The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-13
How to Fit Spectacles.

In all the recent works the eye, the subject of
accommodating or the fitting of glasses receives
the largest share of attention. Considering its ex-
tensve application it is regarded the most valua-
BILE result derived from the recent advances in oph-
tha]mic science.
Structure of the Eye.
The eye is simply optical instru emit, CON-
posed of an object glass the cormea) an eve glass
1.-THE crystalline lens) and screen 1.-THE retina),
which Is the organ of vision. L, order that any
object may be distinctly visible 1t is necessary that
perfect pic ture of it should be formed upon the
rettnG, 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 fundamentally determined and
Their detects 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 absolute
y perfect fit, it is only necessary to devise some
means of measuring the retroactive power of the eve
Near md Far Sight,
when In Its normal condition, the eye is capable of
seeing near as welt as remote objects. This is ac-
complished through the accommodating action of
the crystalline lens which becomes more or less re-
active according to the direction in which rays of
light enter the eyc-whether parallel or divergent,
when pres6JoptG comes on, which results In what
IS called far-sighedness, the crystalline becomes
incapable of uniting divergent rays upon the retina
and they would, if continued, meet at a point be-
hind it, In the myopic 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 reina Both of these results are wholly incom-
Tafi NTH sgff,,a .,s,s1-
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 retina, without calling into ex
ercise the accommodative action of the cry staIine
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 up ON the
ciliary muscles which operate the crystallin This
not only prevents the fall development of the siabt
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 Snelens test
types, subtending at different distances an anoe ot
one minute, can be read. Prof. Donders promises
that the near point of distinct vision thus ascertain
ed shall be designated by p and the far n ojnt
by Il. Representing the adaptive power by,
its value in any case can be determined by the for
mula,
If therefore We have normal eye able to see dis
tinctly from four inches a. its near point p to i'll
finite distance ol its far mint z ,ra Laua