PHYSIOLOGICAL PARADOXES. 



circle (Fig. 49), and that he is seeing the picture 

 through this hole in the hand. 



This appearance, which is well marked 

 and very striking, depends upon the ocular 

 habit of attending to the chief thing, and dis- 

 regarding others a habit which, though we 

 may contrive special experiments in which its 



FIG. 49. HOW TO SEE A HOLE THROUGH THE HAND. 



guidance fails us is for ordinary circumstances 

 very advantageous. 



There are three respects in which we can 

 let our eyes disregard unimportant matters, 

 which are, nevertheless, quite visible if we care 

 to attend to them. Firstly, when we want to 

 observe an object, we direct our eyes straight 

 towards it, so that each eye has it in the centre 

 of its field of vision, and then we neglect 



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