182 OF VERTIGO. SECT. XX. 6< 



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

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

 after he flops But if he opens his eyes again, the objedts before 

 him inflantly appear to move in a retrograde direction, and he 

 lofcs 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 flill clofed ; but the inflant 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- 

 jecls. 



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 objedls 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 fome affedlion of this fenfe. Now, 

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

 tinue to change, when we ftand flill, like the fpeclra defcribed 

 in Seft. III. 3. 6. fuch changes muft fugged to us the idea or 

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

 when we revolve in a li^ht room, and clofe our eyes before we 

 Hop And lalily, on opening our eyes in the fituation above 

 defcribed, the objc~c~ls we chance to view amid thefe changing 

 fpe&ra in the eye, mufl ietm to move in a contrary direction ; 

 as the moon fometinje^ appears to move retrograde, when fwift- 

 gliding clouds are puffing forwards fo much nearer the eye of 

 the beholder. 



To make obfervations on faint occular fpedlra requires fome 

 degree of habit,and compofureof mind, and even patience*, fome 

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

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

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

 motion, as well as thofe of vifion, are in fome degree diflurbed, 

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

 contemplation of fuch weak fenfations, as are occafioned by faint 

 ocular fpeclra. Yet after frequently revolving, both with my 

 eyes clofed, and with them open, and attending to the fpeira 

 remaining in them, by fhading the light from my eyelids more 

 or lefs with my hand, I at length ceafed to have the idea of going 

 forward, after I flopped with my eyes clofed 5 and faw changing 



fpecTra 



