ADDITION. 645 



In them, by fhading the light from my eyelids more 

 or lefs with my hand, I at length ceafed to have the 

 idea of going forward, after I flopped with my eyes 

 clofed ; and faw changing fpectra 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 perceive the fpectra Jn 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 ftand ftill. After more trials I fometimes 

 brought myfelf to believe, that I faw changing fpec- 

 tra 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 fpcak, however, with diffidence of thefe 

 facts, as I could not always make the experiments 

 fucceed, when there was not a ftrong light in my 

 room, or when my eyes were not in the moft pro- 

 per ftate for fiich obfervations. 



The ingenious and learned M. Sauvage has men- 

 tioned other theories to account for the apparent cir- 

 cumgyration 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 cryftalline lens in the eye, by means of involun- 

 tary actions -of the mufcles, which conjlitute the 

 ciliary procefs, Both thefe theories lie under the 

 fame objection as that of Dr. Wells before men- 

 tioned ; namely, that the apparent motions of ob- 

 jects, after the obferver has revolved for fome time, 

 ihould appear to vibrate this way jind that ; and not 

 VOL, I. \J u to 



