50 AN ESSAY ON SINGLE VISION. 



whole chord was in the plane of the horopter, 

 but in every other, only that point of it to which 

 both eyes happened to be turned. A conclusion 

 from this experiment is, that no object, which 

 is truly perpendicular to the horizon, will ap- 

 pear to be so, while our bodies are erect, unless 

 we direct our eyes to a point in it exactly upon 

 a level with themselves. 



It was once my intention to subjoin here 

 several instances, from the most approved 

 authors, of inaccurate descriptions of the single 

 and double appearances of objects ; in order to 

 show, that the theory of visible direction, which 

 I have advanced, is not only consistent with the 

 universally received facts, but that it also dis- 

 covers to us, some minute errors, which un- 

 guided sense has committed upon this subject ; 

 it being, perhaps, one of the surest tests of the 

 soundness, as well as one of the chief uses, of 

 theories in philosophy, that they lead to the 

 knowledge of what, otherwise, might have re- 

 mained for ever hidden. But fearing I have 

 already proved tiresome, I give up this design, 

 and hasten to the consideration of some conse- 

 quences from my theory, which seem to me 

 both curious and important, and which, when 

 first mentioned, may appear to carry with them 

 their own refutation. 



