The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-23
~ r. r. Nc III.
How to Fit Spect7cles.

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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-
tsnsive application it is regarded as the most valua-
Bile result derived from the recent advances in oph-
thalmic science.
Structure of the Eye.
Inc eye is simply an optical instru nent, COM
posed OF an object glass (the cornea) an eye glass
1.-THE crystalline ieas) and screen 1.-THE retina),
winch Is the organ of vision. In order that any
object may be distinctly visible, it is necessary
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 ard eye glass of
telescope, may be mechanically determined, and
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
means OF measuring the retractve power of the eye.
Near and Far Sight,
When in lis normal condition, the eye is capable OF
seeing near as well as remote objects. This is ac-
complished through tue accommodatve action of
the crystalline lens, which becomes more or less re-
fractive according 10 the direction in which rays of
light enter the eyc-whetber parallel or divergent,
When preSbJopio comes on, which results in what
is called "far-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 near.sighted eye the
eongaon of the ball and the high refractive power
of the cornea bring the rays TO focus in front of
the retna. Both of these results are wholly incom-
patible 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 sghtedness is to bring the rays to
focus exactly on the retna without calling into ex-
ercise the accommodative action of the cry staline
why Accuracy is Required.
1t 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, a constant tension is kept 0 on the
ciliary muscles which operate the crystallin This
not only prevents the tail development of the sight
but it hastens the deterioration Of the eye and ne
cessitates gradual increase in the strength of the
glasses
Mathematical FormuIn,
The rules for accomplishing this object were first
devised by Professor Donders, of Utrecht, whose
method is the only 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 point of distinct vision thus ascertain
cd shall be designated by p and the far p oint
by r. Representing the adaptive power by,
4
its value in any case be determined by the for
mula,