Sect. XX. 6. OF VERTIGO, 1B3 



ipe&ra in my eyes, which feemed to move, as it were, over the 

 field of vifion ; till at length, by repeated trials on funny days, 

 I perfuaded myfelf, on opening my eyes, after revolving fome 

 time, on a fhelf of gilded books in my library, that I could per- 

 ceive the fpectra in my eyes move forwards over one or two of 

 the books, like the vapours in the air of a fummer's day ; and 

 could fo far undeceive myfelf, as to perceive the books to fland 

 (till. After more trials I fometimes brought myfelf to believe 5 

 that I fow changing fpectra of lights and fhades moving in my 

 eyes, after turning round for fome time, but did not imagine 

 either the fpectra or the objects to be in a ftate of gyration. "I 

 fpeak, however, with diffidence of thefe facts, as I could not al- 

 ways make the experiments fucceed, when there was not a ftrong 

 Ught in my room, or when my eyes were not in the moll proper 

 ftate for fuch obfervations. 



The ingenious and learned M. Sauvage has mentioned other 

 theories tb account for the apparent circumgyration of objects in 

 vertiginous people. As the retrograde motions of the particles 

 of blood in the optic arteries, by fpafm, or by fear, as is feen in 

 the tails of tadpoles, and membranes between the fingers of frogs* 

 Another caufe he thinks may be from the librations to one fide, 

 and to the other, of the crystalline lens in the eye, by means of 

 involuntary actions of the mufcies, which conftitute the ciliary 

 procefs. Both thefe theories lie under the fame objection as 

 that of Dr. Wells^before mentioned ; namely, that the apparent 

 motions of objects, after the obferver has revolved for fome time, 

 mould appear to vibrate this way and that ; and not to circulate 

 [uniformly in a direction contrary to that, in which the obferver 

 had revolved. 



M. Sauvage has, laftiy, mentioned the theory of colours left 112 

 i the eye, which he has termed impreflions on the retina. He fays, 

 " Experience teaches us, that impreflions made on the retina by 

 U vifible object remain fome feconds after the object is removed ; 

 as appears from the circle of fire which we fee, when a fire-ftick 

 is whirled round in the dark j therefore when we are carried 

 round our own axis in a circle, we undergo a temporary vertigo* 

 when we flop ; becaufe the impreffions of the circumjacent ob- 

 jects remain for a time afterwards on the retina." Nofologc 

 (Method. Claf. VIII. 1. 1. We have before obferved, that the 

 changes of thefe colours remaining in the eye, evinces them to 

 be motions of the fine terminations of the retina, and not impref- 

 fions on it ; as impreffions on a paffive fubftance mud either re- 

 main, or ceafe intirely. 



Having reperufed the ingenious EfTay of Dr. Wells on Single 

 Vifion, and his additional obfervations in the Gentleman's Mag- 



azinc 



