The Variability of the Refraction of the Eye - Better Eyesight,

Discussion in 'Optometry Archives' started by Zetsu, Jun 19, 2009.

  1. Zetsu

    Zetsu Guest

    [...]

    The theory that errors of refraction are due to permanent deformations
    of the eyeball leads naturally to the conclusion, not only that errors
    of refraction are permanent states, but that normal refraction is also
    a continuous condition. As this theory is almost universally accepted
    as a fact, therefore, it is not surprising to find that the normal eye
    is generally regarded as a perfect machine which is always in good
    working order. No matter whether the light is good or imperfect,
    whether the surroundings are pleasant or disagreeable, even under
    conditions of nerve strain or bodily disease, the normal eye is
    expected to have normal refraction and normal sight all the time. It
    is true that the facts do not harmonize with this view, but they are
    conveniently attributed to the perversity of the ciliary muscle. The
    muscle is believed to control the shape of the lens, and is credited
    with a capcity for interfering with the refraction in some very
    curious ways. In hypermetropia (farsight), it is believed to alter the
    shape of the lens sufficiently to compensate, in whole or in part, for
    the shortness of the eyeball. In myopia, or nearsight, on the
    contrary, we are told that it actually goes out of its way to produce
    the condition, or to make an existing condition worse. In other words,
    the muscle is believed to get into a more or less continuous state of
    contraction, thus keeping the lens continuously in a state of
    convexity, which, according to accepted theories, it ought to assume
    only for vision at the near-point. This theory serves the purpose of
    explaining to the satisfaction of most eye specialists why persons who
    at times appear to have myopia, or hypermetropia, appear at other
    times not to have them. After people have reached the age at which the
    lens is not supposed to change it does not work so well, while in
    astigmatism it is available only to a limited extent even at the
    earlier ages; but these facts are quietly ignored.

    When we understand how the shape of the eyeball is controlled by the
    external muscles, and how it responds instantaneously to their action,
    it is easy to see that no refractive state, whether normal or
    abnormal, can be permanent. This conclusion is confirmed by the
    retinoscope and I had observed the facts long before my experiments
    upon the eye muscles of animals, reported in 1915 [1] and to be
    described again in my forthcoming book, had offered a satisfactory
    explanation for them. During thirty years devoted to the study of
    refraction, I have found few people who could maintain perfect sight
    for more than a few minutes at a time, even under the most favorable
    conditions; and others I have seen the refraction change half a dozen
    times or more in a second, the variations ranging all the way from
    twenty diopters of myopia to normal.

    Similarly I have found no eyes with continuous or unchanging errors of
    refraction, all persons with errors of refraction having, at frequent
    intervals during the day and night, moments of normal vision, when
    their myopia, hypermetropia, or astigmatism, wholly disappears. The
    form of the error changes, myopia even changing into hypermetropia and
    one form of astigmatism into another.

    Of twenty thousand school children examined in one year more than half
    had normal eyes, with sight which was perfect at times; but not one of
    them had perfect sight in each eye at all times of the day. Their
    sight might be good in the morning and imperfect in the afternoon, or
    imperfect in the morning and perfect in the afternoon. Many children
    could read one Snellen test card with perfect sight, while unable to
    see a different one perfectly. Many children could also read some
    letters of the alphabet perfectly, while unable to distinguish other
    letters of the same size under similar conditions. The degree of this
    imperfect sight varied within wide limits, from one-third to one-
    tenth, or less. Its duration was also variable. Under some conditions
    it might continue for only a few minutes, or less; under others it
    might prevent the subject from seeing the blackboard for days, weeks,
    or even longer. Frequently all the pupils in a classroom were affected
    to this extent.

    Among babies a similar condition was noted. Most investigators have
    found babies hypermetropic. A few have found them myopic. My own
    observations indicate that the refraction of infants is continually
    changing. One child was examined under atropine on four successive
    days, beginning two hours after birth. A three per cent solution of
    atropine was instilled into both eyes, the pupil was dilated to the
    maximum, and other physiological symptoms of the use of atropine were
    noted. The first examination showed a condition of mixed astigmatism.
    On the second day there was compound hypermetropic astigmatism, and on
    the third compound myopic astigmatism. [2] On the fourth one eye was
    normal and the other showed simple myopia. Similar variations were
    noted in many other cases.

    What is true of children is equally true of adults of all ages.
    Persons over seventy years of age have suffered losses of vision of
    variable degree and intensity, and in such cases the retinoscope
    always indicated an error of refraction. A man eighty years old, with
    normal eyes and ordinarily normal sight, had periods of imperfect
    sight which would last from a few minutes to half an hour or longer.
    Retinoscopy at such times always indicated myopia of four diopters or
    more.

    During sleep the refractive condition of the eye is rarely, if ever,
    normal. Persons whose refraction is normal when they are awake will
    produce myopia, hypermetropia and astigmatism when they are asleep,
    or, if they have errors of refraction when they are awake, they will
    be increased during sleep. This is why people waken in the morning
    with eyes more tired than at any other time, or even with severe
    headaches. When the subject is under ether or chloroform, or
    unconscious from any other cause, errors of refraction are also
    produced or increased.

    When the eye regards an unfamiliar object an error of refraction is
    always produced. Hence the proverbial fatigue caused by viewing
    pictures, or other objects, in a museum. Children with normal eyes who
    can read perfectly small letters a quarter of an inch high at ten feet
    always have trouble in reading strange writing on the blackboard,
    although the letters may be two inches high. A strange map, or any
    map, has the same effect. I have never seen a child, or a teacher, who
    could look at a map at the distance without becoming nearsighted.
    German type has been accused of being responsible for much of the poor
    sight once supposed to be peculiarly a German malady; but if a German
    child attempts to read Roman print, it will at once become temporarily
    myopic. German print, or Greek or Chinese characters, will have the
    same effect on a child, or any other person, accustomed to Roman
    letters. Cohn repudiated the idea that German lettering was trying to
    the eyes. [3] On the contrary, he always found it "pleasant, after a
    long reading of the monotonous Roman print, in return to 'our beloved
    German'." Because the German characters were more familiar to him
    than any others he found them restful to his eyes. "Use," as he truly
    observed, "has much to do with the matter." Children learning to read,
    write, draw, or sew, always suffer from defective vision, because of
    the unfamiliarity of the lines or objects with which they are
    working.

    A sudden exposure to strong light, or rapid or sudden changes of
    light, are likely to produce imperfect sight in the normal eye,
    continuing in some cases for weeks and months.

    Noise is also a frequent cause of defective vision in the normal eye.
    All persons see imperfectly when they hear an unexpected loud noise.
    Familiar sounds do not lower the vision, but unfamiliar ones always
    do. Country children from quiet schools may suffer from defective
    vision for a long time after moving to a noisy city. In school they
    cannot do well with their work, because their sight is impaired. It
    is, of course, a gross injustice for teachers and others to scold,
    punish, or humiliate, such children.

    Under conditions of mental or physicial discomfort, such as pain,
    cough, fever, discomfort from heat or cold, depression, anger, or
    anxiety, errors of refraction are always produced in the normal eye,
    or increased in the eye in which they already exist.

    The variability of the refraction of the eye is responsible for many
    otherwise unaccountable accidents. When people are struck down in the
    street by automobiles or trolley cars, it is often due to the fact
    that they were suffering temporary loss of sight. Collisions on
    railroads or at sea, disasters in military operations, aviation
    accidents, etc., often occur because some responsible person suffered
    temporary loss of sight.

    [1] Bates: The Cure of Defective Eyesight by Treatment without
    Glasses, N. Y. Med. Jour., May 8, 1915.

    [2] In astigmatism the eye is lopsided. In simple hypermetropic
    astigmatism one principal meridian is normal, and the other, at right
    angles to it, is flatter; hence the eye is farsighted in one curvature
    and normal in another. In simple myopic astigmatism the contrary is
    the case, one principal meridian is normal and the other, at right
    angles to it, more convex, making the refraction normal in one
    curvature and shortsighted in another. In mixed astigmatism one
    principal merdian is too flat, the other too convex. In compound
    hypermetropic astigmatism, both principal meridians are flatter than
    normal, one more so than the other. In compound myopic astigmatism
    both are more convex than normal, one more so than the other.

    [3] Eyes and School-Books, Pop. Scl. Monthly, May, 1881, translated
    from Deutsche Rundschsu.

    [...]
     
    Zetsu, Jun 19, 2009
    #1
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  2. Zetsu

    Neil Brooks Guest

    Zetsu has long ago reached the level where he/she/it is nothing
    more than the online equivalent of one of those psychotic homeless
    people who stands on the corner, SHOUTING Bible passages, to ...
    nobody.

    What a pathetic little creature.

    Almost SURELY the illegitimate love child of Otis Brown (and ... who
    else?? Desperate people DO do desperate things....).
     
    Neil Brooks, Jun 19, 2009
    #2
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