7 
when deprived of the Crystalline Lens. 
a line marked with ink, a thread of black silk was stretched 
along the middle of the board. With this instrument, he found 
that his eye could make the lines cross at two different points, 
at several inches distance from each other. The readiest mode 
of making the experiment succeed, was first fixing his eye 
upon some near object, held above and a little on one side of 
the silk thread, and, when the focus of his eye was adapted 
to that distance, then to look at the thread ; afterwards to look 
at some distant object, and, when that had become very dis- 
tinct, again to look at the thread. Upon trying the instru- 
ment with my own eye, in this way, I found the crossing of 
the lines changed its situation, with every change of adjust- 
ment ; and, after being accustomed to make this experiment, I 
was enabled to produce a similar change in the optometer with 
the lens, but by no means in so satisfactory a manner, nor did 
it last more than an instant ; my eye probably not being so well 
fitted as many others, for experiments of this kind. 
The optometer without the lens was hence admitted to be 
the most easily managed, by the eye of a person unaccustomed 
to such experiments, and therefore it was determined to make 
use of it in the trials upon Henry Miles's eye ; which we were 
enabled to do, as his vision was sufficiently distinct without the 
aid of glasses, and as, from never having used them, he saw 
much better with his naked eye. 
The following experiments were made with the optometer 
without the lens, on the 27th of August, 1801. 
The first trials were upon Sir Henry Englefield's eye ; 
which, being most familiar with the use of the instrument, be- 
came a standard with which the others might be compared. 
Sir Henry Englefield’s eye made the lines to intersect 
