py of the original map. Let B.
take the same map to study for the same time; but instead of keeping his
eyes undeviatingly fixed to the object, desire him to view it only for a
few seconds; and then, shutting his eyes, let him endeavour to bring the
picture of the map before his mind: his first efforts will convey a very
confused notion of the actual and relative positions; but he will become
sensible of his defects, and reinspect the map for their correction. If
this successive ocular examination and review by the mind, be continued
during the half hour, or even for a less time, B. will be competent to
make a drawing of the map with superior accuracy to A., who endeavoured
to fix his attention for the whole of the time allotted. In conducting
this experiment some very curious phenomena may be observed. If A. had
directed his eyes to the object intensely and undeviatingly, especially
in a strong light, and had then covered or shut his eyes, in order to
recollect the relative situations in the map, the straining of the organ
to the object would defeat his endeavours; and instead of being able to
bring the picture before his mind, he would be annoyed and interrupted
by the intrusion of ocular spectra, undergoing the succession of changes
described by Dr. Darwin.[14] Thus there are limits to the duration of
our effective attention: if the organ of vision be too long directed to
the object of perception, ocular spectra arise, fatigue and confusion
ensue in the other senses; and if the subjects of recollection be too
long and intensely contemplated, delirium will supervene.

In page 52, after enumerating the wonderful productions of the hand, an
objection was foreseen, which may be conveniently examined in the
present chapter. That all the performances of the human hand, and of the
other members of the body, which are not the result of involuntary
movements, must have been the consequence of the direction of the will,
is indisputable: it is, in fact, the common relation of cause and
effect: but the creation of this distinction, would assign separate
offices to the mind and to the organ;--or to the power directing, and to
the instrument by which the command is executed. Sufficient has been
already adduced, to render it obvious, that mind or organ _alone_ would
be inadequate for the purposes of intelligence. Perception, without its
record or memory, would be a useless endowment; muscles or organs of
motion, without a power to d