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the part where the impression is made, and the brain, no sensation
will take place; for instance, if the nerves be cut or compressed.

In a sound body, sensation is caused, whenever a change takes place
in the state of the nervous power, whether that change be produced by
an external, or an internal cause. The former kind of sensation is
said to arise from impression or impulse, the latter from
consciousness.

Every impression or impulse is not, however, equally calculated to
produce sensation; for this purpose, a middle degree of impulse
appears the best. An impulse considerably less produces no sensation,
and one more violent may cause pain, but no proper sensation denoting
the presence or properties of external objects. Thus too small a
degree of light makes no impression on the optic nerve; and if the
object be too strongly illuminated, the eye is pained, but has no
proper idea of the figure or colour of the object. In the same way,
if the vibrations which give us an idea of sound, be either too quick
or too slow, we shall not obtain this idea. When the vibration is too
quick, a very disagreeable and irritating sensation is perceived, as
for instance, in the whetting of a saw: and on the other hand, when
the vibrations are too slow, they will not produce a tone or sound.
This might be proved of all the senses, and shows, that a certain
degree of impression is necessary to produce perfect sensation.

There is another circumstance likewise requisite to produce
sensation: it is not enough, that the impression should be of the
proper strength; it is necessary likewise, that it should remain for
some time, otherwise no sensation will be produced. There are many
bodies whose magnitude is amply sufficient to be perceived by the
eye; yet, by reason of their great velocity, the impulse they make on
any part of the retina is so short, that they are not visible. This
is proved by our not perceiving the motions of cannon and musket
balls, and many other kinds of motion. On this principle depends the
art of conjuration, or legerdemain; the fundamental maxim of those
who practise them, is, that the motion is too quick for sight.

If the impulse be of a proper degree, and be continued for a
sufficient length of time, the impression made by it will not
immediately vanish with the impulse which caused it, but will remain
for a time proportioned to the strength of the impulse. This, with
respect to sight, is proved by whirli