on the Mechanism of the Eye. 6*5 
actual focal distance is totally unchangeable. This will appear 
from a selection of the most decisive observations. 
1. Mr. R. can read at four inches and at six only, with the 
same glass. He saw the double lines meeting at three inches, 
and always at the same point ; but the cornea was somewhat 
irregularly prominent, and his vision not very distinct ; nor had 
I, at the time I saw him, a convenient apparatus. 
I afterwards provided a small optometer, with a lens of less 
than two inches focus, adding a series of letters, not in alpha- 
betical order, and projected into such a form as to be most legi- 
ble at a small inclination. The excess of the magnifying power 
had the advantage of making the lines more divergent, and 
their crossing more conspicuous ; and the letters served for 
more readily naming the distance of the intersection, and, at 
the same time, for judging of the extent of the power of distin- 
guishing objects too near or too remote for perfect vision. 
(Plate V. Fig. 23.) 
2. Mr. J. had not an eye very proper for the experiment ; 
but he appeared to distinguish the letters at 2} inches, and 
at less than an inch. This at first persuaded me, that he 
must have a power of changing the focal distance : but I after- 
wards recollected that he had withdrawn his eye considerably, 
to look at the nearer letters, and had also partly closed his 
eyelids, no doubt contracting at the same time the aperture of 
the pupil ; an action which, even in a perfect eye, always ac- 
companies the change of focus. The slider was not applied. 
3. Miss H. a young lady of about twenty, had a very narrow 
pupil, and I had not an opportunity of trying the small opto- 
meter : but, when she once saw an object double through the 
slits, no exertion could make it appear single at the same dis- 
MDCCCI. K 
