ADDITION. 



Atiit tonal Observations on VERTIGO, which ought to 

 haw been infer ted in Sefi. XX. 6. after the Words 

 " cplic nerve," at the End of the fecond Paragraph. 



AFTER revolving with your eyes open till you 

 become vertiginous, as loon as you ceafe to revolve, 

 not only the circum-ambient objedts appear to circu- 

 late round you in a direction contrary to that, in 

 which you have been turning, but you are liable to 

 roll your eyes forwards and backwards ; as is well 

 obierved, and ingenioufly ilemonft rated by Dr. Wells 

 in a late publication on vifion. The fame occurs, 

 if you revolve with your eyes clofed, and open them 

 immediately at the time of your ceafmg to turn; 

 and even during the whole time of revolving, as 

 may be felt by your hand prefled lightly on your 

 clofed eyelids. To thefe movements of the eyes, of 

 which he fuppofes the obferver to be inconfcious, 

 Dr. Wells afcribes the apparent circumgyration of 

 objects on ceafiug to revolve. 



The caufe of thus turning our eyes forwards, and 

 then back again, after our body is at reft, depends, 

 I imagine, on the fame circumilance, which induces 

 us to follow the indiftinct fpectra, which are formed 

 on one fide of the center of the retina, when we 

 obierve them apparently on clouds, as defcribed in 

 Sect. XL. 2. 2. ; and then not being able to gain a 

 more diflinct vifion of them, we turn our eyes back, 

 and again and again purfue the flying fhade. 



But 



