er; and the center of the
pink silk with a circle of yellow silk, about one inch in diameter; and the
center of this with a circle of blue silk, about half an inch in diameter;
make a small spot with ink in the center of the blue silk, as in Plate
III.; 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
eye-lids, and 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.

From all these experiments it appears, that these 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 supposed, perhaps erroneously, to take place in
calcined shells and other phosphorescent bodies, after having been exposed
to the light: for in all these cases the spectra in the eye should either
remain of the same colour, or gradually decay, when the object is
withdrawn; and neither their evanescence during the presence of their
object, as in the second experiment, nor their change from dark to
luminous, as in the third experiment, nor their rotation, as in the fourth
experiment, nor the alternate presence and evanescence of them, as in the
fifth experiment, nor the perpetual change of colours of them, as in the
last experiment, could exist.

IV. The subsequent articles shew, that these animal motions or
configurations of our organs of sense constitute our ideas.

1. If any one in the dark presses the ball of his eye, by applying his
finger to the external corner of it, a luminous appearance is observed; and
by a smart stroke on the eye great slashes of fire are perceived. (Newton's
Optics.) So that when the arteries, that are near the auditory nerve, make
stronger pulsations than usual, as in some fevers, an undulating sound is
excited in the ears. Hence it is not the presence of the light and sound,
but the motions of the organ, that are immediately necessary to constitute
the perception or idea of light and sound.

2. During the time of