108 EXPERIMENTS, &c. 



experiments I have met with in books, which 

 have been made, with any tolerable show of ac- 

 curacy, to determine this matter, are those of 

 Dr. Porterfield. I shall not here say what 

 they are, as his Treatise is in every body's 

 hands, but shall only mention the principal 

 conclusions which he drew from them ; .first, 

 that objects could be distinctly seen by him, 

 that is, the pencils of rays which came from 

 them could be accurately collected to points 

 upon the retina, when their distances from his 

 eye did not exceed twenty-seven inches, and 

 were not less than seven ; and secondly, that, 

 as often as the axes of both eyes were directed 

 to any one point, situated within those distances, 

 the rays proceeding from it had their focus in 

 each retina. 



As the results of some experiments, which I 

 have made upon the same subject, differ from 

 these conclusions of Dr. Porterfield, I have read 

 over what he has written upon the matter with 

 more than ordinary attention, and I think I can 

 thence show reason, why they should not be 

 received without caution. For, in the first 

 place, his experiments are related so circum- 

 stantially, and with such an appearance of ac- 

 curacy in the making of them, that you would 

 scarcely suppose he left the least possible room 

 for error. And yet, after finishing his account 



