PAPERS ON PSYCHOLOGY AND PHYSIOLOGY 117 



Observers. Observers were recruited from tlie staff, 

 from the graduate students in the Department of Psy- 

 chology, from the students in the intermediate labora- 

 tory, and from the members of a few elementary classes. 

 There were more than a dozen all told; and they ranked 

 in training from naive observation to the capacity for 

 careful analysis of mental processes. With one or two 

 exceptions, which were noted in our results, the observ- 

 ers were uninformed concerning the problem of the ex- 

 periment or the character and disposition of the ap- 

 paratus. 



Procedure. The experiments lasted through the great- 

 er part of the year 1919-1920 in the course of which time 

 several modifications in the procedure were made. In 

 the early series only five lights were used. They were 

 flashed on in irregular order and no two in unison. Since 

 we were working with a limited number of lamps, at most 

 only twenty, to produce an effect equivalent to that un- 

 der natural conditions we had to increase the speed of 

 flashing to about one per second ; the duration of the flash 

 equal to .5 sec. and the intervals between flashes of the 

 same duration as the flashes. The Snyders give the most 

 reliable value as to flash and interval: 15 flashes per min. 

 and a 6 sec. duration for the interval. We are now con- 

 tinuing the experiment with these longer time values. 



In every experimental series the observer was seated 

 in the dark-room, allowed to rest for about five minutes 

 to permit after-images to disappear and to become ' ' dark 

 adapted," and he was then told to describe as accurately 

 and analytically as possible the effect produced. He was 

 told that a warning signal would appear at about the 

 center of the field which he was to observe and at about 

 three second interval before the observation was to begin. 

 Cotton was inserted in his ears so that no possible noise 

 from the apparatus in the adjoining room could form 

 the basis of grouping even though heard through the 

 closed door. At the end of the series before the light was 

 again turned on the curtain was drawn over the frame- 

 work suspended on the wall. 



From a simple series of five lights, each one flashing 

 at its o^\Ti peculiar rate, the number of lights flashing in 



