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The conditions of this judgment are light and shade. In large and complex objects, like mountain ranges, as form involves position, our judgments in this particular are mingled with and modified by those of distance.

A third judgment in vision is that of size. Though we infer distance from size, and equally well size from distance, the former is the more common case in experience. Well established sizes, settled by close contact, are our ordinary data. Yet it not infrequently happens that distances are known, and we thence infer the dimensions of strange objects. The variety and vagueness of our impressions as to the size of the moon are due to the fact that distance does not enter in as a measurable factor. The primitive data, then, of the eye are color, light and shade, angles; its acquired data distance, involving position, form and size.

Two eyes in vision give us an advantage besides that of protection against accident, or even that of stronger sight. The circles of vision in the two eyes of man do not quite correspond; the one includes portions not found in the other. Also, by virtue of distinct positions, the two alter slightly, each as compared with the other, the relations of objects. These facts make the perspective more definite, especially as regards objects just at hand, in reference to which exactness is important. We secure by two eyes a triangulation available in defining distance and position. So important is this aid that the loss of one eye is attended with considerable confusion of perspective. An intervening object, also, brings less obstruction to two eyes than to one.

The ear, the second leading organ of perception, deals with sound, with the undulations of the air within a certain range of rapidity. This range, however, is not identical in all persons, a rate of vibration being in some instances audible to one and inaudible to another. The physical characteristics of sound are quality or timbre, pitch and quantity