THE PERCEPTION OF SPACE. 173 



an irresistible sensation of tlie latter being itself in motion 

 in the opposite direction to the head. So in abducting the 

 fingers from each other ; some may move and the rest be still 

 still, but the still ones will feel as if they were actively sep- 

 arating from the rest. These illusions, according to Vierordt, 

 are survivals of a primitive form of perception, when 

 motion was felt as such, but ascribed to the whole content 

 of consciousness, and not yet distinguished as belonging ex- 

 clusively to one of its parts. When our perception is fully 

 developed we go beyond the mere relative motion of thing 

 and ground, and can ascribe absolute motion to one of these 

 components of our total object, and absolute rest to another. 

 When, in vision for example, the whole background moves 

 together, we think that it is ourselves or our eyes which 

 are moving ; and any object in the foreground which may 

 move relatively to the background is judged by us to be 

 still. But primitively this discrimination cannot be per- 

 fectly made. The sensation of the motion spreads over all 

 that we see and infects it. Any relative motion of object 

 and retina both makes the object seem to move, and makes 

 us feel ourselves in motion. Even now when our whole ob- 

 ject moves we still get giddy ; and we still see an apparent 

 motion of the entire field of view, whenever we suddenly 

 jerk our head and eyes or shake them quickly to and fro. 

 Pushing our eyeballs gives the same illusion. We know in 

 all these cases what really happens, but the conditions are 

 unusual, so our primitive sensation persists unchecked. So 

 it does when clouds float by the moon. We knoiv the moon 

 is still ; but we see it move even faster than the clouds. 

 Even when we slowly move our eyes the primitive sensation 

 persists under the victorious conception. If we notice 

 closely the experience, we find that any object towards 

 which we look appears moving to meet our eye. 



But the most valuable contribution to the subject is 

 the paper of G. H. Schneider,* who takes up the matter 

 zoologically, and shows by examples from every branch of 

 the animal kingdom that movement is the quality by which 

 animals most easily attract each other's attention. The in- 



* Vierteljahrsch. fur wiss. Philos., n. 377. 



