land, if they make the steps which their number indicates. A red digit
3 which is 4 steps from the track is to be disregarded, because it
would not reach the track. A red digit 3 which is only 1 or 2 steps
from 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highest card is withdrawn; the second is exposed, and from below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