August 1,
1S81.]Dr.
C. MACNAMARA ON ERRORS OF REFRACTION &
ORIGINAL COMMUNICATIONS.
I
V
ECTIJUE I.?ERLlORS OF REFRACTION
ACCOMMODATION OF THE
AND
EYE.
By C.
Surgeon
to
Macnamara, the Royal Westminster Ophthalmic Hospital, ?-c. cj-c.
The eye considered as an
optical instrument, has been
obscura, the retina corresponding to the ground glass screen upon which the image appears ; the object lens being formed by the aptly compared
association of the crystalline
to a
camera
juxtaposed, the cornea and diaphragm (the iris) interposed
two lenses
lens,
with
a
between them. a
simple experiment.
in front of such a lens at a certain distance from a screen it will be easy to adjust the candle, lens and screen, so that a well-defined image o? the flame of the candle, upside down, should be formed upon the screen. The spot on which the image of the candle is thus formed is called the focus of the lens. If the screen is now brought near to the lens the image of the flame of the candle thrown upon it will become enlarged and ill-defined.
It may be restored to its former distinctness a short distance away from the
by moving the lens
By this simple experiment we learn that a lens forms a distinct picture of a luminous object, but only at the exact focus on the side of the lens opposite to the object, the focus is nearer the lens
screen.
convex
when the object is distant, (the >-n.-3 0f light falling on it being parallel rays,) and fu her from the lens when the object is near (divergent rays.) Suppose, however, that the candle is fixed and a stronger lens, or one having an increased convexity, is substituted for the first, the image of the flame of the candle on the screen will be blurred and the lens will have to be moved nearer the screen to give the image definition. If, on the other hand, a weaker lens, or one with a less convex
surface,
be
substituted for the first, it will have
candle, in order that the flame may be accurately focused on the screen. In other words, the greater the convexity of the lens, the nearer is its focus. Supposing the lens we were experimenting with were a perfectly transparent and elastic body ; if we were to stretch it from its circumference in all directions equally we should render its surface flatter and
to be
placed
nearer
thereby lengthen
to the
its focus.
Directly
the force
employed
discontinued the surfaces of the elastic body would bulge outwards, the lens would become more convex, and its focal distance would be shortened. The lens in the human subject is an elastic substance, which is compressed when at rest, because it is contained in a case which is kept stretched from its circumference by its suspensory ligament. When the ciliary muscle was
it relaxes this ligament and so suspends the traction which it otherwise exercises on the lens, and
contracts
the anterior surface of the lens immediately bulges forwards, becoming flatter directly the muscle ceases to act. By the relaxation and contraction of the ciliary muscle therefore, the anterior surface of the lens is rendered more or less convex, and so the power of the lens is
proportionately increased or diminished ; and in this way it is enabled to focus parallel rays of light or those proceeding from a distant object, and also divergent from a near object on the retina. rays or those emanating An eye is said to be emmetropic if when it is in a state of perfect repose (the ciliary muscle being passive) and it is directed to a distant object, the rays of light proare brought to an exact focus ceeding from that object That is, the accommodation of the eye on the retina. rays of light are accurately focused at being
rest, parallel
of the retina. The vision of such is disturbed by even a slightly eye for distant objects of it. It is not improved by front in lens held convex on
bi convex lens is competent to produce a picture of the external world upon a properly arranged screen is a fact of which every one can assure himself If a lighted candle is placed by a That
ACCOMMODATION^ 7
the
yellow spot
an
slightly concave glass. Ametropia.?All eyes which are not emmetropic are on cimetropic. In ametropia parallel rays are not united the retin i, but are brought to a focus either in front of a
else behind the retina. in the experiment above referred Supposing however, to, that the candle is brought close to the lens, the flame will not be accurately focused on the screen, because the rays proceeding from it are divergent ; and in order to briug these rays accurately to a focus we must remove the screen further away from the lens, or else interpose the candle and the screen. In a stronger lens between this object is attained by the action of the human eye the ciliary muscle 011 the lens. Supposing in the caie of it is required to focus rays an emmetropic eye that of light proceeding from a near object, as for instance from printed matter in a book, the observer fixes his the same time the ciliary muscle eyes on the letters at contracts and the suspensory ligament is relaxed, the anterior surface of the lens bulges forwards, and, its
or
convexity being increased, divergent a through it are brought to focus on the
rays passing retina ; the oband directs them towards the server raises his eyes horizon, the ciliary muscle ceases to contract, the suspentraction is exercised on the sory ligament becomes tense, circumference of the lens, its anterior surface is flattened, and parallel rays of light are brought to a focus on the retina. This power, which exists in every perfect eye, is known as its accommodation, and is the power which the eye possesses of accurately focusing parallel anil the retina. divergent rays of light on The influence of the ciliary muscle 011 the accommodation of the eye is paralysed by the action of atropine, of the sulphate of atropine has so that when a solution been applied to the surface of the conjunctiva the ciliary muscle fails to contract, and when near objects are under observation indistinct, because the anterior
they appear
surface of the lens does not become more convex and Under these cirso focus rays of light 011 the retina. cumstances distant objects are still seen, parallel rays to a focus on the retina in an emmetropic
being brought
muscle is at rest. We not. 1111 freeye when the ciliary with patients who tire weak and out of meet quently
THE INDIAN MEDICAL GAZETTE.
203 health
complaining
that their
sight
is
defective ; after
reading for a short time the words become indistinct, they have difficulty in threading a needle, and so on, but can see distant objects clearly. A condition such as this may arise from loss of tone in the ciliary muscle, which unable to contract effectively for any length of time so as to keep the suspensory ligament relaxed and enable the anterior surface of the lens to maintain its is
convexity sufficiently
retina.
This state of
to focus
things
divergent
is known
as
asthenopia.
rays
upon the
accominodatory
But this leads us to another point with reference to vision To be accurate it must be binocular, the rays of light proceeding from any object falling upon corresponding portions of the retina in both eyes. To effect this the eyes converge simultaneously and to an exactly extent upon any object that is gradually approximated to the observer's face. Under these circumstances the pictures formed in both eyes are blended into one in the sensorium, producing a single image of accurate geometrical proportions. Vision by means of one eye is not accurate, for the retina only gives us an account as to direction, so that on looking at a hollow medal with one eye, the idea produced is that the medal stands boldly out. In order to give an accurate perception of relief the two optic axes must co-operate, so that the image shall fall on identical spots of the retina) of both eyes, the object so seen at different angles is associated in the brain so as to produce a picture of its true form and situation in space. The convergence of the eyes however, and their accom-
equal
modation,
are
provoking
the
acts
which take
place simultaneously
one
in
other, consequently
emmetropic eyes convergence and accommodation are exactly balanced. Supposing, however, from the weakness of the internal rectus of one eye the patient cannot converge both eyes accurately on an object, or having brought his eyes to bear on the point, he is unable to keep them steadily fixed on it, because the muscle of one eye being weak the external rectus everts this eye : directly this movement occurs the patient's sight necessarily becomes confused. This condition is known as muscular asthenopia. I must that the
again surface
candle and the
instance, let us (or in the eye
experiment and suppose intervening between the not perfectly spherical, for
refer to our
lens
of the screen
is
suppose that the of the cornea) in
surface of
place
of
the lens
being spheri-
cal is of greater curvature in the vertical than in the horizontal meridian Evidently rays of light passing through such a cornea or lens cannot be brought to an accurate focus on the screen or retina, the vertical being the stronger meridian will bring the rays of light to a
the lens than the horizontal or weaker the image of the candle formed on and ?meridian, This condition of the screen will thus be blurred. as astigmatism. in if the it exists known is things eye. Irregularities of the surface of the cornea are as a rule the cause of imperfect vision of this description. focus
nearer
Hypermetropia.
patient writing and A
comes
to us
complaining
other close work,
that in
especially by
reading, artificial
[August 1,1881.
light or. in a gloomy atmosphere, his light fails him, it becomes indistinct. After working for a short time he probably experiences a feeling of fatigue, the words he may be reading become confused. It may be he suffers from actual pain over the eyebrows, and symptoms such as these oblige him to discontinue the work upon which he is engaged. After resting his eyes he can see distinctly, and resume his occupation ; but the same train of symptoms is produced by re-application to work. In a considerable number of cases conjunctivitis also exists, and the increased flow of tears thus produced still further impairs the patient's sight. The lachrymal secretion is impeded in its flow through its natural passage by the engorged condition of the mucous membrane
lining the canaliculi and lachrymal
sac.
On
examining
eyes affected in this way, they may appear to be healthy, with the exception of a slight amount of con junctivitis, and the patient not unfrequently describes his a
person's
as being particularly good ; but if you p ace him at distance of twenty feet from a ca