[...The Variability of the Refraction of the Eye

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 object regarded is strange or
familiar, 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. This muscle is
believed to control the shape of the lens, and is credited with a
capacity 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 eve 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 it is 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] (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
often 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 also 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 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 and infants 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 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, to 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 physical 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 from 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.

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[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 meridian 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. Sci. Monthly, May, 1881, translated
from Deutsche Rundscau...]

- Dr. W. H. Bates, January 1920