retina into various
successive spasmodic actions._ 8. _Into fixed spasmodic action._ 9.
_Into temporary paralysis._ 10. _Miscellaneous remarks;_ 1. _Direct and
reverse spectra at the same time. A spectral halo. Rule to predetermine
the colours of spectra._ 2. _Variation of spectra from extraneous
light._ 3. _Variation of spectra in number, figure, and remission._ 4.
_Circulation of the blood in the eye is visible._ 5. _A new way of
magnifying objects. Conclusion._

When any one has long and attentively looked at a bright object, as at the
setting sun, on closing his eyes, or removing them, an image, which
resembles in form the object he was attending to, continues some time to be
visible; this appearance in the eye we shall call the ocular spectrum of
that object.

These ocular spectra are of four kinds: 1st, Such as are owing to a less
sensibility of a defined part of the retina; or _spectra from defect of
sensibility._ 2d, Such as are owing to a greater sensibility of a defined
part of the retina; or _spectra from excess of sensibility_. 3d, Such as
resemble their object in its colour as well as form; which may be termed
_direct ocular spectra_. 4th, Such as are of a colour contrary to that of
their object; which may be termed _reverse ocular spectra_.

The laws of light have been most successfully explained by the great
Newton, and the perception of visible objects has been ably investigated by
the ingenious Dr. Berkeley and M. Malebranche; but these minute phenomena
of vision have yet been thought reducible to no theory, though many
philosophers have employed a considerable degree of attention upon them:
among these are Dr. Jurin, at the end of Dr. Smith's Optics; M. AEpinus, in
the Nov. Com. Petropol. V. 10.; M. Beguelin, in the Berlin Memoires, V. II.
1771; M. d'Arcy, in the Histoire de l'Acad. des Scienc. 1765; M. de la
Hire; and, lastly, the celebrated M. de Buffon, in the Memoires de l'Acad.
des Scien. who has termed them accidental colours, as if subjected to no
established laws, Ac. Par. 1743. M. p. 215.

I must here apprize the reader, that it is very difficult for different
people to give the same names to various shades of colours; whence, in the
following pages, something must be allowed, if on repeating the experiments
the colours here mentioned should not accurately correspond with his own
names of them.

I. _Activity of the Retina in Vision._

From the subsequent exper