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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

 

 

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VISUAL ILLUSIONS

I
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