The Portland daily press., 1869-09-06
MISCELLANEOUS.
How to Fit Spectacles.

~, ~
=. as ,ne recent works the eye the subject of
~ aoooii,inn' or the fitting Of glasses, receives
.I,e-,. 49, o' attention. Considering its ex-
tsnsive application, 1t is regarded as The mos IL,
Bile result derived from the recent ,dv,o,
thalmic science. " 'P"-
Structure of the Eve.
The eye is simply an optical instru n emit, COM
posed OF object glass 1.-THE cornea) an eve glass
1.-THE crystalline ieRsl and a screen (the retinal.
which Is the organ OF vision. Ia order that any
object may be distinctly visible 1t 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 crystalline, like the torn and rela-
tive position of the object glass and eye glass Of
telescope. may be mathematicaly determined, and
their defects may be remedied by auxiliary lenses
OF 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, 1t 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 of
seeing near as well as remote objects. This is 40,
eomplished through the accommodating 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 or divergent
When presbyopid comes on, which results In what
Is called "ar-sightedness" the crystalline becomes
incapable of uniting divergent rays upon the retipa
and they would if continued, meet at a point be-
hind it. In the myopic or near.sghted 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 vision, which is only produced
when the rays are brought to focus the retina.
The object sought then in fitting glasses, whether
for near or far sIghtedness is to bring the rays to
focus exactly on the retina without calling into ex-
ercise the accommodative action of the cry stalne
,a Why Accuracy is Required,
1t is important that when the eye is being used,
the crystalline should remain i'm a passive state
II an effort is made to read with glasses too strong
or too weak, a constant tension is kept u 0 on the
cliary muscles which operate the crystalline This
not only prevents the full development of the sight,
but hastens the deterioration of the eye and ne
cesstates gradual increase In the strength of the
glasses. ....-...
Mathematical FOrmuIm.
The rules for accomplishing this object we're 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 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 adaptiye mower bv-.
its value in anv casa can no detarmnad Inv Tho for.
mula, .
SOL SCR II - BIS
3a, 4 p ..f
tinctly from four inches a. its near point p to in
finite distance (oo). its far point r. we have
ts.7a- since iof.-
eight inches (8) and its near point at four t4l we
..t ,tsA" ~ II
..i,i.i.. o sf- - -
I@ n SIS
If however 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 tho eduya-
lent formula: ~ |
-...t- |
|
"
VI -.-44.
7-t
..
Hf par
Ing results mus OBTAINED represent the locus OF |
d lens whch, ii paced upon the crystalline would |
give TO rays coming from the near point direction
as II coming from the far point.
for determining the degree OF presbyopla we rep
resent Tue assumed normal power when accom-
=, dated Co. 49, ,,;oo.s, ;n.4
.soI s.Es;