memory, or imagination, as an aid to sight; and when the sight is imperfect it can be demonstrated, not only that the eye itself is at fault, but that the memory and imagination are impaired, so that the mind adds imperfections to the imperfect retinal image. No two persons with normal sight will get the same visual impressions from the same object; for their interpretations of the retinal picture will differ as much as their individualities differ, and