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SCIENCE. 



[N. S. Vol. XII. No. 293. 



in other words, in the same directions as 

 the currents of the illusory dust-drift. And, 

 taking all into account, it cannot be very 

 far out of the way to conjecture that the 

 same fundamental factors are at work here 

 as in the familiar cases of the artificial 

 waterfall and the rotating spiral. Mere 

 impulses to movement have taken the place 

 of actual movements in the production of 

 after-effects. 



An attempt to obtain the illusion with 

 monocular vision is attended with quite sur- 

 prising results. For even after a full min- 

 ute's fixation of the cloth, no ' drift' is to be 

 seen on the black background. And the result 

 is the same whether both eyes, or the one 

 eye only, be open at the moment of trans- 

 ferring the gaze to the black surface. There 

 is instead an interesting set of phenomena 

 which do not appear in the binocular ex- 

 periments. Now the illusory dust-masses 

 come to view during the fixation . They are 

 not wholly like those above described, but 

 present rather the appearance of fine meshes 

 formed of light gray cobwebby lines. Some- 

 times these meshes appear to lie slightly in 

 front of the cloth, and if the eifort is made 

 to fixate them they temporarily disappear. 

 Movements are by no means wanting, but 

 there is an intermittence about them which 

 the binocular phenomeua never show. A 

 closer examination of this net-work charac- 

 ter of the illusion reveals the fact that each 

 eye, the closed as well as the open, is con- 

 tributing to the total effect. This may be 

 readily demonstrated as follows. Let either 

 eye, the left for example, be entirely screened 

 from the cloth by a tiny box, or something 

 similar, blackened within, the eye remain- 

 ing open and free to move. Let the right 

 eye fixate the lines. Now while this right 

 eye remains open, the most prominent illu- 

 sory movements are decidedly those run- 

 ning jKrpendicular to the direction of the 

 lines. This is true no matter how the lines 

 may lie in the field of vision. But if this 



right eye be closed after a brief fixation, an- 

 other set of movements is seen projected 

 into the dark field of the covered left eye. 

 These movements, though possessing neither 

 vigor nor great vividness, are invariably in 

 the same direction as the objective lines. 

 That which moves here is less a dust-cloud 

 than a set of fleecy or worsted-like bands, 

 in the midst of which the ' crossed' after- 

 image of the lines of the cloth soon appears. 

 In addition then to the regular transfer- 

 rence of an after-image to the field of 

 the unstimulated eye, we have here the 

 transference also of an illusory after-effect. 

 The illusion is to be sure not wholly the 

 same for the two eyes, but neither are 

 crossed after-images entirely identical in 

 character with the direct after-images. The 

 interesting features then of the monocular 

 experiments are that the illusion appears 

 for the stimulated eye during the period of 

 fixation only, and that the unstimulated eye 

 also presents illusory effects of the same 

 general character as those experienced by 

 the open eye. 



It can hardly be said with full certainty 

 that these monocular phenomena have 

 contributed anything decisive towards the 

 explanation of the binocular form of the 

 illusion. Nevertheless there is a point of 

 difference between the two forms which 

 cannot be wholly without meaning. There 

 is, namely, in the monocular experiments 

 a relative absence of the feeling of unrest 

 during the period of fixation. The single 

 eye seems to fixate the chosen point with 

 far less effort. Solicitations to its move- 

 ment are noticeably absent, and the time 

 of stimulation can be prolonged without 

 discomfort to a point where the binocular 

 stimulation would have become exceedingly 

 disagreeable. Now whether this absence of 

 vivid impulses to movement may be re- 

 garded as alone responsible for the difference 

 in the illusion can of course not be affirmed 

 with complete confidence. But it seems 



