524- THE POPULAR SCIENCE MONTHLY. 



and with apparent angular velocities increasing toward the center of 

 vision. Furthermore, I have arranged two such disks so that they 

 could be simultaneously in the field of view while rotating in opposite 

 directions. When the gaze was directed first at a point between them 

 and then at fixed objects, there appeared to be two portions of the field 

 of view rotating, and animated with rotations in opposed senses. 

 Clearly, the eye can not slip round in opposite directions at the same 

 time. In all these illusions, moreover, it is found that this illusory 

 complementary motion only occurs over limited parts of the field of 

 view namely, those which correspond to the portions of the retina 

 which previously received the moving images. Thus, if a waterfall be 

 looked at as in Addams's observation the u.pward illusory after- 

 motion is confined to a vertical streak across the field of vision. This 

 fact alone is sufiicient to negative the theory of muscular slip. 



The final test to which I have appealed is, if possible, even more 

 conclusive. It is probably a familiar observation that the end of the 

 last carriage of a retreating railway-train appears to shrink down small- 

 er and smaller as it subtends a decreasing angular magnitude in the 

 field of view. After looking at this motion for a sufiicient number of 

 seconds to fatigue the eye, stationary objects appear to be expanding. 

 To produce this illusion more effectually, I take a disk like that shown 

 in Fig. 4 (the figure is quarter actual size), marked out in spirals of 

 white and black. If this is slowly rotated say at about one revolu- 

 tion in two seconds the whole pattern apjDcars either to be running 

 into, or running out of, the center of the disk : thei'e is a motion of 

 convergence or divergence, according to the sense of the rotation. 

 Let the disk be turned so as to cause an apparent convergence from all 

 sides to the center, and let the eye steadily watch the center for about 

 a minute, or until the fatigue becomes almost unendurable. Then look 

 at any fixed object the pattern of the wall-paper, or the dial of a 

 clock the object so regarded will for some two, or three, or more 

 seconds, appear to be expanding from the center outward. The effect 

 is still more startling if the object thus viewed be the face of a famil- 

 iar friend. It is quite evident that the eyeball can not slip in all direc- 

 tions at once. 



I have, therefore, somewhat reluctantly been led to propound an 

 explanation for these illusions, embodying the theory of them in an 

 empirical law based upon the physical fact of retinal fatigue, and on 

 the psychological fact of association of contrasts. It is as follows : The 

 retina ceases to perceive as a motion a steady successio?i of images 

 that pass over a particular regioyx for a sufficient time to induce fa- 

 tigue ; and, on a portion of the retina so affected, the image of a body 

 not in motion ap>pears by contrast to be moving in a complementary 

 direction. This law is precisely similar to that of the complementary 

 subjective colors seen after fatiguing the retina by the image of a col- 

 ored body. Similar laws of physico-psychological after-effects are 



