110 EXPERIMENTS, &c. 



inches, my vision of it by the left eye is in- 

 distinct, from the rays of light tending to fo- 

 cuses behind the retina ; secondly, that my vision 

 by the same eye is perfect, if the object seen, 

 and to which both axes are turned, be from 

 about seventeen to about nineteen inches di- 

 stant ; thirdly, that the vision of my left eye 

 becomes again imperfect, if the object be moved 

 to a greater distance than that of nineteen 

 inches, the rays being now collected to focuses, 

 previously to their falling upon the retina ; and 

 fourthly, that I have, by my right eye, imperfect 

 vision of all objects, to which I direct both axes, 

 unless their distances be so great, ' that the rays 

 of each pencil, proceeding from them, may be 

 regarded as parallel. , 



A conclusion is furnished by these experi- 

 ments, similar to one, which was drawn by Mr. 

 Delahire *, from some made by himself; namely, 

 that each eye sees objects distinctly only at one 

 distance ; as I take for granted, that, in every 

 case of ordinary vision, both axes are directed 

 to the object which is viewed. But Mr. Dela- 

 hire drew a second conclusion from his experi- 

 ments, which he seems to have regarded only as 

 another expression of the first, but which, in 

 truth, includes a very different fact. It was, 



* Memoires de Mathematiquc et de Physique, 4to. p. 2Q8. 



