step was,
like the preceding one, a reaction to a visual cue. (Hans's tapping
was almost quite inaudible). Both stop-watches were carefully
regulated. In order to eliminate also the constant error which might
possibly arise as a result of some difference in the functioning of
their pressure-mechanism, the two watches were always exchanged in
the different series of tests, by the observer of the man and the
observer of the horse. The two time-measurements obtained by the two
observers contained, of course, the reaction-times of the observers
themselves. In order to equalize the constant error which thereby
arose, it was arranged that each observer should react alternately
now to the man, now to the horse. In order to be perfectly safe, the
reaction-times of those concerned, (von Hornbostel, Pfungst,
Schumann and Stumpf), were later determined in the laboratory by
means of the carefully regulated Hipp chronoscope. Separate
determinations were made of the reactions to the head-jerk and to an
imitation of the horse's back-step. Then the time which one observer
took to react upon a head-jerk, was compared with the reaction-times
of the other observers to the back-step. Since the greatest
difference which was found in this comparison, did not exceed
one-tenth second, the results obtained in the courtyard required no
correction.]

Measurements of this kind were taken for Mr. von Osten, Mr. Schillings
and myself. In the case of the first two it was taken without any
knowledge on their part. They did not even know that they were being
observed, having been told that the measurements were for the sake of
determining the horse's rate. In my case, to be sure, the time could not
be taken without my knowledge. I succeeded, however, in eliminating the
effect of this knowledge on my part. (Cf. pages 88 and 145.) Since the
results obtained in the case of Mr. Schillings quite agree with those
obtained in my case, it is evident they may be considered as being of
equal value.

With regard to the number of tests the following table may be referred
to. The first vertical column gives the name of the questioner, i. e.
the person operating with the horse. The four other columns give the
number of tests made upon each of these. The name of the person who made
the observation in each series is indicated at the head of the column.
It is unnecessary to give the n