the track is also to be disregarded, because it would cross
beyond the track, if it took 3 steps. But a red 3 which is 3 units
from the track, a red 2 which is 2 units from the track, and a red 1
which is 1 unit from the track would land on the track itself; and the
aim is quickly to find these points. The task is difficult, as the
many black figures divert the attention, and as the red figures too
near or too far are easily confused with those which are just at the
dangerous distance.

As soon as this principle for the experiment was recognized as
satisfactory, it was necessary to find a technical device by which a
movement over this artificial track could be produced in such a way
that the rapidity could be controlled by the subject of the experiment
and at the same time measured. Again we had to try various forms of
apparatus. Finally we found the following form most satisfactory.
Twelve such cards, each provided with a handle, lie one above another
under a glass plate through which the upper card can be seen. If this
highest card is withdrawn; the second is exposed, and from below
springs press the remaining cards against the glass plate. The glass
plate with the 12 cards below lies in a black wooden box and is
completely covered by a belt 8 inches broad, made of heavy black
velvet. This velvet belt moves over two cylinders at the front and the
rear ends of the apparatus. In the centre of the belt is a window
4-1/2 inches wide and 2-1/2 inches high. If the front cylinder is
turned by a metal crank, the velvet belt passes over the glass plate
and the little window opening moves over the card with its track and
figures. The whole breadth of the card, with its central track and its
4 units on either side, is visible through it over an area of 5 units
in the length direction. If the man to be experimented on turns the
crank with his right hand, the window slips over the whole length of
the card, one part of the card after another becomes visible, and then
he simply has to call the letters of those units in the track at which
the red figures on either side would land, if they took the number of
steps indicated by the digit. At the moment the window has reached Z
on the card, the experimenter withdraws that card and the next becomes
visible, as a second window in the belt appears at the lower end when
the first disappears at the upper end. In this way the subject can
turn his crank uninterruptedly until he has gone throug