Fig. 31
Since the sun and moon occupying the same space on the retina are, because of this apparent difference of distance, judged to be farther away at the horizon than they are at the zenith, we are inevitably forced to the conclusion that they are larger in size than when in the other position. The over-estimation of filled space as compared with [{770}] the unfilled is mainly due to the interrupted muscular action of the eyes in traveling over the space requiring more effort. This makes it seem longer. Probably physiological processes on the retina also contribute to the illusion. A series of objects, even dots, will cause a greater physiological excitation of the retina than an equal amount of space, the boundaries of which alone are brought to our attention.
Illusions of size are even more startling than illusions of distance. It is perfectly possible to take three spaces, each exactly a square inch, and by drawing lines in two of them in different directions to make the figures appear of [{771}] very different size. This is a rather disturbing illusion, particularly for women who are apt to think that perpendicular lines make them appear tall and thin, while horizontal lines have the opposite effect. This is true if the lines are not placed quite close together. The reason why women wear many ribbons, however, whether they themselves recognize it or not, is that the attraction of attention to these makes the space in which they are seem longer. Hussars are dressed in uniforms with many rows of gilt cord or braid running across their chests in order to increase their apparent height. As a rule, a cavalry man must not weigh over 140 pounds or his horse will break down in long, forced marches. Such men are often of small stature and their apparent height must be increased by their uniform, so as to make them look formidable. Advantage is taken of this optical illusion of filled space to produce this effect.
Fig. 32
Other illusions of size are quite frequent. It is rather hard for the ordinary observer to think that the half circles, a and a' (Fig. 32), are the same size, or that b and b' in the same chart are the same curve. The interruption in the circles c and c' produce very curious erroneous impressions which require a knowledge of this illusion to correct.
Optical illusions with regard to directions of lines are extremely common. Quite unconsciously we translate directions into special meanings. This is what enables perspective to be effective in drawings. It has many disturbing features, however. Some of these are striking illustrations of the defects of our vision.
Fig. 33
