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POPULAR SCIENCE MONTHLY. 



and natural view of this we focus the attention upon the black lines 

 and observe the familiar illusion, that the four vertical lines seem far 

 from parallel. That they are parallel can be verified by measure- 

 ment, or by covering up all of the diagram except the four main 



Fig. 8. — This drawing may be viewed as 

 the representation of a book standing 

 on its half-opened covers as seen from 

 the back of the book ; or as the inside 

 view of an open book showing the pages. 



Fig. 10. — The smaller square may be re- 

 garded as either the nearer face of a 

 projecting figure or as the more distant 

 face of a hollow figure. 



Fig. 9. — When th"s figure is 

 viewed as an arrow, the up- 

 per or feathered end seems 

 flat ; when the rest of the 

 arrow is covered, the feath- 

 ered end may be made to 

 project or recede like the 

 book cover in Fig. 8. 



lines. But if the white part of the diagram is conceived as the de- 

 sign against a black background, then the design is no longer the 

 same, and with this change the illusion disappears, and the four 

 lines seem parallel, as they really are. It may require a little effort 

 to bring about this change, but it is very marked when once realized. 

 A curious optical effect which in part illustrates the change in 

 appearance under different aspects is reproduced in Fig. 7. In this 

 case the enchantment of distance is necessary to produce the trans- 

 formation. Viewed at the usual reading distance, we see nothing 



