3122 
NEAR AND DISTANT SIGHT, 
Changes in the The alteration which has taken place in my range of vision, 
organ of sijit. ^ reason to believe, is not unusual. Dr. Wells, in his 
paper on this subject, mentions the case of a gentleman, who, 
like me, was near-sighted, and whose sight, as he advanced 
in life, had undergone a similar change. The following is also 
an instance of this kind that is still more remarkable. Mr. L., 
sixty- 
fifteen years of age I perceived that many persons saw distant objects 
better than myself ; but the difference being productive of no incon- 
venience, I did not use concaves till about six years afterwards, and 
have never used tliem at all but at the theatre, where 1 cannot see fea- 
tures without them, or when travelling in the country, or upon similar 
occasions. The same virtual focus has conslanlly suited me ; and 
now that my eye has lost its power of adjustment by age, it would 
be attended with pain to use either a shallower or deeper glass, lliis 
focus, ascertained by the distance at which the divergent light from 
the sun, after passing through the lens, and falling upon paper, forms 
a circle of twice the diameter of the lens itself, is about three feet j 
which is the shallowest number of the opticians but one. At all times, 
before the age of thirty, the habitual distance for reading with the 
naked eye was six inches, and, when employed upon minute subjects, 
four. Rather before fifty, it became necessary to use convex glasses, 
and at present, (sixty) vision at seven inches and a half distant is 
effected by the assistance of lenses of eleven inches focus. Distant 
vision by the naked eye requires a distance of tsvo feet. The time at 
which the power of focal adjustment was lost was not nolicedj but 
from varieus circumstances this must liavc happened soon after the age 
of fifty. No effort can now alter the distinctness of objects either 
way by the change of which the eye was formerly capable, though 
spectacles seem less necessary in the morniug than later. The power 
of inclining the optical axes of the eyes appears likewise to have un- 
dergone some change : for the separation of objects by squinting is un- 
comfortable if carried beyond tw'enty degrees. The act of squinting, 
however, is in itself uncomfortable with a single object. For if two 
objects, for example caudles, be thus made to afford three images by 
the coincidence of the two inner images, the eyes will acquire a str.te 
of repose, altlioiigh tlie axes be much more inclined. In this in:u:rier 
1 can produce the coincidence at sixty degrees, whether with distant 
candles, or by single vision of an object, at the distance of U‘6 inchei 
from the cyc.«, w hich is also the distance from pupil to pupil, when the 
axes are parallel.— \V. N. 
