ADDITION, 



this experiment, is, becaufe the images of objects 

 are fanned in rotation round the axis of the eye, 

 and not from one fide to the other of the axis of 

 k y fo that, as the eyeball has not power to turn in 

 its focket round its own axis, k cannot follow the 

 apparent motions of thefe evarrefceru fpectra, either 

 before or alter the body is at reft. From all which 

 arguments it is manifeft y that thefe apparent retro- 

 grade gyrations of objects are not caufed by the roll- 

 ing of the eye-balls ; ftrft, becaufe no apparent re- 

 trogreilion of objects is obferved in other rollings 

 of the eyes : fecondly, becaufe the apparent retro- 

 greffion of objects continues many feconds after the 

 Foil ing of the eyeballs ceafes. Thirdly, becaufe the 

 apparent retrogreilion of objects i& fometimes one 

 way r and fomctimes another, yet the rolling of the 

 eyeballs is the lame. Fourthly, becaufe the rolling 

 of the eyeballs exitts before the apparent retrograde 

 motion of objects is obferved ; that is, before the 

 revolving peifon flops. And fifthly, becaufe the ap- 

 parent retrograde gyration of objects is produced, 

 when there is no rolling of the eyeballs at all. 



Doctor Wells imagines, that no fpectra can be 

 gained in the eye, it a per on levolves with his eye- 

 lids do fed, and thinks this a fufficient argument 

 sgainft the opinion, that the apparent progrcffion 

 of the fj.c&ra of light or colours in the eye can 

 caufe the apparent retrogreflion of objects in 

 i he vertigo above defcribccl ; but it is certain, 

 when any perfon revolves in a light room with 

 his eyes clofed, that he nevertheleis perceives dif- 

 erenees of light both in quantity and colour 

 through his eyelids, as he turns round; and readily 

 gains Ipeclra of thofe differences. And thefe fpec- 

 tra are not very d'Hercnt except in Vivacity from 

 thofe, which he acquire*, when he. revolves wilh 

 uBelofed eyes, fince if he ihcn revolves very rapidly 



the 



