d on a six-inch square of
yellow paper, the spectrum of the central paper in the closed eye was
yellow, incircled with a blue halo. On looking long on the meridian sun,
the disk fades into a pale blue surrounded with a whitish halo.

These circumstances, though they very much perplexed the experiments till
they were investigated, admit of a satisfactory explanation; for while the
rays from the bright internal object in exp. a. fall with their full force
on the center of the retina, and, by fatiguing that part of it, induce the
reverse spectrum, many scattered rays, from the same internal pink paper,
fall on the more external parts of the retina, but not in such quantity as
to occasion much fatigue, and hence induce the direct spectrum of the pink
colour in those parts of the eye. The same reverse and direct spectra occur
from the violet paper in exp. b.: and in exp. c. the scattered rays from
the central pink paper produce a direct spectrum of this colour on the
external parts of the eye, while the scattered rays from the external blue
paper produce a direct spectrum of that colour on the central part of the
eye, instead of these parts of the retina falling reciprocally into their
reverse spectra. In exp. d. the colours being the reverse of each other,
the scattered rays from the exterior object falling on the central parts of
the eye, and there exciting their direct spectrum, at the same time that
the retina was excited into a reverse spectrum by the central object, and
this direct and reverse spectrum being of similar colour, the superior
brilliancy of this spectrum was produced. In exp. e. the effect of various
quantities of stimulus on the retina, from the different respective sizes
of the internal and external areas, induced a spectrum of the internal area
in the center of the eye, combined of the reverse spectrum of that internal
area and the direct one of the external area, in various shades of colour,
from a pale green to a deep blue, with similar changes in the spectrum of
the external area. For the same reasons, when an internal bright object was
small, as in exp. f. instead of the whole of the spectrum of the external
object being reverse to the colour of the internal object, only a kind of
halo, or radiation of colour, similar to that of the internal object, was
spread a little way on the external spectrum. For this internal blue area
being so small, the scattered rays from it extended but a little way on the