The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-04

VI thc AttiHS VI SidoScs, iSSS''>N
the largest share of attention. Considering its ex-
tensive application it is regarded the most valua-
Die result derived from the recent advances in oph-
thalmic science.
Structure of the Eye.
The eye is simply optical instru ent, COM
posed OF an object glass 1.-THE cornea) eye glass
(the crystalline iens) and screen 1.-THE retinal,
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
fefind, which is simply mirror ing the back
part of the eye. The form and relative position of
the cornea and crystal line, like the form and rela
tive position of the object glass and eye glass of
telescope, may be mechanically determined, and
their defects may be remeded by auxiliary lenses,
or spectacles. These auxiliary lenses should
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 retroactive power of the eye.
Near and Far sight. a...4
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
the crystalline lens, which becomes more or less re-
fractve according to the direction in which rays of
light enter the eyc-whether parallel divergent.
When presbyopia comes on, which results in what
Is called "far-sghtedness," the crystalline becomes
incapable of uniting divergent rays upon the retna,
and they would, if continued, meet at a point be-
hnd 1t. In the nyopic or near-sighted eye the
elongation of the ball and the high retroactive power
of the cornea bring the rays to focus in front of
the retina. Both of these results are wholly incom-
patible with distinct vison which is only produced
whenth the rays are brought to focus on the retna.
The object sought then i'll fitting glasses, whether
or near or tar slghedness, Is to bring the rays to
focus exactly on the retina without calling into ex-
ercise the accommodative action of the cry stane,
Why Accuracy is Required.
1t is important that when the eye is being used,
the crystalline should remain in a passive state.
an effort is made to read with glasses too strong
or too weak, a constant tension is kept up on the
ciliary muscles which operate the crystalin 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 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 Sneen's 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 shall be designated by P and the far oint
by Il. Representing the adaptive power by,
Il
lts value IN any case can be determined by the for.
mua,
If therefore we have normal eye able to see dis
tincty from four inches AZ its near point p, to in-
finite distance { ''L its far point r, we have
i
-, since 80
Should the eye be myopc, having its far point at
eight inches 10y and its point at four IN we
should find
l.
....- ..-...,.. ~, ...- ~
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: i4f -i. r--.
1 1