A handbook of the diseases of the eye and their treatment

Front matter

SWANZY

A HANDBOOK

AND THEIR

Preface To The Eighth Edition

In the preparation of this Edition the book has been again revised throughout. Moreover, some fresh matter has been introduced, and some obsolete matter omitted, in order to bring the work up to date, without materially increasing its size. The following are some of the additions : — In Chapter IV., A Description of Conjunctivitis Petrificans; in Chapter VI., A Description of Grating Keratitis, Guttate Keratitis, Keratitis Aspergillina, Recurrent Abrasion of the Cornea, and of Injuries of the Cornea caused by blows and Burns ; in Chapter VII., A More Detailed Account of Herpes Zoster Ophthalmicus, A Description of Lymphangiodes of the Eyelids, and of Pfliiger's Method of Tarsorrhaphy; in Chapter VIII., A More Detailed Account of Kuhnt's Method of Extirpation of the Lacrimal Sac ; in Chapter X., in view of the vast importance of the subject, A More Detailed Account of Sympathetic Ophthalmitis ; in Chapter XIV., A More Detailed Account of the Use of the Magnet for Foreign Bodies in the Eye ; in Chapter XVL, A More Detailed Account of Tumors of the Optic Nerve; in Chapter XVII. , A Description of the Eye Symptoms attendant upon Myasthenia Gravis; in Chapter XVIIL, A Description of Mr. P. W. Maxwell's Operation for Shrunken Socket, and of Professor Kronlein's Temporary Resection of the Outer Wall of the Orbit for Orbital Tumors, etc. ; in Chapter XIX., A Description of Amaurotic Family Idiocy.

I am indebted to Dr. Kathleen Lynn for the great care she has given to the preparation of the Index.

23 Merrion Square, June, 1903.

Preface To The Fourth Edition

The Third Edition of this book was pubhshed in October, 1890, and I am gratified that the work continues to find favor, not only with students, for whom it is mainly intended, but also with practitioners.

The book has now again been revised throughout, and brought up toi date.

In an Appendix, Holmgren's Method for Testing the Color Sense has been described in greater detail than before.

Some new illustrations have been added.

The great difficulty of an author in the preparation of a book like this consists in " Saying not all he might, but all he ought." It is his duty to give a succinct and practical account of his subject in its most modern aspect, without weighting his pages with excessive detail and prolonged discussion. This has been my aim. For deeper and wider information, larger handbooks and original monographs must be consulted.

23 Mf.rrton Square, October, 1892.

Chapter I. Page

Numbering of Trial-Lenses and Spectacle Glasses — Normal Refraction and Accommodation — The Meter Angle — The Angle Gamma — The Sense of Sight (Light-Sense, Color-Sense, Form- Sense) — The Field of Vision . . . . = . . .17

Chapter II. Abnormal Refraction And Accommodation

Hypermetropia — Correction of H. — Amplitude of Accommodation in H. — Angle Gamma in H. — Cramp of Ciliary Muscle in H. — Accommodative Asthenopia in H. — Internal Strabismus in H. — The Prescribing of Spectacles in H 2>^

Myopia — Determination of Degree of M.— Amplitude of Accommodation in M. — Angle Gamma in M. — Complications of Progressive M. — Functional Anomalies Attendant upon M. — Management of M. — The Prescribing of Spectacles in M. — Operative Cure of M. 42

Astigmatism — Symptoms of As. — Estimation of Degree of, and Correction of As. — The Astigmometer — Lental As. — Irregular As. . 52

Anisometropia 64

Anomalies of Accommodation — Presbyopia — Paralysis of Accommodation — Cramp of Accommodation 64

CHAPTER HI.

THE OPHTHALMOSCOPE.

Why Necessary — Helmholtz's Ophthalmoscope — Modern Ophthalmoscope — Direct Method — Indirect Method 70

Estimation of the Refraction by Aid of the Ophthalmoscope — Direct

Method — Retinoscopy 76

Focal Illumination 87

The Normal Fundus Oculi as Seen with the Ophthalmoscope — The Optic Papilla, or Optic Disc— The Retina— The Macula Lutea— The General Fundus Oculi — The Retinal Vessels . . . .88 lo CONTENTS.

Chapter IV. Diseases Of The Conjunctiva

PAGE

Hyperemia — Conjunctivitis — Catarrhal Conjunctivitis — Acute Blennorrhea of the Conjunctiva, or Purulent Ophthalmia — Follicular Conjunctivitis — Spring Catarrh — Hay Fever — Trachoma, Granular Conjunctivitis, or Granular Ophthalmia — Acute Trachoma, or Acute Granular Ophthalmia — Chronic Trachoma, or Chronic Granular Ophthalmia — Lymphoma of the Conjunctiva — Croupous Conjunctivitis — Diphtheritic Conjunctivitis — Conjunctival Complications of Smallpox — Amyloid Degeneration — Tubercular Disease of the Conjunctiva — Lupus — Pemphigus — Xerosis, or Xerophthalmos — Pterygium — Pinguecula — Subconjunctival Ecchymosis — Nevus — Polypus — Dermoid Tumors — Lipoma — Syphilitic Disease of the Conjunctiva — Papilloma, or Papillary Fibroma — Epithelioma — Sarcoma — Simple Cysts — Subconjunctival Cysticercus — Lithiasis — Conjunctivitis Petrificans — Uric Acid Deposits — Injuries of the Conjunctiva 93

Chapter V. Phlyctenular, Or Strumous, Conjunctivitis And Keratitis

Solitary, or Simple, Phlyctenula of the Conjunctiva — Multiple, or Miliary, Phlyctenula of the Conjunctiva — Modes of Secondary Corneal Affection — Primary Phlyctenular Keratitis — Different Forms of Same — Symptoms of Phlyctenular Keratitis — Causes of Phlyctenular Ophthalmia — Treatment 133

Chapter VI. Diseases Of The Cornea

Inflammations of the Cornea — (a) Ulcerative Inflammations of the Cornea — Simple Ulcer — Deep Ulcer — Serpiginous Ulcer — Rodent Ulcer — Marginal Ring Ulcer — Absorption Ulcer — Neuro-Paralytic Keratitis — Infantile Ulceration of the Cornea with Xerosis of the Conjunctiva — Herpes' — Filamentary Keratitis — Bullous Keratitis — Dendriform Keratitis — Keratitis Aspergillina .... 140 (b) Non-Ulcerative Inflammations of the Cornea — Abscess — Diffuse Interstitial, or Parenchymatous, Keratitis — Keratitis Profunda— Grating-like Keratitis— Guttate Keratitis— Keratitis Punctata — Sclerotizing Opacity — Riband-like Keratitis . . . 161

Ectasies of the Cornea — Staphyloma Cornese (Evisceration and Mules'

Operation) — Conical Cornea, or Keratoconus .... i6g

Tumors of the Cornea .... 177

CONTENTS. II

Injuries of the Cornea — Foreign Bodies — Losses of Substance — Recurrent Abrasion, or Disjunction — Blows — Burns — Perforating Injuries 177

Opacities of the Cornea — Nebula, ^lacula, Leukoma — Arcus Senilis . 183

Chapter VII. Diseases Of The Eyelids

Eczema — Herpes Zoster Ophihalmicus — Primary Syphilitic Sores — Secondary Syphilitic Sores — Tertiary Syphilitic Affection — Vaccine Vesicles — Rodent Ulcer — Marginal Blepharitis (Ophthalmia Tarsi) — Phtheiriasis Ciliorum — Hordeolum (Stye) — Chalazion (Meibomian Cyst, Tarsal Tumor) — Milium — Molluscum — Nevus — Xanthelasma — Chromidrosis — Epithelioma, Sarcoma, Adenoma and Lupus — Solid Edema, or Elephantiasis Lymphangioides — Clonic Cramp of the Orbicularis JMuscle — Blepharospasm — Ptosis — Operations for its Cure — Lagophthalmos — Symblepharon — Blepharophimosis — Canthoplastic Operation — Distichiasis and Trichiasis — Operations for their Cure — Entropium — Operations for its Cure — Spastic Entropium — Senile Entropium — Operations for its Cure — Ectropium — Operations for its Cure — Ankyloblepharon — The Restoration of an Eyelid — Injuries — Ecchymosis — Epicanthus — Congenital Coloboma 187

Chapter VIII. Diseases Of The Lacrimal Apparatus

Malposition of the Punctum Lacrimale — Stenosis and Occlusion of the Punctum Lacrimale — Obstruction of the Canaliculus — Stricture of the Nasal Duct — Blennorrhea of the Lacrimal Sac — Obliteration and Extirpation of the Lacrimal Sac — Acute Dacryocystitis — Dacryoadenitis — Hypertrophy of the Lacrimal Gland . . . 233

Chapter IX. Diseases Of The Sclerotic

Inflammations of the Sclerotic — Periodic Transient Episcleritis, or Hot Eye — Episcleritis — Deep Scleritis — Injuries of the Sclerotic — Tumors of the Sclerotic — Pigment Spots 244

Chapter X. Diseases Of The Uveal Tract

Iritis — Symptoms — Syphilitic — Rheumatic — Gonorrheal — Causes

— Prognosis — Treatment . . 250

12 CONTENTS.

Cyclitis 259

Chorioiditis — Disseminated — Syphilitic — Central Senile Guttate — Central — Purulent 261

Sympathetic Ophthalmitis, and Sympathetic Irritation — Introductory — Sympathetic Irritation — Sympathetic Ophthalmitis — Diagnosis — Prognosis — Treatment — Prophylaxis — Therapeutic and Optical Operations used in Sympathetic Ophthalmitis — Pathogenesis . 265

Injuries of the Iris — Punctured Wounds — Foreign Bodies — Iridodialysis — Retrofleijcion — Rupture — Dehiscence — Traumatic Mydriasis 284

Injuries of the Ciliary Body — Punctured Wounds — Foreign Bodies . 287

Injuries of the Chorioid — Foreign Bodies — Incised Wounds — Rupture — Extravasation of Blood 288

New Growths of the Iris — Cysts — Granuloma — Tubercle — Sarcoma —

Ophthalmia Nodosa 289

New Growths of the Ciliary Body — Sarcoma — Myosarcoma — Carcinoma 292

New Growths of the Chorioid — Sarcoma — Carcinoma — Tubercle —

Sarcoma Carcinomatosum — Myosarcoma — Osteosarcoma . . 292

Other Diseases of the Chorioid — Posterior Staphyloma — Detachment

— Central Senile Areolar Atrophy . 297

Malformations of the Iris . . . . . . . . , 297

Malformations of the Chorioid 298

Operations on the Iris 300

CHAPTER XL

MOTIONS OF TflE PUPIL IN HEALTH AND DISEASE.

The Size of the Pupil in Health — Contraction of the Pupil — Dilatation of the Pupil 303

The Action of the Mydriatics and Myotics on the Pupil . . . 309 The Size of the Pupil in Disease — Myosis — Mydriasis . . . 309

Tabular Arrangement of Mydriatics, Myotics, and Anesthetics . 316

Chapter XII. Glaucoma

Primary Glaucoma — Chronic, or Non-Inflammatory, Glaucoma — Acute, or Inflammatory, Glaucoma — Glaucoma Fulminans — Subacute Glaucoma — Etiology — Pathology — Treatment . . . 320

Congenital Hydrophthalmos 340

Chapter XIII. Diseases Of The Crystalline Lens

Complete Cataracts — Senile Cataract — Progress, Pathogenesis, and

Etiology — Treatment 342

CONTENTS. 13

Complete Cataract of Young People— Diabetic Cataract — Complete

Congenital Cataract — Black Cataract 349

Partial Cataracts — Central Lental Cataract — Zonular, or Lamellar, Cataract — Anterior Polar, or Pyramidal, Cataract — Fusiform, or Spindle-Shaped, Cataract 35o

Secondary Cataract — Posterior Polar Cataract — Total Secondary

Cataract 352

Capsular Cataract 353

Traumatic Cataract 353

Operations for Cataract — Extraction of Cataract — Linear Extraction The Modified Peripheral Linear Extraction — The Three Millimeter Flap Operation — Cataract Extraction without Iredectomy . 355

Discission, or Dilaceration — Suction Operation — Secondary Cataract

and its Operation — Capsulotomy — Iridotomy 37^

Dislocation of the Crystalline Lens — Lenticonus — Aphakia . . . 3^1

Chapter XIV. Diseases Of The Vitreous Humor

Purulent Inflammation — Other Inflammatory Affections — Opacities — Muscle Volitantes — Fluidity (Synchysis) — Synchysis Scintillans — Foreign Bodies in the Vitreous Humor and Interior of the Eye in General — Use of Rontgen Rays in Detecting Foreign Bodies — The Sideroscope — Removal of Foreign Bodies — The Magnet — Cysticercus — Blood- Vessels — Persistent Hyaloid Artery — Detachment ........ 384

Chapter XV. Diseases Of The Retina

Alterations in Vascularity — Hyperemia — Anemia 400

Retinitis — Syphilitic — Hemorrhagic — Albuminuric — Diabetic — Leukemic — Punctata Albescens — Development of Connective Tissue, or Retinitis Proliferans — Retinitis Circinata — Purulent Retinitis . 401

Atrophy of the Retina — Retinitis Pigmentosa — Gyrate Atrophy of the

Retina and Chorioid 407

Diseases of the Retinal Vessels — Apoplexy of the Retina — Embolism of the Central Artery — Thrombosis of the Central Artery — Thrombosis of the Central Vein — Aneurysm of the Central Artery —Sclerosis of the Retinal Vessels (Perivasculitis)— Quinin Amaurosis 4^9

Injury of the Retina by Strong Light — Sunlight — Snow-Blindness —

Effect of Electric Light on the Eyes 4I5

Tumor of the Retina — Glioma 416

Parasitic Disease of the Retina— Cysticercus . . , . • 4^7

14 CONTENTS.

Detachment of the Retina 4^^

Traumatic Affections of the Retina — Traumatic Anesthesia — Commotio Retinae, or Traumatic Edema of the Retina .... 422

Chapter XVI. Diseases Of The Optic Nerve

Optic Neuritis (Papillitis), due to: Cerebral Tumors — Tubercular Meningitis — Hydrocephalus — Tumors of the Orbit — Inflammatory- Processes in the Orbit — Exposure to Cold — Suppression of Menstruation — Chlorosis — Syphilis — Rheumatism — Lead Poisoning — Peripheral Neuritis — Multiple Sclerosis — Tabes Dorsalis — Acute Myelitis — and to Hereditary and Congenital Predisposition . . 424

Toxic Central Amblyopia, or Central Scotoma — Retrobulbar Neuritis — Optic Neuritis, Associated with Persistent Dropping of Watery Fluid from the Nostril 429

Atrophy of the Optic Nerve, due to : Optic Neuritis — Pressure — Embolism of the Central Artery of the Retina — Syphilitic Retinitis, Retinitis Pigmentosa, Choriodo-Retinitis, and to Disease of the Spinal Cord (Spinal Amaurosis) — Optic Atrophy as a purely Local Disease 433

Tumors of the Optic Nerve — Hyaline, or Colloid, Outgrowths — Injuries of the Optic Nerve 436

Amblyopia due to Hemorrhage from the Stomach, Bowels, or Uterus

— Glycosuric Amblyopia 437

Chapter XVII. The Motions Of The Eyeballs, And Their Derangements

Actions of the Orbital Muscles — Inclination of the Vertical Meridian in the Several Principal Positions — Muscles called into Action in the Several Principal Positions — The Field of Fixation — Strabismus 441

Paralyses of the Orbital Muscles — General Symptoms — Paralysis of the External Rectus — Paralysis of the Superior Oblique — Paralsis of the Internal Rectus, Superior Rectus, Inferior Oblique, and Levator Palpebrse — Ophthalmoplegic Migraine — Ophthalmoplegia Externa, or Nuclear Paralysis — Fascicular Paralysis — Myasthenia Gravis — Cerebral Paralysis of Orbital Muscles — The Localizing Value of Paralysis of Orbital Muscles in Cerebral Disease . 447

Convergent Concomitant Strabismus — Causes — Single Vision in — Amblyopia of Squinting Eye — Clinical Varieties of — Measurement of — Mobility of Eye in — Treatment — Orthoptic Treatment — Operative Treatment — Tenotomy — Advancement of External Rectus — Dangers of the Strabismus Operation — Treatment Subsequent to Operation 471

CONTENTS. 15

Insufficiency of the Internal Recti, and Divergent Concomitant Strabismus — Muscular Asthenopia — Treatment — Operative Treatment 494 Nystagmus 499

Chapter XVIII. Diseases Of The Orbit

Orbital Cellulitis — Periostitis of the Orbit — Caries of the Orbit — Injuries of the Orbit — Orbital Tumors — Implication of Neighboring Cavities — Pulsating Exophthalmos — Tumors of the Lacrimal Gland — Hernia Cerebri — Exophthalmic Goiter — Enophthalmos . 501

Part I. Ocular Diseases And Symptoms Liable To Accompany Focal Diseases

OF THE BRAIN.

Hemianopsia — Arrangement of the Cortical Visual Centers, their Relations to the Retina, and the Course of the Optic Fibers between these Two Points — Localization of the Lesion in Hemianopsia — Alexia, or Word-BHndness — Visual Aphasia — Dyslexia — Amnestic Coior-Blindness — Visual Hallucinations — ]\Iind-Bhndness, or Optic Amnesia 522

Part II

OCULAR DISEASES AND SYMPTOMS LIABLE TO ACCOMPANY DIFFUSE ORGANIC DISEASES OF THE BRAIN.

Disseminated Sclerosis of the Brain and Spinal Cord — Diffuse Sclerosis of the Brain — General Paralysis of the Insane — Amaurotic Family Idiocy — Meningitis — Traumatic Meningitis — Hydrocephalus — Infantile Paralysis — Paralysis Agitans — Encephalopathia Saturnia — Epilepsy — Chorea 53^

Part III

OCULAR DISEASES AND SYMPTOMS LIABLE TO ACCOMPANY DISEASES AND INJURIES OF THE SPINAL CORD.

Tabes Dorsalis — Hereditary Ataxy — Myelitis — Syringomyelia, and Morvan's Disease — Myotonia Congenita — Acute Ascending Paralysis — Injuries of the Spinal Cord ....... 543 i6 CONTENTS.

Part IV. Nervous Amblyopia, Or Nervous Asthenopia

PAGE

Nervous Amblyopia in Neurasthenia — Nervous Amblyopia in Hysteria — Nervous Amblyopia in Traumatic Neurosis . . . 547

Part V. Various Forms Of Amblyopia

Transitory Hemianopsia, or Scintillating Scotoma — Congenital Amblyopia — Reflex Amblyopia — Night-Blindness — Uremic Amblyopia Pretended Amaurosis — Erythropsia . . . » . . 552

APPENDIX I.

Holmgren's Method for Testing the Color-Sense . . . . . 559

APPENDIX II.

Regulations as to Defects of Vision which Disqualify Candidates for Admission into the Civil, Naval, and Military Government Services, the Royal Irish Constabulary, and the Mercantile Marine . 56?

DISEASES OF THE EYE.

Chapter I. The Numbering Of Trial-lenses

The lenses in trial cases and in spectacles are numbered according to the metrical system.

The lens of one meter (39 1-2 inches) focal length is called the Dioptric Unit, or the Dioptry (i D), of the metrical system. 2 D, 3 D, 4 D, etc., indicate the number of meter lenses, or dioptrics, contained in each of these lenses. 2 D is therefore twice as powerful a lens (its focal length only half as long) as I D.

Convex lenses are indicated by the + sign placed before their number, thus, -]- 5 D; and concave lenses by the — sign, thus, -5D.

If it be required to ascertain the focal length of a given lens, 100 (i meter = 100 cm.) is to be divided by the dioptric number of the lens, and the answer will give the focal length in centimeters. For example, the focal length of 10 D is W^ = 10 cm.

If the focal length of the lens be known, and it be desired to ascertain its dioptric number, we find it by dividing 100 cm. by the focal length. For example, if the focal length be 33 cm., then VV = 3 D-

THE POWERS OF NORMAL REFRACTION AND ACCOMMODATION OF THE EYE.

The eye is a dark chamber with a series of convex refracting surfaces — namely, the cornea, and the anterior and posterior surfaces of the crystalline lens ; and certain intraocular or dioptric media — namely, the aqueous humor, the substance of the crystalline lens, and the vitreous humor. By aid of this apparatus, which is called the dioptric system of the eye, distinct inverted images of external objects are formed on the retina.

The refracting media are centered on the optical axis {O A, Fig. i) an imaginary line which, passing through the optical center (A^) of the eye, meets the retina at a point {A) slightly to the inner side of the macula lutea (M).

In treating of the eye we have to consider two sets of visual objects — viz., distant objects and near objects. Distant objects

Fig. I {right eye).

are those at 6 meters and more from the eye ; near objects are those closer to the eye than 6 meters. For practical purposes the rays which pass through the pupil, coming from any given point of a distant object, are as good as parallel, their divergence being so very slight when they reach the eye, and we regard them as being parallel.

Refraction.

By the Refraction of the Eye is meant the faculty it has when at rest {i. e., without an effort of accommodation) of altering the direction of rays of light which pass into it, making parallel rays convergent, and divergent rays less divergent.

In Normal Refraction, or Emmetropia (e'jajuerpov, the standard; ojip, the eye), as it is termed, parallel rays (see Fig. 2, in which the object from which the rays come is supposed to be 6 meters or more from the eye) in passing through the dioptric media are given such a convergence that they are brought to a focus on the layer of rods and cones of the retina, and form on it a distinct inverted image of the point or object from which they come. In other words, the retina is placed at the principal focus of the dioptric system of the normal eye, which is thus adapted for parallel rays, and its far point {vide infra) is at infinity.

ACCOMMODATION.

Accommodation.

The eye can see near objects distinctly as well as distant objects, although the rays from any given point (a, Fig. 3) of a near object reach the eye with a divergence so considerable that they could not be brought to a focus on the retina by the unaided refraction, but would converge towards a point (their conjugate

Fig. 2.

focus a') behind the retina, and would not form a distinct image on the latter, but merely a blurred image or circle of diffusion (at & c). It is obvious, therefore, that an increase of refracting power in the eye is necessary, in order that near objects may be

Fig. 3.

distinctly seen. It is this increase in the refracting power for the purpose of near vision which is called Accommodation.

The Mechanism of Accommodation is as follows : The ciliary muscle (m, Fig. 4) contracts, thus drawing forward the chorioid and the ciliary processes, and relaxing the zonuh of

Zinn (2), which is attached to the latter. The lens (/), which was flattened by the tension of the zonula, is now free to assume a more spherical shape, in response to its own elasticity. The posterior surface of the lens scarcely alters in shape, being fixed in the patellary fossa; but the anterior surface becomes more convex, thus increasing its refracting power. Associated with the act of accommodation is a contraction of the pupil. The acompanying figure (Fig. 4) represents the changes which take place in accommodation, the dotted lines indicating the latter state.

Tscherning^ has shown that the increased curvature of the anterior surface of the lens occurs mainly in the center of that surface — in other words, that in accommodation the anterior surface becomes somewhat conical, and not merely more spherical. He

Fig. 4. — c, cornea ; a, anterior chamber ; /, lens ; v, vitreous humor ; i, iris ; 2, zonula of Zinn; m, ciliary muscle.

holds that this is due to a tightening, and not to a relaxation of the zonula. This theory has been vigorously combated by other observers, and its true value remains to be determined.

The Far Point and the Near Point.— It is possible for the eye to see objects accurately at every distance from its Far Point — i e., its most distant point of distinct vision (Punctum Remotum, — R.), up to a point only a few centimeters from the eye, called the Near Point (Punctum Proximum, — P.). We can find the latter by directing the patient to look at a page printed in small type, and by bringing it slowly closer and closer to his

ACCOMMODATION. 21

eye, until a point is reached where he cannot distinguish the words and letters, which become blurred. A point very slightly more removed from the eye than this, where he can read distinctly, is the near point. Between the near point and the eye vision is indistinct, because no effort of the ciliary muscle can produce the amount of convexity of the lens required for so short a distance.

The Amplitude of Accommodation. — This is the amount of accommodative effort of which the eye is capable — i. e., the effort it makes in order to adapt itself from its Far Point (R.) up to its Near Point (P.). The amplitude of accommodation {a), therefore, is equal to the difference between the refracting power of the eye when at rest (r), and when its accommodation is exerted to the utmost (/>), as expressed by the formula a =

Fig. 5.

p — r. It may be represented by that convex lens placed close in front of the eye, which would take the place of the increased convexity of the lens, or, in other words, which would give to rays coming from the nearest point of distinct vision a direction as if they came from the far point. The number of this lens expresses the amplitude of accommodation in a given eye.

For example: if, in an emmetropic eye {E, Fig. 5) the near point be situated at 20 cm., then a convex lens (L) of 20 cm. focal length placed close to the eye (between that point and the eye) would give to rays coming from the near point a direction {i. e., would make them parallel) as though they came from a distant object, and this normally refracting eye would then be enabled by aid of its refraction alone to bring these rays to a focus on the retina. Making use of the above equation, we find in this case — since a focal length of 20 cm. represents a lens of

5 D — that a = 5 — r, but R. being situated at infinity we designate it by the sign 00 ; hence, r =z — - = — = o ; therefore a

The amount of ampHtude of accommodation (i. e., the number of the lens which would represent it) is the same in every kind of refraction, according to the age of the individual, but in emmetropia alone is a = p as above, because in it alone is r = o.

Under the head of " Anomalies of Accommodation," chap, ii., will be found Professor Bonders' diagram representing the amplitude of accommodation at different ages.

Connection betv^een Accommodation and Convergence (Relative Accommodation). — With every degree of convergence of the visual lines a certain effort of accommodation is associated. Thus, if the object be situated 2 meters from the eye the visual lines converge to that point, and a certain effort of accommodation is made. But this connection between accommodation and convergence is somewhat elastic, for the accommodative effort may be increased or decreased, while the object is kept distinctly in view, and the same convergence maintained. That it may be increased is shown by the experiment of placing a weak concave glass before the eye, when it will be found that the object is still distinctly seen ; and if a weak convex glass be then held before the eye the object will also be clearly seen, showing that the accommodative effort may be lessened without affecting vision or convergence. This amplitude of accommodation for a given point of convergence of the visual lines, found by the strongest concave and strongest convex glasses with which the object can still be distinctly seen, is called the Relative Amplitude of Accommodation. That part of it which is already in use, and

* It must be observed that R represents the distance of the Far Point from the eye, while r represents the refractive power which is added to the eye by accommodation or by a lens in order to adapt it for the distance R. Hence it is evident that r = ^, because the strength, or refractive power, of a lens is inversely as its focal length — e. g., a lens of the strength of 4 D will have a focal length of 1-4 that of a lens of i D— -/. e.,

i^^2-^^ =0.25 cm. (see above. Numbering of Trial-Lenses). Similarly,

4 /J = - and a = - ; P representing the distance of the Near Point,, and

A the focal length of the lens which represents the Accommodation.

im 4

ACCOMMODATION.

is represented by the convex lens, is termed the negative part; while the positive part is represented by the concave lens, and has not been brought into play. For sustained accommodation at any distance it is necessary that the positive part of the relative amplitude of accommodation be considerable in amount.

Moreover, the convergence may be altered, while the same effort of accommodation is maintained, as is shown by the experi-

FiG. 6.

Fig. 7.

ment of placing a weak prism with its base inwards before one eye. In order that the object may then be seen singly, it will be necessary for the eye before which the prism is placed to rotate somewhat outwards ; and it will be found that the individual can do this, while at the same time he sees the object with the same distinctness, showing that the same effort of accommodation has been maintained, although the angle of convergence of the visual axis is less than before.

The Meter Angle.

If the visual line (£ i, Fig. 6) of an eye (E) have to be brought to bear on a point (i, Fig. 6) i meter distant from it in the median line (M i), the angle of convergence (E i M) which the visual line thus makes with the median line is called the Meter Angle. It expresses the degree of convergence necessary for binocular vision at that distance, and is employed as the unit for expressing other degrees of convergence. If, for example, an object be situated i-2 a meter (i-2, Fig. 6) from the eye, the angle of convergence {E i-2 M) must be practically twice as large as at I meter: C. (Convergence = 2 meter angles. If the object be only 1-3 of a meter distant, 3 meter angles are required: C. = 3 meter angles. If the object be situated 2 meters from the eye, the angle of convergence will be only one-half as great as at i meter, and here C. = 1-2 meter angle; while if the eye be directed towards a distant object (D) there will be no angle of convergence, and if the visual lines be divergent the meter angle A^ill be negative.

Now the average normal emmetropic eye requires for each distance of binocular vision as many meter angles of convergence as it requires dioptrics Q>i accommodation. For a distance of i meter an efifort of accommodation of I dioptry is required, and also i meter angle of convergence; at 1-2 meter from the eye 2 D of accommodation is required and 2 meter angles; at 1-3 meter from the eye 3 D of accommodation and 3 meter angles, and so on; while for distant objects neither angle of convergence nor effort of accommodation is required.

The Angle Gamma.

The Optic Axis is an imaginary line (P' P, Fig. 7) which pas'ses through the center (C) of the cornea and the posterior pole (F) of the globe — a point situated between the macula lutea (M) and the optic papilla (D). The Visual Line {M O) unites the point of fixation (O) — the object looked at — with the macula lutea; it does not coincide with the optic center {K) of the eye. The Line of Fixation {R 0) joins the center of rotation {R) of the eye with the point of fixation. The Angle y is the angle O R P' formed at the center of rotation by the optic axis and the line of fixation.

The line of fixation and the visual line so nearly coincide that in practice we regard them as identical ; and hence, in practice, the angle y is the angle O K P' . It should not be confounded, as is often the case, with The Angle Alpha, which is the angle O K C formed at the nodal point by the visual line and the major axis (C K) of the corneal ellipse. This axis rarely passes through the center of the cornea ; but, as it never lies far from the latter, the difference in dimension between the two angles is very slight.