102 TITCHENER AND PVLE— JUDGMENT OF DISTANCE. [April 18 



and darkened the two lamps in front by curtains of black cloth. 

 Under these conditions, the shadows stood out sharp and crisp upon 

 the thread line. To render the shadows faint, we reduced the in- 

 tensity of the single light at the back, as in the previous experi- 

 ments, until the angles were barely perceptible. These faint 

 shadows were, of course, stronger than the shadows of the earlier 

 experiments. In the latter, the light was so far reduced that, at 

 best, only shapeless patches of bright grey could be discerned upon 

 the white background. In the present series, the shadows were 

 still seen as angular strips of very light grey. At the same time, 

 they were so faint that they frequently faded out, in whole or part, 

 during the progress of a series. If, then, the efficacy of the illusion 

 motive varies with intensity of stimulus, there should be a wide 

 difference in the results of experiments carried out at these two 

 extremes of the intensive scale. 



As the illusory effect of the Miiller-Lyer figure may decrease 

 with practice,^ we thought it well to secure the services of naive 

 and untrained observers, in order that we might compare their 

 judgments with the judgments of some of the trained observers 

 already at our disposal. Experiments were made with two un- 

 practised observers. Miss G. M. Fairlamb and Mr. G. W. Hau. 

 Of H's results we shall speak presently. The average effect of the 

 illusion motive in the first eight series taken with F was 



Shadows Weak. Shadows Strong. 



30.6 mm. 41 mm. 



There was thus a distinct difference in favor of the stronger shodows. 



Nevertheless, the high value of the MV in the series with weak 



shadows, the length of time required for the passing of judgment 



in the critical zone, and the observer's complaints of the fluctuating 



character of the shadows, showed that the two series were not 



^ C. H. Judd, Philosophical Reviezv, ix., 1902, 27 ff. Jiidd's law of de- 

 crease with practice is not universal, as is proved by the fact that the magni- 

 tude of the Miiller-Lyer illusion-effect in the case of one of the writers (T) 

 has shown a slight but constant increase with increase of practice. Cf. V. 

 Benussi, " Zur Psychologic des Gestalterfassens," in A. Meinong's " Unter- 

 suchungen zur Gegenstandstheorie und Psychologic," 1904, 331 f. In general, 

 however, we agree with Judd and F. Schumann (Zeits. f. Psych, u. Physiol. 

 d. Sinnesorg., XXX., 1902, 263 f.) that, with spontaneous reaction to the 

 figure, practice tends to reduce the illusion-effect. 



