must therefore consider more specifically what are the
determining factors that make for success or failure of the response.
First of all, the innocent questioner very often did not designate the
direction with sufficient clearness. Furthermore, Hans presumably was
not able to discriminate sufficiently between the direction of the
experimenter's eye and that of his head, which two directions did not
always coincide. Finally the horse's attention was often diverted, while
he was running toward the piece indicated, by the other pieces lying to
the right and to the left, and for this reason the addition of a single
piece to the otherwise unchanged row of five pieces tended to decrease
greatly the chances of success.

The case is different with the perception of the directive signs for
tapping, for nodding and shaking the head, etc., all of which require
the perception of movements. This is not necessarily more difficult on
account of the imperfect constitution of the tissues that serve for the
refraction of light. Some authors even aver that this facilitates the
perception of moving objects. This view was first advanced by the
excellent ophthalmologist, R. Berlin[39] of Stuttgart. In arriving at
this view he was guided by the following considerations. The peculiar
form of astigmatism of the lens of the horse's eye, which Berlin has
described as "butzenscheibenfoermig",[Y] because it appears in the form
of a series of glossy concentric circles around the lens nucleus, has
the property of enlarging the pathway (and with it the rapidity) of
moving retinal images. If we take a speculum by means of which a view
may be had of the interior of the eye, and fixate a definite point on
the retina of the horse, and then make a slight movement of the head
horizontally, we find that the point fixated moves--apparently at
least--toward the border of the pupil. In a normally constructed eye
this seeming movement will be in a straight line, while in the eye of
the horse, (according to Berlin), its path is curved, and therefore
longer. Berlin believes that the same thing which here occurs in the
case of this merely apparent movement, must also happen when an external
moving object is imaged on the horse's retina. Its pathway, too, will be
curved, and therefore longer, so that if the head of Mr. von Osten moves
past the animal's eye, then the image on the horse's retina will take a
longer, more circuitous route than it would if the eye were n