Fig. 85.—Exaggerated illusions in architecture.
One of the conditions which is responsible for certain illusions and has been compensated for on occasions is represented in [Fig. 86]. On the left are a series of squares of equal size placed in a vertical row. If these are large so that they might represent stories in a building they will appear to decrease in size from the bottom upward, because of the decreasing projection at the eye. This is obvious if the eye is considered to be at the point where the inclined lines meet. In order to compensate for the variation in visual angle, there must be a series of rectangles increasing considerably in height toward the top. The correction is shown in the illustration. It is stated that an inscription on an ancient temple was written in letters arranged vertically, and in order to make them appear of equal size they were actually increased in size toward the top according to the law represented in [Fig. 86]. Obviously a given correction would be correct only for one distance in a given plane.
Fig. 86.—Illustrating the influence of visual angle upon apparent vertical height.
In Chapter VIII the phenomenon of irradiation was discussed and various examples were presented. It exerts its influence in the arts as elsewhere. Columns viewed against a background of white sky appear of smaller diameter than when they are viewed against a dark background. This is illustrated in [Fig. 87] where the white and the black columns are supposed to be equal in diameter.
The careful observer will find numberless optical illusions and occasionally he will recognize an attempt on the part of the architect to apply an illusory effect to his advantage. In [Fig. 88] some commonplace illusions are presented, not for what they are worth, but to suggest how prevalent they may be. Where the pole or column intersects the arches or circle, there is an apparent change in the direction of the curved lines. The different types of arches show different degrees of the illusion. It may be of interest for the reader to refer to preceding chapters and to ascertain what types of illusions are involved.
Fig. 87.—Irradiation in architecture.