I907.] TITCHENER AND PVLE— JUDGMENT OF DISTANCE. 95 



and the training and temperament of the writers markedly diverse, 

 much variety of opinion will ensue, and dogmatism is altogether out 

 of place. But the case now under consideration is unique.^ It is 

 asserted that in straightforward psychophysical method-work, done 

 under strict conditions, the eye may be solicited by lines which it 

 cannot see, the judgment warped by a motive which is neither in 

 consciousness at the time nor ingrained by habit in the nervous 

 system of the observer. 



A principle so revolutionary — for the whole environment is 

 full of subliminal influences, which experimental psychologists have 

 systematically neglected ! — must, one would think, be based upon 

 unequivocal evidence. Hardly less surprising, now, than the con- 

 clusion itself are the numerical results which claim its acceptance. 

 The standard length of line, throughout the experiments, was 25 

 cm. The errors of judgment ascribed to the illusion motive vary 

 between the extreme limits of 1.05 mm. and o.io mm.^ ''The 

 difference," as the author admits, '' is slight ; but," he adds, " we 

 should hardly expect to get more than a slight efifect from the 

 shadows under the circumstances."'^ The efifectiveness of an optical 

 illusion, that is to say, stands in direct proportion to the clearness 

 of contour of the figure shown. Is it not more reasonable to sup- 

 pose that an illusion motive, if effective at all, will be effective at 

 full strength? Or, at any rate, is not this alternative supposition 

 worthy of experimental test? 



It is, however, easier to accept a statement once made — especially 

 if the content of the statement fall in with our immediate intellec- 

 tual purpose — than critically to estimate the value of the evidence 

 upon which the statement rests. And Dunlap's thesis has, accord- 



^Dunlap finds a parallel to his own results in the experiments of C. S. 

 Pierce and J. Jastrow on small differences of sensation (Memoirs of the 

 National Academy of Sciences, III., i., 1884, 75 ff.). There is however, no 

 resemblance whatever between the two investigations. As the latter has been 

 discussed by G. E. Miiller in 1904 ("Die Gesichtspunkte und die Tatsachen 

 der psychophysischen Methodik") and by Titchener in 1905 ("Experimental 

 Psychology," II.), and as it contains nothing which can invalidate the canons 

 of psychophysical method, we leave it here out of account. 



2 Dunlap, op. cit., 448. The figures given are: 0.62, 0.40, 0.17, 0.83, 0.18, 

 0.10, 1.05 and 0.72 mm.: an average of half a millimetre! 



3 Op. cit., 450. 



