4 
NATURE 
| May 5, 1870 
on the skin for a galvanic shock, the physiological time of 
touch was lengthened. But there is one element, that of 
intensity (which we have every reason to think makes 
itself felt in sensory impressions, and especially in cerebral 
actions even more than in motor impulses), that disturbs all 
these calculations, and thus causes the matter to be left in 
considerable uncertainty. How can we, for instance, 
compare the intensity of vision with that either of hearing 
or of touch? 
The sensory term, therefore, of a complete mental action 
is far less clearly understood than the motor term ; and 
we may naturally conclude that the middle cerebral term 
is still less known. Nevertheless, here too it is possible to 
arrive at general results. We can, for instance, estimate 
the time required for the mental operation of deciding 
between two or more events, and of willing to act in 
accordance with the decision. Thus, if a galvanic shock 
be given to one foot, and the signal be made with the hand 
of the same side, a certain physiological time is consumed 
in the act. But if the apparatus be so arranged that the 
shock may be given to either foot, and it be required that 
the person experimenting, not knowing beforehand to 
which foot the shock is coming, must give the signal with 
the hand of the same side as the foot which receives {the 
shock, a distinctly longer physiological time is found to 
be necessary. The difference between the two cases, 
which, according to Donders, amounts to zits, or about 
}; of a second, gives the time taken up in the mental 
act of recognising the side affected and choosing the side 
for the signal. 
A similar method may be employed in reference to 
light. Thus we know the physiological time required for 
any one to make a signal on seeing a light. But Donders 
found that when matters were arranged so that a red 
light was to be signalled with the left hand and a white 
with the right, the observer not knowing which colour 
was about to be shown, an extension of the physiological 
time by jib of a second was required for the additional 
mental labour. This of course was after a correction 
(amounting to ;%o Of a second) had been made for the 
greater facility in using the right hand. 
The time thus taken up in recognising and willing, was 
reduced in some further observations of Donders, by the 
use of a more appropriate signal. The object looked for 
was a letter illuminated suddenly by an electric spark, and 
the observer had to call out the name of the letter, his cry 
being registered by a phonautograph, the revolving cylinder 
of which was also marked by the current giving rise to 
the electric spark. 
When the observer had to choose between two letters, 
the physiological time was rather shorter than when the 
signal was made by the hand ; but when a choice of five 
letters was presented, the time was lengthened, the duration 
of the mental act amounting in this case to qf of a second. 
When the exciting cause was a sound answered by a 
sound, the increase of the physiological time was much 
shortened. Thus, the choice between two sounds and the 
determination to answer required about 7$§5 of a second ; 
while, when the choice lay between five different sounds, 
ziSs of a second was required. In these observations two 
persons sat before the phonautograph, one answering the 
other, while the voices of both were registered on the 
same revolving cylinder. 
| 
These observations may be regarded as the beginnings 
of a new line of inquiry, and it is obvious that by a proper 
combination of changes various mental factors may be 
eliminated and their duration ascertained. For instance, 
when one person utters a sound, the nature of which has 
been previously arranged, the time elapsing before the 
answer is given corresponds to the time required for 
simple recognition and volition. When, however, the 
first person has leave to utter any one, say of five, given 
sounds, and the second person to make answer by the 
same sound to any and every one of the five which he 
thus may hear, the mental process is much more com- 
plex. There is in this case first the perception and recog- 
nition of sound, then the bare volition towards an answer, 
and finally the choice and combination of certain motor 
impulses which are to be set going, in order that the 
appropriate sound may be made in answer. All this latter 
part of the cerebral labour may, however, be reduced to a 
minimum by arranging that though any one of five sounds 
may be given out, answer shall be made to a particular 
one only, The respondent then puts certain parts of his 
brain in communication with the origin of certain out- 
going nerves; he assumes the attitude, physical and 
mental, of one about to utter the expected sound. To use 
a metaphor, all the trains are laid, and there is only need 
for the match to be applied. When he hears any of the 
four sounds other than the one he has to answer, he has 
only to remain quiet. The mental labour actually em- 
ployed when the sound at last is heard is limited almost 
to a recognition of the sound, and the rise of what we may 
venture to call a bare volitional impulse. When this is 
done, the time is very considerably shortened. In this 
way Donders found, as a mean of numerous observations, 
that the second of these cases required 73%, of a second, and 
the third only ;3%5 over and above the first. That is to: 
say, while the complex act of recognition, rise of volitional 
impulse, and inauguration of an actual volition, with the 
setting free of co-ordinated motor impulses, took z$$q Of 
a second, the simple recognition and rise of volitional im- 
pulse took 7325 only. We infer, therefore, that the full 
inauguration of the volition took $3339 =73$5- In rough 
language, it took 4; of a second to think, and rather 
less to will. 
We may fairly expect interesting and curious results 
from a continuation of these researches. Two sources of 
error have, however, to be guarded against. One, and 
that most readily appreciated and cared for, refers to 
exactitude in the instruments employed ; the other, far 
more dangerous and less readily borne in mind, is the 
danger of getting wrong in drawing averages from a 
number of exceedingly small and variable differences. 
M. FOSTER 
CHOICE AND CHANCE 
Choice and Chance. By the Rev. Wm. Allen Whitworth, 
M.A., Fellow of St. John’s College, Cambridge. 2nd 
ed. Enlarged. 1870. (Deighton, Bell & Co.) 
E shou!d think that not a few copies of the first edition 
of this work must have been purchased under the im- 
pression that it was an interesting story ; and itis surprising 
that so neat and suggestive a title had not been long ago 
appropriated by some needy novelist. This work, how- 
ever, is a very able elementary treatise on those puzzling 
branches of mathematics which treat of combinations 
