13.  By the word stimulus is not only meant the application of external bodies to our organs of sense and muscular fibres, which excites into action the sensorial power termed irritation; but also pleasure or pain, when they excite into action the sensorial power termed sensation; and desire or aversion, when they excite into action the power of volition; and lastly, the fibrous contractions which precede association; as is further explained in Sect.  XII. 2. 1.

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SECT.  III.

THE MOTIONS OF THE RETINA DEMONSTRATED BY EXPERIMENTS.

I. Of animal motions and of ideas. II. The fibrous structure of the 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; for the goad of a spur on the skin of a horse shall induce him to move a load of hay.  They differ from the gravitating motions there mentioned as they are exerted with equal facility in all directions, and they differ from the chemical class of motions, because no apparent decompositions or new combinations are produced in the moving materials.