246 



PSYCIIOLOGT. 



direction to that in wliicli, a moment previously, we had 

 been seeing the water move, whilst on either side of this 

 band another band of planks will move as the water did. 

 Looking at a waterfall, or at the road from out of a car- 

 window in a moving tram, produces the same illusion, which 

 may be easily verified in the laboratory by a simple piece 

 of apparatus. A board with a window five or six inches 

 wide and of any convenient length is supported upright on 

 two feet. On the back side of the board, above and below 

 the window, are two rollers, one of which is provided with 

 a crank. An endless band of any figured stuff is passed 

 over these rollers (one of which can be so adjusted on its 

 bearings as to keep the stuff always taut and not liable to 

 slip), and the surface of the front board is also covered with 

 stuff or paper of a nature to catch the eye. Turning the 

 crank now sets the central band in continuous motion, 

 whilst the margins of the field remain really at rest, but 

 after a while appear moving in the contrary way. Stopping 

 the crank results in an ilhisorj- appearance of motion in 

 reverse directions all over the field. 



A disk with an Archimedean spiral drawn upon it, 

 whirled round on an ordinary rotating machine, produces 

 still more startling effects. 



Fig. 65. 



"If the revolution is in the direction in which the spiral line 

 approaches the centre of the disk the entire surface of the latter .seems to 

 expand during revolution and to contract after it has ceased ; and 



