79. Accurate tracings from a photograph (short exposures at intervals) of the sun setting 172 80. Explanation offered by Smith of the apparent enlargement of heavenly bodies near the horizon 174 81. Explanation of a common mirage 176 82. Illustrating the apparent distortion of a picture frame in which the grain of the wood is visible 190 83. Another example similar to Fig. 82 191 84. From actual photographs of the end-grain of a board 192 85. Exaggerated illusions in architecture 198 86. Illustrating the influence of visual angle upon apparent vertical height 199 87. Irradiation in architecture 200 88. Some simple geometrical-optical illusions in architecture 201 89. By decreasing the exposed length of shingles toward the top a greater apparent expanse is obtained 202 90. An example of a mirror "illusion" 207 91. Another example of "mirror magic" 208 92. A primary stage in the evolution of the use of geometrical-optical illusions on ships 226 93 and 94. Attempts at distortion of outline which preceded the adoption of geometrical-optical illusions 228 95 and 96. Illustrating the use of models by the Navy Department in developing the geometrical-optical illusion for ships 229 97 and 98. Examples of the geometrical-optical illusion as finally applied 231 99. Representative earth backgrounds for an airplane (uncamouflaged) as viewed from above 235 100. Illustrating the study of pattern for airplanes. The photograph was taken from an altitude of 10,000 feet. The insert shows the relative lengths (vertical scale) of an airplane of 50-foot spread at various distances below the observer 239 VISUAL ILLUSIONS I INTRODUCTION