[...]
Much of my information about the eye has been obtained by means of
simultaneous retinoscopy.
The retinoscope is an instrument used to measure the refraction of the
eye. It throws a beam of light into the pupil by reflection from a
mirror, the light being either outside the instrument - above and
behind the subject - or arranged within it by means of an electric
battery. On looking through the sight-hole one sees a larger or
smaller part of the pupil filled with light, which in normal human
eyes is a reddish yellow, because this is the color of the retina, but
which is green in a cat's eye, and might be white if the retina were
diseased. Unless the eye is exactly focussed at the point from which
it it being observed, one sees also a dark shadow at the edge of the
pupil, and it is the behavior of this shadow when the mirror is moved
in various directions which reveals the refractive condition of the
eye. If the instrument is used at a distance of six feet or more, and
the shadow moves in a direction opposite to the movement of the
mirror, the eye is myopic. If it moves in the same direction as the
mirror, the eye is either hypermetropic or normal; but in the case of
hypermetropia the movement is more pronounced than in that of
normality, and an expert can usually tell the difference between the
two states merely by the nature of the movement. In astigmatism the
movement is different in different meridians. To determine the degree
of error, or to distinguish accurately between hypermetropia and
normality, or between the different kinds of astigmatism, it is
usually necessary to place a glass before the eye of the subject.
This exceedingly useful instrument has possibilities which have not
been generally realized by the medical profession. It is commonly
employed only under certain artificial conditions in a dark room; but
it is possible to use it under all sorts of normal and abnormal
conditions on the eyes both of human beings and of the lower animals.
I have used it in the daytime and at night; when the subjects were
comfortable and when they were excited; when they were trying to see
and when they were not; when they were lying and when they were
telling the truth. I have also used it, under varying conditions, on
the eyes of many cats, dogs, rabbits, birds, turtles, reptiles and
fish.
Most ophthalmologists depend upon the Snellen test card, supplemented
by trial lenses, to determine whether the vision is normal or not, and
to determine the degree of any abnormality that may exist. This is a
slow, awkward and unreliable method of testing the vision, and
absolutely unavailable for the study of the refraction of the lower
animals and that of human beings under the conditions of life. The
test card can be used only under certain favorable conditions, but the
retinoscope can be used anywhere. It is a little easier to use in a
dim light than in a bright one, but it may be used in any light, even
with the strong light of the sun shining directly into the eye. It is
available whether the subject is at rest or in motion, asleep or
awake, or even under ether or chloroform. It is also available when
the observer is in motion. It has been used successfully when the
eyelids were partly closed, shutting off part of the area of the
pupil; when the pupil was dilated; also when it was contracted to a
pin-point; when the subject was reading fine print at six inches, or
at a greater distance; and when the eye was oscillating from side to
side, from above downward, or in other directions.
It takes a considerable time, varying from minutes to hours, to
measure the refraction with the Snellen test card and trial lenses.
With the retinoscope, however, the refraction can be determined in a
fraction of a second. With the Snellen test card and trial lenses it
would be impossible to get any information about the refraction of a
baseball player at the moment he swings for the ball, at the moment he
strikes it, and at the moment after he strikes it. With the
retinoscope, however, it is quite easy to determine whether his vision
is normal, or whether he is myopic, hypermetropic, or astigmatic, when
he does these things; and if any errors of refraction are noted, one
can guess their degree pretty accurately by the rapidity of the
movement of the shadow.
With the Snellen test card and trial lenses conclusions must be drawn
from the patient's statements as to what he sees; but the patient
often becomes so worried and confused during the examination that he
does not know what he sees, or whether different glasses make his
sight better, or worse; and moreover, visual acuity is not reliable
evidence of the state of the refraction. One patient with two diopters
of myopia may see twice as much as another with the same error of
refraction. The evidence of the test card is, in fact, entirely
subjective; that of the retinoscope entirely objective, depending in
no way upon the statements of the patient.
By means of simultaneous retinoscopy it has been demonstrated that the
refraction of the eye is never constant; that all persons with errors
of refraction have, at frequent intervals during the day and night,
moments of normal vision when their myopia, hypermetropia, or
astigmatism, disappears completely; and that all persons, no matter
how good their sight may ordinarily be, have moments of imperfect
sight when they become myopic, hypermetropic, or astigmatic. It has
also been demonstrated that when the eye makes an effort to see, an
error of refraction is always produced, and that when it looks at
objects without effort, all errors of refraction disappear, no matter
how great their degree, or how long their duration. It has been
further demonstrated that when the eye strains to see distant objects
myopia is always produced in one or all merdians, and when it strains
to see near objects hypermetropia is always produced in one or all
meridians.
The examination of the eyes of persons while asleep, or under the
influence of ether or chloroform, has shown that the eye is rarely at
rest during sleep, or while the subject is unconscious from any cause.
Persons whose sight was normal while awake were found to have myopia,
hypermetropia and astigmatism when asleep, and if these errors were
present when they were awake, they were increased during sleep. This
explains why so many people are unable to see as well in the morning
as at other times, and why people waken with headaches and pain in the
eyes. Under ether or chloroform, errors of refractioon are also
produced or increased, and when people are sleepy they have invariably
been found to have errors of refraction.
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. In a dim light,
in a fog, or in the rain, the retinoscope may indicate no error of
refraction in eyes which ordinarily have normal sight; but a pilot on
a ship on a rainy night usually has an error of refraction, because he
is straining to see, and it is rare to find persons in positions of
responbility under unfavorable conditions with normal vision.
In order to obtain reliable results with the retinoscope it must be
used at a distance of six feet or more from the subject. When used at
a distance of three feet or less, as it commonly is, the subject
becomes nervous and unconsciously strains, thus altering his
refraction.
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