iments it appears, that the retina is in an active
not in a passive state during the existence of these ocular spectra; and it
is thence to be concluded, that all vision is owing to the activity of this
organ.

1. Place a piece of red silk, about an inch in diameter, as in plate 1, at
Sect. III. 1., on a sheet of white paper, in a strong light; look steadily
upon it from about the distance of half a yard for a minute; then closing
your eyelids cover them with your hands, and a green spectrum will be seen
in your eyes, resembling in form the piece of red silk: after some time,
this spectrum will disappear and shortly reappear; and this alternately
three or four times, if the experiment is well made, till at length it
vanishes entirely.

2. Place on a sheet of white paper a circular piece of blue silk, about
four inches in diameter, in the sunshine; cover the center of this with a
circular piece of yellow silk, about three inches in diameter; and the
center of the yellow silk with a circle of pink silk, about two inches in
diameter; and the center of the pink silk with a circle of green silk,
about one inch in diameter; and the centre of this with a circle of indigo,
about half an inch in diameter; make a small speck with ink in the very
center of the whole, as in plate 3, at Sect. III. 3. 6.; look steadily for
a minute on this central spot, and then closing your eyes, and applying
your hand at about an inch distance before them, so as to prevent too much
or too little light from passing through the eyelids, you will see the most
beautiful circles of colours that imagination can conceive, which are most
resembled by the colours occasioned by pouring a drop or two of oil on a
still lake in a bright day; but these circular irises of colours are not
only different from the colours of the silks above mentioned, but are at
the same time perpetually changing as long as they exist.

3. When any one in the dark presses either corner of his eye with his
finger, and turns his eye away from his finger, he will see a circle of
colours like those in a peacock's tail: and a sudden flash of light is
excited in the eye by a stroke on it. (Newton's Opt. Q. 16.)

4. When any one turns round rapidly on one foot, till he becomes dizzy, and
falls upon the ground, the spectra of the ambient objects continue to
present themselves in rotation, or appear to librate, and he seems to
behold them for some time still in motion.

From all these ex