THE PERCEPTION OF SPACE. 



263 



tion to the production of that percept which oiiv judgment tells us is 

 right, our knowledge strives in vain to conjure up the feeling of it ; 

 we then know that we see something to which no reality corresponds, 

 but we see it all the same." * 



Note that no object not probable, no object ichich ive are not 

 incessantly practised in reproducing, can acquire this vividness 

 in imagination. Objective corners are ever changing their 

 angles to the eyes, spaces their apparent size, lines their 

 distance. But by no transmutation of position in space 

 does an objective straight line appear bent, and only in one 

 position out of an infinity does a broken line look straight. 

 Accordingly, it is impossible by projecting the after-image 



A B 





D 



Fig. 77. 



of a straight line upon two surfaces which make a solid 

 angle with each other to give the line itself a sensible 

 * kink.' Look with it at the corner of your room : the 

 after-image, which may overlap all three surfaces of the 

 corner, still continues straight. Volkmann constructed a 

 complicated surface of projection like that drawn in Fig. 

 77, but he found it impossible so to throw a straight after- 

 image upon it as to alter its visible form. 



* Hermann's Handl). der Pliysiologie. in. 1, p. 565-71. 



