Angles, influence of, 76 various effects of, 81 contours and, 87 apparent effect on length, 91 theories, 98 Animals, protective coloration, 211 Architecture, 195 balance in, 201 Arcs, circular, illusions due to, 86 Areas, juxtaposed, illusions of, 96 Artist, 179 Attention, fluctuation of, 65, 106, 141, 164 Aubert, 49 Auerbach's indirect vision theory, 100 Aureole, 178 Balance in architecture, 201 Bas-relief, 143 Battleships, 222 Binocular disparity, 105 Binocular vision, 29, 31 Blending of colors in camouflage, 216 Blind spot, 21 Blue light on war-vessels, 230 Boswell, varieties of irradiation, 122 Brightness, illusions due to variations in, 107 and color contrasts, 114 apparatus, 115 and hue, 125 sky, 241 Brucke's theory, 37 Brunot's mean distance theory, 101 Camouflage, 210 terrestrial, 210 detection of, 215 marine, 222 airplane, 234 Carr, observations on distance illusions, 108 Chromatic aberration, 19, 135 Chlorophyl, 214 Circle, 11 arcs of, illusion, 86 contracting and expanding illusion, 98 Clouds, 235 Color, 124 after-images, 128 blending in camouflage, 216 contrasts and brightness, 114 growth and decay of sensation, 131 illusions of, 156 retiring and advancing, 138 saturation, 154 sensibility of retina, 138 warm and cold, 158 Confusability, 226 Confusion theory of angular illusions, 100 Contour, illusions of, 52 and angles, illusions, 87 Contracting and expanding circles, illusion of, 98 Contrasts, illusions of, 53 simultaneous, 124, 154 apparatus for, 115, 125 color, 114, 154, 188 brightness, 114 Convergence, illusions of, 108, 191 Cord, twisted, illusion, 88 Daylight, artificial, 189 Decoration, painting and, 179 Decorator, 188 Dember, 167 Depth and distance, illusions, 102 Direction, illusions of Zöllner's, 76 Wundt's, 79 Hering's, 80 Disk, Mason, 132 Distance and depth, illusions, 102 and size, 35, 104, 166 Distance illusions, Carr's observations, 108 Double images, 37 Dynamic theory of angular illusions, 99 Enlargement of sun and moon at horizon, apparent, 169 Equivocal figures, 64 Euclid, 8 Extent, interrupted, illusions of, 48 External image, 15, 17, 34 Eye, physiology, 13 position, 30 adjustments, 33 defects, 19 Fatigue, 128 Field, visual, effect of location in, 44 Figures, equivocal, 64 Filters, color, 214 Fluctuation of attention, 65, 106, 141, 164 Focusing, 14 Form of sky, apparent, 166 Fovea centralis, 22, 23, 139 Frames, picture, effect of wood grain, 190 Geometrical illusions, 44 Glare, 119 Grain of wood, apparent distortions due to, 190 Grecian art, 196 Growth and decay of color sensation, 131 Guttman, 175 Hallucination, 4, 72 Halo, 178