.EXPERIMENTS AND OBSERVATIONS 



ON 



SEVERAL SUBJECTS IN OPTICS. 



ARTICLE I. 



On Visible Position, and Visible Motion. 



IN the estimates we make by sight of the 

 situation of external objects, we have always 

 some secret reference to the position of our 

 own bodies, with respect to the plane of the 

 horizon ; and from this cause, we often judge 

 such to be at rest, whose relative places to us 

 are continually changing ; and others to be in 

 motion, though they may constantly preserve, 

 in regard to us, the same distance and direc- 

 tion. To give an instance, let us suppose our 

 eyes first directed to a star near to the horizon ; 

 should we afterward, by a mere motion of the 

 head, point them to another, some degrees 

 above the former, this second star will appear 

 higher than the first did. Were we now, while 

 the eyes are kept fixed in relation to the head, 



