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 nevertheless 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