The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-25
How to Fit. Spectacles.

| ~ ~ o " ssS<-eezv1c.-H
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 appcation.isis regarded as the most valua
BILE result derived from the recent advances in oph
| thalmic science. ....a--.y rti,g .....ti
- fs Structure of the Eye. II
The eye is simply an optical instru ent, COM
posed of an object glass the cornea) an eye glass
1.-THE crystalline leas) and screen 1.-THE reti,aa),
which is the organ of vision. Iu order that any
object may be distinctly visible it is necessary that
perfect pic ture of 1t should be formed upon the
Tetsnd, 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 defects may be remeded by auxiliary lenses,
or spectacles. These auxiliary lenses should cor
respond with mathematical accuracy to the defects
OF tile lenses of the eye and to insure an absolute
ly perfect fit, 1t is only necessary to devise some
means of measuring the reractive power of the eye
| Nenr md Far Sight,
When in its normal condition, the eye is capable OF
seeing near we'll as remote objects. This is ac-
complshed through the accommodative action of
the crystalline lens, which becomes more or less re-
ractive according 10 the direction in which rays OF
light enter the eye-whether parallel or divergent.
| When prestyopio comes on, which results in what
called "far-sightedness, the crystalline becomes
Incapable of uniting divergent ray upon the retna
| and they would, if continued, meet at a point be-
hind it. In the myopic or near.sighted eye the
eongatOn Of the ball and the high refractive power
of the cornea bring the rays to focus in front of
the retina. Both Of these results wholly ineom-
patibe with distinct vision, which is only produced
when the rays are brought to a focus ON the retna
The object sought then in fitting glasses, whether
for near or far sghedness, is to bring the rays to
focus exactly the retina without calling into ex-
ercise the accommodative actcn Of the cry staline
Why Accuracy is Required.
It's important that when the eye is being used,
the crystalline should remain in q passive state
II an effort is made to read with glasses too strong
or too weak, constant tension is kept u o on the
ciliary muscles which operate the crystallin 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.
|9sh .IIE Mathematical Formulse. fTE
The rules for accomplishing this object were first
devised by Professor Donders, of Utrecht, whose
method Is the only one in use. 1t is necessary,
to ascertain frst at what distances Snellen's test
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 y,
its value in any can be determined by the for
| mula, # tg42 2EEE5E,E WfE
1 -
su oj ao...2eY A. p 71R e.p ps79oHh
II therefore We have normal eye able to see dis
tinctly from four inches A. its near point p, to in
finite distance % its far point r, we have
b 1-aqr11 .s1 1 zssyavs 4 .,-.4. III
sod' - - since -.=oapH
Should the eye be myoric, having Its far point at
eight inches B) and its point at four (4l we
i1i istd Il 4qi 1iri II 4 essk 4iele
| bag ais ~ -a=-- -=- eiuo hsr
u1m sif.t EaH p s5sHs sa4zua 5saaa
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 equiva
| lent formula:
| 1 4 1g1i .lisdr
| + p-R
| or .b9su8Eg EsaIe fHs
i.ii ifi sai ,isl,3l1 laag r = p til.t.1. .r
| s,..s4..- II RP
The results thus obtained represent the focus Of
lens which, il placed upon the crystalline would
give to rays coming from the point direction
as ii coming from the far point. -s FT
For determining the degree of presbyopia we rep
resent the assumed normal power when accom-
arl-
modated for near objects by -, eight inches being
| F4FEftE gf4H8tg
regarded as tine starting point of presbyopa and the
observed power of the eye VIC express by
-. We have then the formula: 4 .i
n = i-,saAslA sti >. l.f t...- ra"t-
| ajtf-a Pra---s.< a} 4.,s-.ss II
Now If by testing eye VIC find its near point at
eleven inches, we have III, or =. The
value Of Pr then will be N II
| st f-.1' $685.. Pr = -,
| %. -Hir'fitiff it.' II i'H pol
which simply means that 91-inch glass is reqnired
to neutralize the presbyopia at inches.
This method is here exhibited because it is simple
and carries its proof with it. 1t is by means the
whole of the process but enough is shown to estab-
lsh Its soundness. Itdiscovers and corrects anom-
aties of vision and restores the sight by artificial
means TO Its normal power. .t ...........t ...
The undersigned will fit glasses according to this
method without extra charge.
Spectaces and Eye Glasses, OF every variety and
price, for sale by a., .,..t t.1...
| ~, C.H. FARLEY,
sep3d1mG w2w No. Exchange ST.
b +7of3RS