AuGrsT 10, 1900.] 



SCIENCE. 



209- 



still see traces of the ' dust ' after a lapse of 

 30 sec, while in another case everything 

 had disappeared at the end of 4 sec. Per- 

 haps 10 sec. would be a fair average dura- 

 tion. At about this time the regular after- 

 image is apt to make its appearance, and 

 this tends strongly to drive the illusion 

 away. 



But the really interesting point in the 

 matter is the direction of the moving drift. 

 This turns out to be directly dependent 

 upon the direction of the lines in the field 

 of fixation. The most general statement of 

 the matter is that, however these lines may 

 lie, the illusory dust currents run in a di- 

 rection perpendicular to them. Quite often, 

 however, it is impossible to speak of the 

 direction as strictly perpendicular, since the 

 course of the drift may be along curved lines, 

 as if a tiny whirlwind had caught up a bit 

 of light, fluffy snow. Or, further — and this 

 is perhaps most often the case — there are 

 secondary currents visible whose directions 

 do not coincide with that of the main stream. 

 Nevertheless some particular direction is 

 almost invariably more prominent than any 

 other, and the statements of various ob- 

 servers show that the direction of the most 

 vivid stream is most decidedly perpendicu- 

 lar to that of the lines. If the lines are 

 vertical, the drift is usually to the left, 

 though some subjects see it always to the 

 right. If the lines are horizontal, the 

 tendency to see the drift running downioards 

 seems to be slightly more marked. Nearly 

 as many subjects, however, see an upward 

 drift, and quite often currents are seen to 

 run side by side in both directions. 



If the experiment be so arranged that 

 half the field of fixation is occupied by ver- 

 tical and the other half by horizontal lines, 

 two clearly separated currents will appear in 

 the illusion with horizontal and vertical 

 directions respectively. Or if the usual field 

 of fixation be divided by a vertical strip of 

 some uniform color, no ' drift ' will be seen 



in that portion of the field corresponding to 

 the strip. If the centre of a set of concen- 

 tric black circles upon a white ground be 

 fixated, the resulting illusion suggests a 

 confused boiling movement, sometimes run- 

 ning in converging lines towards the cen- 

 ter, sometimes passing in diverging lines 

 towards the periphery of the field. 



Now the oddity of this illusion consists 

 precisely in this : that without inten- 

 tional movement either of eyes or of object, 

 there is yet an after-effect in the form of 

 a definite and unmistakable perception 

 of motion. An ordinary after-image of 

 motion requires a previous objective move- 

 ment of some sort. Here, on the other hand , 

 we can only say that the resulting percep- 

 tion is as if there had been a previous and 

 actually perceived motion. And this latter 

 is exactly the case with another peculiarity 

 not directly connected with the illusion 

 itself. After steadily viewing the cloth for 

 say 30 sec. , the closely set lines begin to 

 appear beaded. They are no longer straight, 

 but wavy. And even the after-image when 

 it appears presents the same aspect. Now 

 this result is identical with that pi'oduced 

 by actual movement of parallel lines across 

 the retina.* Accordingly we have in con- 

 nection with this illusion, two phenomena 

 that ordinarily follow actual movement. 

 This fact would seem to indicate the direc- 

 tion in which an explanation is to be sought. 

 For while there is no intended movement 

 of the eyes during the fixation of the cloth, 

 there are certainly imjnd-ses to movement 

 aroused by the various lines about the fix- 

 ation-point. Every one knows how hard it 

 is to let the eyes come to rest in a field 

 occupied by such lines. Each one of the 

 latter solicits the center of fixation to rest 

 upon it. The impulses to movement are 

 then in directions perpendicular to the lines, 



^This has been well described by too Fleischl, 

 SitzuDgsber. d. Weiner Acad. 1882, Bd. 86, III. Abth, 

 S. 8. 



