The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-10-01
How to Fit Spectacles.

I @/
In all the recent works on the eye, the subject of
accommodation, the fitting of glasses, receives
the largest share of attention. Considering its ex-
tensve application, it is regarded as the most valua
BILE result derived from the recent advances in oph
thalmfc science.
Structure ef tf,e Eye.
The eye is simply an optical instru nent, COM
posed of an object glass 1.-THE cormea), eye glass
uhe crystalline leas) and a screen (the retina(
which is the organ of vision. Ia order that any
object may be distinctly visible it is necessary that
a 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 mathematically determined, and
their detects may be remedied by auxiliary lenses,
or spectacles. These auxiliary lenses should cor
respond with mathematical accuracy to the defects
of the lenses of the eye, and to insure absolute
ly perfect tit it is only necessary to devise some
means OF measuring the reiractive power of the eye.
Near and Far Sight,
When in its normal condition. the eye is capable OF
seeing near as we'll as remote objects. This is ac-
complished through the aceommodative action of
the crystalline lens, which becomes less
fractive according TO the direction in which rays of
light enter the eye-whether parallel or divergent.
When preSbyopiG comes on, which results in what
is called "ar-sightedness the crystalline becomes
incapable of uniting divergent rays upon the retna
and they would, if continued, meet at point be-
hind it. In the myopic or near-sighted eye the
elongation of the ball and the high refraetve power
of the cornea bring the rays to focus in front of
the retina. Both of these results wholly incom
patible with distinct vision, which is only produced
when the rays brought to a focus on the retina.
The object sought then in fitting glasses whether
for near or far slghtedness is to bring the rays to
focus exactly on the retna, without calling into
ercse the accommodatye action of the crystalline.
why Accuracy is Required.
II is important that when the eve is being used,
the crystalline should lemain in passive state.
If an effort is made to read with glasses too strong
or too weak, q constant tension is kept u on the
ciary muscles which operate the crystalline. This
not only prevents the full development of the sight,
but it hastens the deterioration of the eye and ne
cessitates gradual increase in the strength of the
glasses.
Mathematical Formula.
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 Sneen's test
types subending at different distances an angle of
one minute, can be read. Prof. Donders proposes
that the point of distinct vision thus ascertain
ed shall be designated by p and the far point
by p. Representing the adaptive power by,
4
its value in any case can be determined by the for
mua,
PR
If therefore we have normal eye able to see dis
tinctly from four inches az its near point p, 10 in
finite distance 10y, its far point r, we have
1=4-.=4' since %
Should the eye be myopic having its far point at
eight inches 49, and its near point at four (4l we
I 1
If however we test the eye by lens of high power
the whole range is then brought within the limits of
few inches, and is readily obtained by the equva-
1p
OF
R-P
A RIP
The results thus obtained represent the focus Of
a lens which, il placed upon the crystalline would
give to rays coming from the point direction
as il coming from the far point.
For determining the degree Of presbyopia we rep
resent the assumed normal power when accom-
m5dated for near objects by =, eight inches being
regarded the starting point of presbyopa, and the
| observed power of the eye we express by
| We have then the formula:
Now If by testing an eye we find its near point al
eleven inches, VIC have III, or =. The
value of Pr then will be n II
Pr
| 8 II POI
which simply means that 291ineh glass is 1tqnre'
| to neutralize the presbyopia at inches.
This method is here exhibited, because it is simple
and carries its proof with it. 1t is by tht
W1oleOI Tue PROCESS, out enough Is SHOWN 10 estdp
| lsh It's soundness. Itdiscovers and corrects anon
alies of vision and restores the sight by artificial
| means to its normal power. -r.s
The undersigned will lit glasses according to th
method, without extra charge.
Spectacles and Eye Glasses, OF every variety am
price, for sale by
g. ii. FARLEY,
sep3dlan&w2sy No. Exchange ST.
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