SECT. XX. 6. OF VERTIGO. 185 



Whence I conclude that vertigo may have for its caufe ei- 

 ther the ocular fpeftra of the fenfe of vifion, when a perfon 

 revolves with his eyes open ^ or the auricular murmurs of the 

 fenfe of hearing, if he is revolved near a cafcade ; or the evanef- 

 cent titillations of the fenfe of touch, if he revolves blindfold. 

 All thefe I (hould wi(h to call vanifhing ideas, or fenfual motions, 

 of thofe organs of fenfe ; which ideas, or fenfual motions, have 

 lately been aflbciated in a circle, and therefore for a time con- 

 tinue to be excited. And what are the ideas of colours, when 

 they are excited by imagination or memory, but the repetition 

 of finer ocular fpe&ra ? What the idea of founds, but the repe- 

 titon of finer auricular murmurs ? And what the ideas of tangi- 

 ble objects, but the repetition of finer evanefcent titillations ? 



The tangible, and the auricular, and the vifual vertigo, are all 

 perceived by many people for a day or two after long travelling 

 in a boat or coach ; the motion of the veflel, or vehicle, or of 

 the furrounding obje&s, and the noife of the wheels and oars, 

 occur at intervals of reverie, or at the commencement of flsep. 

 See :>el. XX. 5. Thefe ideas, or fenfual motions, of fight, of 

 hearing, and of touch, are fucceeded by the fame effedls as the 

 ocular ipeftra, the auricular murmurs, and the evanefcent titil- 

 lations above mentioned ; that is, by a kind of vertigo, and can- 

 not in that refpeft be diftinguiihed from them. Which is a fur- 

 ther confirmation of the truth of the dodrine" delivered in SeK 

 III. of this work, that the colours remaining in the eyes, which 

 are termed ocular fpeftra, are ideas, or fenfual motions, belong- 

 ing to the fenfe of vifion, which for too long a time continue 

 their aHvity. 



Any one, who ftands alone on the top of a high tower, if he 

 has not been accullomed to balance himfelf by objefts placed at 

 fuch diftances and with fuch inclinations, begins to dagger, and 

 endeavours to recover himfelf by his mufcular feelings. During 

 this time the apparent motion of objects at a diftance below him 

 is very great, and the fpectra of thefe apparent motions continue 

 a little time after he has experienced them ; and he is perfuaded 

 to incline the contrary way to counteract their effecfh ; and 

 either immediately falls, or applying his hands to the building, 

 ufes his mufcular feelings to preferve his perpendicular attitude, 

 contrary to the erroneous perfuafions of his eyes Whild the 

 pevfon, who walks in the dark, daggers, but without dizzinefs; 

 for he neither has the fenfation of moving objefts to take off his 

 attention from his mufcular feelings, nor has he the fpeftra of 

 thofe motions continued on his retina to add to his confufion. 

 It happens indeed fometimes to one dandirig on a tower, that 

 the idea of his not having room to extend his bafe by moving 

 VOL. I. A A one 



