ions; which will likewise be fully
explained in the succeeding section.

3. The time taken up in performing an idea is likewise much the same as
that taken up in performing a muscular motion. A musician can press the
keys of an harpsichord with his fingers in the order of a tune he has been
accustomed to play, in as little time as he can run over those notes in his
mind. So we many times in an hour cover our eye-balls with our eye-lids
without perceiving that we are in the dark; hence the perception or idea of
light is not changed for that of darkness in so small a time as the
twinkling of an eye; so that in this case the muscular motion of the
eye-lid is performed quicker than the perception of light can be changed
for that of darkness.--So if a fire-stick be whirled round in the dark, a
luminous circle appears to the observer; if it be whirled somewhat slower,
this circle becomes interrupted in one part; and then the time taken up in
such a revolution of the stick is the same that the observer uses in
changing his ideas: thus the [Greek: dolikoskoton enkos] of Homer, the long
shadow of the flying javelin, is elegantly designed to give us an idea of
its velocity, and not of its length.

4. The fatigue that follows a continued attention of the mind to one object
is relieved by changing the subject of our thoughts; as the continued
movement of one limb is relieved by moving another in its stead. Whereas a
due exercise of the faculties of the mind strengthens and improves those
faculties, whether of imagination or recollection; as the exercise of our
limbs in dancing or fencing increases the strength and agility of the
muscles thus employed.

5. If the muscles of any limb are inflamed, they do not move without pain;
so when the retina is inflamed, its motions also are painful. Hence light
is as intolerable in this kind of ophthalmia, as pressure is to the finger
in the paronychia. In this disease the patients frequently dream of having
their eyes painfully dazzled; hence the idea of strong light is painful as
well as the reality. The first of these facts evinces that our perceptions
are motions of the organs of sense; and the latter, that our imaginations
are also motions of the same organs.

6. The organs of sense, like the moving muscles, are liable to become
benumbed, or less sensible, from compression. Thus, if any person on a
light day looks on a white wall, he may perceive the ramifications of the
optic artery, a