THE PERCEPTION OF SPACE. 259 



and the image combine so completely that the twain are 

 one flesh, as it were, and cannot be discriminated in the 

 result. If the complement be, as we have called it (on pp. 

 237-8), a set of imaginary absent eye-sensations, they seem 

 no whit less vividly there than the sensation which the eye 

 now receives from without. 



The case of the after-images distorted by projection upon 

 an oblique plane is even more strange, for the imagined 

 perspective figure, lying in the plane, seems less to combine 

 with the one a moment previously seen by the eye than to 

 suppress it and take its place.* The point needing explana- 

 tion, then, in all this, is how it comes to pass that, when 

 imagined sensations are usually so inferior in vivacity to real 

 ones, they should in these few experiences prove to be 

 almost or quite their match. 



The mystery is solved when we note the class to which 

 all these experiences belong. They are ' perceptions ' of 

 definite 'things,' definitely situated in tridimensional space. 

 The mind uniformly uses its sensations to identify things by. 

 The sensation is invariably apperceived by the idea, name, 

 or ' normal ' aspect (p. 238) of the thing. The peculiarity of 

 the optical signs of things is their extraordinary mutability. 

 A ' thing ' which we follow with the eye, never doubting of 

 its pliA'sical identity, will change its retinal image inces- 

 santly. A cross, a ring, waved about in the air, will pass 

 through every conceivable angular and elliptical form. All 

 the while, however, as we look at them, we hold fast to the 

 perception of their ' real ' shape, by mentally combining 

 the pictures momentarily received with the notion of peculiar 

 positions in space. It is not the cross and ring pure and 

 simple which we perceive, but the cross so held, the ring so 

 held. From the day of our birth we have sought every hour 

 of our lives to correct the apparent form of things, and trans- 



* I ought to say that I seem always able to see the cross rectangular at 

 will. But this appears to come from an imperfect absorption of the 

 rectangular after-image by the inclined plane at which the eyes look. The 

 cross, with me, is apt to detach itself from this and then look square. I get the 

 illusion better from the circle, whose after-image becomes in various ways 

 elliptical on being projected upon the different surfaces of the room, and 

 cannot then be easily made to look circular again. 



