3 
Discrimination of shades of gray for different intervals of time. 
white light, and partly, through varying the number of thicknesses 
of muslin to prevent too great changes in intensity of light from day 
to day, or even parts of the same experimental hour. The time 
intervals were measured by a metronome, and the exposures given 
by raising and dropping a black cotton-flannel curtain which hung 
in front of the color-mixer. The experiments took place under three 
different conditions. (A) Byes closed during interval between norm (N) 
and comparison (F) whilst an effort was made to hold fast a visual 
image of the disc, [a) Eyes open with relaxed attention during the 
time interval. (D) Distraction experiments in which a simultaneous 
discrimination of two grays of about the shade of the successive 
stimuli, was introduced into the middle of the time interval. An 
attempt to use one disc as a distraction failed through the tendency 
of the reagents to use the disc as a mediate term of comparison 
between the two main stimuli. When, however, the reagents had to 
pass judgment on a pair of »distraction* discs, simultaneously ex¬ 
posed, the tendency to form mediate judgments disappeared. 
The norm in all cases was 180 degrees of white plus 180 degrees 
of black; there was, however, no danger of the formation of an »ab¬ 
solute memory* of the norm partly on account of variations of atmo¬ 
spheric brightness, and partly on account of variations in the reagents 
subjective estimates of the norm. The method used was the mixed 
method of right and wrong cases and of minimal changes, and to 
this end the norm was compared with itself and 4 other brightnesses : 
— viz. 200, 190, 170, 160 deg. of black. The order of exposure was 
in the first instance determined by chance, and then this order, and 
its reverse were carried through the entire series of intervals for both 
time orders, after which a second arrangement was determined by 
chance. The time of work was in the late afternoon between five 
and. six thirty — a disadvantageous period, as it sometimes happened 
that the sun went down behind the university buildings during the 
last series. The reagents were Prof. Külpe (Ke) and Privat Dozent 
of Philosophy W. Kinkel (Kl) of Giessen, a member of the Würz¬ 
burg Institute, but at that time a beginner in experimental work. 
As regards introspection, the reagents were asked to note such 
mental imagery as seemed relevant to the work — especially verbal 
and visual imagery. From time to time they were questioned in 
1*