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 through the 12 cards.
The experimenter notes down the numbers of the cards and the letters
which the subject calls. Besides this, the number of seconds required
for the whole experiment, from the beginning of the first card to the
end of the twelfth, is measured with a stopwatch. This time is, of
course, dependent upon the rapidity with which the crank is turned.
The result of the experiment is accordingly expressed by three
figures, the number of seconds, the number of omissions, that is, of
places at which red figures would land on the track which were not
noticed by the subject; and, thirdly, the number of incorrect places
where letters were called in spite of the fact, that no danger
existed. In using the results, we may disregard this third figure and
give our attention to the speed and the number of omissions.

The necessary condition for carrying out the experiments with this
apparatus is a careful, quiet, practical explanation of the device.
The experiment must not under any circumstances be started until the
subject completely understands what he has to do and for what he has
to look out. For this purpose I at first always show the man one card
outside of the apparatus and explain to him the differences between