ADDITION. 6.;i 



But this rolling of the eyes, after revolving till 

 we become vertiginous, cannot caufe the apparent 

 circumgyration of objects, in a direction contrary 

 to that in which we have been revolving, for the fol- 

 lowing reafons. I. Becaufe in purfuing a fpectrum 

 in the iky, or on the ground, as above mentioned, 

 we perceive no retrograde motions of objects. 2. 

 Becaufe the apparent retrograde motions of objects, 

 when we have revolved till we are vertiginous, conti- 

 nues much longer than the rolling of the eyes above 

 defcribed. 



3. When we have revolved from right to left, the 

 apparent motion of objects, when we flop, 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 thefe cafes the eyes of the 

 revolver are feen equally to roll forwards and back- 

 wards. 



4. Becaufe this rolling of the eyes backwards and 

 forwards takes place during our revolving, as may 

 be perceived by the hand lightly prefled on the 

 clofed eyelids, and therefore exifts before the effect 

 afcribed to it. 



And fifthly, I EOW come to relate an experiment, 

 in which the rolling of the eyes does not take place 

 at all after revolving, and yet the vertigo is morg 

 diftrefling than in the fituations above mentioned* 

 If any one looks fteadily at a fpot in the ceiling over 

 his head, or indeed at his own finger held up high 

 over his head, and in that fituation turns round till 

 he becomes giddy ; and then (lops, and looks hori- 

 zontally ; he now finds, that the apparent rota i ion 

 of objects is from above downwards, or from below 

 upwards; that is, that the apparent circulation of 

 objects is now vertical inftead of hotizontal, making 

 part of a circle round the axis of his eye ; and this 

 without any rolling of his eyeballs. The reafon of 

 there being no rolling of the eyeballs peiceived after 



this 



