828 



SCIENCE. 



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



sion as the overestimation of the upper 

 half of a verticah Such a theory Wuudt 

 long ago propounded. His recent work is 

 only an especially thoroughgoing attempt, 

 from a novel standpoint, to defend the old 

 thesis. Especial emphasis is placed upon 

 the consideration of equivocal figures and 

 upon the secondary character of ' perspec- 

 tive.' And everywhere attention is called 

 to the effect of particular positions and 

 movements of the eyes. The essence of 

 the theory is that everj^ visual spatial per- 

 ception is a complex formed by the assimi- 

 lation of visual qualities with sensations 

 coming from the muscles of the eye. What- 

 ever, therefore, increases the intensity of 

 the muscular sensations that enter into the 

 complex occasions the perception of a 

 greater spatial magnitude. The particular 

 conditions necessarj' for this increase of 

 muscular intensity are to be found both in 

 the assymmetry of the eye-muscles and in 

 those cases to which the general mechan- 

 ical principle can be applied, that brief 

 movements require relatively more energy 

 than those of longer duration, since it is 

 harder to start and stop a movement than 

 to maintain one already under way. This 

 theorjf is called 'physiological,' to call at- 

 tention to the fact that the conditioning 

 factors are of physiological rather than 

 psj^chological origin. "With the exception 

 of a few cases, such as the illusion of con- 

 trast shown in Fig. 12, this principle of 

 muscular energy finds universal applica- 

 bility. One may be imwilling to accept the 

 wide-reaching implications that this theory 

 has for the general doctrine of space percep- 

 tion. Yet one must frankly admire Wundt' s 

 masterljr efl'ort at unification and acknowl- 

 edge the compelling power of his argumen- 

 tation, especially as it appears in this new 

 form. 



In conclusion, attention may be called to 

 the illusion of Fig. 13, in which the oblique 

 line ah appears to curve slightly at its point 



of intersection with the vertical. The illu- 

 sion is not marked, but it can usually be 

 seen bj^ all observers. For some it may be 

 more distinct if the three figures be held 



Fig. 13. 



horizontally, and it may be more apparent 

 in some one of the figures than in the other 

 two. But if the eye carefully follow the 

 line from a to 6 the line will probably 

 be seen to bulge on either side of the point 

 of intersection in such a way that it forms 

 an extremely attenuated S. Each of the 

 three principles of explanation considered 

 above is applicable here. The first would 

 say that the points of the oblique and the 

 vertical are mutually antagonistic in the 

 immediate vicinitj' of the point of intersec- 



