Now the motions of an organ of sense are a succession of configurations of
that organ; these configurations succeed each other quicker or slower; and
whatever configuration of this organ of sense, that is, whatever portion of
the motion of it is, or has usually been, attended to, constitutes an idea.
Hence the configuration is not to be considered as an effect of the motion
of the organ, but rather as a part or temporary termination of it; and
that, whether a pause succeeds it, or a new configuration immediately takes
place. Thus when a succession of moving objects are presented to our view,
the ideas of trumpets, horns, lords and ladies, trains and canopies, are
configurations, that is, parts or links of the successive motions of the
organ of vision.

[Illustration: Plate I.]

These motions or configurations of the organs of sense differ from the
sensorial motions to be described hereafter, as they appear to be simply
contractions of the fibrous extremities of those organs, and in that
respect exactly resemble the motions or contractions of the larger muscles,
as appears from the following experiment. Place a circular piece of red
silk about an inch in diameter on a sheet of white paper in a strong light,
as in Plate I.--look for a minute on this area, or till the eye becomes
somewhat fatigued, and then, gently closing your eyes, and shading them
with your hand, a circular green area of the same apparent diameter becomes
visible in the closed eye. This green area is the colour reverse to the red
area, which had been previously inspected, as explained in the experiments
on ocular spectra at the end of the work, and in Botanical Garden, P. 1.
additional note, No. 1. Hence it appears, that a part of the retina, which
had been fatigued by contraction in one direction, relieves itself by
exerting the antagonist fibres, and producing a contraction in an opposite
direction, as is common in the exertions of our muscles. Thus when we are
tired with long action of our arms in one direction, as in holding a bridle
on a journey, we occasionally throw them into an opposite position to
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