IN OPTICS. 109 



of them, he tells us, that he would have repeated 

 them with more care and exactness*, had he not 

 been interrupted. Secondly, his experiments 

 were made upon one eye only, though his con- 

 clusions apply to both eyes ; an inaccuracy 

 which gives occasion to suspect others. Lastly, 

 he says, that he could not see an object distinctly 

 at the distance of seven inches, unless both axes 

 were pointed to another object, at only half that 

 distance. Had he then directed both axes to 

 an object seven inches distant, which he does 

 not mention he ever did, it must consequently 

 have been seen indistinctly ; and yet one of his 

 conclusions states, that objects, distant from 

 about seven, to about twenty-seven inches, were 

 always distinctly seen, when the axes of both 

 eyes were directed to them. Such are the rea- 

 sons which lead me to think, that the whole of 

 the difference, between the results of the expe- 

 riments of Dr. Porterfield and myself, is not to 

 be attributed to a difference in the structure of 

 our eyes. 



The experiments, which I made upon this 

 subject, were with luminous points. They 

 proved to me,Jlrst, that, when both optic axes 

 are directed to any object, placed at a less 

 distance from my eyes than about seventeen 



Treatise on the Eye, Vol. I. p. 423.' 



