relieve the fatigued muscles.

Mr. Locke has defined an idea to be "whatever is present to the mind;" but
this would include the exertions of volition, and the sensations of
pleasure and pain, as well as those operations of our system, which
acquaint us with external objects; and is therefore too unlimited for our
purpose. Mr. Lock seems to have fallen into a further error, by conceiving,
that the mind could form a general or abstract idea by its own operation,
which was the copy of no particular perception; as of a triangle in
general, that was neither acute, obtuse, nor right angled. The ingenious
Dr. Berkley and Mr. Hume have demonstrated, that such general ideas have no
existence in nature, not even in the mind of their celebrated inventor. We
shall therefore take for granted at present, that our recollection or
imagination of external objects consists of a partial repetition of the
perceptions, which were excited by those external objects, at the time we
became acquainted with them; and that our reflex ideas of the operations of
our minds are partial repetitions of those operations.

II. The following article evinces that the organ of vision consists of a
fibrous part as well as of the nervous medulla, like other white muscles;
and hence, as it resembles the muscular parts of the body in its structure,
we may conclude, that it must resemble them in possessing a power of being
excited into animal motion.--The subsequent experiments on the optic nerve,
and on the colours remaining in the eye, are copied from a paper on ocular
spectra published in the seventy-sixth volume of the Philos. Trans. by Dr.
R. Darwin of Shrewsbury; which, as I shall have frequent occasion to refer
to, is reprinted in this work, Sect. XL. The retina of an ox's eye was
suspended in a glass of warm water, and forcibly torn in a few places; the
edges of these parts appeared jagged and hairy, and did not contract and
become smooth like simple mucus, when it is distended till it breaks; which