On the Concept of 'Formless Invariants' in Visual Perception

One of the many meanings of the term form is the face of a solid object [1]. The face of a polyhedron is a flat surface, a polygon. One of the faces of a polyhedron is the front face, that is the surface facing the point of observation. When an observer moves (or when the object itself is moved) the projectedform of this surface-form undergoes a continuous sequential perspective transformation [2]. The optical form undergoes change but the substantial form does not. The mathematician can think of this change as a family of geometrical forms, each linked to every other by a specific transformation and this conception is implied by projective geometry, a conception of forms outside of time. The perceptionist, however, thinks of it as a change in time. If he displays a continuous sequence of this sort, obtained by projecting a transforming shadow on a translucent screen, he finds that it will arouse the perception of a rigid surface in motion [3]. It was first concluded from this experiment that such a sequence (or geometrical family) of optical forms was a 'stimulus' for the perception of the face of an object. The term stimulus was being used in a very general sense, in a sense of the term that I have now abandoned, and I would now say instead that the sequence contained information for the perception. Note that only one face of an object is being considered. How one goes from the perception of the face of an object to the perception of the whole object was not explained at that time. More of this question later. In these first experiments, the hypothesis of invariants in the sequence of perspective transformations was implicit. It gradually became more