SECT. XX. 7. OF VERTIGO. 275 



jo revolve, the longer will continue thefe fucceffive 

 motions of the parts of the optic nerve. See an 

 additional note at the end of this volume. 



Any one who (lands alone on the top of a high 

 , tower, if he has not been accuftomed to balance 

 himfelf by objects placed at fuch diftances and with 

 fuch inclinations, begins to dagger, and endea- 

 vours 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 perfua- 

 ded to incline the contrary way to counteract their 

 effects ; 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. Whilft the 

 perfpn, who walks in the dark, daggers, but with- 

 out dizzinefs ; for he neither has the fenfation of 

 moving objects to take off his attention from his 

 mufcular feelings, nor has he the fpectra of thofe 

 motions continued on his retina to add to his con- 

 fufion. It happens indeed fometimes to one (land- 

 ing on a tower, that the idea of his not having 

 room to extend his bafe by moving one of his feet 

 outwards when he begins to incline, fuperadds 

 fears to his other inconveniences ; which like fur- 

 prife, joy, or any great degree of fenfation, ener- 

 vates him in a moment^ by employing the whole 

 fenforial power, and by thus breaking all the affo- 

 ciated trains and tribes of motion. 



j. The irritative ideas of objects, whilft we are 

 awake, are perpetually prefent to our fenfe of fight ; 

 as we view the furniture of our rooms, or the 

 ground, we tread upon, throughout the whole day 

 without attending to it. And as our bodies are 

 never at perfect reft during our waking hours, 

 phefe irritative ideas of objects are attended perpe- 

 tually 



