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fixed, the eye carries forward the process by observing their angular or apparent size in various positions. Size and distance mutually contain each other; if we know the one we can infer the other.

We also judge of distance by intervening objects, themselves interpreted in their relations by experience. Again we infer it from depth of color. This test, however, is applicable only to remote objects, and, in comparative judgments, to objects quite unequally remote. The ridges of mountains, rising in succession beyond each other, are separated to the eye by the different shades of blue that rest upon them. The degree in which, in these estimates, we are dependent on our own experience is indicated by our wild conclusions under novel conditions. The pure atmosphere and the unaccustomed dimensions of high mountains make the impressions of one who visits a country like Switzerland for the first time exceedingly deceptive. Weeks and months of laborious walking must be passed before these objects assume their true dimensions, and take on their real grandeur. In like manner the inexperienced landsman loses all accuracy when called on to estimate distances on the water.

A fourth aid in determining distances is the muscular adjustment of the eyes in bringing the image to the focal point on the retina. This test affords but slight assistance, however, and is applicable to objects comparatively close at hand. We are not conscious of a readjustment of the eye, except under a sudden change of vision at ranges whose inequality is very obvious. If an object near by is suddenly thrust upon the attention, the effort to see it becomes even painful. The judgments of ordinary vision are vague, giving the general relation of objects in position more than their direct distance from the observer.

A second perceptive judgment in sight is that of form.