Representation at the visual receptors of such properties of the object as its size, shape, orientation, and movement undergo considerable variation as the distance, bearing, posture, and motion of the observer, relative to the object, changes. However, despite these gross and frequent deformations of the image, perceived properties remain extraordinarily stable. Such constancy has obvious biological utility; the observer perceives his world according to its fixed physicalfeatures rather than in terms of its variable sensory representation.

Constancy of apparent size, shape, orientation, and movement is dependent on information for the distance, bearing, lateral tilt, and movement of the observer, respectively. Thus, as the retinal image shrinks with distance, constancy of apparent size is maintained by five classes of distance, stimuli that operate singly or in various combinations. Likewise, visual orientation and visual stimuli for the orientation of the observer, and movement constancy on visual and nonvisual stimuli for the movement of the observer. Illusion occur when stimuli that normally preserve constancy are operative but with the image of the object not varied. Thus if retinal disparity, convergence-accommodation, projected stimuli, or other distance stimuli are varied with the image not varied, illusions of size occur. Those rsluting from variation of projected stimuli are the. well-known geometrical size illusions and include the Mü llerLyer group. In essentially the same manner, independent manipulation of stimuli for the orientation and the motion of the observer, with the orientation and the motion of the image at the retina not varied, gives rise to illusory orientations and movements of the object.

Limited attempts to explain size illusions in terms of the projected stimuli that preserve perceptual constancy are by no means new; Thiéry (51) proposed such a view in the later part of the last century, and in recent times there has been a spate of such proposals including the " misapplied constancy hypothesis" advanced by Gregory (2). However, Gregory's theory is confined largely to geometrical size illusions and invoks only distance scaling given by a limited number of projected stimuli. Furthermore, the Müller-Lyer illusion is seen by him to be a consequence of distance scaling resulting from the converging arrows. There is no recognition of the range of such effects with various attached elements, as shown in Fig3, and no attention is accorded the recently established difference between the illusions with inward and outward-directed elements. The theory of spatial illusions outlined here distinguishes between classes of illusory effect and, in linking each to its particular class of spatial constancy, offers a general and testable explanation. Failure to recognize classes of illusion(and perceptual constancy), such as those of size, orientation, and movement, can be regarded as among the major deficiencies of recent attempts (2, 44) to explain illusory effects. I do not claim that this explanation, which I call the general constancy theory, satisfactorily encómpasses all known illusions, but merely that it is more comprehensive than alternative explantions I conclude that any stimulus which serves to maintain perceptual constancy of a property of an object as the visual representation of that property varies will, when independently manipulated with the retinal image not varied, produce an illusion. This general principle predictS the conditions under which illusory effects will occur and has wide explanatory application.