and distant Sight of different Persons, 4^ 



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 

 reads with ease, (which, with my left or best eye, is five in- 

 ches,) by that at which he wishes to read, which may be said to 

 be twelve inches; divide the product, sixty, by seven, the differ- 

 ence between the two, and it leaves 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 distinguish 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, my 

 eye has varied from what it was a few years ago, when I was 

 able to distinguish both near and distant objects correctly, 

 through No. 8. This is ground to a radius of eight inches on 

 one side, and six inches on the other, and with it I can still 

 read a type like that in which the Transactions of the Royal 

 Society are printed ; but am unable to distinguish through it 

 many distant objects, which I formerly used to see distinctly. 

 — Hence it appears that my eyes have a confined range of 

 distinct vision, extending only to an inch, or an inch and a 

 quarter; and that they remain nearly in the same state in 

 which they were many years ago with regard to near ob- 

 jects, but have lost a part of the power which they formerly 



