of the central circle, as by the
unsteadiness of the eye a part of the fatigued retina falls on the white
paper; and as by the increasing fatigue of the eye the central part of the
silk appears paler, the edge on which the unfatigued part of the retina
occasionally falls will appear of a deeper red than the original silk,
because it is compared with the pale internal part of it. M. de Buffon in
making this experiment observed, that the red edge of the silk was not only
deeper coloured than the original silk; but, on his retreating a little
from it, it became oblong, and at length divided into two, which must have
been owing to his observing it either before or behind the point of
intersection of the two optic axises. Thus, if a pen is held up before a
distant candle, when we look intensely at the pen two candles are seen
behind it; when we look intensely at the candle two pens are seen. If the
sight be unsteady at the time of beholding the sun, even though one eye
only be used, many images of the sun will appear, or luminous lines, when
the eye is closed. And as some parts of these will be more vivid than
others, and some parts of them will be produced nearer the center of the
eye than others, these will disappear sooner than the others; and hence the
number and shape of these spectra of the sun will continually vary, as long
as they exist. The cause of some being more vivid than others, is the
unsteadiness of the eye of the beholder, so that some parts of the retina
have been longer exposed to the sunbeams. That some parts of a complicated
spectrum fade and return before other parts of it, the following experiment
evinces. Draw three concentric circles; the external one an inch and a half
in diameter, the middle one an inch, and the internal one half an inch;
colour the external and internal areas blue, and the remaining one yellow,
as in Fig. 4.; after having looked about a minute on the center of these
circles, in a bright light, the spectrum of the external area appears first
in the closed eye, then the middle area, and lastly the central one; and
then the central one disappears, and the others in inverted order. If
concentric circles of more colours are added, it produces the beautiful
ever changing spectrum in Sect. I. Exp. 2.

From hence it would seem, that the center of the eye produces quicker
remissions of spectra, owing perhaps to its greater sensibility; that is,
to its more energetic exertions. These remissio