the stopping of the car as soon as the
danger of an accident threatens is evidently effective only if the
movement controlling the lever is carried out with sufficient
rapidity. We should accordingly be justified in examining the
quickness with which the individual reacts on optical stimuli. If a
playing child suddenly runs across the track of the electric railway,
a difference of a tenth of a second in the reaction-time may decide
his fate. But I may say at once that I did not find characteristic
differences in the rapidity of reaction of those motormen whom the
company had found reliable and those who have frequent accidents. It
seems that the slow individuals do not remain in the service at all.
As a matter of course certain other indispensable single functions,
like sharpness of vision are examined before the entrance into the
service and so they cannot stand as characteristic conditions of good
or bad service among the actual employees.

For this reason, in the case of the motormen I abstracted from the
study of single elementary functions and turned my attention to that
mental process which after some careful observations seemed to me the
really central one for the problem of accidents. I found this to be a
particular complicated act of attention by which the manifoldness of
objects, the pedestrians, the carriages, and the automobiles, are
continuously observed with reference to their rapidity and direction
in the quickly changing panorama of the street. Moving figures come
from the right and from the left toward and across the track, and are
embedded in a stream of men and vehicles which moves parallel to the
track. In the face of such manifoldness there are men whose impulses
are almost inhibited and who instinctively desire to wait for the
movement of the nearest objects; they would evidently be unfit for the
service, as they would drive the electric car far too slowly. There
are others who, even with the car at high speed, can adjust themselves
for a time to the complex moving situation, but whose attention soon
lapses, and while they are fixating a rather distant carriage, may
overlook a pedestrian who carelessly crosses the track immediately in
front of their car. In short, we have a great variety of mental types
of this characteristic unified activity, which may be understood as a
particular combination of attention and imagination.

My effort was to transplant this activity of the motormen into
laborato