I now come to relate an experiment, in which the rolling
of the eyes does not take place at all after revolving, and yet the
vertigo is more distressing than in the situations above mentioned. If
any one looks steadily at a spot in the ceiling over his head, or
indeed at his own finger held up high over his head, and in that
situation turns round till he becomes giddy; and then stops, and looks
horizontally; he now finds, that the apparent rotation of objects is
from above downwards, or from below upwards; that is, that the apparent
circulation of objects is now vertical instead of horizontal, making
part of a circle round the axis of his _eye_; and this without any
rolling of his eyeballs. The reason of there being no rolling of the
eyeballs, perceived after this experiment, is, because the images of
objects are formed in rotation round the axis of the eye, and not from
one side to the other of the axis of it; so that, as the eyeball has
not power to turn in its socket round its own axis, it cannot follow
the apparent motions of these evanescent spectra, either before or
after the body is at rest. From all which arguments it is manifest,
that these apparent retrograde gyrations of objects are not caused by
the rolling of the eyeballs; first, because no apparent retrogression
of objects is observed in other rollings of the eyes: secondly, because
the apparent retrogression of objects continues many seconds after the
rolling of the eyeballs ceases. Thirdly, because the apparent
retrogression of objects is sometimes one way, and sometimes another,
yet the rolling of the eyeballs is the same. Fourthly, because the
rolling of the eyeballs exists before the apparent retrograde motions
of objects is observed; that is, before the revolving person stops. And
fifthly, because the apparent retrograde gyration of objects is
produced, when there is no rolling of the eyeballs at all.

Doctor Wells imagines, that no spectra can be gained in the eye, if a
person revolves with his eyelids closed, and thinks this a sufficient
argument against the opinion, that the apparent progression of the
spectra of light or colours in the eye can cause the apparent
retrogression of objects in the vertigo above described; but it is
certain, when any person revolves in a light room with his eyes closed,
that he neverthe