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This eBook from the Gutenberg Project consists of approximately 678 pages of information about Harvard Psychological Studies, Volume 1.

Why such perspective lines should enter into the process of judgment it is not difficult to infer.  The plane of perspective for human beings is characteristically horizontal, in consequence of the distribution of important objects within the field of visual perception.  Roughly, the belt of the earth’s horizon contains the loci of all human perspective planes.  Both natural and artificial arrangements of lines converge there.  The systems of visual objects on the earth and in the sky are there broken sharply off in virtue of their practically vast differences in quality and significance for the observer.  The latter perspective probably never extends downward illusorily to points on the earth’s surface; and the former system of objects is carried continuously upward to skyey points only on relatively rare occasions, as when one mistakes clouds for mountains or the upper edge of a fog-belt on the horizon for the rim of sea and sky.  The point of convergence of the fundamental lines of perspective thus becomes assimilated with the idea of the visual horizon, as that concept has fused with the notion of a subjective horizon.  There can be little doubt that the disposition of such lines enters constantly into our bodily orientation in space along with sensations arising from the general body position and from those organs more specially concerned with the static sense.

Upon the misinterpretation of such objective planes depends the illusion of underestimation of the height or incline of a hill one is breasting, and of the converse overestimation of one seen across a descending slope or intervening valley.  The latter illusion is especially striking, and in driving over forest roads (in which case the correction of a wider range of view is excluded) the stretch of level ground at the foot of a hill one is descending is constantly mistaken for an opposing rise.  This illusion is put into picturesque words by Stevenson when he describes the world, seen from the summit of a mountain upon which one stands, as rising about him on every side as toward the rim of a great cup.  The fitness of the image may be proved by climbing the nearest hill.  In all such cases a reconstruction of the sensory data of judgment takes place, in which the most significant factor is the plane determined by the positions of the observing eye and the perspective focus.  In these judgments of spatial relationship, as they follow one another from moment to moment, this plane becomes a temporary subjective horizon, and according as it is positively or negatively rotated do corresponding illusions of perception appear.

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If a small rod is passed slowly before a rotating disc composed of two differently colored sectors, the rod appears to leave behind it on the disc a number of parallel bands of about the width of the rod and of about the colors, alternately arranged, of the two sectors.  These appear not to move, but gradually to fade away.

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