thout giving any voluntary sign of any sort. Hans tapped
2, whereupon Mr. Grabow put a number of similar questions; but I no
longer thought of the answers, and all of Hans's responses went wrong.

Although Hans was not influenced by others so long as a suitable
experimenter was present, yet he might be disturbed and under certain
conditions might be led to make the back-step in response to certain
movements in his environment. The person to whom he responded would have
to be close to the experimenter and would necessarily have to execute a
movement greater in extent than the experimenter's. In such instances
the raising of the head, arm or trunk, was a sufficient stimulus. Thus
we made the following two series of tests. Mr. Stumpf stood with trunk
bent forward before the horse, and at a moment decided upon beforehand,
assumed an erect position. I myself stood beside Hans and asked him to
tap. When I stood at the horse's neck, then Mr. Stumpfs interruption was
effective. When I stood at the horse's flank, the interruption effected
only a seeming hesitation, and when I moved still farther back, the
horse continued to tap despite any attempted disturbance. In the second
series the questioner remained constantly at the right shoulder of the
horse, while the one who attempted to distract him, changed positions.
When the latter stood to the right immediately in front of or beside the
questioner, the disturbance was effective in 10 out of 13 cases. But
when he stood back of, and to the right of, the questioner, the attempts
at disturbance were seldom successful. If he chose a place before and to
the left of the horse, there was hardly any distraction (in 4 cases
only, out of 13), and if he stood to the left and behind the animal, he
exerted no influence whatever. Hans manifestly turned his attention,
almost exclusively, to the side at which the questioner stood.

That knowledge of this _modus operandi_ made it possible for those
persons to get responses from the horse, who hitherto had been
unsuccessful, is shown in the case of Mr. Stumpf when he began to
control his movements voluntarily on the basis of observations which had
been made.

_II. Problems which Hans solved by movements of the head._

We are here concerned with the horse's head movements upward, downward,
to the right and to the left, and also with nodding and shaking of the
head to signify "yes" and "no". We soon discovered that these
experiments, also, were