This is the fifth in a series of technical research reports by Harvard Project Zero which study artistic creation and comprehension as a means toward better art education. The three papers in this report all concern the bearing of projective geometry on the perceptual processes by which pictures are "read" for spatial information. The first paper describes the human inclination to interpret pictures as representing rectilinear forms when, from a geometric standpoint, they might represent nonrectilinear forms. The second paper studies how the visual system compensates for pictures seen at an oblique angle, rather than perpendicularly. The third paper examines how viewers readily and consistently interpret simple line drawings as space forms, even though conventional depth cues such as perspective, occlusion, or "familiarity" may be absent. Throughout these papers, the logical ambiguity of line drawings--their lack of distinct three-dimensional information--is stressed together with the active role of the visual system in making assumptions to resolve this ambiguity. (Author/DE)