The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-27
How v. to Fit Spectacles.

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-
tensive application it is regarded as the most valua
BILE result derived from the recent advances in oph-
thalmic science
'''t Structure of the Eye.
The eye is simply an optical instru nent, COM
posed of an object glass the cornea), an eye glass
1.-THE crystalline leas) and screen 1.-THE retina),
which is the organ of vision. Ia order that any
object may be distinctly visible it 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 crystal line, like the form and rela
tive position of the object glass and eye glass of
telescope, may be mathematical'y 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 absolute
" perfect fit, it is only necessary to devise
means of measuring the reractive 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 tie aceommodative action of
the crystalline lens which becomes less re-
dcCoIuiHS 10 tne urectlOn IN which rays OF
light enter the eyc-whether parallel or divergent
When preSb5opio 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 point be-
hind it. In the myopic or near-sighted eye the
elongation of the ball and the high reractve power
of the cornea bring the rays to a focus in front of
the retina. Both Of these results are wholly ineom-
patible with distinct vsion which is only produced
when the rays are brought to focus en- the retina
The object sought then in fitting glasses, whether
for near or tar slghedness, is to bring the rays to
focus exactly on the retna, without calling into ex-
ercise the accommodative action of the cry stallne
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
ciiary muscles which operate the crystalline This
not only prevents the full development of the sight,
but it hasens the deterioration of the eye, and ne
cessitates gradual increase in the strength of the
glasses. ....-. ~ -.
Mathematical FOrmulH.
The rules for accomplishing this object were first
devised by Professor Donders, of Utrecht, whose
method is the only one in use. 1t is necessary
to ascertain rst at what distances Snellen's test
types, subending at different distances an angle of
one minute, can be read. Prof. Donders proposes
that the point of distinct vision thus ascertain
ed shall be designated by p and the far p oint
bhA ihie ht -'A iiti @!"' ' +i-41
by r. Representing the adaptive power by, Be- '
!. bYi4yl.a aa4d tfj 54l,1t I..ast Yi.'I A ll 81.
its value in any case can be determined by the for
mula, +.,. ..t..., t.,.t.ti l.stI ...f sAl .ss
ttl--I bs
LI. 49alal P R,iui -uiat 9.i
II therefore have normal eye able to see dis