OF VERTIGO. SECT. XX. a, 



want of perpendicularity, is by attending to the apparent mo- 

 tion of the objects within the fphere of diftinct vifion ; for this 

 apparent motion of objects, when we incline from our perpen- 

 dicularity, or begin to fall, is as much greater than the real mo- 

 tion of the eye, as the diameter of the fphere of diftinct vifion is 

 to our perpendicular height. 



Hence no one, who is hood-winked, can walk in a ftraight 

 line for a hundred fteps together ; for he inclines fo greatly, 

 before he is warned of his want of perpendicularity by the fenfc 

 of touch, not having the apparent motions of ambient objects 

 to meafure this inclination by, that he is neceffitated to move 

 one of his feet outwards, to the right or to the left, to fupport 

 the new centre of gravity, and thus errs from the line he en- 

 deavours to proceed in. 



For the fame reafon many people become dizzy, when they 

 look from the fummit of a tower, which is raifed much above 

 all other objects, as thefe objects are out of the fphere of dif- 

 tinct vifion, and they are obliged to balance their bodies by the 

 lefs accurate 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 black lozenges or rhomboids, 

 many people become dizzy ; for when they begin to fall, the 

 next and the next lozenge fucceed upon the eye*, which they mif- 

 take for the firft, and are not aware, that they have any apparent 

 motion. But if you fix a iheet of paper, or draw any other fig- 

 ure, in the midft of thefe lozenges, the charm ceafes, and no 

 dizzinefs is perceptible. The fame occurs when we ride over 

 a plain covered with fnow without trees or other eminent objects. 



2. But afrer having compared vifible objects at reft with the 

 fenfe of touch, and learnt to diflinguifli their fhapes and (hades, 

 and to meaiure our want of perpendicularity by their apparent 

 motions, we come to confider them in real motion. Here a new 

 difficulty 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 perpen- 

 dicularity. Thus fome people become dizzy at the fight of a 

 whirling wheel, or by gazing on the fluctuations of a river, if 

 no fteady objects are at the fame time within the fphere of their 

 diftinct vifion : and when a child firft can {land erecl upon his 

 legs, if you gain his attention to a white hankerchief fteadily 

 extended like a fajl, and afterwards make it undulate, he in- 



ftantly 



