The Portland daily press., 1869-09-29
How to Fit Spant2nlnn-


In all the recent works on the eye, the subject of
accommodation, or the fitting of glasses, receives
the largest share of attention. Considering its
tensve application it regarded as the most valua
BILE result derived from the recent advances in oph-
thalmic science.
structure of the Eye.
The eye is simply optical nstru ncnt, COM
posed OF an object glass the cormeal an eye glass
Qhe crystalline ieas) and screen (the retiaa)
which IS the organ OF vision. I order that any
object may be distinctly visible, it is necessary that
perfect pic ture of it should be formed upon the
retina, which is simply mirror covering the back
part OF the eye. The form and relative position OF
the cornea and crystalline like the form and rela-
tive position of the object glass and eye glass of
telescope may be mathematicaly y determined, and
their defects may be remedied by auxiliary lenses,
or spectacles. These auxiliary lenses should cor
respond wi h mathematical accuracy to the defects
OF the lenses OF the eye and to insure an absolute
ly perfect fit, it is only necessary to devise some
means of measuring the reractve power of the eye
Near and Far sight,
When in its normal condition, the eye is capable OF
seeing near as well as remote objects. This is ac-
complished through tue accommouative action of
the crystalline lens which becomes or less re-
lractive according TO The direction in which rays OF
light enter the eJe-whether parallel divergent.
When presd5o opio comes on, which results in what
is called "iar-sightedness,' the crystalline becomes
incapable of uniting divergent RAYS upon the retina
and they would, if continued, meet at point be-
hind It. IN the aagopic or near-sighted eye the
elongation of the bali ana the gh refrattYe power
of the cornea bring the rays to focus in front OF
the retna. Both of these results are wholly neom-
patible with distinct vsion which is only produced
when the rays brought to focus on the retina
The object sought then i'll fitting glasses, whether
for near or far slghtedness is to bring the rays to
focus exactly on the retna, without calling intoex-
ercise the accommodatve action of the crystalline
Why Accuracy is Required.
1t is important that when the eve is being used.
the crystalline should remain in passive state.
Ii an effort is made to read with glasses too strong
UT 700 ~eak' -Hanstant tension is kept u on the
cillary muscles which operate cryatallin This
not only prevents the full development OF the sight,
but it hastens the deterioration of the eye and ne
cessates gradual increase in the strength of the
| glasses.
Mathematical Formulm.
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 Snelen's test
types, subtending at different distances an angle of
one minute, can be read. Prof. Donders proposes
that the near point of distinct vision thus ascertain
| ed shall be designated by P and the far oint
| by II. Representing the adaptive power by,
| mula,
-
If therefore we have normal eve able to see dis
tnctly from four inches AZ its point P, to in
finite distance ''), its far point r, we have
1-4s-s-1' since 80 !.
Should the eye be myopic, having its far point at
eight inches (s) and its near point at four (4l we
| should find
A-4-s=
If however test the eye by lens of high power,
the whole range is then brought within the limits
few inches, and is readily obtained by the equiva-
lent formula:
a
R-P
RP
The results thus obtained represent the focus Of
a lens which, il placed upon the crystalline would
give to rays coming from the near point direction
as II coming from the far point.
For determining the degree of presbyopia we rep
resent the assumed normal power when accom-
m dated for near objects by :, eight inches being
|
regarded the starting pc of presbyopia and the
observed power of the eye we express by
We have then the formula:
| 8
Now If by testing an eye we find its point al
eleven inches, we have III, or - :. The
value OF Pr then will be II
| Pr
II pol
which simply means that 9.1-nch glass is reqnret
to neuralize the presbyopa at inches.
his method is here exhibited, because ii is simple
and carries its proof with it. Itis by no means the
whole OF the process, but enough is shown to estab
nsh lis SCUM mess. 1t discovers and corrects anon
alt1es of vision and the sight by artihcia
means to its normal power.
The undersigned will tit glasses ac ,orIipg to thi:
method, without extra charge.
Spectacles and Eye Glasses, OF every variety ant
price, for sale by
G. II. FARLEY,
sep3d1m & w2w No. A Exchange ST.
Maine SaVinUS Ronl