OPTICAL ILLUSIONS OF MOTION. 525 



abundant. A steady sound of one constant pitch ceases to be heard 

 until we become aware of it by its cessation. A steady light of one 

 color, such as the yellow light of gas-flaraes, ceases to be noticed as a 

 yellow light until some other color-sensation break the illusion. The 

 same is true of smells, of tastes, of the sensations of temjjerature, of 

 the sensation of rotation after a waltz, and of many others. All these 

 are probably only different instances of the operation of some much 

 more general physico-psychological law. It is quite consonant with 

 these kindred phenomena that, when any region of the retina is affected 

 by an image of objects moving steadily across the corresponding por- 

 tion of the field of view in any given direction, that portion of the 

 retina gradually loses consciousness of the motion, and perceives it 

 only as a steady sensation, or as one of approximate rest. When, how- 

 ever, an object really at rest is looked at, the associative faculty seizes 

 upon the contrast in the sensations affecting that region, and interprets 

 the new sensation by imputing a motion in the opposite sense to the 

 objects occupying the corresponding portion of the field of vision. I 

 have proposed to give to the empirical law expressing these matters 

 the name of the laio of subjective co^nplementary motion. 



It is impossible to quit the subject without pointing out two lines 

 of thought suggested by that which has been advanced. 



Firstly, it is conceivable that the explanation here propounded may 

 at some future time be superseded by a better hypothesis of a more 

 purely physical character. Suppose, for example, that it could be 

 shown what I have reason to suspect, but have been foiled in all 

 attempts to prove in any experimental fashion that the eye has the 

 power of altering at will the actual size of the retinal images by a 

 double muscular adjustment between the magnifying power of the 

 lenses of the eye and the distance of their equivalent optical center 

 from the surface of the retina, such a fact, once established, would 

 entirely cut away the significance of my crucial test with the rotating 

 spirals ; and the apparent expansions and contractions of objects would 

 be merely due to the continuous attempts of the eye to retain the reti- 

 nal images of one constant size. If this were so (though I have failed 

 in every kind of attempt to devise some satisfactory test), it might 

 also explain one little matter that is still very mysterious and unex- 

 plainable, namely, that in these illusions of expansion and contraction 

 the changes of apparent magnitude often appear to take place by dis- 

 continuous jumps rather than by steady motions. 



Secondly, it is found that these different illusions affect different 

 individuals with very different degrees of success, some persons being 

 much more sensitive than others to the after-workings of the subjective 

 motion ; and, indeed, there are individuals in whose case it is almost 

 impossible to produce the illusions. Doubtless some of these differ- 

 ences may be accounted for by defects of vision, astigmatism, achro- 

 matopsy, myopy, and the like. But there is also a time-element in 



