Haze, illusions, etc., 103, 166, 183 earth and high, 240 Helmholtz, 13, 74 Hering, 74 illusion of direction, 80 Hue and brightness, 125 Illusions, geometrical, 44 equivocal figures, 64 influence of angles, 76 of depth and distance, 102 irradiation and brightness contrast, 114 color, 124 light and shadows, 144 in nature, 164 in painting and decoration, 179 mirror, 205 camouflage, 210 Image after-, 24, 25, 59, 128, 129, 186 double, 37 external, 15, 17, 34 retinal, inversion of, 16 of airplane, size at various altitudes, 238 Indirect vision theory, Auerbach's, 100 Intaglio, 143 Interrupted extent, illusions of, 48 Iris, 15 Irradiation, 120 and brightness contrast, 114 varieties of (Boswell), 122 in architecture, 199 James, 170 Jastrow, 80 Jones, L. A., 223 Judd, 86, 93 Judgment, 1 Karrer, 160 Kepler, 8 Light, effect of spectral character, 189 Lighting, illusions of depth and distance due to, 102 contrasts, 154 diffusion, effect of, 145 direction, effect of, 144, 151 ending of searchlight beam, 160 warm and cold colors, 158 Lipps, 10, 11 dynamic theory of, 99 Location in visual field, effect, 44 Mean distance theory, Brunot's, 101 Mechanical, esthetic unity, 11 Magician, 3 Magic, mirror, 205 Marine camouflage, 222 Mason disk, 132 Mirage, 3, 176 Mirror magic, 205 Miscellaneous color effects, 140 Moon, apparent size at horizon, 8, 36, 169 theories of, 173 apparent radius of crescent, 121 Müller-Lyer illusion, 53, 93 National Research Council, Committee on Camouflage, 234 Nature, 164 Necker, 74 Oppel, 9 Painting and decoration, 179 Painter, 2, 179, 186 Parallax, 105 Parthenon of Athens, 196 Persistence of vision, 131 Perspective, 58 in architecture, 198 aerial 165, 183 theory, 98 Photographer, 152 Photography, use in detection of camouflage, 220 Photometer, 156, 217 Pigments, 184 Poggendorff illusion, 85 Protective coloration, animals, 211 Psychology, 2, 6, 157 of light, 193 Purkinje phenomenon, 26, 139 Reflection-factors, 236 Retina, 14, 20 inertia, 130 color sensibility, 138 Retinal rivalry, 140 Retiring and advancing colors, 136 Reversal of mirror image, 205 Rods and cones, 21 Schröder's staircase, 70 Sculpture, 204 Searchlight beam, ending of, 160 Sensation, color, growth and decay, 131 Sense, 1 Shading, counter, for vessels, 224 Shadows, importance in camouflage, 215 Size and distance, 35, 36 illusions of, 104, 166 Sky apparent form of, 166 brightness, 241 Skylight and sunlight, relative proportions of, 215, 243 Smith, Robert, 173 Smoke-screens, 230 Spectral character of light, 189 Sphere, illusions, 145, 150, 151 Spherical aberration, 19 Sphinx illusion, 206