NKAR AND DISTANT SIGHT. 
2sr 
that persons wlio have had the crystalline humour removed, 
have had less power to ascertain the distance of an obj;.ct when 
they look through a conveK glass, than when they view it 
without this assistance ; in consequence of which such persons 
seldom make use of glasses when they are w’alking : and the 
inconvenience of glasses is particularly experienced w hen they 
• descend a flight of steps, or pass over uneven ground. 
Near sighted persons do not appear to possess the same Near-sif;litcd 
lexteut of vision that is enjoyed by those who have a distant 
' to liave a U.s* 
sight. Being near sighted, I have repeatedly endeavoured to ranee of focal 
.. c ■ ■ •• ailjustuicut. 
ascertain my own range of vision ; and I find, by examining 
the focus of my right eye through the abovementioned opto- 
I meter, that I see two converging lines, which appear to meat, 
'with very slight variations, at the distance of three inches from 
I the eye; and no effort I am able to make can keep these 
dines united further than the distance of four inches and a 
• quarter. They then separate, and continue to diverge. With 
I my left eye, the lines do not appear to meet nearer than four 
I inches, and they continue united as far as five inches and a 
quarter, after which they also separate and diverge ; so tliat 
(he range of distinct vision in me does not extend further than 
i an inch and a quarter in either eye; and within these dis- 
Itances I always hold a book when I read.— I find also the 
I following rule, for determining the concavity of the glass 
(that is best adapted for near sighted persons, to be perfectly 
• correct with respect to myself, and, I believe, it may be safely' 
I adopted by those who, from distance or any other cause, are 
unable to suit themselves at the shop of an expert optician. 
’The rule is this. Multiply the distance at which the person to 
ireads with ease, (which, with my left or best eye, is five in- 
ot spectacles 
.ches,) by that at wh ich he wishes to read, which may be said to for near-sighted 
Ibe twelve inches ; divide the product, sixty, by seven, the diflFer- 
lence betw’een the two, and it leaves nearly nine inches for the 
ifocus of the concave glass that shall produce the desired effect. 
'I'his is the exact concavity of the glass that I am obliged to 
I use, to enable me to read with ease; and it answers to that 
liold under the name of No. 0 ; which, I am informed by Mr, 
Blunt 
