retina._ III. _The activity of the retina in vision._ 1. _Rays of light
have no momentum._ 2. _Objects long viewed become fainter._ 3. _Spectra
of black objects become luminous._ 4. _Varying spectra from gyration._
5. _From long inspection of various colours._ IV. _Motions of the
organs of sense constitute ideas._ 1. _Light from pressing the
eye-ball, and sound from the pulsation of the carotid artery._ 2.
_Ideas in sleep mistaken for perceptions._ 3. _Ideas of imagination
produce pain and sickness like sensations._ 4. _When the organ of sense
is destroyed, the ideas belonging to that sense perish._ V. _Analogy
between muscular motions and sensual motions, or ideas._ 1. _They are
both originally excited by irritations._ 2. _And associated together in
the same manner._ 3. _Both act in nearly the same times._ 4. _Are alike
strengthened or fatigued by exercise._ 5. _Are alike painful from
inflammation._ 6. _Are alike benumbed by compression._ 7. _Are alike
liable to paralysis._ 8. _To convulsion._ 9. _To the influence of old
age._--VI. _Objections answered._ 1. _Why we cannot invent new ideas._
2. _If ideas resemble external objects._ 3. _Of the imagined sensation
in an amputated limb._ 4. _Abstract ideas._--VII. _What are ideas, if
they are not animal motions?_

Before the great variety of animal motions can be duly arranged into
natural classes and orders, it is necessary to smooth the way to this yet
unconquered field of science, by removing some obstacles which thwart our
passage. I. To demonstrate that the retina and other immediate organs of
sense possess a power of motion, and that these motions constitute our
ideas, according to the fifth and seventh of the preceding assertions,
claims our first attention.

Animal motions are distinguished from the communicated motions, mentioned
in the first section, as they have no mechanical proportion to their cause;