OBSERVATIONS AND EXPERIMENTS ON VISION. gg^ 



f^erving any change in my sight, beyond what I referred to 

 the increased size of the pupils; but as I had not looked 

 for any other, I thought it possible, that some additional 

 One might have happened, without my having perceived it. 

 1 resolved therefore to make the experiment anew. But to 

 contluct it with precision, it was previously necessary to 

 know, to what extent I possessed the faculty of adapting 

 my eyes to different distances. On this subject I had made State of the 

 many experiments with great care, nearly twenty years be- ^"*"°'^'* ^y^^' 

 fore, and had ascertained*, that with my left eye, which 

 was more perfect than the right, I could bring to single 

 points on the retina pencils of rays, which flowed froiA 

 every distance, greater than that of seven inches from the 

 cornea. In the mean time, however, my eyes had altered 

 considerably, with respect to their seing near objects dis* 

 tinctly, and I had, in consequence, been obliged, not only 

 to use convex glasses, but to change them several times for 

 others of higher power. No dependance therefore being Power of adapt- 

 now to be placed on my former experiments, in regard to diftancelost 

 the present state of my sight, I repeated them, and found, 

 to my great surprise, that the power I once possessed of 

 adapting my eyes to diiferent distances was entirely gone ; 

 in other words, that I was now obliged to regard all ob- 

 jects, whether near or remote, in the same refractive state 

 of those organs. I found also, that my eyeSy considered 

 as mere optical instruments, were nearly the same as they 

 had been in my youth, and that the convex glasses which I 

 nsed did very little more than supply, with respect to / 



near objects, the place of a living power which I had lost, 

 without compensating, except in a very small degree, for 

 any alteration in the external shape of the eye, or any 

 change in the configuration of its interior parts. I as- 

 certained, for instance, that to give my left eye the refrac- 

 tive power which it formerly possessed while in its most re- 

 laxed state, that by which it was enabled to bring a pencil' 

 of parallel rays to a point on the retina, a glass of thirty- 

 six inches focus was fully sufficient; whereas to produce aa 

 equal effect upon rays proceeding from a point at the dis- 



* Essay on Single Vision with two eyes, &c. p. 137. / 



Y 2 tance 



