76 EXPERIMENTS, &c. 



becomes dizzy, and falls upon the ground, the 

 spectra of the ambient objects continue to pre- 

 sent themselves in rotation, or appear to librate, 

 and he seems to behold them for some time in 

 motion." I do not indeed pretend to under- 

 stand his opinion fully; but this much seems 

 clear, that, if such an apparent motion of the 

 surrounding objects depends, in any way, upon 

 their spectra, or the illusive representations of 

 those objects, occasioned by their former im- 

 pressions upon the retinas, no similar motion 

 would be observed, were we to turn ourselves 

 round with our eyes shut, and not to open them 

 till we became giddy ; for in this case, as the 

 surrounding objects could not send their pic- 

 tures to the retinas, there would, consequently, 

 be no spectra to present themselves afterward 

 in notation. But whoever will make the expe- 

 riment, will find, that objects about him appear 

 to be equally in motion, when he has become 

 giddy by turning 'himself round, whether this 

 has been done with his eyes open or shut. I 

 shall now venture to propose my own opinion 

 upon this subject. 



If the eye be at rest, we judge an object to 

 be in motion when its picture falls in succeed- 

 ing times upon different parts of the retina ; 

 and if the eye be in motion, we judge an object 

 to be at rest, as long as the change in the place 



