After revolving with your eyes open till you become vertiginous, as
soon as you cease to revolve, not only the circum-ambient objects
appear to circulate round you in a direction contrary to that, in which
you have been turning, but you are liable to roll your eyes forwards
and backwards; as is well observed, and ingeniously demonstrated by Dr.
Wells in a late publication on vision. The same occurs, if you revolve
with your eyes closed, and open them immediately at the time of your
ceasing to turn; and even during the whole time of revolving, as may be
felt by your hand pressed lightly on your closed eyelids. To these
movements of the eyes, of which he supposes the observer to be
inconscious, Dr. Wells ascribes the apparent circumgyration of objects
on ceasing to revolve.

The cause of thus turning our eyes forwards, and then back again, after
our body is at rest, depends, I imagine, on the same circumstance,
which induces us to follow the indistinct spectra, which are formed on
one side of the center of the retina, when we observe them apparently
on clouds, as described in Sect. XL. 2. 2.; and then not being able to
gain a more distinct vision of them, we turn our eyes back, and again
and again pursue the flying shade.

But this rolling of the eyes, after revolving till we become
vertiginous, cannot cause the apparent circumgyration of objects, in a
direction contrary to that in which we have been revolving, for the
following reasons. 1. Because in pursuing a spectrum in the sky, or on
the ground, as above mentioned, we perceive no retrograde motions of
objects. 2. Because the apparent retrograde motions of objects, when we
have revolved till we are vertiginous, continues much longer than the
rolling of the eyes above described.

3. When we have revolved from right to left, the apparent motion of
objects, when we stop, is from left to right; and when we have revolved
from left to right, the apparent circulation of objects is from right
to left; yet in both these cases the eyes of the revolver are seen
equally to roll forwards and backwards.

4. Because this rolling of the eyes backwards and forwards takes place
during our revolving, as may be perceived by the hand lightly pressed
on the closed eyelids, and therefore exists before the effect ascribed
to it.

And fifthly,