December 9, 1898.] 



SCIENCE. 



815 



informed about the simplest forms of the 

 most interesting types of illusion. The 

 signal for the scientific discussion of the 

 subject seems to have been given in 1889 

 when Miiller-Lyer first published the ' fig- 

 ures ' which now bear his name. The in- 

 vestigations and discussions that grouped 

 themselves about these particular figures 

 soon spread to the whole field of optical 

 illusions, until there grew up that body of 

 technical literature which today has as- 

 sumed very respectable dimensions. It is 

 not so much that new forms of illusions 

 have been devised or discovered — although 

 more than one valuable contribution has 

 been made — as that the heretofore well- 

 known illusions have been subjected to a 

 closer scrutiny than ever before. Like 

 everything else accessible to experimenta- 

 tion, illusions have been taken into the 

 laboratory. Variants, possessing character- 

 istics that difier somewhat from the original 

 form, have been devised. Each figure has 

 been dissected and analyzed that it might be 

 reduced to the lowest terms. . And, above 

 all, each figure has been subjected to a 

 quantitative investigation, the amount of 

 the illusion being accurately measured under 

 the widest possible variety of conditions. 

 These results in turn have been made the 

 basis of theoretical considerations, and the 

 end sought has, of course, always been some 

 satisfactory explanation which shall furnish 

 adequate grounds for the presence of an il- 

 lusion in any given case. It is here that 

 the war has waged. For while, in general, 

 there has been sufficiently close agreement 

 in reference to the results of experimental 

 observation, there has been small uniformity 

 in the theoretical conclusions reached. The 

 one great attempt of to-day is, therefore, to 

 bring harmony into this field ; to establish, 

 if possible, some single point of view which 

 shall be applicable to all geometrical optical 

 illusions alike, and which shall furnish that 

 wished-for, comprehensive unity among the 



seemingly scattered and unrelated facts. 

 That this attempt has met with complete 

 success can hardly be asserted. Theories 

 fundamentally antagonistic stand side by 

 side with others that seek to combine and 

 reconcile, and the day of perfect agree- 

 ment seems yet distant enough. The splen- 

 did attempt of Wundl* to connect all illu- 

 sions with actual or attempted movements of 

 the eyes ; the no less earnest attempt of 

 Thieryf and FilehneJ to establish an ex- 

 planation in terms oi perspective ; the classic 

 attempt of Helmholtz and, more recently, of 

 Heymans§ and Loeb to apply in one form 

 or another the principle of contrast, and the 

 very pretensions effort of Lipps^ to define 

 and utilize an aesthetic principle of unre- 

 strained and victoriously striving activities, 

 or their opposites, are all cases in point. 



Perhaps the clearest way to give those 

 readers of Science who, while interested in 

 the subject, are unable to follow the tech- 

 nical literature closely and at first hand, 

 some impression of the more recent work 

 that has been done, will be to consider, in 

 the first place, the discussions that have 

 centered about some of the best known il- 

 lusions, leaving until the end the account of 

 the various explanatory principles that 

 have been advanced and vigorously de- 

 fended. 



A. 



1. Zollner's Pattern. — In one or another of 

 its many forms every one is familiar with 

 the illusion of Fig. 1, in which a set of 

 parallels is made to appear alternately con- 

 vergent and divergent, by the addition 

 to them of transverse cutting lines. But 



* Wundt. Die geometrisch-optischen Tiiuschungen, 

 1898. 



t Thiery. Philosophische Studien., XI and XII. 



IFilebne. Zeitsch. f. Psych., etc., XVII (1898): 

 15. 



§ Heymans. Ditto., XIV., 118. 



If Lipps. EauniEesthetili u. geometrisch-optischen 

 Tauschungen, 1897. 



