SECT. XX. 6. OF VERTIGO. 1 8 1 



the apparent motions of thefe evanefcent fpeftra, either before 

 or after the body is at reft From all which arguments it is 

 rnanifeft, that thefe apparent retrograde gyrations of objects are 

 no: caufed by the rolling of the eye-balls ; firft, becaufe no ap- 

 parent retrogreflion of objects is obferved in other rollings of 

 the eyes. Secondly, becaufe the apparent retrogreflion of ob- 

 j<-cls continues many feconds after the rolling of the eyeballs 

 ceafes. Thirdly, becaufe the apparent retrogreilion of objects is 

 fometimes one way, and fometimes another, yet the rolling of 

 the eyeballs is the fame Fourthly, becaufe the rolling of the 

 eyeballs exifts before the apparent retrograde motions of objects 

 i i obferved ; that is, before the revolving perfon (lops And 

 fifthly, becaufe the apparent retrograde gyration of objects is 

 pr duced, when there is no rolling of the eyeballs at all. 



Dodor Wells imagines, that no fpeclracan be gained in the 

 eye, if <i perfon revolves with his eyelids clofed, and thinks this 

 a tuflicient argument againft the opinion, that the apparent pro- 

 grrflion of the fpedlra of light or colours in the eye can caufe 

 the apparent retrogreflion of objects in the vertigo above defcri- 

 bed * but it is certain, when any perfon revolves in a light room 

 with his eyes clofed, that he neverthelefs perceives differences of 

 light both in quantity and colour through his eyelids, as he turns 

 round ; and readily gains fpedtra of rhofe differences. And thefe 

 fpectra are not very different except in vivacity from thofe, which 

 he acquires, when he revolves with unclofed eyes, fmce if he 

 then revolves very rapidly the colours and forms of furrounding 

 objects are as it were mixed together in his eye ; as when the 

 prifmatic colours are painted on a wheel, they appear white as 

 they revolve. The truth of this is evinced by the daggering or 

 vertigo of men perfectly blind, when they turn round ; which 

 is not attended with apparent circulation of objects, but is a ver- 

 tiginous diforder of the fenfe of touch. Blind men balance them- 

 felves by their fenfe of touch ; which, being lefs adapted for 

 perceiving fmall deviations from their perpendicular, occafions 

 them to carry themfelves more erect in walking. This method 

 of balancing themfelves by the direction of their preflure againft 

 the floor, becomes difordered by the unufual mode of action in 

 turning round, and they begin to lofe their perpendicularity, 

 that is, they become vertiginous ; but without any apparent cir- 

 cular motions of vifible objects. 



It will appear from the following experiments, that the appar- 

 ent progreflion of the ocular fpectra of light or colours is the 

 caufe of the apparent retrogreflion of objedts, after a perfon has 

 revolved, till he is vertiginous. 



when a perfon turns round in a light Toom with his 



eyes 



