ADDITION. 643 



the colours and forms of furrounding objects are 

 as it were mixed together in his eye ; as when the 

 prifmatic colours are painted on a wheel, they ap- 

 pear white as they revolve. The truth of this is 

 evinced by the ftaggering or vertigo of men per- 

 fectly blind, when they turn round ; which is not 

 attended with apparent circulation of objects, but 

 is a vertiginous diforder of the fenfe of touch. 

 Blind men balance themfelves by their fenfe of 

 touch ; which, being lefs adapted for perceiving 

 fmall deviations from their perpendicular, occafions 

 them to carry themfelves more erect in walking. 

 This method of balancing themfelves by the direc- 

 tion of their preffure againft the floor, becomes dif- 

 ordered by the unufual mode of action in turning 

 round, and they begin to lofe their perpendicula- 

 rity, that is, they become vertiginous ; but without 

 any apparent circular motions of viiible objects. 



It will appear from the following experiments, 

 that the apparent progreflion of the ocular fpectra 

 of light or colours is the caufe of the apparent re- 

 trogreflion of objects, after a perfon has revolved, 

 till he is vertiginous. 



Firft, when a perfon turns round in a light room 

 with his eyes open, but clofes them before he (lops, 

 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 objects before him 

 inflantly appear to move in a retrograde direction, 

 and he lofes 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 feerns to 

 be for a time carried forward, if his eyes are ftill 

 clofed ; but the inftant he opens them, the furround- 

 ing 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 



