should wish to call vanishing ideas, or
sensual motions, of those organs of sense; which, ideas, or sensual
motions, have lately been associated in a circle, and therefore for a time
continue to be excited. And what are the ideas of colours, when they are
excited by imagination or memory, but the repetition of finer ocular
spectra? What the idea of sounds, but the repetition of finer auricular
murmurs? And what the ideas of tangible objects, but the repetition of
finer evanescent titillations?

The tangible, and the auricular, and the visual vertigo, are all perceived
by many people for a day or two after long travelling in a boat or coach;
the motions of the vessel, or vehicle, or of the surrounding objects, and
the noise of the wheels and oars, occur at intervals of reverie, or at the
commencement of sleep. See Sect. XX. 5. These ideas, or sensual motions, of
sight, of hearing, and of touch, are succeeded by the same effects as the
ocular spectra, the auricular murmurs, and the evanescent titillations
above mentioned; that is, by a kind of vertigo, and cannot in that respect
be distinguished from them. Which is a further confirmation of the truth of
the doctrine delivered in Sect. III. of this work, that the colours
remaining in the eyes, which are termed ocular spectra, are ideas, or
sensual motions, belonging to the sense of vision, which for too long a
time continue their activity.

ADDITION IV. OF VOLUNTARY MOTIONS.

A correspondent acquaints me, that he finds difficulty in understanding how
the convulsions of the limbs in epilepsy can be induced by voluntary
exertions. This I suspect first to have arisen from the double meaning of
the words "involuntary motions;" which are sometimes used for those
motions, which are performed without the interference of volition, as the
pulsations of the heart and arteries; and at other times for those actions,
which occur, where two counter volitions oppose each other, and the
stronger prevails; as in endeavouring to suppress laughter, and to stop the
shudderings, when exposed to cold. Thus when the poet writes,

------video meliora, proboque,
Deteriora sequor.----

The stronger volition actuates the system, but not without the
counteraction of unavailing smaller ones; which constitute deliberation.

A second difficulty may have arisen from the confined use of the words "to
will," which in common discourse generally mean to choose after
deliberation; and hence our wil