ILLUSIONS OF THE SENSES 



155 



accurate measurement will show that they are strictly 

 parallel. 



By changing the direction of the oblique lines, as shown 

 in Fig. 32, the horizontal lines appear to be crooked although 

 they are perfectly straight. 



Fig. 3*. 



All these curious illusions are, however, far surpassed by 

 an experiment which we will now proceed to describe. 



