818 



SCIENCE. 



[N. S. Vol. VIII. No. 206. 



experiences are had while we are in motion. 

 In such cases the angle made by any up- 

 right object that is perpendicular to the 

 earth changes as we approach it or recede 

 from it. This change is always of such a 

 kind that, as we approach, the angle ap- 

 pears to diminish from an obtuse to a right 

 angle, and as we reach and pass beyond the 

 object the angle seems to increase, the per- 

 pendicular appearing, that is, to fall gradu- 

 ally away from the moving observer. But 

 whetlier the observer be approaching or 

 receding, the apparent movement — which 

 may be regarded as a rotation about the 

 point of contact with the ground — is alwaj's 

 opposite to the direction of the observer's 

 progress, and always towards that position 

 where it shall seem to tip away from the 

 observer as he has passed by. Most strik- 

 ingly, possibi}', is this seen in railway 

 travel. The telegraph poles seem to rotate 

 as they fly past, and alwaj'sin the direction 

 of that position where they shall appear 

 to overhang. "Well, countless experiences 

 of this kind have stored up such a mass 

 of memory-images that when, as in the 

 Zollner diagram, similarlj' overhanging ob- 

 liques are viewed, these latent images are 

 brought to the threshold of consciousness 

 and the diagram itself becomes enlivened 

 with an illusory motion, occurring in strict 

 accord with actual objective experience. 

 Psychologically considered, the language 

 used is not wholly free from objection, but 

 the meaning of the theory is on the whole 

 clear enough. It has seemed well to report 

 these new observations at some length, 

 since their importance for the theory of 

 optical illusions is evident. Filehne asserts 

 the complete lack of connection between 

 the illusory movements described by him- 

 self and those mentioned by Helmholtz. 

 To the writer, however, it would seem that 

 the two sets of phenomena are very closely 

 related. Whether this be so, only a care- 

 ful and more extended examination of the 



matter can determine. The alleged move- 

 ments, or their lack, are too difficult of ob- 

 servation and too elusive of rigid verifica- 

 tion to admit of any positive statements at 

 present. Nevertheless the question may be 

 asked very pertinently, how any visual im- 

 pression, of whatever characteristics, should 

 be capable of causing illusory perceptions 

 of movement at a moment when every 

 actual movement of the eyes is excluded. 

 Certainly, if the observations recorded be 

 true, we have something novel in the realm 

 of psychology— a perception of motion, but 

 a motionless object and a motionless eye. 



2. The Poggendorf Illu- 

 \ sion. — The secondary illu- 



sion to be seen on the ob- 

 lique lines of the heavy 

 Zollner pattern, the more 

 usual form of which is 

 shown in Fig. 3, still con- 

 tinues to be the object of 

 ^ experimental and theoret- 



j,-jy 3 ical inquiry. We meet here 



tiie same contrasted theo- 

 ries as before, though we find them some- 

 what less sharply stated. 



The explanation that rests upon the over- 

 estimation of acute angles has, as usual, 

 little difficulty with the matter. The free 

 ends of the transversal are simply rotated 

 about the points of contact with the ver- 

 ticals, with the result that a new line may 

 be drawn at either end in apparent continua- 

 tion of the other. It would seem also, as 

 Wundt has pointed out, that other factors 

 cooperate to produce the illusion, since its 

 amount is much diminished by giving the 

 figure a horizontal position. One such fac- 

 tor is doubtless the universal tendency to 

 overestimate the upper as opposed to the 

 lower half of any vertical dimension. 

 Another factor may well be the likewise 

 universal tendency to underestimate empty 

 as opposed to filled spaces. Underestimating 

 the open space between the inner ends of 



