"In the horizontal center strip there are birds and fish equivalent to eachother. We associate flying with sky, and so for each of the black birds thesky in which it is flying is formed by the four white fish which encircleit. Similarly swimming makes us think of water, and therefore the fourblack birds that surround a fish become the water in which it swims." M.C.E.

This print has been used is physics, geology, chemistry, and in psychology for the study of visual perception. In the picture a number of visual elements unite into a simple visual representation, but separately each forms a point of departure for the elucidation of a theory in one of these disciplines. The basis of this print is a regular division of the plane consisting of birds and fish. We see a horizontal series of these elements - fitting into each other like the pieces of a jigsaw puzzle - in the middle, transitional portion of the print. In this central layer the pictorial elements are equal: birds and fish are alternately foreground or background, depending on whether the eye concentrates on light or dark elements. The birds take on an increasing three-dimensionality in the upward direction, and the fish, in the downward direction. But as the fish progress downward they gradually lose their shapes to become a uniform background of sky and water, respectively.

We can think of a number of paired concepts applying to this picture: light-dark, top-bottom, flat-rounded, figure-background, interlocking pictorial elements-independent pictorial elements, geometric structure-realistic form; and, with respect to the subject of the print, birds-fish, sky-water, immobility-movement. Any scientist who uses a print to illustrate a theory selects one or more elements from the print that are analogous to components of the theory, and in this way the print becomes a model for the theory.C.H.A. BROOS