52 AN ESSAY ON 



inclinations to each other in such a manner, that 

 the common axis, may, notwithstanding, remain 

 fixed, an object placed in either optic axis, and 

 following it in every motion, will possess but 

 one visible direction,, in all this variety of real 

 positions. That these conclusions from my 

 theory, or rather parts of it, are true in fact, 

 I can assert upon the authority of observations, 

 and I shall now attempt to trace them both to 

 a common principle, by means of some experi- 

 ments, which were instituted with a very dif- 

 ferent view. 



When we have looked steadily for some time 

 at the flame of a candle, or any other luminous 

 body, a coloured spot will appear upon every 

 object, to which we shortly after direct our 

 eyes, accompanying them in all their motions, 

 and exactly covering the point, which we desire 

 to see the most accurately. Whatever there- 

 fore can be proved concerning the apparent 

 direction of such a spot, in any given position 

 of the eyes, must likewise be true in the same 

 position of the eyes, with regard to the appa- 

 rent direction of an object, situated at the con- 

 currence of the optic axes ; as its pictures must 

 occupy, in this case, the very -parts of the re- 

 tinas, upon the affections of which the illusion of 

 the spot depends. This being premised, I shall 

 now relate one or two observations, respecting 



