The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-22
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 the most vaua
Bile result derived from the recent advances in oph-
thalmic science.
Structure of the Eye.
The eye is simply an optical instru nent, COM
posed of an object glass 1.-THE corneal 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, 1t is necessary that
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 rea-
tive position Of the object glass and eye glass Of
telescope. may be Mechanically determined, and
their defects may be remedied by auxiliary lenses
or spectaces. 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 refractive 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-
complished through the accommo6atve action of
the crystalline lens which becomes more less re-
fracve according 10 the direction in which rays of
| light enter the eye-whether parallel or divergent
When presbyopia comes on, which results In what
is called ar-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
of the coInea bring the rays to focus in front of
the reina Both Of these results are wholly inecm-
patble with distinct vision, which is only produced
when the rays are brought to focus the retina.
the object sought then i'll fitting glasses, whether
for near or tar slghtedness, is to bring the rays to
focus exactly on the retna, without calling into ex
ercse the accommodatlve action of the cry stalIine
. Why Accuracy is Required.
1t is important that when the eye is being used,
the crystalline should remain in passive state.
Ii an effort is made to read with glasses too strong
or too weak constant tension is kept u on the
clliary 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.
Mathematical F0rmuIR.
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, subtending at different distances an ANGLE of
one minute, can be read. Prof. sDonders proposes
that the near point Of dsiD vision thus ascertain
ed shall be desis1dted by p and the far p oint
~
by n. Representing the adaptive power by,
Il
its value in any case can be determined by the for
mula,