512 PSYCHOLOOT. 



adequate jyhysiological ground why sometimes one, sometimes the other 

 part of them is felt to move. In order to investigate the matter con- 

 veniently, I had the simple 

 apparatus constructed which 

 is represented in Fig. 86. Au 

 oil-cloth with a simple pattern 

 is horizontally stretched over 

 two cylinders (each 2 metres 

 long and 3 feet apart) and kept 

 in uniform motion by the help 

 of a crank. Across the cloth, 

 Fig- 86. ^^^ some 30 cm. above it,^ is- 



stretched a string, with a knot x, which serves as a fixation-point for 

 the eye of the observer. If the observer follow with his eyes the 

 pattern of the cloth as it moves, he sees it in movement, himself and the 

 surroundings at rest. But if he looks at the knot, he soon feels as- 

 if the entire room were moving contrary to the direction of the cloth, 

 whilst the latter seems to stand still. This change in the mode of 

 looking comes about in more or less time according to one's momentary 

 disposition, but usually it takes but a few seconds. If one once under- 

 stands the point, one can make the two appearances alternate at will. 

 Every following of the oil-cloth makes the observer stationary ; every 

 fixation of the knot or inatteyition to the oil-doth^ so that its pattern 

 becomes hlwred, sets him in apparent motion." * 



Professor Macli proceeds to explain the phenomenon as- 

 follows: 



" Moving objects exert, as is well known, a peculiar motor stimulation 

 upon the eye, they draw our attention and our look after them. If the 

 look really follows them ... we assume that they move. But if the 

 eye, instead of following the moving objects, remains steadfastly at 

 rest, it must be that the constant stimulus to motion which it receives 

 is neutralized by an equally constant current of innervation flowing 

 into its motor apparatus. But this is just what would happen if the 

 steadfastly fixated point were itself moving uniformly in the other di- 

 rection, and we were following it with our eyes. When this comes 

 about, whatever motionless things are looked at must appear in mo- 

 tion." t 



The knot x, the string, we ourselves, and all our sta- 



* BeitrSge zur Analyse der Enipfindungen, p. 65. 

 + P. rs. 



