e a revolution in eight
thirds of time, it presented to the eye a complete circle of fire; from
whence he concludes, that the impression continues on the organ about the
seventh part of a second. (Mem. de l'Acad. des Sc. 1765.) This, however, is
only to be considered as the shortest time of the duration of these direct
spectra; since in the fatigued eye both the direct and reverse spectra,
with their intermissions, appear to take up many seconds of time, and seem
very variable in proportion to the circumstances of fatigue or energy.

4. It sometimes happens, if the eyeballs have been rubbed hard with the
fingers, that lucid sparks are seen in quick motion amidst the spectrum we
are attending to. This is similar to the flashes of fire from a stroke on
the eye in fighting, and is resembled by the warmth and glow, which appears
upon the skin after friction, and is probably owing to an acceleration of
the arterial blood into the vessels emptied by the previous pressure. By
being accustomed to observe such small sensations in the eye, it is easy to
see the circulation of the blood in this organ. I have attended to this
frequently, when I have observed my eyes more than commonly sensible to
other spectra. The circulation may be seen either in both eyes at a time,
or only in one of them; for as a certain quantity of light is necessary to
produce this curious phenomenon, if one hand be brought nearer the closed
eyelids than the other, the circulation in that eye will for a time
disappear. For the easier viewing the circulation, it is sometimes
necessary to rub the eyes with a certain degree of force after they are
closed, and to hold the breath rather longer than is agreeable, which, by
accumulating more blood in the eye, facilitates the experiment; but in
general it may be seen distinctly after having examined other spectra with
your back to the light, till the eyes become weary; then having covered
your closed eyelids for half a minute, till the spectrum is faded away
which you were examining, turn your face to the light, and removing your
hands from the eyelids, by and by again shade them a little, and the
circulation becomes curiously distinct. The streams of blood are however
generally seen to unite, which shews it to be the venous circulation,
owing, I suppose, to the greater opacity of the colour of the blood in
these vessels; for this venous circulation is also much more easily seen by
the microscope in the tail of a tadp