8 
SOME NEW OPTICAL ILLUSIONS. 
Further, I have noticed that if a strobic circle be “rotated,” 
while a number of other circles are lying stationary within the 
field of view, when the eye was directed to the moving circle the 
others also began to “ rotate ” ( Plate Lifig, 3 ). 
This last observation cannot, I think, be explained on any 
supposition of unconscious muscular movement. In fact I en- 
tirely doubt the validity of this hypothesis in the case of Addams’s 
observation upon the waterfall before cited, 
I am inclined rather to attribute these effects, and those of 
“ compensation ” in general, to waves of nervous disturbance 
moving over the retina; these waves, being of two orders, — one 
primary, and in the same direction as the objective motion of the 
images upon the retina ; the other secondary and later in time, — 
giving rise to the subjective motions of compensation. I do not 
see how on any other supposition the phenomena noted in an 
earlier paragraph relative to compensative shrinking or expanding 
of objects can be explained. Such a hypothesis, will, I believe, 
also embrace all the other phenomena of apparent motion, except 
those which are the result of mental associations alone — such 
illusions, in fact, as those of the windmill and the flying crow. 
Such waves of nervous disturbance have, it would seem, a definite 
rate of propagation, probably not independent of the nature of 
the moving image with respect to colour, relative luminosity, and 
apparent magnitude. But whether these waves of sensible im- 
pression are due to a physical motion of any structures of the 
retina I am not yet prepared to offer an opinion. 
