3°4 



POPULAR SCIENCE MONTHLY. 



make them visible. It is difficult to illustrate this process inten- 

 tionally, 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 reproduc- 

 tion 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 con- 

 tours of the letters which in the original are suggested but not really 

 present; the original outline, Fig. da, becomes something like Fig. 3fr, 



Fig. 5. — The black and white por- 

 tions 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, al- 

 though 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 disap- 

 pears (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 fol- 

 lowed this by an experiment of a similar character. I placed upon 



