OPTICAL ILLUSIONS OF MOTION. 



521 



ster's hypothesis is, indeed, extremely vague, and is neither physical 

 nor psychological in any exact sense. If understood physically, it 

 means that there is actually motion in the retina itself, which is hardly 

 conceivable, since the structure of the rods and cones almost precludes 

 even any idea of vibration, or of propagation of waves of motion by 

 vibration, much less any movements of them as a whole. And, if the 

 explanation is intended as a psychological one, something further is 

 needful before the principle of compensation here laid down could be- 

 come intelligible. 



The first exj)eriments made by the writer of this article upon illu- 

 sions of motion arose from a casual observation in 1876. He had been 

 preparing, for the purpose of testing astigmatism, a set of concentric 

 circles in black and white, such as those shown in Fig. 1. Happening 



Fig. 1. 



to shake the sheet on which the circles were drawn, he noticed an ap- 

 parent motion of rotation to be set up. The illusion is easily produced 

 by imparting to the pattern a slight motion of the same character as 

 that adopted in rinsing out a pail, but with a very minute radius of 

 motion. All the circles will appear to rotate with the same angular 

 velocity as that imparted. Now, undoubtedly the persistence of visual 

 impressions has a good deal to do with the production of this illusion, 

 which, by the way, succeeds best when the circles make from two to 

 four turns in a second, and when the radius of the imparted motion is 

 equal to the thickness of one ring, so that each black or white band is 

 displaced through a distance equal to its own width in all directions 

 successively. Nevertheless, the persistence of visual impressions will 



