The Portland daily press., 1869-09-03
How to Fit Spectacles.

In all the recent works tho 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-
BILE result derived from therecent advances In oph-
thalmic science.
Structure Of the Kyo.
The eye is simply an optical instru n ent, com
posed OF an object glass 1.-THE corneo), an eye glass
1.-THE crystalline iens) and q screen 1.-THE retina),
which Is the organ OF vision. In order that any
object may be distinctly visible it is necessary that
a perfect pic ture Of 1t should be formed upon the
retina, which ID 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 mathematically 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
y perfect fit, it is only necessary to devise some
means OF measuring the retroactive power Of the eye
Near and Far Sight,
When in Its normal condition, the eye is capable O,
seeing near as we'll as remote objects. This is ac-
complished through the accommodating action of
the crystalline lens which becomes more or less re-
hactive according to the direction in which rays of
light enter the eyc-whether parallel or divergent
When presbyopia 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 or near.sighted eye the
eongaton of the ball and the high refractive power
Of the cornea bring the rays to focus in front of
the retina Both of these results wholly ineom-
patibe 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 ghtedness is to bring the rays to
focus exactly on the retna, without calling into
ercse the accemmodative action Of the cry stalne.
Why Accuracy is Required.
1t is important that when the eye Is being used
the crystalline should remain in q passive state
If an effort is made to read with glasses too strong
or too weak, constant tension is kept no on the
ciary muscles which operate the crystalin e. This
not only prevents the full development OF the sisht
but 1t hastens the deterioration of the eve and
cesstates gradual increase In the strength of the
glasses.
Mathematical Formula.
The rules for accomplishing this object were FIRST
desised by Professor Donders, of Utrecht. whose
metnod Is the only one now in use 1t is necessary,
70 ascertain first, ct what distances Snellens test
types, subtending at different distances 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 tar oint
by r. Representing the adaptive power by.
Its value in any case can be determined by the for
mula,
p r
n tberefore We have normal eye able to see dis
tinctly from four inches a. its point p to In
finite distance o. Its tar point r, We have
1 1 1
-, since 80
Should the eye be myopic, having Its far point at
eight inches CS) and its near point at four (4), we
should find
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 formula:
1
R-P
RIP
mi A. -.