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 the black and the red figures, and the counting of the steps, and show to him in a number of cases how some red figures do not reach the track, how others go beyond the track, and how some just land in danger on the track.  As soon as he has completely understood the principle, we turn to the apparatus and he moves the window slowly over a test card, and tries to find the dangerous spots, and I turn his attention to every case in which he has omitted one or has given an incorrect letter.  We repeat this slowly until he completely masters the rules of the game.  Only then is he allowed to start the experiment.  I have never found a man with whom this preparation takes more than a few minutes.

After developing this method in the psychological laboratory, I turned to the study of men actually in the service of a great electric railway company which supported my endeavors in the most cordial spirit.  In accordance with my request, the company furnished me with a number of the best motormen in its service, men who for twenty years and more had performed their duties practically without accidents, and, on the other hand, with a large number of motormen who had only just escaped dismissal and whose record was characterized by many more or less important collisions or other accidents.  Finally, we had men whose activity as motormen was neither especially good nor especially bad.