The relational determination of perceived size.

One generalization that is now rather widely accepted in the field of perception is that perceptual qualities are often determined by relational rather than by absolute characteristics of the stimulus. Of the many examples that could be given to illustrate the point we will restrict ourselves to two which have certain formal similarities to the problem we wish to discuss in this paper. The first example is concerned with the problem of neutral or achromatic color. One might ask, what is the stimulus for the experience of a given shade of gray? In the achromatic series it would seem there is no simple stimulus for neutral color corresponding to the role that wave length plays in accounting for chromatic color. Intensity of stimulation is inadequate because this changes with every change in illumination whereas the particular gray perceived remains constant. It is true that the albedo or reflectance property of an object is fixed (i.e. a white surface is one that reflects roughly 80% of the light it receives, a black, one that reflects roughly 5%), but this is a statement about the external object. The problem remains of uncovering the invariant characteristic of the stimulus that makes possible the invariant perceptual experi-