6 Mr. Home’s Lecture on the Rower of the Eye , 
Mr. Cary, the optician, made exactly in - the same manner as 
that which had been executed under Dr. Young’s direction. I 
first, however, tried the experiments upon my own eye ; but had 
the mortification to find myself unable to make the lines cross 
in two different situations. This led me to try the eyes of 
several of my friends ; who were equally unable to make the 
lines cross any where, except at one point. Young people, 
indeed all those under thirty years of age, were capable of vary- 
ing the place of intersection ; but none who were above forty, 
could produce any change in it. 
As I could not doubt of my own eye having the power of 
varying its adjustment, I was led to believe that the instrument 
required some address in the management, which I had not 
acquired; and therefore despaired of making Henry Miles 
'Sufficiently master of it, to do justice to my views. 
To obviate these difficulties, I adapted the optometer, without 
the lens, to presbyopic eyes, by making a line 4 feet long, 
upon strong paper, divided into inches, and having the same 
slits to look through as in the other. This instrument, and 
Dr. Young’s, I put into the hands of my friend Sir Henry 
Englefield, with a request that he would examine them, and, 
when he had become perfectly master of them, and of the best 
mode of using them, that he would assist me in making expe- 
riments with them ; for, as he was more in the habit of chang- 
ing the focus of his eye, in using optical instruments, he would 
more readily detect the circumstance which prevented me from 
succeeding in the experiment. 
After several trials with this optometer, and seeing its de- 
fects, Sir Henry Englefield improved it, by having the 
paper pasted upon a strong board, 4 feet long, which rendered 
the surface free from the slightest inequalities ; and, instead of 
