326 
KEAR AND DISTANT SIGHT, 
known, yet sometimes it has been induced by the use of eva- 
cuating remedies, particularly of leaches applied to the temples, 
and sometimes by looking through a microscope, for a conti- 
nued length of time in several successive days. 
Fourthly ; instances are not uncommon, in which persons, 
far advanced in life, (viz. between eighty and ninety, whose 
eyes have been accustomed for a long time to the use <}£ 
deeply convex glasses, when they have read or written, have 
ceased to derive benefit from these glasses, and they haye 
become able, without any assistance, to see both near and 
distant objects almost as well as when they were young. 
Although it be not easy to ascertain the cause of this amended 
vision, it seems not improbable, that it is occasioned by an ab- 
✓ 
sorption of part of the vitreous humour, in consequence of 
•which the sides of the eye collapse, and its axis, /lom the 
cornea to the retina, is lengthened, by which alteration the 
length of this axis is brought into the same proportion to the 
flattened state of the cornea or crystalline, or both, which it 
had to these parts before the alteration took place. 
Additional 
In the same volume of the Transactions, Sir Charles Blagden 
facts respect- ^3* given an appendix to this paper. He was not near-sighted 
ing tlie jji yj. years of age, when he learned to read, but was 
changes which / b » 
take place in perceptibly so at nine or ten. He did not become uncom- 
the eve l)v Sir 
Charles’ Blag- ^o^'f^^ly near-sighted till beyond thirty, when the number two 
or three of the opticians suited his eyes. Since that time, 
during a few more years, the near-sightedness increased till he 
was obliged to use No. 5 ; and at 'this point his vision has 
Other obser stationary between fifteen and twenty years. Sir 
vatioiis on the Charles also mentions an observation on the vertical fluting of 
distance \n* a marble chimney-piece, which, in the judgment derived from 
near objects habit, seemed more remote when the eyes were made to pro- 
duce a coincidence of different flutings by widening the angle 
of* direction of the optical axes. This circumstance, combined 
with others, mentioned in Priestley's Optics, and elsewhere, 
shews 
from the posi- 
tion of the 
optical axes. 
