It is hoped that this condensed discussion, which is ambitious only in scope, will be of interest to the general reader, to painters, decorators, and architects, to lighting experts, and to all interested in light, color, and vision. It is an essential supplement to certain previous works. M. LUCKIESH November, 1920. CONTENTS CHAPTER PAGE I. Introduction 1 II. The eye 13 III. Vision 29 IV. Some types of geometrical illusions 44 V. Equivocal figures 64 VI. The influence of angles 76 VII. Illusions of depth and of distance 102 VIII. Irradiation and brightness-contrast 114 IX. Color 124 X. Lighting 144 XI. Nature 164 XII. Painting and decoration 179 XIII. Architecture 195 XIV. Mirror Magic 205 XV. Camouflage 210 LIST OF ILLUSTRATIONS FIGURE PAGE 1. Principal parts of the eye 14 2. Stereoscopic pictures for combining by converging or diverging the optical axes 41 3. Stereoscopic pictures 41 4. The vertical line appears longer than the equal horizontal line in each case 46 5. The vertical dimension is equal to the horizontal one, but the former appears greater 47 6. The divided or filled space on the left appears longer than the equal space on the right 49 7. The three lines are of equal length 50 8. The distance between the two circles on the left is equal to the distance between the outside edges of the two circles on the right 50 9. Three squares of equal dimensions which appear different in area and dimension 51 10. The vertical distance between the upper circle and the left-hand one of the group is equal to the overall length of the group of three circles 52 11. Two equal semi-circles 53 12. Arcs of the same circle 53 13. Three incomplete but equal squares 53 14. Middle sections of the two lines are equal 54 15. An effect of contrasting areas (Baldwin's figure) 54