THE PERCEPTION OF SPACE. 255 



sion, after long fixation, that a friend who painted such a 

 mask for me told me it soon became difiicult to see how to 

 apply the brush. Bend a visiting-card across the middle, 

 so that its halves form an angle of 90° more or less ; set it 

 upright on the table, as in Fig. 72, and view it with one eye. 



Fig. 72. 



You can make it appear either as if it opened towards you 

 or away from you. In the former case, the angle ah lies 



upon the table, b being nearer to you than a ; in the latter 



case oh seems vertical to the table — as indeed it really is — 



with a nearer to you than b* Again, look,with either one or 



* Cf. E. Mach, Beitriige zur Analyse der Euiptindungen, p. 87. 



