In order to develop effective tools for scientific visualization, consideration must be given to the perceptual competencies, limitations, and biases of the human operator. Perceptual psychology has amassed a rich body of research on these issues, and can lend insight to the development of visualization techniques. Within a perceptual psychological framework, the computer display screen can best be thought of as a special kind of impoverished visual environment. Guidelines can be gleaned from the psychological literature to help visualization tool designers avoid ambiguities and/or illusions in the resulting data displays.