Part 5

75. A concave hemispherical cup on the left and a convex hemisphere on the right lighted by a light-source of large angle such as a window 150

76. The same as Fig. 75, but lighted by a very small light-source 151

77. Apparent ending of a searchlight beam 161

78. An accurate tracing from a photograph (continual exposure) of the moon rising 171

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

INTRODUCTION

Seeing is deceiving. Thus a familiar epigram may be challenged in order to indicate the trend of this book which aims to treat certain phases of visual illusions. In general, we do not see things as they are or as they are related to each other; that is, the intellect does not correctly interpret the deliverances of the visual sense, although sometimes the optical mechanism of the eyes is directly responsible for the illusion. In other words, none of our conceptions and perceptions are quite adequate, but fortunately most of them are satisfactory for practical purposes. Only a part of what is perceived comes through the senses from the object; the remainder always comes from within. In fact, it is the visual sense or the intellect which is responsible for illusions of the various types to be discussed in the following chapters. Our past experiences, associations, desires, demands, imaginings, and other more or less obscure influences create illusions.

An illusion does not generally exist physically but it is difficult in some cases to explain the cause. Certainly there are many cases of errors of judgment. A mistaken estimate of the distance of a mountain is due to an error of judgment but the perception of a piece of white paper as pink on a green background is an error of sense. It is realized that the foregoing comparison leads directly to one of the most controversial questions in psychology, but there is no intention on the author's part to cling dogmatically to the opinions expressed. In fact, discussions of the psychological judgment involved in the presentations of the visual sense are not introduced with the hope of stating the final word but to give the reader an idea of the inner process of perception. The final word will be left to the psychologists but it appears possible that it may never be formulated.

In general, a tree appears of greater length when standing than when lying upon the ground. Lines, areas, and masses are not perceived in their actual physical relations. The appearance of a colored object varies considerably with its environment. The sky is not perceived as infinite space nor as a hemispherical dome, but as a flattened vault. The moon apparently diminishes in size as it rises toward the zenith. A bright object appears larger than a dark object of the same physical dimensions. Flat areas may appear to have a third dimension of depth. And so on.