We estimate the size of visible objects (by instinct or by experience) from the visual angle and the supposed distance of the object and by comparison with objects of known size. If we are mistaken in the distance of the object, we are often mistaken in our estimate of its size. We estimate the distance of an object by the distinctness with which we see it, by comparison with objects of known distance and by the muscular effort we make in turning the eyes inward so as to direct them upon the object. The axes of the eyes intersect at the object. The angle between the axes is called the optical angle. The greater the optical angle, the less the distance. (a.) The more obscure an object, the more distant (and, consequently, the larger) it seems to be. Hence, the apparent enormous size of objects seen in a fog. When the moon appears on the horizon, we see that she is beyond all terrestrial objects in that direction and she seems farther off (and, consequently, larger) than when she is overhead, there being then no intervening objects for comparison. But the moon is actually nearer us when she is in the zenith than when in the horizon and the visual angle is, consequently, greater.