represent an electric railway track on a street.  On either side the 4 rows of squares are filled in an irregular way with black and red figures of the three first digits.  The digit 1 always represents a pedestrian who moves just one step, and that means from one unit into the next; the digit 2 a horse, which moves twice as fast, that is, which moves 2 units; and the digit 3 an automobile which moves three times as fast, that is, 3 units.  Moreover, the black digits stand for men, horses, and automobiles which move parallel to the track and cannot cross the track, and are therefore to be disregarded in looking out for dangers.  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.  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  If this highest card is withdrawn; the second is exposed, and from below springs press the remaining cards against the glass plate.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