retina 12.5s, if it were to be distinguished. It seems, therefore, probable that the violet light had not
been converted into a nervous impulse within this interval, and if this is the case it would give us a
minimum time for this process. The familiar experiment with rotating discs shows that light-
impressions of moderate intensity following one another at intervals of 25s are just fused together. It
seems, therefore, that the retina is excited, and begins to resume its normal condition in about 25s .
If this assumption is correct we have the maximum time for the period under consideration. We may
be tolerably sure that the time passing before a light is converted into a nervous impulse varies with
the intensity of the light, and may perhaps assume the time to be 15-20s for daylight reflected from a
white surface.
These considerations lead us to suppose that, when a reaction is made on light, only about half the
time, that is 75s , is taken up by the cerebral operations. We naturally ask what happens in the brain
after the nervous impulse reaches it. It has generally been assumed that the largest factors of the
reaction-time are taken up by the processes of perception and willing. I think however that if these
processes are present at all they are very rudimentary. Perception and volition are due, we may
assume, to changes in the cortex of the cerebrum, but reflex motions in answer to sense-stimuli, as
in contraction of the pupil and in winking, can be made after the cortex has been removed, and an
animal in this condition can carry out motions adapted to the nature of the stimulus. If a pigeon from
which the cerebral hemispheres have been removed is thrown into the air, it will not only fly, but also
avoid obstacles and alight naturally on the ground. It seems to have consequently sensations of
light, but apparently no perceptions, either because it does not see colour and form, or because it
lacks the intelligence needed to understand their meaning. In the same way a reaction such as we
are considering can probably be made without need of the cortex, that is, without perception or
willing. When a subject has had no practice in making reactions (in which case the reaction-time is
usually longer than 150s ) I think the will-time precedes the occurrence of the stimulus. That is, the
subject by a voluntary effort, the time taken up by which could be determined, puts the lines of
communication between the centre for simple light sensations (in the optic thalami probably), and
the centre for the co-ordination of motions (in the copora striata, perhaps, connected with the
cerebellum), as well as the latter centre, in a state of unstable equilibrium. When therefore a
nervous impulse reaches the thalami, it causes brain-changes in two directions; an impulse moves
along to the cortex, and calls forth there a perception corresponding to the stimulus, while at the
same time an [p. 233] impulse follows a line of small resistance to the centre for the coordination of
motions, and the proper nervous impulse, already prepared and waiting for the signal, is sent from
the centre to the muscle of the hand. When the reaction has often been made the entire cerebral
process becomes automatic, the impulse of itself takes the well-travelled way to the motor centre,
and releases the motor impulse.[14]
I now go on to give the results of my experiments. I only give the determinations made on B (Dr. G.
O. Berger) and C (the writer); I have made similar determinations on other subjects of different age,
sex, occupation, etc., but these can be better considered after we know the results of careful and
thorough experiments on practised observers. We have first to consider the simple reaction-time for
light. When this was to be measured, all being in readiness, as described in the foregoing section,
the experimenter said 'Jetzt,' and the observer fixated the point at which the light was to appear, and
put himself in readiness to make the reaction. The experimenter then set the clock-work of the
chronoscope in motion, and about one second afterwards caused the light to appear by means of
the apparatus described. The observer lifted his hand as soon as possible after the appearance of
the light, and the interval that had elapsed between the occurrence of the light and the
commencement of the muscular contraction was read by the experimenter directly from the
chronoscope. In no single case, as far as I can remember, did the observer make a premature
reaction, that is, lift his hand before the necessary physiological operations had had time to occur.
The only disturbance was caused by the clock-work of the chronoscope sometimes not being
properly controlled by the vibrating spring. If the experimenter noticed this in time he did not produce
the light. This occasional failure of the chronoscope was always noticed, so does not interfere with
the accuracy of the times here given, but the observer was sometimes disturbed so that his
reactions may have been made less regular. Throughout this paper I give every series and every
reaction made; I give, however, in addition to each series, a corrected value reached by the method
above described. This correction simply excludes all abnormal values. In the Tables I give the
average of the variation of each reaction from the average of the series to which it belongs (V); that
is, if A is the average of the n reactions [p. 234] making up the series, and a1, a2, a3, . . an are the
values of the several reactions, then