The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-15
MkSGEE1EAkElsDs.
How to Fit Spectscles.

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In all the recent works on tho eye the subject of
accommodating, or the fitting of glasses receives
the largest share of attention. Considering its ex-
tensve application it is regarded as the most yalua-
BILE 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 cornea) an eye glass
Vhe crystalline lens) and screen (the retina),
wnich is the organ of vision. Ii order that any
ooJect may be distinctly visible it is necessary that
perfect pic ture of 1t 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-
tive position of the object glass and eye glass of
telescope. may be mathematicaliy 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 some
means OF measuring the relractve power of the eye
Near and Far Sight,
When in lis normal condition. the eve is CAPABLE of
seeing near we'll as remote objects. This is ar
complished through the accommodatiye action of
the crystalline lens which becomes more or less re-
iactlve according TO the direction in which rays of
light enter the eyc-whetber parallel or divergent
When presogop'G comes on, which results in what
Is called "far-sghtedness,n the crystalline becomes
incapable of uniting divergent rays upon the retina
and they would, if continued, meet at a point he
hind It, In the myopic near-sighted eve the
eiongation of the ball and the high retroactive power
nf fl,a Y.g-
Oh 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 in 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 accommodatve action of the cry stalne.
2 Why Accuracy is Required.
1t is important that when the eye is being used,
the crystalline should remain in passive state.
If an effort is made to read with glasses too strong
or too weak a constant tension is kept up on the
ci'iary muscles which operate the crystalline. 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 tbe
glasses.
mathematical Formula. a..
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 Snellens test
types, subtendng 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 r. Representing the adaptive power BY.
its value in any case be determined by the for
mula,
Il
Ii therefore we have normal eye able to see dis
tnctly from four inches A. its near point p to in
finite distance (', its far point r, we have
-, since 80
Should the eye be myopic, having its far point at
eight inches (S) and its near point at four az we
should find
-
8 8
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 fnrmula.