98 TITCHENER AND PYLE— JUDGMENT OF DISTANCE. [Aprl .8, 



series was thrown out, and repeated later on. We wished, how- 

 ever, to give the subHminal shadows every chance to produce their 

 effect, and were therefore content to run this risk of additional 

 work. 



Having regulated the intensity of the light behind the apparatus, 

 the experimenter affixed the Miiller-Lyer angles to the sheet and 

 rods. The constant or standard line, which was placed as often 

 to the right as to the left, was 25 cm. in length. The variable 

 line was set, at the beginning of a series, to appear either as dis- 

 tinctly longer or as distinctly shorter than the standard; and the 

 limiting strip was moved in or out, by steps of i mm. and at intervals 

 of 15 seconds, until several judgments had been recorded of the 

 opposite tenor to that with which the series began. Every series 

 taken with the angles was paired with a precisely similar series 

 taken without them. 



The order of experimentation was, so far as possible, left to 

 chance. Thus, the position of the angles for a given paired series, 

 as open or closed, was determined by lot. There were as many 

 series beginning with " longer " as with '' shorter," but their dis- 

 tribution was also determined by lot. Finally, while the members 

 of a paired series were always given together, chance was allowed 

 to decide whether the shadow-series should precede the shadowless, 

 or conversely. 



The judgments of the observer referred always to the variable 



line, and took the form " longer," " shorter," and " equal " or 



" doubtful." The middle point of the region of doubt and equality 



was taken as the mean equality point of the single series, and this 



was compared with the mean equality point of the other member 



of the pair^. Five observers took part in the experiments : i\Ir. 



L. R. Geissler, Mr. C. R. Hugins, Professor T. A. Hunter, Mr. W. 



H. Pyle and Miss E. A. Smith. All except Mr. Hugins had had 



extended training in psychological observation. 



^ In the foregoing, we have closely followed Dunlap : op. cit., 436 ff. The 

 principal differences of procedure appear to be these : that we gave our ob- 

 servers a fairly extensive preliminary practice ; that we allowed a period for 

 adaptation of the eyes to the dark; and that we placed the standard line as 

 often on the one side of the figure as on the other (for Dunlap, the standard 

 was always on the left). 



