when deprived of the Crystalline Lens . 5 
with, as arising from the smallness of the aperture, and not from 
any power of adjustment in the iye. Dr. Young, therefore, 
with a view to obviate all possibility of deception in future, 
constructed an optometer, upon the principle of that of Dr. 
Porterfield. In this instrument, when applied to presbyopic 
eyes, the eye, by looking along a line through a small convex 
lens, before which is placed a card with two narrow slits in it, 
near enough to each other to be within the limits of the pupil, 
will see the line as two lines, crossing each other at the point 
of perfect vision ; and every eye that has the power of adjust- 
ment, will make the lines cross in different places, when adjusted 
to different distances. 
With this instrument, Dr. Young made experiments upon 
several eyes which had been deprived of the crystalline lens; 
and with all of them found, that the crossing of the lines was 
seen only at one point ; he therefore concludes, that the power 
of adjustment was lost. 
These experiments of Dr. Young led me to reconsider the 
subject; and it was matter of regret that Benjamin Clerk 
was not in this country, as making a trial with the optometer 
on his eye, would have determined, in the most satisfactory 
manner, whether there had been a fallacy in the former expe- 
riments. 
This not being in my power, I made inquiry after Henry 
Miles, upon whom the second experiments were tried ; and I 
had the pleasure to hear, that he was in good health, and that 
his eyes continued to have very distinct vision, so much so, that 
he never had occasion to make use of any glasses, from the time 
the operation had been performed. 
With the view of making some experiments on this man’s eyes, 
with Dr. Young’s optometer, I procured that instrument from 
