their light upon the same screen, we
should require ten more candles to be added before our eyes could
perceive any difference in the amount of illumination. But if we begin
with only 100 candles shining upon the screen, we should perceive an
increase in the illumination by adding a single candle. And what is true
of sight is equally true of all the other senses: if any stimulus is
increased, the smallest increase of sensation first occurs when the
stimulus rises one per cent, above its original intensity. Such being
the law on the side of sensation, suppose that we place upon the optic
nerve of an animal the wires proceeding from a delicate galvanometer, we
find that every time we stimulate the eye with light, the needle of the
galvanometer moves, showing electrical changes going on in the nerve,
caused by the molecular agitations. Now these electrical changes are
found to vary in intensity with the intensity of the light used as a
stimulus, and they do so very nearly in accordance with the law of
sensation just mentioned. So we say that in sensation the cerebral
hemispheres are, as it were, acting the part of galvanometers in
appreciating the amount of molecular change which is going on in sensory
nerves; and that they record their readings in the mind as faithfully as
a galvanometer records its readings on the dial.

* * * * *

Hitherto we have been considering certain features in the physiology of
nervous action, so far as this can be appreciated by means of
physiological instruments. But we have just seen that the cerebral
hemispheres may themselves be regarded as such instruments, which record
in our minds their readings of changes going on in our nerves. Hence,
when other physiological instruments fail us, we may gain much
additional insight touching the movements of nervous matter by attending
to the thoughts and feelings of our own minds; for these are so many
indices of what is going on in the cerebral hemispheres. I therefore
propose next to contemplate the mind, considered thus as a physiological
instrument.

The same scientific instinct which led Hobbes so truly to anticipate the
progress of physiology, led him not less truly to anticipate the
progress of psychology. For just as he was the first to enunciate the
fundamental principle of nerve-action in the vibration of molecules, so
was he likewise the first to enunciate the fundamental principle of
psychology in the assoc