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THE POPULAR SCIENCE MONTHLY. 



not explain all the facts of this curious illusion : for, in the first place, 

 it is found that for increasing distances from the eye the concentric 

 rings must be made wider if the illusion is to succeed ; there being 

 apparently one particular magnitude of their images on the retinae 

 which favors the production of the illusion. Again, if two such " stro- 

 bic circles " (as I have called them) are printed side by side on one 

 card, that set of circles seems to turn most effectively at which the eye 

 is not looking. On stopping the " rinsing motion " suddenly, there 

 appears to be, for an instant, a reverse motion. Finally, if a set of 

 circles is " rotated " while another set lies motionless within the field 

 of view, the second set will appear to rotate when the first are " ro- 

 tated " in the manner described above. It is possible, also, to have a 

 number of such apparent motions going on at once independently in 

 one field of view. Fig. 2 shows a compound pattern, containing an 



Fig. 2. 



interior set of concentric circles and six internally-toothed wheels. 

 When a very minute " rinsing " motion is imparted to this figure, the 

 circles appear to whirl round while the toothed-wheels work slowly 

 backward, moving through one tooth while the circles whirl round 

 once. Here, again, persistence of vision is concerned but not exclu- 

 sively. 



Dr. Emile Javal, the able director of the Ophthalmological Labo- 

 ratory of the Sorbonne, has recently advanced an explanation of these 

 illusions different from that adopted by the writer, and in substance 

 identical with that advanced by R. Addams in the case of the water- 

 fall illusion. He avers that the eye, in order to observe a movement, 

 follows the moving body for an instant and then suddenly slips back ; 

 that this oscillation, frequently repeated, is associated with a sensation 

 of motion in the particular direction in question ; and that when the 



