The Portland daily press., 1869-09-08
How to Fit Spectacles.

90
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-
tensve application it is regarded as the most valua-
bic result derived from the recent advances in oph-
thalmic science.
Structure of the Eye.
The eye is simply an optical instru n ent, COM
posed of an object glass 1.-THE cormea) an eve glass
Uhe crystalline iens) and screen 1.-THE ret;ns,.
winch IS the organ of vision. II order that any
OBJECT may be distinctly visible it is necessary that
aperfect pic ture of it should be formed upon the
Tet'nd, 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-
1--WE position OF the object glass ard eye glass of
telescope, may be mathematicalIy 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 an absolute
iy perfect fit, it is only necessary to devise some
means OF measuring the rcfraotue power of tho oxe
Near and Far Sight,
Wheu in Its normal condition, the eye is capable of
seeing near as we'll remote objects. This is ac
compiished through the accommodative action of
the crystalline lens, which becomes more or less re-
ractve according 10 the direction in which ravs of
light enter the eye-whether parallel or divergent
when pres65opia comes on, which results in what
is called -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 myopic or near-sighted eye the
elongation of the ball and the high refractve power
OF the cornea bring the rays to focus in front of
the retna. Both of these results are wholly incom-
pable with distinct vision, which is only produced
when the rays are brought to focus on the retina
The object sought then i'll fitting glasses, whether
for near or far sIghedness, is to bring the rays to
focus exactly on the retna, without calling into ex
ercse the accommodative action of the cry stalIne
Why Accuracy is Required.
IL Is important that when the eye is being used
the crystalline should 1emain in a passive state
If an effort is made to read with glasses too strong
or too weak, constant tension is kept u on the
ciary muscles which operate the crystalline This
not only prevents the full DEVELOPMENT of the s,bt
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 Snellens 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 snail be designated by p and the far oint |
by r. Representing the adaptive power by.
Il |
its value in any case can be determined by the for
mula,
|
Ii therefore we have normal eve able to see dis
fincty from four inches AZ its near point p to in
finite distance t'), its far point r, we have
/
a., since 30
90
Should the eye be myopic, having its far point at
eight inches s) and its point at four az we
should find |
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-
lent formula:
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 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 point of presbyopa and the
observed power of the eye VIC express by
-. We have then the formula:
Pr=--
Now If by testing an eye find It's near point at |
eleven inches, we have III, or = =. The |
value OF Pr then will be ~ II
Pr
IL 291
which simply means that 1-inch glass is reonired
70 neutralize the presbyopia at inches
This method is here exhibited because it is simple
md carries its proof with it. 1t is by no means the
whole of the process, but enough is shown to estab
lsh its soundness. 1t discovers and corrects anom-
ltes of vision and restores 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
G. ii. FARLEY,
,mSw2w No A Exchange st.
MISS U0~ES