December 9, 1898.] 



SCIENCE. 



817 



arouse discussion most seriously. To be 

 sure, there are other theories in the field, 

 but they may perhaps remain unmen- 

 tioned here. How these two theories fare 

 when they descend to details we must con- 

 sider later. 



Attention has been called recently in a 

 very interesting way to certain phenomena 

 of movement that are to be observed upon 

 the heavy original Zollner pattern shown 

 in Fig. 2.* In his Physiologische Optih 



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Helmh'oltz called attention to certain facts 

 that appear to be less universally known 

 than their importance would seem to 

 justify. He noticed that if any small object, 

 as the head of a pin, be steadily fixated 

 while drawn horizontally across this dia- 

 gram the heavy verticals appear to be set 

 into motion in the direction of their own 

 lengths, adjacent lines moving in opposite 

 directions. The necessary rate of move- 

 ment for the fixated point can be deter- 

 mined readily for each observer. If this 

 motion be from left to right those lines 

 bearing upward running transversals will 

 seem to shoot upwards, those bearing down- 

 ward running transversals downwards, and 

 if the carefully fixated point be moved back 

 * Filehne, Loc. Cit. 



and forth over the diagram a most startling 

 unsteadiness is produced in it. Now the 

 movements recently noticed by Filehne are 

 very similar to these in appearance. They 

 are to be seen thus : Placing the diagram so 

 that the main lines lie horizontally, view it, 

 not too fixedly, through a tube blackened 

 within. Movements of diagram and tube 

 being obviated in some way, a few mo- 

 ments' careful gazing will reveal the fact 

 that the horizontals, two, three or more at 

 a time, are darting about to the right and 

 left, during a period of from one to three 

 seconds. The motion of any particular hori- 

 zontal is seen to be always in the direction 

 of the overhanging ends of the obliques. Though 

 it is affirmed that these phenomena may 

 be seen even when the possibility of eye- 

 movements is excluded, the writer finds 

 slight movements absolutely essential 

 such, for example, as are produced by be- 

 ginning to close the eye. Though most 

 easily seen, perhaps, when the lines of the 

 diagram are placed horizontallj^, these 

 movements are none the less to be seen in 

 the vertical position. Those lines that be- 

 fore darted to the left now pass upwards. If 

 placed in an oblique position a peculiar fact 

 may be noted. If the lines run upwards 

 from right to left the observed motion will 

 be upwards for those lines that before 

 moved to the left and upwards respectively. 

 But if the lines run upwards from left to 

 right no motion is to be seen, since, while 

 the tendencies to motion found in the hori- 

 zontal and vertical positions have reinforced 

 each other in the first oblique position, they 

 are such as to cancel each other here. But 

 in any case, whenever motion occurs, it 

 takes place in the direction of the over- 

 hanging ends of the obliques The reason 

 for these phenomena Filehne claims to find 

 in the consideration of ' memory-images of 

 motion.' His theory is supplementary to 

 the general perspective theory of which he 

 is an earnest advocate. Many of our visual 



