rse (see page 42), and had the advantage that the measurement was
not influenced by knowledge on the part of the time-keeper. Two
observers were required, one watching the horse, the other the
questioner. Both observers had fifth-second stop-watches. The larger
face of this watch shows the fifth-second and a hand on the smaller face
indicates the minute. By pressing upon the stem the watch may be set in
motion at any moment desired, and by pressing it once more it may be
instantly stopped, and the time elapsing between the setting in motion
and the stopping may be read on the face. By pressing upon the stem a
third time the hands are brought back to zero, and the watch is ready
for another test. At a moment agreed upon beforehand--usually the third
tap of the horse--both observers started their watches. Practice tests
had shown that this could be done with all the accuracy necessary in
this case. As soon as the observer of the questioner noticed the
latter's head movement he stopped his watch, and as soon as the observer
of the horse noticed the latter's back-step he stopped his watch. Since
the movement of the horse's foot does not occur as a jerk, but is of
greater extent than a jerk would be, it was agreed that the observer was
to stop the watch as soon as he recognized the back-step as such, not
when the foot was being raised from the ground, because it was not then
evident whether the horse would bring it back to the original position
or whether he was preparing to give another tap, nor when he had brought
his foot completely back, but at the moment in which it was evident that
the horse intended to make the back-step. Experimentation had shown that
an agreement as to this moment was possible. A tap with the left foot,
which might possibly follow upon the back-step, could be left out of
account. The difference in time between the two watches would show the
time between the head-jerk of the questioner and the back-step of the
horse,[J] and if the back-step was indeed a reaction upon the
head-jerk, then the watches would have to show a later time for the
back-step than for the head-jerk.

[Footnote J: For the benefit of those who are familiar with
reaction-time experiments of this kind, I would state the following:
The reaction to the head-jerk, on account of the minuteness of the
latter, was sensory throughout, and therefore all precipitate
reactions are entirely wanting. The reaction to the back-