The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-30
How to Fit Spect acles.

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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 is regarded as the most vua-
BILE result derived from the recent advances in oph-
thalmic science.
Structure of the Eye.
The eye is simply optical nstru nent, COM
posed of an object glass the cornea) an eye glass
1.-THE crystalline ieas) and screen 1.-THE retina),
which is the organ OF vision. In order that any
object may be distinctly visible it is necessary that
perfect pic ture of it should be formed upon the
1etina, which is simply mirror covering the back
part of the eye. The form and relative position of
the cornea and crystal line like the form and rea-
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 spectaeles. These auxiliary lenses should cor
respond with mathematical accuracy to the defects
of the lenses of the eye, and TO insure absolute
ly perfect fit, it is only necessary to devise some
means OF measuring the reractive power of the eye.
Ncnr 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 the accommodative action of
fractve according to the direction in which rays OF
light enter the eye-whether parallel or divergent
When presb5opia comes on, which results in what
is called "iar-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 nayopic or near-sighted eye the
elongation of the ball and the high refractve power
of the coInea bring the rays to focus in front of
the retna. Both of these results are holly ineo-
patible with distinct vision, which is only produced
when the rays are brought to focus the retna.
The object sought then i'll fitting glasses, whether
for near or tar slghtednes is to bring the rays to
focus exactly on the retna without calling into ex-
ercise the accommodatve action of the crystalline.
WHY Accuracy is Required.
1t is important that when the eye is being used,
the crystalline sh uld Iemain in a passive state.
If an effort is made to reld with glasses too strong
or too weak, a constant tension is kept uo on the
ciliary muscles which operate the crystallin This
not only prevents the fall development OF the sight,
but it hastans the deterioration of the eye aud me-
cessitates gradual increase in the strength of the
glasses.
Mathematics! Formula.
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 Snelen's test
types, subendins at different distances an angle of
one minute, can be read. Prof. Donders proposes
that the point of distinct vision thus
ed shall be designated by p and the tar point
by q. Representing the adaptive power by,
its value in any case be determined by the for
mua,
T. qI0
4 Pr aitaa
If therefore we have normal eve able to see dis
tincty from four inches a, its point p, to in
finite distance { y, its tar point r, we have
- -, since !
Should the eye be myopc, having its far point at
eight inches (s) and its near point at four ~ we
should find
Hata a s
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 formua . .lt.u. .'. .r II.
.a a p n
n-p
The results thus obtained represent the focus Of
lens which, il placed upon the crystalline would
give to rays coming from the near point direction
as If 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 as the starting pont presbyopa and the
observed power of the eye we express by Isl a
We have then the formula si.
pr -
Now If by testing an eye we find its point at
eleven inches, we have I'll, or -. The
value of Pr then will be ~ II
Pr - . @
8 IL POI
which simply means that 29.1-nch glass is reqnired
to ueuralize the presbyopia at inches.
This method is here exhibited, because it is simple
and carries its proof with it. is by no means the
whoeo the process, but enough is shown to estab-
Ish its soundness. 1t discovers and correcs anom-
alties of vision and the sight by artificial
means to its normal power.
The undersigned will fit glasses according to this
method, without extra charge.
Spectacles and Eye Glasses, OF every variety and
price, for sale by as. .l a.a ...
Hill @. III. FARLEY, ue
scp3df.n Zw2w No. a Exchange ST.