The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1869-09-17


accommodation, or the fitting of glasses receives
the largest share of attention Considering IFS ex
tensye application it is regarded as the most vaua-
Die result derived from the recent advances in cpl-
thamic science. ,..
Structure of the Eve.
The eye is simply an optical instru n ent COM
posed OF an object glass 1.-THE cornea), eye glass
tlhe crystalline teas) and screen 1.-THE retina),
winch is the organ of vision. La order that any
object may be distinctly VISIBLE is necessary that
perfect pic ture of it should be formed upon the
Tettad, which is simply mirror covering the lack
part OF the eye. The form and relative position of
the cornea and crystalline like the form and rela-
i've position of the object glass ard eye glass of
telescope, may be mathematically determined and
their detects may be remeded 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 tit, it is only necessary to devise some
means OF measuring the relractive power of tie eye
Near and Far Sight,
is rs ond71on, the eye is capable of
seeing near as well as remote objects. This is &C.
complisbed through tie accommodative action of
the crystalline lens which becomes more or less re-
fracve according TO the direction in which rays of
light enter the eye-whether parallel or divergent.
When presb5opia comes on, which results in what
is called "far-sighedness' the crystalline becomes
incapable of uniting divergent rays upon the retipa
ana they would, If continued, meet at point be-
hind 1t. In the myopic or near-sighted eve the
eongaton of the ball and the high refractive power
OF the cofnea bring the rays to focus in front of
the retina. Both of these results are wholly ncom
patlble with distinct vision, which is only produced
when the rays are brought to a focus on the retna.
The object sought then in fitting glasses, whether
ior near or tar sghtedness is to bring the rays to
locus exactly on the retina, without calling into ex
ercise the accommodatve action of the cry staline
Why Accuracy is Required.
1t is important that when the eye is being used
the crystalline should remain in a passive state
Ii an effort is made to read with glasses too strono
or too weak, a constant tension is kept u on the
ciliary muscles which operate the crystalline This
not only prevents the full development of the siebt
but it hastens the deterioration of the eye and ne
cessitates gradual increase in the strength of tbe
glasses.
Mathematical Formula.
Ihe 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, 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 D oint
by r. Representing the adaptive power by.
Its value in any case can be determined by the for
mua, t,-- ..r.-- II -
Ii therefore We have normal eve able to see dis
Inctly from four inches a. its point p TO in