Srri.
/
1,1881.]
Dr C. MACN AMARA ON ERRORS OP
REFRACTION & ACCOMMODATION. 235 vision,
ORIGINAL COMMUNICATIONS.
eyes
/LECTURE
II.?errors of refraction
AND ACCOMMODATION OF THE EYE. By C.
Surgeon
to the
Macnamara,
Royal Westminster Ophthalmic Hospital, fyc. fyc. MYOPIA.
In cases of myopia the eyeball is too long from before backwards, so that when the accommodation is at rest rays of light are brought to a focus in front of the retina. The posterior part of the globe of the eye in myopia protruding so far backward that although the cornea and crystaline lens may be perfectly normal, nevertheless, rays of light passing through them are
focused anterior to the retina. This condition of things by means of a candle, lens and screen. If the light and the lens are so placed that parallel rays from the candle are accurately focused on the screen which is then withdrawn a short distance backwards, the image of the candle at once becomes blurred. Under these circumstances, if a suitable concave glass is placed between the candle and the lens it will disperse the rays of light falling upon the lens to such an ex-
may be illustrated
and in a considerable number of cases affected with a central scotoma
one or
both
are
arising from choroidal haamorrhage, or from a posterior staphyloma which has invaded tlie polar region of the eye ; detachment of the retina, softening of the vitreous or a cataract sometimes complicate cases of myopia. In order to guard a myopic eye from these complications it is neces-
sary to overcome the error of refraction from which the patient suffers in early life by means of proper concave
Many short-sighted persons have a peculiar habit of half closing their eyes when looking at distant objects ; the reason for this is that by nearly closing the eyelids the circles of diffusion of light otherwise formed on the retina are lessened. This fact may be demonstrated by rendering an emmetropic eye myopic by means of a convex lens held in front of the eye ; such a lens makes all distant objects appear blurred, because the rays of light passing through it are focused anterior to the retina. Under these circumstances if_ the eyelids are half closed an improvement in vision is at once apparent for the reason above referred to. As
rule
tent that
myopic eyes are large and prominent in feel somewhat hard on pressure, but the lengthening of the globe posteriorly is the essential feature of by far the majority of cases of This
and the
condition is
they are accurately focused upon the screen, image of the candle is then clearly defined. In some instances of myopia the length of the eye from before backwards has been known to measure a third
than that of the normal eye, so that the patient's accommodation being at rest, it is impossible for parallel rays to be brought to a focus on the retina. The pathological changes which give rise to posterior more
have been described, so that at present we refer to the symptoms and the appropriate only treatment of errors of refraction arising from this condition.
staphyloma
need
The diagnosis of myopic- is a matter of no great difficulty ; the patient's distant vision is either absent, or if present it is confused, but his sight is immediately improved if a suitable concave glass is held before his ?eyes.
In the
majority
cases
although
the distant
sight is very defective, vision for near objects ia excellent, and even appears to be better than in an emmetropic eye. The reason of this is, that a myopic patient, from the peculiar conformation of his eyes, is able to define small objects brought close to his face, and the nearer the object is to the eye the larger is the image it forms on the retina. Myopic patients therefore can often see very small objects which other people are unable to define, because if they were to attempt to bring the object as near their faces as myopic persons do, they would be unable to see them. The power which short-sighted persons possess of seeing very small objects is increased because their pupils are always largely dilated, and so they require less light to illuminate an object than people ordinarily do. But "when we come to examine more closely into the condition of their sight, we find that scarcely twenty-eight o per cent, of myopic persons have a normal acuity
a
front, they
frequently
myopia.
augmented through the increased pressure exercised by the ocular muscles on the globe of the eye. As I have before the conver-
remarked,
gence of the eyes on a near object and their accommodation take place simultaneously, the one act provoking the other; but in cases of myopia to see near objects it is only necessary for a person to use a very small amount of his accommodation in order that he may focus rays of light over his retina. The effort of accommodation being feeble, the act of convergence is also weak, and it requires an inordinate exertion on the part of a patient to force the internal recti muscles to converge the eyes upon a near
object. This increased effort wearies the muscles, and symptoms of asthenopia In myopia, therefore, the conare then established. not only to be maintained has of the vergence habieyes tually because the patient is short-sighted, but this
convergence of the eyes has to be kept up by an inordinate action on the part of the internal recti muscles for the reasons above referred to. This increased muscular emotion produces pressure on the globe and so tends to protrude still further backwards the weak posterior hemisphere of the eyeball. A
short-sighted
from muscular asthenopia complains of symptoms very similar to those of a patient affected with accommodatory asthenopia. His vision is perhaps clear when he commences his work, for he can then bring his optic axes accurately to converge upon it. But after a certain amount of work his eyes fill with person
suffering
The patient experiences a sensation of weariness and sometimes pain, and he is compelled to discontinue his occupation ; after a period of rest he can resume his work for a limited time. Inflammations of the margins of the eyelids, styes, and epiphora with tears
are?together
THE INDIAN MEDICAL GAZETTE.
236
headache?frequently met with in myopia ; these sympby abnormal muscular tension, and are be to cured only by the use of proper glasses. Another result arising from the necessity for increased convergence of the optic axis in myopia proceeding from insufficiency of the internal recti muscles is, that the patient unconsciously rotates one of his eyes outwards, so as to prevent an image of the object under observation being formed on the retina. In this way toms are induced
he gets rid of the difficulty which would otherwise arise from double vision. But an eye which is rotated outwards in this way gradually loses its power of sight. The recti are not the only mnscles however which are a source of trouble in myopia. We frequently find, young people affected with perhaps
especially among only a slight amount of myopia, that symptoms of accommodatory asthenopia exist, and on examining their eyes with the ophthalmoscope we find that the optic disc is extremely congested ; in the greater number of such cases spasm of the ciliary muscle exists, and must be overcome by keeping the muscle paralysed for
six weeks by the instllation of a strong atropine. After the effect of this drug has passed away the patient must be supplied with proper a
month
or
solution of
glasses. Ophthalmoscopic
concave
examination in
myopia.?The
direct
method of examination being employed, in consequence of the abnormal length of the eyeball from before backwards the rays of light emerging from the cornea are convergent rays which intersect immediately in front of the eye, so the image they form on the observer's retina is an inverted one. If the observer moves his head in one direction the retinal vessels will appear to travel in the opposite direction. If a proper concave lens is
placed behind the sight-hole
of the
ophthalmoscope,
image of the fundus of the eye will be obtained?the strength of the concave lens so employed will give us an idea of the amount of myopia from which erect
an
the
patient
suffers. Treatment
of
Myopia.
complaining that he distant objects clearly, but can see small type held close to his eyes, we direct him to place himself at twenty feet distance from Snellen's test types, and we examine first one eye and then the other ; it may be at this distance the patient can make out letters Supposing
a
patient applies
to us
is miable to define
No. 100 with the right eye, but with the left can only see No. 1G9 ; we then ascertain the lowest concave glass with which the patient can distinctly make out No. 25 at twenty feet, first the right eye and then the left being examined. The loioest concave glasses which the
give
best attainable sight at 20 feet may be ordered for the patient, and these with a few exceptions (to be described) are the only glasses he will require either for near or for distant vision. It is necessary to be careful to order the lowest concave glasses which give the best attainable
sight
at 20
feet, because with
a
stronger glass the patient
may see equally well by making use of his accommodation and so neutralizing the divergent action on the
of
light
of
the too powerful
glasses.
It is
rays in this
way that an
[September 1,
emmetropic,
or
even
an
1881,
hypermetropic
eye may see clearly with a weak concave glass held before it. We must bear in mind the fact that the treatment of short-sighted patients by means of glasses is to be directed towards increasing the distance at which near and far objects may be clearly seen. It requires for an individual first usinofor near objects?to overcome the habit he has formed of exerting an inordinate amount of convergence of his eyes on the objects, so that for near sight be may object to glasses which neutralize his myopia for distance. Unless, however, a shortsighted person can see ordinary size print, or work at a
little
concave
practice, however, glasses?especially
12 inches, he is bound to use concave glasses, because it is when employed on near work that accurate images are necessary, and these the myopic patient can only obtain by harmful approximation of the object to his eyes ; the only way to overcome this difficulty is by the use of concave glasses. Spectacles are less necessary for distance, because a short-sighted person may, if he
chooses, pass through the world and only see a small portion of it ; by so doing he will not exert injurious pressure upon the eyeballs through the constant action of the ocular muscles which he exercises in converging his eyes upon near objects. The question frequently arises with reference to the advisability of neutralizing a difference which may exist in the eyes of myopic patients?are we to attempt to overcome this difference by appropriate glasses for tho right and left eye. The rule ia *n cases of this kind
not to order glasses of different power for the two eyes, but to furnish both eyes with glasses that suit the best eye, for instance, concave 4 dioptics is the weakest glass which gives the best sight to the right eye, but the myopia in the left eye is neutralized by concave 5 dioptics. Do not order No. 4 for the right eye, and 5 for the left, but give the patient concave 4 dioptics for both eyes. In some few cases it may be necessary to obtain the best possible vision and to try and overcome the difference between the two myopic eyes with appropriate glasses for each eye ; under these circumstances,
the vision is not unfrequently rendered indistinct in consequence of the unequal size of the retinal images formed by lenses of different powers. As I have before remarked, there are exceptions to the above rule ; in consequence of the existence of excessive posterior staphyloma or some other complication the
however,
vision is so indistinct that the patient has to bring objects that he may see them ; very close to his eyes in order to overcome this condition a very strong concave glass may be necessary, but the image of the object is so much reduced in size that an imperfect retina is unable
appreciate it. Under these conditions insufficiency of the internal recti muscles may complicate the case. If a myopic patient's sight therefore is so defective that ho is unable to read No 3 of Snellen unless lie brings the type to within three or four inches of his eyes, it is not desirable to order spectacles for reading or work
to
of that
description.
Suppose that
a
myopic patient
who
is
somewhat
Sept. 1,
1881.]
Db. C. MACNAMARA ON ERRORS OF REFRACTION & ACCOMMODATION. 237
advanced in life, can read about 5 of Snellen at 12 but not beyond that distance, he is apt to suffer from a certain amount of discord between the accommodation and convergence of his eyes on a near object. If we
inches,
give such a patient the weakest glasses for reading which neutralize his myopia for distance, he is apt to complain of pain in his eyes when he attempts to use these glasses
objects ; because he has, while working at near objects, been accustomed to relax his accommodation. If we give him strong concave glasses which entirely neutralize his myopia, and which would therefore necessitate his employing his accommodation for near objects, the ciliary muscle not being accustomed to work refuses
on near
to
respond to the demand made upon it. Under these circumstances order glasses by which the patient can see 2 or 3 of Snellen at 14 inches. After he has worn these for a time, and brought his ciliary muscle into working order, he will probably take to stronger glasses. It is generally supposed that as short-sighted persons advance in years their myopia will be neutralized by presbyopia ; but when we remember that the latter condition is simsenile insufficiency of accommodation, it follows that myopics, although they may not be aware of the change, are nevertheless affected by it. As a rule however, short-sighted persons do not require spectacles before they are 60 years of age, and are then often benefited by weak convex glasses for reading and concave glasses for dis-
piy
a
Should their myopia have been of high degree, that the patient is unable to read No. 2, or 3, of Snellen beyond 8 inches, then as complete loss of accommodation is brought about by advancing years, they will still read clearly at about 8 inches, but will not see distinctly either beyond, or within that distance ; and so
tance. so
no glasses are required in such presbyopic without knowing it. Myopia is largely due to civilization ;
rule
as a
a
case, the pa-
tient is
that such is the is evident to those who have had to deal with masses of people removed fromtbe centres of civilization; among such people myopia is almost unknown, whereas, from statistics elaborated in Germany, of 10,000 students no less than 1,800 were found to be suffering from defects case
1,000 were due to myopia ; in ten the affection was hereditary. Among the children of village schools the percentage of children affected with myopia was 1*4, whereas it rose to ll'4 per cent in the town schools. Of these latter in the primary schools myopia affected 6*7 per cent of the scholars, in medium schools 10-3 and in the schools 26'2 per of refraction. Of these cases
per
cent.
In the
high
cent,
high schools
in the upper classes
are
more
than half of the
pupils
myopic.
These facts speak for themselves : it is evident that among young people the constant application of the eyes to near objects, such as reading and writing, causes myopia. This result is increased in proportion as the light is or from faulty position of the scholars the
oblige young persons to seek a larger retinal image by the approximation of the object under observation close to their eyes, consequently a good side light and properly arranged desks, together with well printed books, should be provided by all school authorities ; and if myopic pupils are found who cannot read small type with comfort at 12 or 14 inches from the eye, they should wear concave glasses. Presbyopia.
Presbyopia.?From the age of 20 to three score years and ten the acuity of vision decreases, but in addition to
this the accommodative energy diminishes with advancing age, the ciliary muscle gradually losing its power and the elasticity of the lens diminishing, so that by the time
a
person is 45 years of age he is unable to focus
his retina. It is evident, however, that must be held within a distance of from 12 to 13 inches from a person's eyes in order that a sufficiently large image of the letters may be formed on his
objects small print near
on
retina to enable him to see them distinctly. The necessity for approximating an object like this to the eye is increased if the sensibility of the retina to light diminishes, as it does with advancing years; consequently in aged persons as the nerve becomes less sensi-
the individual to bring small objects close of accommodation fails him, and eyes, his power Such a person so symptoms of presbyopia are induced. can see clearly at a distance, and with a proper convex lens can define the smallest type at 12 inches from his
tive, obliging to his
eyes. A patient 45 years of age or upwards, comes to us stating that he has had excellent sight all his life, but that he now finds difficultly in reading small type, specially by candle-light. He can see No. 25 of Snellen at 20 feet, but cannot read No. 1 at 12 inches. He can perhaps make out No. 2 if the type ig held 12 or 16 inches from his eyes. With convex 1 dioptics this indi-
vidual sees No. 1 of Snellen at 12 inches from his eyes, but these glasses disturb his distant vision, in fact he sees better without glasses, unless for reading, writing or such like work. Patients suffering from presbyopia instinctively seek so that tbey can sometimes see small type without glasses if a bright light is placed between their eyes and the book they are reading. The reason of this is, that in a very bright light the pupil contracts so that the circles of diffusion otherwise formed a
bright light,
completely
on the retina are diminished, and the individual's sight is in this way improved. That the contraction of the pupil is the cause of the improvement of vision is demonstrated by the fact that presbyopic persons when looking through a pin hole in a card find that their sight is of this kind is useful in many ; a test
causes are
vastly improved to distinguish errors of refraction from loss of sight depending on disease of the retina or some of the deep structures of the eye, which would not be assisted by looking through a minute hole. The correct glasses to overcome presbyopia depend
ness
upon the age of the
imperfect, book or writing too close to the
bad
upon which they are engaged is brought over work therefore, especially in a
face;
is the direct cause of myopia. The secondary bad print, too small a type, and slight haziof the cornea. In fact all those conditions which
light,
cases
individual, the
rule is at 45 years
THE INDIAN MEDICAL GAZETTE.
238 age order convex No. 1
of
of the glasses
dioptics, and increase the dioptics for every five years
strength by of age beyond 45, thus, at 50 years order convex No. 2 dioptics, at 55 No. 3, at 65 No. 5 and so on. There are however exceptions to this rule, for instance a person has been slightly hypermetropic all his life, but through the aid afforded him by his ciliary muscle he one
has overcome his defective
refraction,
and
managed
to
focus both parallel and divergent rays of light on his But when this person arrives at about the age of 4',) or 45 his ciliary muscle becomes less active, and his lens firmer, so that he now has difficulty in overcoming his hypermetropia by an effort of accommodation, and so both his near and far sight is defective. In a case of this kind at 20 feet the patient may be unable to see any of the letters beyond No. 100, but with a convex retina.
he reads No. 25, so that we may presume dioptics will cover his hypermetropia, but as this individual is 45 years of age, he will require No. 1 convex to neutralize his presbyopia, consequently we order him glasses of convex 1 X 1 *50=2*50 dioptics. No. 1
dioptic
As the
patient
advances in life he will
require
an
increase
dioptic for every five years, and in addition For convex 1*50 dioptics to cover his hypermetropia. instance, a patient has always worn No. 4 convex glasses one
hypermetropia, and he is now sixty years his for of age ; presbyopia he requires convex 4 dioptics, add this to No. 4 for his hypermetropia, and we order convex 8 dipotics for reading, writing and so on, but for ordinary use at a distance this individual only requires No. 4 dioptics, No. 8 would greatly inconvenience him, and to overcome this condition of things his spectacles of each glass may be so arranged that the upper portion contains a No. 4 convex lens, and the lower half No. 8 to overcome his
Under these conditions in raising his eyes for the patient looks throught No. 4 convex glasses, and in casting down his eyes for reading and writing he sees through No. 8 convex. But supposing a patient has been myopic all his life, will he require to alter his glasses as he advances in life ? As a rule in low degrees of myopia it not uofrequently
convex.
distant
object
of age a person will for distance and with weak convex glasses for near objects. But if the myopia has been higher, let us suppose to the extent of the
happens that
see
at about
sixty
better with concave
years
glasses
1881.
faivas possible, to enlarge the retinal image of all small objects, such as printed matter upon which they maybe engaged ; and to increase the size of the retinal image they are in the habit of bringing the type close to their eyes although using powerful convex glasses. The habitual approximation of small objects to the face necessitates an increased convergence of the eyes upon the object, and as in these cases the accommodation is in abeyance, discord in what should be harmonious actions is the result with a proportionate amount of strain on the eyes, and not unfrequently increased intra, ocular blood stasis and faulty contortion of the retina and other structures of the eye. We may,
howeverj
this difficulty by directing the patient to read with a hand (convex) glass through which he looks with one eye, and thus prevent the necessity for converging his eyes on the object upon which he is engaged. overcome
that about No. 1*50
of
[September 1,
Astigimatism.
that condition of one or both in which the of their vertical, horizontal, curvature eyes or some one or more of the meridians of the eyes is not perfectly spherical ; it may he one of these meridians is more convex or more concave than some other meridian ; in fact there can be endless varieties of this
By astigmatism
we mean
but for all practical purposes we may confine our attention to irregularities thus caused in the refraction of the eyes by congenital anomalies in the curvature of the cornea. I say congenital anomalies, because I need not here refer to those cases of astigmatism resulting from a wound of the cornea such as that which is sometimes met with after extraction ?f a cataract, conical For the same reason I may exclude cornea, and so onthose obscure cases of astigmatism resulting from defects in the lens, whether congenital ?r arising from irregular action of certain fibres of the ciliary muscle.
kind,
define astigmatism (in the sense in shall use the term) as a fault in the refracting media of the eye depending on irregularities of the cur. vature of the cornea, the various meridians of the cornea deviating to such a degree from the spherical form as to interfere with the perfection of sight. It is evident if an irregularity in the curvature of the So that we may
which
we
clearly beyond eight inches, relaxed that eight inches
meridians of the cornea exists that rays of light passing such a cornea must be irregularly refracted, and cannot all be brought to the same focus on the retina, so that an object seen by such an eye is blurred and
glasses the upper half of which contain concave and the lower half convex lenpes. Lastly, with advancing years the near point of vision not only recedes from the eyes, but in different degrees among various people the retina becomes less sensitive to lighf-,80 that some presbyopic people are obliged, as
small type, and his distant vision is imperfect, but on holding a card close to one of his eyes and looking through a pin hole he finds that his vision is improved. This individual notices that when he is standing at some distance from a clock, and closing one eye, with the other he can see some of the figure on the dial, but not all of them, at any rate with the same clearness. The horizontal bars of a window are distinct and the vertical
patient or
not
being
able to
see
when his accommodation was
was his far point, so that at that distance parallel rays of light were focused on his retina, he will still evidently see small objects at eight inches, although the whole of his accommodation may have been destroyed by advancing years. Practically, therefore, unless engaged on very near objects such a person does not require to use convex glasses, his myopia covers his presbyopia. In cases of this kind it is often an advantage to order spectacles with
through
indistinct. For instance, a person about twenty-five years of age consults us, complaining that so long as he can remember, his sight has been defective for both near and distant objects. On examining his eyes we find their tension is normal, the media transparent. The patient cannot read
Sept.
ones one
1, 1881.]
Dr. C. MACNAMARA ON ERRORS OF REFRACTION & ACCOMMODATION. 239
less so, but if this individual inclines his side or the other the bars which he first saw
head
to
clearly
lose their distinctness, while those first seen dimly appear clear. It is not uncommon for patients affected in this way to be able in some degree, for a time, to overcome their defect of sight by means of an effort of accommodation ; but if this power fails them their vision
latter hypermetropic. Let us being emmetropic, the of one or both eyes exists ; condition suppose that this of it is evident that the rays
patients affected with astigmatism not unfrequently complain of symptoms of asthenopia in addition to the more permanent defect of
becomes very
imperfect,
and
so
vision above referred to, A case of this kind does not depend on amblyopia, in other words on defect of sight depending on serious changes in the deeper tissues of the eye ; for if such were the case the patient's vision would not be improved when
he looks through a small hole drilled in a card. On the other hand, if the surface of his cornea is not perfectly spherical, rays of light falling upon it cannot all be brought to the same focus on the retina ; if, however, the
patient under
these circumstances looks through a pin card held close to his eye, the small opening only gives passage to a minute pencil of light which falls upon an equally small extent of cornea, and passing through this, is accurately focused on the retina, being uninfluenced by the surrounding irregularities of the
hole in
a
In this way the patient's sight is improved when through a pin hole in a card. Again, a person suffering from astigmatism may see the vertical bars of the window at 20 feet distance, but cannot see the horizontal bars, because the refracting media of the vertical meridian of his eye is normal, but its
cornea.
he looks
horizontal meridian is too flat to admit of the rays which pass through it being focused on the retina, and the individual consequently cannot see the vertical, but only
of the window. If under these conditions the patient bends his head over to one side so as to bring the vertical meridian of his eyes into the horizontal position, he thus sees the cross bars of the window, but not the upright ones. It is evident that faults in the asymmetry of any of the other meridians of the cornea would cause symptoms of astigmatism equally
the horizontal bars
the
formed on the nerve and so the picture of any object a case we must order glasses would be blurred. In such with the refraction of the rays which will not interfere vertical meridian, but which will be the passing through in the horizontal to bring the rays convex sufficiently also to a focus on the retina. passing through this meridian must be plain in the vertical the glasses In other words, meridian. Lenses which and convex in the horizontal the of meridians of refraction are intended to equalize the segments the eye are called cylindrical lenses, being such as ordinary lenses a of a cylinder instead of sphere The surface of these cylindrical lenses are usually are but are either concave or convex parallel in one direction, and consequently they only in the other direction, passing through the latter plain. of light the refract rays same reason that the picture It is evident for the of a person suffering from astigformed on the retina the rays of light reflected from matism is imperfect, so will be unequally refracted on the back of such an eye cornea, consequently the optic disc of the from emerging with the ophthalmoscope such a person's eye examined to be circular it looks of appearing instead is distorted, By the direct method of examination more or less oval. the long axis of the disc corresponds to the meridian of the eye having the greatest refraction ; and the reason often appear to unduretinal vessels for the same the direction of the long method indirect the late. By more or less horizontal, axis of the disc is generally the lens from before withdrawing on its shape varying either absolutely the patient's eye, the image enlarging of the of greatest meridian direction in the
with imperfections in its horizontal or vertical meridians. The patient can see large objects, but their outline is distorted and often surrounded with greyish coloured lialos consequent on the irregular refraction of the rays of light in their passage through the cornea. But it is important to bear in mind the fact that the absence of clear vision dates back as long as the patient can reas I have before remarked, he may have been able to overcome some of the error of refraction by means of his of accommodation, especially
member, although,
power
before and for some few years after the adult period of life. It is remarkable that few persons' eyes are absolutely free from astigmatism ; but so long as the defect only amounts to such a degree as to slightly interfere with vision, it is not recognised. As a rule those cases of that come under the care of the surgeon for defects of sight depend on the vertical meridian being of a shorter focus than the horizontal one, the former
astigmatism
light passing through
be brought to a focus on the vertical meridian will through the horizontal those passing that but retina, to a focus behind the retina, meridian would be brought
or
relatively
refraction. the To ascertain the presence of Astigmatism.?In must be tested for hypermefirst place the patient's sight the defect of sight from which the tropia or myopia, for he due to errors of lefraction dependsuffers may patient of these causes. If such is not ing on one or other close one eye, and standing the case, the patient must half circle of radiating Snellen's about 15 feet from to approach the card until slowly be directed must lines, he sees at least some of the radiating lines clearly. The direction of these lines gives us the direction of the meridian of the greatest refraction of the patient's eye. Convex and concave spherical lenses are then held before the patient's eye until the highest convex or lowest conthe cave lens is found through which the individual sees Ihe card. indistinct on the otherwise were which lines spherical lens which thus brings the indistinct lines into view will indicate the strength of the cylin-
clearly
drical glasses required to overcome the defect in the refraction of the faulty meridian of the patient's eye. Astigmatism may exist in addition to myopia or in which case it is necessary to correct
hypermetropia, these
errors
of refraction
by
means
of proper
glasses
THIS INDIAN MEDICAL GAZETTE.
240
which must be applied to the eyes when testing them with cylindrical lenses to overcome their astigmatism. Tests such as those I have referred to are sufficient to overcome by far the greater number of cases of astig. matism, the more complicated instances (compound astigmatism) are comparatively rare, in which both meridians are singularly myopic or hypermetropic. I know of no more simple or effective means of testing these cases than
by
the
use
of Mr. Tvveedy's optometer.
[September 1,
1881.
/
1,1881.]
Dr C. MACN AMARA ON ERRORS OP
REFRACTION & ACCOMMODATION. 235 vision,
ORIGINAL COMMUNICATIONS.
eyes
/LECTURE
II.?errors of refraction
AND ACCOMMODATION OF THE EYE. By C.
Surgeon
to the
Macnamara,
Royal Westminster Ophthalmic Hospital, fyc. fyc. MYOPIA.
In cases of myopia the eyeball is too long from before backwards, so that when the accommodation is at rest rays of light are brought to a focus in front of the retina. The posterior part of the globe of the eye in myopia protruding so far backward that although the cornea and crystaline lens may be perfectly normal, nevertheless, rays of light passing through them are
focused anterior to the retina. This condition of things by means of a candle, lens and screen. If the light and the lens are so placed that parallel rays from the candle are accurately focused on the screen which is then withdrawn a short distance backwards, the image of the candle at once becomes blurred. Under these circumstances, if a suitable concave glass is placed between the candle and the lens it will disperse the rays of light falling upon the lens to such an ex-
may be illustrated
and in a considerable number of cases affected with a central scotoma
one or
both
are
arising from choroidal haamorrhage, or from a posterior staphyloma which has invaded tlie polar region of the eye ; detachment of the retina, softening of the vitreous or a cataract sometimes complicate cases of myopia. In order to guard a myopic eye from these complications it is neces-
sary to overcome the error of refraction from which the patient suffers in early life by means of proper concave
Many short-sighted persons have a peculiar habit of half closing their eyes when looking at distant objects ; the reason for this is that by nearly closing the eyelids the circles of diffusion of light otherwise formed on the retina are lessened. This fact may be demonstrated by rendering an emmetropic eye myopic by means of a convex lens held in front of the eye ; such a lens makes all distant objects appear blurred, because the rays of light passing through it are focused anterior to the retina. Under these circumstances if_ the eyelids are half closed an improvement in vision is at once apparent for the reason above referred to. As
rule
tent that
myopic eyes are large and prominent in feel somewhat hard on pressure, but the lengthening of the globe posteriorly is the essential feature of by far the majority of cases of This
and the
condition is
they are accurately focused upon the screen, image of the candle is then clearly defined. In some instances of myopia the length of the eye from before backwards has been known to measure a third
than that of the normal eye, so that the patient's accommodation being at rest, it is impossible for parallel rays to be brought to a focus on the retina. The pathological changes which give rise to posterior more
have been described, so that at present we refer to the symptoms and the appropriate only treatment of errors of refraction arising from this condition.
staphyloma
need
The diagnosis of myopic- is a matter of no great difficulty ; the patient's distant vision is either absent, or if present it is confused, but his sight is immediately improved if a suitable concave glass is held before his ?eyes.
In the
majority
cases
although
the distant
sight is very defective, vision for near objects ia excellent, and even appears to be better than in an emmetropic eye. The reason of this is, that a myopic patient, from the peculiar conformation of his eyes, is able to define small objects brought close to his face, and the nearer the object is to the eye the larger is the image it forms on the retina. Myopic patients therefore can often see very small objects which other people are unable to define, because if they were to attempt to bring the object as near their faces as myopic persons do, they would be unable to see them. The power which short-sighted persons possess of seeing very small objects is increased because their pupils are always largely dilated, and so they require less light to illuminate an object than people ordinarily do. But "when we come to examine more closely into the condition of their sight, we find that scarcely twenty-eight o per cent, of myopic persons have a normal acuity
a
front, they
frequently
myopia.
augmented through the increased pressure exercised by the ocular muscles on the globe of the eye. As I have before the conver-
remarked,
gence of the eyes on a near object and their accommodation take place simultaneously, the one act provoking the other; but in cases of myopia to see near objects it is only necessary for a person to use a very small amount of his accommodation in order that he may focus rays of light over his retina. The effort of accommodation being feeble, the act of convergence is also weak, and it requires an inordinate exertion on the part of a patient to force the internal recti muscles to converge the eyes upon a near
object. This increased effort wearies the muscles, and symptoms of asthenopia In myopia, therefore, the conare then established. not only to be maintained has of the vergence habieyes tually because the patient is short-sighted, but this
convergence of the eyes has to be kept up by an inordinate action on the part of the internal recti muscles for the reasons above referred to. This increased muscular emotion produces pressure on the globe and so tends to protrude still further backwards the weak posterior hemisphere of the eyeball. A
short-sighted
from muscular asthenopia complains of symptoms very similar to those of a patient affected with accommodatory asthenopia. His vision is perhaps clear when he commences his work, for he can then bring his optic axes accurately to converge upon it. But after a certain amount of work his eyes fill with person
suffering
The patient experiences a sensation of weariness and sometimes pain, and he is compelled to discontinue his occupation ; after a period of rest he can resume his work for a limited time. Inflammations of the margins of the eyelids, styes, and epiphora with tears
are?together
THE INDIAN MEDICAL GAZETTE.
236
headache?frequently met with in myopia ; these sympby abnormal muscular tension, and are be to cured only by the use of proper glasses. Another result arising from the necessity for increased convergence of the optic axis in myopia proceeding from insufficiency of the internal recti muscles is, that the patient unconsciously rotates one of his eyes outwards, so as to prevent an image of the object under observation being formed on the retina. In this way toms are induced
he gets rid of the difficulty which would otherwise arise from double vision. But an eye which is rotated outwards in this way gradually loses its power of sight. The recti are not the only mnscles however which are a source of trouble in myopia. We frequently find, young people affected with perhaps
especially among only a slight amount of myopia, that symptoms of accommodatory asthenopia exist, and on examining their eyes with the ophthalmoscope we find that the optic disc is extremely congested ; in the greater number of such cases spasm of the ciliary muscle exists, and must be overcome by keeping the muscle paralysed for
six weeks by the instllation of a strong atropine. After the effect of this drug has passed away the patient must be supplied with proper a
month
or
solution of
glasses. Ophthalmoscopic
concave
examination in
myopia.?The
direct
method of examination being employed, in consequence of the abnormal length of the eyeball from before backwards the rays of light emerging from the cornea are convergent rays which intersect immediately in front of the eye, so the image they form on the observer's retina is an inverted one. If the observer moves his head in one direction the retinal vessels will appear to travel in the opposite direction. If a proper concave lens is
placed behind the sight-hole
of the
ophthalmoscope,
image of the fundus of the eye will be obtained?the strength of the concave lens so employed will give us an idea of the amount of myopia from which erect
an
the
patient
suffers. Treatment
of
Myopia.
complaining that he distant objects clearly, but can see small type held close to his eyes, we direct him to place himself at twenty feet distance from Snellen's test types, and we examine first one eye and then the other ; it may be at this distance the patient can make out letters Supposing
a
patient applies
to us
is miable to define
No. 100 with the right eye, but with the left can only see No. 1G9 ; we then ascertain the lowest concave glass with which the patient can distinctly make out No. 25 at twenty feet, first the right eye and then the left being examined. The loioest concave glasses which the
give
best attainable sight at 20 feet may be ordered for the patient, and these with a few exceptions (to be described) are the only glasses he will require either for near or for distant vision. It is necessary to be careful to order the lowest concave glasses which give the best attainable
sight
at 20
feet, because with
a
stronger glass the patient
may see equally well by making use of his accommodation and so neutralizing the divergent action on the
of
light
of
the too powerful
glasses.
It is
rays in this
way that an
[September 1,
emmetropic,
or
even
an
1881,
hypermetropic
eye may see clearly with a weak concave glass held before it. We must bear in mind the fact that the treatment of short-sighted patients by means of glasses is to be directed towards increasing the distance at which near and far objects may be clearly seen. It requires for an individual first usinofor near objects?to overcome the habit he has formed of exerting an inordinate amount of convergence of his eyes on the objects, so that for near sight be may object to glasses which neutralize his myopia for distance. Unless, however, a shortsighted person can see ordinary size print, or work at a
little
concave
practice, however, glasses?especially
12 inches, he is bound to use concave glasses, because it is when employed on near work that accurate images are necessary, and these the myopic patient can only obtain by harmful approximation of the object to his eyes ; the only way to overcome this difficulty is by the use of concave glasses. Spectacles are less necessary for distance, because a short-sighted person may, if he
chooses, pass through the world and only see a small portion of it ; by so doing he will not exert injurious pressure upon the eyeballs through the constant action of the ocular muscles which he exercises in converging his eyes upon near objects. The question frequently arises with reference to the advisability of neutralizing a difference which may exist in the eyes of myopic patients?are we to attempt to overcome this difference by appropriate glasses for tho right and left eye. The rule ia *n cases of this kind
not to order glasses of different power for the two eyes, but to furnish both eyes with glasses that suit the best eye, for instance, concave 4 dioptics is the weakest glass which gives the best sight to the right eye, but the myopia in the left eye is neutralized by concave 5 dioptics. Do not order No. 4 for the right eye, and 5 for the left, but give the patient concave 4 dioptics for both eyes. In some few cases it may be necessary to obtain the best possible vision and to try and overcome the difference between the two myopic eyes with appropriate glasses for each eye ; under these circumstances,
the vision is not unfrequently rendered indistinct in consequence of the unequal size of the retinal images formed by lenses of different powers. As I have before remarked, there are exceptions to the above rule ; in consequence of the existence of excessive posterior staphyloma or some other complication the
however,
vision is so indistinct that the patient has to bring objects that he may see them ; very close to his eyes in order to overcome this condition a very strong concave glass may be necessary, but the image of the object is so much reduced in size that an imperfect retina is unable
appreciate it. Under these conditions insufficiency of the internal recti muscles may complicate the case. If a myopic patient's sight therefore is so defective that ho is unable to read No 3 of Snellen unless lie brings the type to within three or four inches of his eyes, it is not desirable to order spectacles for reading or work
to
of that
description.
Suppose that
a
myopic patient
who
is
somewhat
Sept. 1,
1881.]
Db. C. MACNAMARA ON ERRORS OF REFRACTION & ACCOMMODATION. 237
advanced in life, can read about 5 of Snellen at 12 but not beyond that distance, he is apt to suffer from a certain amount of discord between the accommodation and convergence of his eyes on a near object. If we
inches,
give such a patient the weakest glasses for reading which neutralize his myopia for distance, he is apt to complain of pain in his eyes when he attempts to use these glasses
objects ; because he has, while working at near objects, been accustomed to relax his accommodation. If we give him strong concave glasses which entirely neutralize his myopia, and which would therefore necessitate his employing his accommodation for near objects, the ciliary muscle not being accustomed to work refuses
on near
to
respond to the demand made upon it. Under these circumstances order glasses by which the patient can see 2 or 3 of Snellen at 14 inches. After he has worn these for a time, and brought his ciliary muscle into working order, he will probably take to stronger glasses. It is generally supposed that as short-sighted persons advance in years their myopia will be neutralized by presbyopia ; but when we remember that the latter condition is simsenile insufficiency of accommodation, it follows that myopics, although they may not be aware of the change, are nevertheless affected by it. As a rule however, short-sighted persons do not require spectacles before they are 60 years of age, and are then often benefited by weak convex glasses for reading and concave glasses for dis-
piy
a
Should their myopia have been of high degree, that the patient is unable to read No. 2, or 3, of Snellen beyond 8 inches, then as complete loss of accommodation is brought about by advancing years, they will still read clearly at about 8 inches, but will not see distinctly either beyond, or within that distance ; and so
tance. so
no glasses are required in such presbyopic without knowing it. Myopia is largely due to civilization ;
rule
as a
a
case, the pa-
tient is
that such is the is evident to those who have had to deal with masses of people removed fromtbe centres of civilization; among such people myopia is almost unknown, whereas, from statistics elaborated in Germany, of 10,000 students no less than 1,800 were found to be suffering from defects case
1,000 were due to myopia ; in ten the affection was hereditary. Among the children of village schools the percentage of children affected with myopia was 1*4, whereas it rose to ll'4 per cent in the town schools. Of these latter in the primary schools myopia affected 6*7 per cent of the scholars, in medium schools 10-3 and in the schools 26'2 per of refraction. Of these cases
per
cent.
In the
high
cent,
high schools
in the upper classes
are
more
than half of the
pupils
myopic.
These facts speak for themselves : it is evident that among young people the constant application of the eyes to near objects, such as reading and writing, causes myopia. This result is increased in proportion as the light is or from faulty position of the scholars the
oblige young persons to seek a larger retinal image by the approximation of the object under observation close to their eyes, consequently a good side light and properly arranged desks, together with well printed books, should be provided by all school authorities ; and if myopic pupils are found who cannot read small type with comfort at 12 or 14 inches from the eye, they should wear concave glasses. Presbyopia.
Presbyopia.?From the age of 20 to three score years and ten the acuity of vision decreases, but in addition to
this the accommodative energy diminishes with advancing age, the ciliary muscle gradually losing its power and the elasticity of the lens diminishing, so that by the time
a
person is 45 years of age he is unable to focus
his retina. It is evident, however, that must be held within a distance of from 12 to 13 inches from a person's eyes in order that a sufficiently large image of the letters may be formed on his
objects small print near
on
retina to enable him to see them distinctly. The necessity for approximating an object like this to the eye is increased if the sensibility of the retina to light diminishes, as it does with advancing years; consequently in aged persons as the nerve becomes less sensi-
the individual to bring small objects close of accommodation fails him, and eyes, his power Such a person so symptoms of presbyopia are induced. can see clearly at a distance, and with a proper convex lens can define the smallest type at 12 inches from his
tive, obliging to his
eyes. A patient 45 years of age or upwards, comes to us stating that he has had excellent sight all his life, but that he now finds difficultly in reading small type, specially by candle-light. He can see No. 25 of Snellen at 20 feet, but cannot read No. 1 at 12 inches. He can perhaps make out No. 2 if the type ig held 12 or 16 inches from his eyes. With convex 1 dioptics this indi-
vidual sees No. 1 of Snellen at 12 inches from his eyes, but these glasses disturb his distant vision, in fact he sees better without glasses, unless for reading, writing or such like work. Patients suffering from presbyopia instinctively seek so that tbey can sometimes see small type without glasses if a bright light is placed between their eyes and the book they are reading. The reason of this is, that in a very bright light the pupil contracts so that the circles of diffusion otherwise formed a
bright light,
completely
on the retina are diminished, and the individual's sight is in this way improved. That the contraction of the pupil is the cause of the improvement of vision is demonstrated by the fact that presbyopic persons when looking through a pin hole in a card find that their sight is of this kind is useful in many ; a test
causes are
vastly improved to distinguish errors of refraction from loss of sight depending on disease of the retina or some of the deep structures of the eye, which would not be assisted by looking through a minute hole. The correct glasses to overcome presbyopia depend
ness
upon the age of the
imperfect, book or writing too close to the
bad
upon which they are engaged is brought over work therefore, especially in a
face;
is the direct cause of myopia. The secondary bad print, too small a type, and slight haziof the cornea. In fact all those conditions which
light,
cases
individual, the
rule is at 45 years
THE INDIAN MEDICAL GAZETTE.
238 age order convex No. 1
of
of the glasses
dioptics, and increase the dioptics for every five years
strength by of age beyond 45, thus, at 50 years order convex No. 2 dioptics, at 55 No. 3, at 65 No. 5 and so on. There are however exceptions to this rule, for instance a person has been slightly hypermetropic all his life, but through the aid afforded him by his ciliary muscle he one
has overcome his defective
refraction,
and
managed
to
focus both parallel and divergent rays of light on his But when this person arrives at about the age of 4',) or 45 his ciliary muscle becomes less active, and his lens firmer, so that he now has difficulty in overcoming his hypermetropia by an effort of accommodation, and so both his near and far sight is defective. In a case of this kind at 20 feet the patient may be unable to see any of the letters beyond No. 100, but with a convex retina.
he reads No. 25, so that we may presume dioptics will cover his hypermetropia, but as this individual is 45 years of age, he will require No. 1 convex to neutralize his presbyopia, consequently we order him glasses of convex 1 X 1 *50=2*50 dioptics. No. 1
dioptic
As the
patient
advances in life he will
require
an
increase
dioptic for every five years, and in addition For convex 1*50 dioptics to cover his hypermetropia. instance, a patient has always worn No. 4 convex glasses one
hypermetropia, and he is now sixty years his for of age ; presbyopia he requires convex 4 dioptics, add this to No. 4 for his hypermetropia, and we order convex 8 dipotics for reading, writing and so on, but for ordinary use at a distance this individual only requires No. 4 dioptics, No. 8 would greatly inconvenience him, and to overcome this condition of things his spectacles of each glass may be so arranged that the upper portion contains a No. 4 convex lens, and the lower half No. 8 to overcome his
Under these conditions in raising his eyes for the patient looks throught No. 4 convex glasses, and in casting down his eyes for reading and writing he sees through No. 8 convex. But supposing a patient has been myopic all his life, will he require to alter his glasses as he advances in life ? As a rule in low degrees of myopia it not uofrequently
convex.
distant
object
of age a person will for distance and with weak convex glasses for near objects. But if the myopia has been higher, let us suppose to the extent of the
happens that
see
at about
sixty
better with concave
years
glasses
1881.
faivas possible, to enlarge the retinal image of all small objects, such as printed matter upon which they maybe engaged ; and to increase the size of the retinal image they are in the habit of bringing the type close to their eyes although using powerful convex glasses. The habitual approximation of small objects to the face necessitates an increased convergence of the eyes upon the object, and as in these cases the accommodation is in abeyance, discord in what should be harmonious actions is the result with a proportionate amount of strain on the eyes, and not unfrequently increased intra, ocular blood stasis and faulty contortion of the retina and other structures of the eye. We may,
howeverj
this difficulty by directing the patient to read with a hand (convex) glass through which he looks with one eye, and thus prevent the necessity for converging his eyes on the object upon which he is engaged. overcome
that about No. 1*50
of
[September 1,
Astigimatism.
that condition of one or both in which the of their vertical, horizontal, curvature eyes or some one or more of the meridians of the eyes is not perfectly spherical ; it may he one of these meridians is more convex or more concave than some other meridian ; in fact there can be endless varieties of this
By astigmatism
we mean
but for all practical purposes we may confine our attention to irregularities thus caused in the refraction of the eyes by congenital anomalies in the curvature of the cornea. I say congenital anomalies, because I need not here refer to those cases of astigmatism resulting from a wound of the cornea such as that which is sometimes met with after extraction ?f a cataract, conical For the same reason I may exclude cornea, and so onthose obscure cases of astigmatism resulting from defects in the lens, whether congenital ?r arising from irregular action of certain fibres of the ciliary muscle.
kind,
define astigmatism (in the sense in shall use the term) as a fault in the refracting media of the eye depending on irregularities of the cur. vature of the cornea, the various meridians of the cornea deviating to such a degree from the spherical form as to interfere with the perfection of sight. It is evident if an irregularity in the curvature of the So that we may
which
we
clearly beyond eight inches, relaxed that eight inches
meridians of the cornea exists that rays of light passing such a cornea must be irregularly refracted, and cannot all be brought to the same focus on the retina, so that an object seen by such an eye is blurred and
glasses the upper half of which contain concave and the lower half convex lenpes. Lastly, with advancing years the near point of vision not only recedes from the eyes, but in different degrees among various people the retina becomes less sensitive to lighf-,80 that some presbyopic people are obliged, as
small type, and his distant vision is imperfect, but on holding a card close to one of his eyes and looking through a pin hole he finds that his vision is improved. This individual notices that when he is standing at some distance from a clock, and closing one eye, with the other he can see some of the figure on the dial, but not all of them, at any rate with the same clearness. The horizontal bars of a window are distinct and the vertical
patient or
not
being
able to
see
when his accommodation was
was his far point, so that at that distance parallel rays of light were focused on his retina, he will still evidently see small objects at eight inches, although the whole of his accommodation may have been destroyed by advancing years. Practically, therefore, unless engaged on very near objects such a person does not require to use convex glasses, his myopia covers his presbyopia. In cases of this kind it is often an advantage to order spectacles with
through
indistinct. For instance, a person about twenty-five years of age consults us, complaining that so long as he can remember, his sight has been defective for both near and distant objects. On examining his eyes we find their tension is normal, the media transparent. The patient cannot read
Sept.
ones one
1, 1881.]
Dr. C. MACNAMARA ON ERRORS OF REFRACTION & ACCOMMODATION. 239
less so, but if this individual inclines his side or the other the bars which he first saw
head
to
clearly
lose their distinctness, while those first seen dimly appear clear. It is not uncommon for patients affected in this way to be able in some degree, for a time, to overcome their defect of sight by means of an effort of accommodation ; but if this power fails them their vision
latter hypermetropic. Let us being emmetropic, the of one or both eyes exists ; condition suppose that this of it is evident that the rays
patients affected with astigmatism not unfrequently complain of symptoms of asthenopia in addition to the more permanent defect of
becomes very
imperfect,
and
so
vision above referred to, A case of this kind does not depend on amblyopia, in other words on defect of sight depending on serious changes in the deeper tissues of the eye ; for if such were the case the patient's vision would not be improved when
he looks through a small hole drilled in a card. On the other hand, if the surface of his cornea is not perfectly spherical, rays of light falling upon it cannot all be brought to the same focus on the retina ; if, however, the
patient under
these circumstances looks through a pin card held close to his eye, the small opening only gives passage to a minute pencil of light which falls upon an equally small extent of cornea, and passing through this, is accurately focused on the retina, being uninfluenced by the surrounding irregularities of the
hole in
a
In this way the patient's sight is improved when through a pin hole in a card. Again, a person suffering from astigmatism may see the vertical bars of the window at 20 feet distance, but cannot see the horizontal bars, because the refracting media of the vertical meridian of his eye is normal, but its
cornea.
he looks
horizontal meridian is too flat to admit of the rays which pass through it being focused on the retina, and the individual consequently cannot see the vertical, but only
of the window. If under these conditions the patient bends his head over to one side so as to bring the vertical meridian of his eyes into the horizontal position, he thus sees the cross bars of the window, but not the upright ones. It is evident that faults in the asymmetry of any of the other meridians of the cornea would cause symptoms of astigmatism equally
the horizontal bars
the
formed on the nerve and so the picture of any object a case we must order glasses would be blurred. In such with the refraction of the rays which will not interfere vertical meridian, but which will be the passing through in the horizontal to bring the rays convex sufficiently also to a focus on the retina. passing through this meridian must be plain in the vertical the glasses In other words, meridian. Lenses which and convex in the horizontal the of meridians of refraction are intended to equalize the segments the eye are called cylindrical lenses, being such as ordinary lenses a of a cylinder instead of sphere The surface of these cylindrical lenses are usually are but are either concave or convex parallel in one direction, and consequently they only in the other direction, passing through the latter plain. of light the refract rays same reason that the picture It is evident for the of a person suffering from astigformed on the retina the rays of light reflected from matism is imperfect, so will be unequally refracted on the back of such an eye cornea, consequently the optic disc of the from emerging with the ophthalmoscope such a person's eye examined to be circular it looks of appearing instead is distorted, By the direct method of examination more or less oval. the long axis of the disc corresponds to the meridian of the eye having the greatest refraction ; and the reason often appear to unduretinal vessels for the same the direction of the long method indirect the late. By more or less horizontal, axis of the disc is generally the lens from before withdrawing on its shape varying either absolutely the patient's eye, the image enlarging of the of greatest meridian direction in the
with imperfections in its horizontal or vertical meridians. The patient can see large objects, but their outline is distorted and often surrounded with greyish coloured lialos consequent on the irregular refraction of the rays of light in their passage through the cornea. But it is important to bear in mind the fact that the absence of clear vision dates back as long as the patient can reas I have before remarked, he may have been able to overcome some of the error of refraction by means of his of accommodation, especially
member, although,
power
before and for some few years after the adult period of life. It is remarkable that few persons' eyes are absolutely free from astigmatism ; but so long as the defect only amounts to such a degree as to slightly interfere with vision, it is not recognised. As a rule those cases of that come under the care of the surgeon for defects of sight depend on the vertical meridian being of a shorter focus than the horizontal one, the former
astigmatism
light passing through
be brought to a focus on the vertical meridian will through the horizontal those passing that but retina, to a focus behind the retina, meridian would be brought
or
relatively
refraction. the To ascertain the presence of Astigmatism.?In must be tested for hypermefirst place the patient's sight the defect of sight from which the tropia or myopia, for he due to errors of lefraction dependsuffers may patient of these causes. If such is not ing on one or other close one eye, and standing the case, the patient must half circle of radiating Snellen's about 15 feet from to approach the card until slowly be directed must lines, he sees at least some of the radiating lines clearly. The direction of these lines gives us the direction of the meridian of the greatest refraction of the patient's eye. Convex and concave spherical lenses are then held before the patient's eye until the highest convex or lowest conthe cave lens is found through which the individual sees Ihe card. indistinct on the otherwise were which lines spherical lens which thus brings the indistinct lines into view will indicate the strength of the cylin-
clearly
drical glasses required to overcome the defect in the refraction of the faulty meridian of the patient's eye. Astigmatism may exist in addition to myopia or in which case it is necessary to correct
hypermetropia, these
errors
of refraction
by
means
of proper
glasses
THIS INDIAN MEDICAL GAZETTE.
240
which must be applied to the eyes when testing them with cylindrical lenses to overcome their astigmatism. Tests such as those I have referred to are sufficient to overcome by far the greater number of cases of astig. matism, the more complicated instances (compound astigmatism) are comparatively rare, in which both meridians are singularly myopic or hypermetropic. I know of no more simple or effective means of testing these cases than
by
the
use
of Mr. Tvveedy's optometer.
[September 1,
1881.
