The Portland daily press., 1869-09-16
Near and Far sight,


When in IFS normal condition the eve is capable of
seeing near as well as remote Objects This is ac-
compllshed through the accommodaiive action of
the crystalline lens, which becomes more or less
Taetive according 10 the direction in which ravs of
light enter the eyc-whether parallel or divergent.
When pres65opad comes on, which results in what
Is called lar-sighedness, the crystalline becomes
incapable of uniting divergent rays upon the retua
ana they would, if continued meet at a point he
hind it, In the myopic or near-sighted eve the
eiongation of the ball and the high refracye power
OF the conea bring the rays to focus in front of
the retina Both of these results are wholly incom
patlDe with distinct vision, which is only produced
Wheu tne rays are brought to focus the retina.
ihe on)ect sought then i'll fitting glasses, whether
Tor near or tar sighedness is to bring the rays TO
FOCUS exactly on the retina without calling into ex
ercise the aceommodative action of the cry salline
- Why Accuracy is Required.
1t is important that when the eye is being used.
the crystalline should remain in passive state.
Ii an effort is made to read with glasses too strong
or too weak, constant tension is kept II o on the
ciiary muscles which operate the crystalline, This
not only prevents the full development of the sebt
but it hastens the deterioration of the eye and ne
cessitates gradual increase in the strength of the
glasses.
The rules for accomplishing this object were first
devised by Professor Donders of Utrecht. whose
method is the only now in use. 1t is necessary
to ascertain first at what distances Snellen's test
types, subtendng at different distances an angle of
one minute, can be read. Prof Donders PROMISES
that the near point of distinct vision thus ascertain
ed shall be designated by p and the far oint
by r.. Representing the adaptive power by,
its value in any can be determined by the for
mula, .E ,..fe.
-
Ii therefore we have normal eve able to see dis
tincty from four inches AZ its near point p to in
finite distance t"), its far point r, VIC have
-, since 80
N 4 %
Should the eye be myopic, having its far point at
eight inches Cs) and its point at four a. we
should find
-
f4lfle ilss
'.,fl3 8 amos
If however we test the eye by lens of high power,
the whole range is then brought within the limits of
a few inches, and Is readily obtained by the equiva-
lent formula:
u"''-''- Il
R-P
RIP
Inc results thus obtained represent the focus of
lens which, il placed upon the crystalline would
give to rays coming the near point n direction
as II coming from the far point.
For determining the deoree Of
resent the assumed normal power when accom-.
Undated for near objects by =, eight inches heinz
regarded as the starting point of presbyopia and tbe
observed power of the eye we express by
-. We have then the formula:
Pr
Now If by testing an eve we find its neor ,o;nt =,
eleven inches, we have III, orr Tho
value of Pr then will be :,
Pr
II POI
which simply means that 29. 1-nch glass is reonred
TO ueuiralize the presbyopia at inches
e ine1hod Is here exhibited, because ii is simple
anu carries Its proof with it, 1t is by no means the
wnoieo 'he process, but enough is shown TO estab-
hsh It's soundness. 1t discovers and correcs apom-
alties OF VISION and restores the sight by artificial
means to its normal power -
The undersigned will fit olossos ,,
method, without extra charge. u 'n''
Spectacles and Eye Glasses, OF every uo.:.... -,
price, for sale by '"''e'y anu
O. III. FARLEY.
sep'dim~wSw No. Exchange ST.
C7LoT HI