less perceives differences of light both in quantity
and colour through his eyelids, as he turns round; and readily gains
spectra of those differences. And these spectra are not very different
except in vivacity from those, which he acquires, when he revolves with
unclosed eyes, since if he then revolves very rapidly the colours and
forms of surrounding objects are as it were mixed together in his eye;.
as when, the prismatic colours are painted on a wheel, they appear
white as they revolve. The truth of this is evinced by the staggering
or vertigo of men perfectly blind, when they turn round; which is not
attended with apparent circulation of objects, but is a vertiginous
disorder of the sense of touch. Blind men balance themselves by their
sense of touch; which, being less adapted for perceiving small
deviations from their perpendicular, occasions them to carry themselves
more erect in walking. This method of balancing themselves by the
direction of their pressure against the floor, becomes disordered by
the unusual mode of action in turning round, and they begin to lose
their perpendicularity, that is, they become vertiginous; but without
any apparent circular motions of visible objects.

It will appear from the following experiments, that the apparent
progression of the ocular spectra of light or colours is the cause of
the apparent retrogression of objects, after a person has revolved,
till he is vertiginous.

First, when a person turns round in a light room with his eyes open,
but closes them before he stops, he will seem to be carried forwards in
the direction he was turning for a short time after he stops. But if he
opens his eyes again, the objects before him instantly appear to move
in a retrograde direction, and he loses the sensation of being carried
forwards. The same occurs if a person revolves in a light room with his
eyes closed; when he stops, he seems to be for a time carried forwards,
if his eyes are still closed; but the instant he opens them, the
surrounding objects appear to move in retrograde gyration. From hence
it may be concluded, that it is the sensation or imagination of our
continuing to go forwards in the direction in which we were turning,
that causes the apparent retrograde circulation of objects.

Secondly, though there is an audible vertigo, as is known