CHAPTER XI. 



OPTICAL ILLUSIONS ZOLLNER'S DESIGNS THE THAUMATROPE PHENO 

 KISTOSCOPE THE ZOOTROPE THE PRAXINOSCOPE THE DAZZLING 

 TOP. 



WE shall now continue the subject by describing some illusions more 

 curious still those of ocular estimation. These illusions depend rather 

 on the particular properties of the figures we examine, and the greater 

 part of these phenomena may be placed in that category whose law we have 

 just formulated : the differences clearly perceived appear greater than the 



differences equal to them y but perceived with greater difficulty. Thus a line 



when divided appears greater than when not divided ; the direct perception 

 of the parts makes us notice the number of the sub-divisions, the size of 

 which is more perceptible than when the parts are not clearly marked off. 

 Thus, in fig. 1-15, we imagine the length ab equals be, although ab is in 



Fig: li$. ~ L 



Fig. 116. 



reality longer than be. In an experiment consisting of dividing a line into 

 two equal parts, the right eye tends to increase the half on the right, and the 

 left eye to enlarge that on the left. To arrive at an exact estimate, we turn 

 over the paper and find the exact centre. 



Illusions of this kind become more striking when the distances to be 

 compared run in different directions. If we look at A and B (fig. 1 1 6), which 

 are perfect squares, A appears greater in length than width, whilst B, on the 

 contrary, appears to have greater width than length. The case is the same 

 with angles. On looking at fig. 117, angles I, 2, 3, 4 are straight, and should 

 appear so when examined. But i and 2 appear pointed, and 3 and 4 

 obtuse. The illusion is still greater if we look at the figure with the right 

 eye. If, on the contrary, we turn it, so that 2 and 3 are at the bottom, i 

 and 2 will appear greatly pointed to the left eye. The divided angles always 

 appear relatively greater than they would appear without divisions. 



