IN OPTICS. 75 



Some of the older writers upon optics ima- 

 gined the visive spirits to be contained in the 

 head, as water is in a vessel, which therefore, 

 when once put in motion by the rotation of our 

 bodies, must continue in it for some time after 

 this has ceased ; and to this real circular move- 

 ment of the visive spirits, while the body is at 

 rest, they attributed the apparent motions of 

 objects in giddiness. Dechales* saw the weak- 

 ness of this hypothesis, and conjectured, that 

 the phenomenon might be owing to a real 

 movement of the eyes, but produced no fact in 

 proof of his opinion. Dr. Porterfieldf, on the 

 contrary, supposed the difficulty of explaining 

 it to consist in showing, why objects at rest 

 appear in motion to an eye which is also at 

 rest. The solution he offered of this repre- 

 sentation of the phenomenon, is not only ex- 

 tremely ingenious, but is, I believe, the only 

 probable one which can be given. It does not 

 apply, however, to the fact which truly exists ; 

 for I shall immediately show, that the eye is 

 not at rest, as he imagined. The last author, 

 I know of, who has touched upon this subject, 

 is Dr. Darwin t. His words are, " When any 

 one turns round rapidly on one foot till he 



* Cursus Mathemat. Tom. ii. p. 422. 

 t Treatise on the Eye, Vol. ii. p. 426. 

 I Philosoph, Transact. Vol. Ixxvi. p. 315. 



