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POPULAR SCIENCE MONTHLY 



the black surfaces represent the bottoms of the blocks, all pointing- 

 downward and to the left, and now the black surfaces have changed 

 and have become the tops pointing upward and to the right. For 

 some the changes come at will; for others they seem to come unex- 

 pectedly, but all are aided by anticipating mentally the nature of the 

 transformation. The effect here is quite striking, the blocks seeming 

 almost animated and moving through space. In Fig. 17 a similar ar- 

 rangement serves to create an illusion as to the real number of blocks 

 present. If viewed in one way — the black surface forming the tops 

 of the blocks — there seem to be six arranged as in Fig. 18; but 



Fig. 16.— This interesting figure (which is reproduced with modifications from Scripture— The 

 New Psychology) is subject in a striking way to interchanges between foreground and 

 background. Most persons find it difficult to maintain for any considerable time either 

 aspect of the blocks (these aspects are described in the text) ; some can change them at 

 will, others must accept the changes as they happen to come. 



when the transformation has taken place and the black surfaces 

 have become the overhanging bottoms of the boxes, there are seven, 

 arranged as in Fig. 19. Somewhat different, but still belonging 

 to the group of ambiguous figures, is the ingenious conceit of the 

 duck-rabbit shown in Fig. 20. When it is a rabbit, the face looks to 

 the right and a pair of ears are conspicuous behind; when it is a 

 duck, the face looks to the left and the ears have been changed into 

 the bill. Most observers find it difficult to hold either interpretation 

 steadily, the fluctuations being frequent, and coming as a surprise. 

 All these diagrams serve to illustrate the principle that when the 

 objective features are ambiguous we see one thing or another accord- 

 ing to the impression that is in the mind's eye; what the objective 

 factors lack in definiteness the subjective ones supply, while familiar- 



