periments it appears, that the spectra in the eye are not
owing to the mechanical impulse of light impressed on the retina, nor to
its chemical combination with that organ, nor to the absorption and
emission of light, as is observed in many bodies; for in all these cases
the spectra must either remain uniformly, or gradually diminish; and
neither their alternate pretence and evanescence as in the first
experiment, nor the perpetual changes of their colours as in the second,
nor the flash of light or colours in the pressed eye as in the third, nor
the rotation or libration of the spectra as in the fourth, could exist.

It is not absurd to conceive, that the retina may be stimulated into
motion, as well as the red and white muscles which form our limbs and
vessels; since it consists of fibres, like those, intermixed with its
medullary substance. To evince this structure, 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 shews that it consists of fibres; and that its fibrous construction
became still more distinct to the sight, by adding some caustic alkali to
the water, as the adhering mucus was first eroded, and the hair-like fibres
remained floating in the vessel. Nor does the degree of transparency of the
retina invalidate the evidence of its fibrous structure, since Leeuwenhoek
has shewn that the crystalline humour itself consists of fibres. (Arcana
Naturae, V. 1. p. 70.)

Hence it appears, that as the muscles have larger fibres intermixed with a
smaller quantity of nervous medulla, the organ of vision has a greater
quantity of nervous medulla intermixed with smaller fibres; and it is
probable that the locomotive muscles, as well as the vascular ones, of
microscopic animals have much greater tenuity than these of the retina.

And besides the similar laws, which will be shewn in this paper to govern
alike the actions of the retina and of the muscles, there are many other
analogies which exist between them. They are both originally excited into
action by irritations, both are nearly in the same quantity of time, are
alike strengthened or fatigued by exertion, are alike painful if excited
into action when they are in an inflamed state, are alike liable to
paralysis, and to the torpor of old age.

II. OF SPECTRA FROM D