27* Of VERTIGO. SECT. XX. *, 



is raifed much above all other objects, as thefe ob- 

 jects are out of the fphere of diftinct vifion, and they 

 are obliged to balance their bodies by the lefs accu- 

 rate feelings of their mufcles. 



There is another curious phenomenon belonging 

 to this place, if the circumjacent vifible objects are 

 fo fmall, that we do not diftinguifh their minute 

 parts ; or fo fimilar, that we do not know them 

 from each other ; we cannot determine our perpen- 

 dicularity by them. Thus in a room hung with a 

 paper, which is coloured over with fimilar fmall 

 txlack lozenges or rhomboids, many people become 

 dizzy ; for when they begin to fall, the next and 

 the next lozenge fucceeds upon the eye; which 

 fhey miftake for the firft, and are not aware, that 

 they have any apparent motion. But if you fix a 

 fheet of paper, or draw any other figure, in the 

 midft of thefe lozenges, the charm eeafes, and no 

 dizzinefs is perceptible.- The fame occurs, when 

 we ride over a plain covered with (ROW without 

 trees or other eminent objects . 



2. But after having compared vifible objets at 

 reft with the fenfe of touch, and learnt to diftinguifh 

 their fhapes and fhades, and to meafure our want of 

 perpendicularity by their apparent motions, we come 

 to confrder them in real motion. Here a new diffi- 

 culty occurs, and we require fome experience to learn 

 the peculiar mode of motion of any moving objects, 

 before we can make ufe of them for the purpofes 

 of determining our perpendicularity. Thus fome 

 people become dizzy at the fight of a whirling wheel, 

 or by gazing on the fluctuations of a river, if no 

 fleady objects are at the fame time within the fphere of 

 their diflin-Ct vifion ; and wh-en a child firft can (land 

 erect upon his legs, if you gain his attention to a 

 white handkerchief fteadily extended like a fail, and 

 afterwards make it undulate, he inftantly lofes his- 

 perpendicularity, and tumbles on the ground. 



3- ^ 



