easy to gain a direct spectrum of any coloured object in the day-time,
where there is much lateral light, except of very bright objects, as of the
setting sun, or by looking through an opake tube; because the lateral
external light falling also on the central part of the retina, contributes
to induce the reverse spectrum, which is at the same time the direct
spectrum of that lateral light, deducting only the colour of the central
object which we have been viewing. And for the same reason, it is difficult
to gain the reverse spectrum, where there is no lateral light to contribute
to its formation. Thus, in looking through an opake tube on a yellow wall,
and closing my eye, without admitting any lateral light, the spectra were
all at first yellow; but at length changed into blue. And on looking in the
same manner on red paper, I did at length get a green spectrum; but they
were all at first red ones: and the same after looking at a candle in the
night.

The reverse spectrum was formed with greater facility when the eye was
thrown from the object on a sheet of white paper, or when light was
admitted through the closed eyelids; because not only the fatigued part of
the retina was 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, was by these rays stimulated at the same
time into those motions which form the reverse spectrum. Hence, when, the
reverse spectrum of any colour became faint, it was wonderfully revived by
admitting more light through the eyelids, by removing the hand from before
them: and hence, on covering the closed eyelids, the spectrum would often
cease for a time, till the retina became sensible to the stimulus of the
smaller quantity of light, and then it recurred. Nor was the spectrum only
changed in vivacity, or in degree, by this admission of light through the
eyelids; but it frequently happened, after having viewed bright objects,
that the spectrum in the closed and covered eye was changed into a third
spectrum, when light was admitted through the eyelids: which third spectrum
was composed of such colours as could pass through the eyelids, except
those of the object. Thus, when an area of half an inch diameter of pink
paper was viewed on a sheet of white paper in the sunshine, the spectrum
with closed and covered eyes was green; but on removing the hands from
before the closed eyelids, the