SECT. XX. 6. OF VERTIGO. 183 



fpeftra 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 forne- 

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

 ceive the fpetra 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 (land 

 ftill. After more trials I fometimes brought myfelf to believe, 

 that I faw changing fpectra of lights and diodes moving in my 

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

 either the fpeclra or the objects to be in a (late 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 

 light in my room, or when my eyes were not in the moil proper 

 {late for fuch obfervations. 



The ingenious and learned M. Sauvage has mentioned other 

 theories to acccount 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 ringers of frogs. 

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

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

 involuntary actions of the mufcles, 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, (hould appear to vibrate this way and that ; and not to cir- 

 culate uniformly in a direction contrary to that, in which the ob- 

 ferver had revolved. 



M. Sauvage has, laftly, mentioned the theory of colours left in 

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

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

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

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

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

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

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

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

 Method. Claf. VIII i. i. 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 j as impreflions on a paflive fubftance muft either re- 

 mam, or ceafe inrirely. 



Having reperufed the ingenious Effay of Dr. Wells on Single 

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

 azine 



