Zoonomia, Vol. I | Page 8

objects consists of a partial
repetition of the perceptions, which were excited by those external
objects, at the time we became acquainted with them; and that our
reflex ideas of the operations of our minds are partial repetitions of
those operations.
II. The following article evinces that the organ of vision consists of a
fibrous part as well as of the nervous medulla, like other white muscles;
and hence, as it resembles the muscular parts of the body in its structure,
we may conclude, that it must resemble them in possessing a power of
being excited into animal motion.--The subsequent experiments on the
optic nerve, and on the colours remaining in the eye, are copied from a
paper on ocular spectra published in the seventy-sixth volume of the
Philos. Trans. by Dr. R. Darwin of Shrewsbury; which, as I shall have

frequent occasion to refer to, is reprinted in this work, Sect. XL. 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 evinced that it consisted of
fibres. This fibrous construction became still more distinct to the light
by adding some caustic alcali 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 this evidence of
its fibrous structure, since Leeuwenhoek has shewn, that the crystalline
humour itself consists of fibres. Arc. Nat. V. I. 70.
Hence it appears, that as the muscles consist of larger fibres intermixed
with a smaller quantity of nervous medulla, the organ of vision consists
of a greater quantity of nervous medulla intermixed with smaller fibres.
It is probable that the locomotive muscles of microscopic animals may
have greater tenuity than these of the retina; and there is reason to
conclude from analogy, that the other immediate organs of sense, as the
portio mollis of the auditory nerve, and the rete mucosum of the skin,
possess a similarity of structure with the retina, and a similar power of
being excited into animal motion.
III. The subsequent articles shew, that neither mechanical impressions,
nor chemical combinations of light, but that the animal activity of the
retina constitutes vision.
1. Much has been conjectured by philosophers about the momentum of
the rays of light; to subject this to experiment a very light horizontal
balance was constructed by Mr. Michel, with about an inch square of
thin leaf-copper suspended at each end of it, as described in Dr.
Priestley's History of Light and Colours. The focus of a very large
convex mirror was thrown by Dr. Powel, in his lectures on
experimental philosophy, in my presence, on one wing of this delicate
balance, and it receded from the light; thrown on the other wing, it
approached towards the light, and this repeatedly; so that no sensible
impulse could be observed, but what might well be ascribed to the
ascent of heated air.

Whence it is reasonable to conclude, that the light of the day must be
much too weak in its dilute state to make any mechanical impression on
so tenacious a substance as the retina of the eye.--Add to this, that as
the retina is nearly transparent, it could therefore make less resistance
to the mechanical impulse of light; which, according, to the
observations related by Mr. Melvil in the Edinburgh Literary Essays,
only communicates heat, and should therefore only communicate
momentum, where it is obstructed, reflected, or refracted.--From
whence also may be collected the final cause of this degree of
transparency of the retina, viz. left by the focus of stronger lights, heat
and pain should have been produced in the retina, instead of that
stimulus which excites it into animal motion.
2. On looking long on an area of scarlet silk of about an inch in
diameter laid on white paper, as in Plate I. the scarlet colour becomes
fainter, till at length it entirely vanishes, though the eye is kept
uniformly and steadily upon it. Now if the change or motion of the
retina was a mechanical impression, or a chemical tinge of coloured
light, the perception would every minute become stronger and
stronger,--whereas in this experiment it becomes every instant weaker
and weaker. The same circumstance obtains in the continued
application of sound, or of sapid bodies, or of odorous ones, or of
tangible ones, to their adapted organs of sense.
[Illustration: Plate II.]
Thus when a circular coin, as a shilling, is pressed on the palm of the
hand, the sense of touch is mechanically compressed; but it is the
stimulus of this pressure that excites