THE EFFECT OF IMPERCEPTIBLE SHADOWS ON 

 THE JUDGMENT OF DISTANCE. 



By EDWARD BRADFORD TITCHENER 



AND 



WILLIAM HENRY PYLE. 

 {Read April i8, 1907.) 



Some six years ago, an investigation was published under the 

 above title from the Psychological Laboratory of the University of 

 California/ Briefly stated, its thesis was 'that a motive to optical 

 illusion, although so faint as to be wholly imperceptible to the ob- 

 server, is nevertheless effective in the determination of judgments 

 of visual distance. One is required to compare, under certain 

 methodical conditions, the lengths of two contiguous sections of a 

 straight line. To all appearances, the stimuli are perfectly simple : 

 one sees, drawn horizontally upon a white background, a thick 

 black line which is bounded and divided by three vertical black 

 marks ; and the problem is, in successive observations, to report 

 upon the equality or difference of the two sections thus displayed. 

 The peculiarity of the experiment lies in the fact that the stimuli are 

 only apparently simple. In certain series, not known to the observer, 

 the experimenter throws upon the white background angular 

 shadows, disposed in such a way as to convert the two lengths of 

 line into the two parts of the Miiller-Lyer illusion. These shadows, 

 be it repeated, are so faint that they are never, even under the 

 greatest strain of attention, visible to the observer. Yet they have 

 their due effect: the judgments of length of line prove to be subject 

 to a constant error, whose sign, plus or minus, reflects the tendency 

 of the motive to illusion. 



This contention is, in itself, startling enough. A great deal 

 has been written, of recent years, about the subliminal and the sub- 

 conscious, and many wonderful things have been declared in their 

 name. Where the phenomena are obscure, the definitions arbitrary, 



^ K. Dunlap, in Psychological Reviezv, VII., 1900, 435 ff. 



94 



