the near and distant Sight of different Persons. 339, 
sons who have had the crystalline humour removed, have 
less power to ascertain the distance of an object when they 
look through a convex glass, than when they view it with- 
out this assistance ; in consequence of which such persons 
seldom make use of glasses when they are walking: and 
the inconvenience of glasses is particularly experienced when 
they descend a flight of steps, or pass over uneven ground. 
Near-sighted persons do not appear to possess the same 
extent of vision that is enjoyed by those who have a distant 
sight. Being near-sighted, I have repeatedly endeavoured 
to ascertain my own range of vision: and I find, by ex- 
amining the focus of my right eye through the abovemen- 
tioned optometer, that I see two converging lines, which 
appear to meet, with very slight variations, at the distance 
of three inches from the eye; and no effort I am able to 
make can keep these lines united further than the distance 
of four inches and a quarter. They then separate, and 
continue to diverge. With my left eye, the lines do not 
appear to meet nearer than four inches, and they continue 
united as far as five inches and a quarter, after which they 
alse. separate and diverge; so that the range of distinct 
vision in me does not extend further than an inch and a 
quarter in either eye; and within these distances | always 
hold a book when I read.—I find also the following rule, 
for determining the concavity of the glass that 1s best 
adapted for near-sighted persons, to be perfectly correct 
with respect to myself, and, I believe, it may be safely 
adopted by those who, from distance or any other cause, 
are unable to suit themselves at the shop of an expert opti- 
cian. The rule isthis. Multiply the distance at which the 
person reads with ease, (which, with my left or best eye, 1s 
five inches,) by that at which he wishes to read, which may 
be said to be twelve inches; divide the product, sixty, by 
seven, the difference between the two, and it Jeaves nearly 
nine inches for the focus of the concave glass that shall 
produce the desired effect. This is the exact concavity of 
the glass that I am obliged to use, to enable me to read 
with ease ; and it answers to that sold under the name of 
No. 6; which, I am informed by Mr. Blunt the optician, 
is a double concave glass, ground on a tool of eight inches 
radius on one side, and eleven inches on the other, the mean 
between which is very nearly nine inches. With a glass 
of this description I can read the smallest print; but to di- 
stinguish distant objects I am obliged to look through that 
denominated No. 9, by opticians, which is ground on a 
tool of nine inches radius on both sides, In this respect, 
Y2 my 
