182 OF VERTIGO. Sect. XX. 6. 



eyes open, but clofes them before he flops, he will feem to be 

 carried forwards in the direction he was turning for a fhort time 

 after he (lops. But if he opens his eyes again> the objects before 

 him inftantly appear to move in a retrograde dire&ion, and he 

 lofes the fenfation of being carried forwards. The fame occurs 

 if a perfon revolves in a light room with his eyes clofed •, when 

 he flops, he feems to be for a time carried forwards, if his eyes 

 are ilill clofed \ but the inftant he opens them, the furrounding 

 objects appear to move in retrograde gyration. From hence it 

 may be concluded, that it is the fenfation or imagination of our 

 continuing to go forwards in the direction in which we were 

 turning, that caufes the apparent retrograde circulation of ob- 

 jects. 



Secondly, though there is an audible vertigo, as is known by 

 the battement, or undulations of found in the ears, which many 

 vertiginous people experience ; and though there is alfo a tangi- 

 ble 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 fight, we are to look for the caufe 

 of our appearing to move forward, when we flop with our eyes 

 clofed after gyration, to forae affe£lion of this fenfe. Now, 

 thirdly, if the fpeclra formed in the eye during our rotation con- 

 tinue to change, when we fland Mill, like the fpettra defcribed 

 in See~l. III. 3. 6". fuch changes mull fugged to us the idea or 

 fenfation of our flill continuing to turn round j as is the cafe, 

 when we revolve in a light room, and clofe our eyes before we 

 flop. And laflly, on opening our eyes in the fituation above 

 defcribed, the objects we chance to view amid thefe changing 

 fpe£tra in the eye, mufl feem to move in a contrary direction ; 

 as the moon fometimes appears to move retrograde, when fwift-t 

 gliding clouds are palling forwards fo much nearer the eye of 

 the beholder. 



To make obfervations on faint occular fpeclra requires forae 

 degree of habit, and compofure of mind, and even patience ; fome 

 of thofe defcribed in feet. XL. were found difficult to fee, by 

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

 the confufioii of vertigo, when all the other irritative tribes of 

 motion, as well as thofe of vifion, are in iom? degree difturbed, 

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

 contemplation of fuchwer. ; r nfations,as are cccaHoned by faint 

 occular fpectra. Yet after frequently revolving, both with my, 

 eyes clofed, and with thern open, and attending to the fpect: 

 remaining in them, by ihaciing the light from my eyelids more^ 

 or lefs with rny hdnd, I at length ecafed to have the idea of going 

 forward, after I ito^ped with my eyes clcfed ; and faw changjr 



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