ADDITION. 



caufes the apparent retrograde circulation of 

 objects. 



Secondly, though there is an audible vertigo, as 

 is known by the battement, or undulations of found 

 in the ears, which many vertiginous people expe- 

 rience ; and though there is alfo a tangible vertigo, 

 as when a blind perfon turns round, as mentioned 

 above ; yet as this circumgyration of objects is an 

 Hallucination or deception of the fenfe of frght, we 

 are to look for the caufe of our appearing to move 

 forward, when we ftop with our eyes elofed after gy- 

 ration, to fome affeftion of this fenfe. Now, thirdly, 

 if the fpeftra formed in the eye during our rota- 

 tion, continue to change, when we ftand ftill, like 

 the fpeftra- defcribed in Seft. III. 3. 6. fuch changes 

 muft fuggeft to us the idea or fenfation of our ftill 

 continuing to turn round ; as is the cafe, when we 

 revolve in a light room, and clofe our eyes before 

 we ftop. And laftly, on opening our eyes in the 

 iituation above defcribedj the objects we chance to 

 view amid thefe changing fpectra in the eye, muft 

 fee m to move in a contrary direction ; as the moon 

 fometimes appears to move retrograde, when fwift- 

 gliding clouds are paifing forwards fo much nearer 

 tfce eye of the beholder, 



To make obfervations on faint ocular fpectra re- 

 quires fome degree of habit, and compofure of 

 mind, and even patience ; fome of thofe defcribed 

 in Sect. XL. were found difficult to fee, by many, 

 who tried them ; now it happens, that the mind, 

 during the confulion of vertigo, when all the other 

 irritative tribes of motion, as well as thofe of vi- 

 fion, are in fome degree difturbed, together with 

 the fear of falling, is in a very unfit ftate for the 

 contemplation of fnch weak fenfations, as are oc- 

 cafioned by faint ocu lar fpectra. Yet afier frequent- 

 ly revolving, boih with my eyes elofed, and with 

 operi, and auendin-g to the fpectra remaining 



in 



