The red digits, on the other hand, are the dangerous
ones. They move from either side toward the track. The idea is that
the man to be experimented on is to find as quickly as possible those
points on the track which are threatened by the red figures, that is,
those letters in the 26 track units at which the red figures would
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.
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