RADIAL MOVEMENT. 



283 



5. Perception of Size. 



(a.) Fix the centre of Fig. 135 at a distance of 3 to 4 cm. 

 from the eye, when by indirect vision the broad white and 

 black areas of the peripheral parts, bounded by hyperbolic 

 curves, will appear as small and the lines bounding them as 

 straight as the smaller areas in the middle zone. 



6. Apparent Movements. 



(a.) Strobic Discs. Give the discs a somewhat circular 

 but rapid movement, and observe that the rings appear to 

 move, each one on its own axis, 



(b.) Radial Movement. While another person rotates a 

 disc like Fig. 136 on the rotating wheel, look steadily at the 



Fig. 136. 



centre of the disc. One has the impression as if the disc 

 were covered with circles which, arising in the centre, and, 

 gradually becoming larger, disappear at the periphery. After 

 long fixation look at printed matter or at a person's face, the 

 letters appear to move towards the centre, while the person's 

 face appears to become smaller and recede. If the disc be 

 rotated in the opposite direction, the opposite results are 

 obtained. 



