make them visible. It is difficult to illustrate this process intentionally, because the knowledge that one's powers of observation are about to be tested places one on one's guard, and thus suppresses the natural activity of the mind's eye and draws unusual attention to objective details. Let the reader at this point hold the page at some distance off—say, eight or twelve feet—and draw an exact reproduction of the letters shown in Fig. 3. Let him not read further until this has been done, and perhaps he may find that he has introduced strokes which were not present in the original. If this is not the case, let him try the test upon those who are ignorant of its nature, and he will find that most persons will supply light lines to complete the contours of the letters which in the original are suggested but not really present; the original outline, Fig. 3a, becomes something like Fig. 3b,
| Fig. 5.—The black and white portions of this design are precisely alike, but the effect of looking at the figure as a pattern in black upon a white background, or as a pattern in white upon a black background, is quite different, although the difference is not easily described. | Fig. 6.—When this figure is viewed as a black pattern on a white background, the four main vertical lines seem far from parallel; when it is viewed as a white pattern on a black background this illusion disappears (or nearly so), and the black lines as well as the white ones seem parallel. |
and so on for the rest of the letters. The physical eye sees the former, but the mental eve sees the latter.
I tried this experiment with a class of over thirty university students of Psychology, and, although they were disposed to be quite critical and suspected some kind of an illusion, only three or four drew the letters correctly; all the rest filled in the imaginary light contours; some even drew them as heavily as the real strokes. I followed this by an experiment of a similar character. I placed upon
