ion 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 the organ of touch into animal action, which
constitutes the perception of hardness and of figure; for in some minutes
the perception ceases, though the mechanical pressure of the object
remains.

3. Make with ink on white paper a very black spot about half an inch in
diameter, with a tail about an inch in length, so as to resemble a tadpole,
as in Plate II.; look steadfastly for a minute on the center of this spot,
and, on moving the eye a little, the figure of the tadpole will be seen on
the white part of the paper; which figure of the tadpole will appear more
luminous than the other part of the white paper; which can only be
explained by supposing that a part of the retina, on which the tadpole was
delineated, to have become more sensible to light than the other parts of
it, which were exposed to the white paper; and not from any idea of
mechanical impression or chemical combination of light with the retina.

4. When any one turns round rapidly, till he becomes dizzy, and falls upon
the ground, the spectra of the ambient objects continue to present
themselves in rotation, and he seems to behold the objects still in motion.
Now if these spectra were impressions on a passive organ, they either must
continue as they were received last, or not continue at all.

5. Place a piece of red silk about an inch in diameter on a sheet of white
paper in a strong light, as in Plate I; look steadily upon it from the
distance of about half a yard for a minute; then closing your eye-lids,
cover them with your hands and handkerchief, and a green spectrum will be
seen in your eyes resembling in form the piece of red silk. After some
seconds of time the spectrum will disappear, and in a few more seconds will
reappear; and thus alternately three or four times, if the experiment be
well made, till at length it vanishes entirely.

[Illustration: Plate III.]

6. Place a circular piece of white paper, about four inches in diameter, in
the sunshine, cover the center of this with a circular piece of black silk,
about three inches in diameter; and the center of the black silk with a
circle of pink silk, about two inches in diamet