INTRODUCTION

We discussed in Chapter IV the tendency of configurations to retain their formal qualities constant as a whole, even when there were changes in the particular elements composing them. We shall now turn to an important and interesting group of phenomena arising from the tendency to perceive as constant and unchanging certain particular sensory qualities of real objects; these phenomena have in fact been termed the ‘constancies’ by the Gestalt psychologists. The phenomena demonstrate quite clearly the tendency, described in Chapter II, to ascribe an unchanging identity to real objects in the world around us; and, assuming that identity, to perceive little or no change in any of their aspects, even when the retinal images to which they give rise vary greatly in shape, size, brightness, and colour. Such changes of the retinal image occur when the object is tilted from the frontal parallel position (at right angles to the line of sight); when its distance from the observer is varied; and when the light falling on it varies in intensity or hue. But as we shall see such changes are, in the ordinary course of events, attributed in whole or in part to changes of position or illumination of the object with respect to its spatial setting; and not to intrinsic changes in the object itself. Koffka (1935) has endeavoured to relate these phenomena together under one universal physical law. But it will be shown that there are considerable differences between the various phenomena in the extent to which ‘constancy’ obtains; and that the manner in which it appears varies both with circumstances and also with the attitudes of the observers. This has been made particularly clear in the recent very extensive studies of the phenomena. Consequently comparatively little space will be devoted to the Gestalt laws which were dealt with at some length in the author's Visual Perception (1937b); and more consideration will be given to the variation of the constancy phenomena as studied by different experimenters.