with which the eye has been fatigued
in making the experiment: thus the reverse spectrum of red must be such a
green as would be produced by a combination of all the other prismatic
colours. To evince this fact the following satisfactory experiment was
made. The prismatic colours were laid on a circular pasteboard wheel, about
four inches in diameter, in the proportions described in Dr. Priestley's
History of Light and Colours, pl. 12. fig. 83. except that the red
compartment was entirely left out, and the others proportionably extended
so as to complete the circle. Then, as the orange is a mixture of red and
yellow, and as the violet is a mixture of red and indigo, it became
necessary to put yellow on the wheel instead of orange, and indigo instead
of violet, that the experiment might more exactly quadrate with the theory
it was designed to establish or confute; because in gaining a green
spectrum from a red object, the eye is supposed to have become insensible
to red light. This wheel, by means of an axis, was made to whirl like a
top; and on its being put in motion, a green colour was produced,
corresponding with great exactness to the reverse spectrum of red.

3. In contemplating any one or these reverse spectra in the closed and
covered eye, it disappears and re-appears several times successively, till
at length it entirely vanishes, like the direct spectra in Sect. V.; but
with this additional circumstance, that when the spectrum becomes faint or
evanescent, it is instantly revived by removing the hand from before the
eyelids, so as to admit more light: because then not only the fatigued part
of the retina is inclined spontaneously to fall into motions of a contrary
direction, but being still sensible to all other rays of light, except that
with which it was lately fatigued, is by these rays at the same time
stimulated into those motions which form the reverse spectrum.

From these experiments there is reason to conclude, that the fatigued part
of the retina throws itself into a contrary mode of action, like oscitation
or pandiculation, as soon as the stimulus which has fatigued it is
withdrawn; and that it still remains sensible, that is, liable to be
excited into action by any other colours at the same time, except the
colour with which it has been fatigued.

VII. _The retina after having been excited into action by a stimulus
somewhat greater than the last mentioned falls into various successive
sp