The differences between the media of books and computers lies partly in the way in which information is displayed, rather than in the content of the information itself. The term "legibility" denotes how specific physical characteristics of a display affect visual fatigue, reading speed, and ultimately comprehension. Some of these physical components include illumination, color, the printing surface, spacing, typography, and illustrations. There are, however, several features of the computer that affect legibility in totally new ways. For example, in a computer-reading environment, one reads "printing" not "print." There is only one surface for the display of information, the cathode ray tube. In addition, both the presentation of new text and the elimination of the old occur during the reading process. Furthermore, a new physical dimension has been added, the dimension of time. Dynamic legibility is a source of concern because of the nearly limitless ways of controlling the third dimension, the temporal features of text and graphic displays. Though the previous research on legibility provides some foundation for making decisions on how to display text and graphics, new research is needed to address the question of when to display text and graphics. (HOD)