The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-28
How to Fit Spect tacles.

< Ss
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 ex-
tensive APPLICATION. it s regarded as the most yalua
BILE result derived from the r,cent advances in oph-
thamc science.
structure Of IN. Eye.
The eye is simply optical hstru nent. COM
posed OF an object glass the coTna), an eye glass
(the crystalline lesas) and screen 1.-THE yetina).
which is the organ of vision. Ia or,cr that any
object may be distinctly visible, it is ssarv that
perfect pic ture of it should be formed mon the
retind, which is simply mirror covering r.e back
part of the eye. The form and relative pesiion of
the cornea and crystalline, like the form ard ,ea-
tive position of the object glass and eye glass u
telescope, may be mathematicay determined, aid
their defects 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 an absolute
ly perfect fit, it is only necessary to devise some
means of measuring the reractive 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-
complshed through the accommodative action of
the crystalline lens which becomes more or less re-
fractive according 10 the direction in which rays of
light enter the eye-whether parallel divergent.
When presbJopia comes on, which results in what
is called 'far-sighedness," the crystalline becomes
incapable of uniting divergent rays upon the retina
and they would, if continued, meet at point be-
hind it. In the myopic or near-sghted eye the
eongation of the ball and the high refractive power
of the cornea bring the rays to a focus in front of
the retina. Both of these results are wholly ineom-
patibe with distinct vision, which is only produced
when the rays are brought to focus on the retina
The object sought then in fitting glasses, whether
for near or far slghedness, is to bring the rays to
focus exactly on the retna, without calling into ex-
ercse the accommodatve action Of the cry stalline.
Why Accuracy is Required.
1t is important that when the eye is being used,
the crystalline should remain in passive state.
If an effort is made to read with glasses too strong
or too weak, constant tension is kept u o on the
~liarY muscles which operate the crystalline. This
iio1 only preve-ts the full development OF the sight
out hastens the deterIorattan of the eye, and
cessitates gradual increase in the strength of tbe
glasses.
Mathematical FormuIn.
The rules for accomplishing this object were first
devised by Professor Donders. of Utrecht, whose
method is the only one now in use 1t is necessary,
to ascertain first at what distances Snellen's test
types, subteudng at different distances an angle of
one minute, can be read. Prof. Donders PROPOSES
that the point of distinct vision thus ascertlin-
ed shall be designated by p and the far point
by II. Representing the adaptive power by.
its value in any case can be determined by the for
mula,
-
Il
If therefore we have norml eve able to see dis
tincty from four inches A. its near point p to in-
finite distance t"l its far point r, we have
| - since 80 .
Should the eye be myopic, having its far point at
eight inches (s) and its point at four (4, we
should find
4 8
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:
Il p
| RP .
The results thus obtained represent the focus Of
lens which, if placed upon the crystalline would
give to rays coming from the near point direction
| as il 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 po mt of presbyopia and the
| observed power of the eye We express by
We have then the formula:
| Pr
Now # by testing eye we find its near point a
eleven inches, we have n III, -. The
| value of Pr then will be II
| 1
| Pr
| IL POI
which simply means that 90 1-inch glass is r,qnirc
to ueutralize the presbyopia at inches.
This method is here exhibited, because it is simp,
and carries its proof with it. 1t is by no means tn
whole of the process, but enough is shown to cstat
lsh its soundness. ltdiscovers and correets anon
alties of vision and restores the sight by arthcis
means to its normal power. ...
The undersigned will fit glasses according to thi
method, without extra charge.
Speetaces and Eye Glasses OF every variety an
price, for sale by
| q. IL. FARLEY,
sepSdlmsw2w NO Exchange ST
Maine Savings Bank
No. 100 Middle Street.