^ \'\>fZ A System of Ocular Skiametry INCLUDING SUCH PORTIONS OF OPTOMETRY* AS [ARE PERTINENT TO THE USE OF THE "SHADOW TEST" WITH THE PLANE MIRROR BY ANDREW JAY CROSS Author of " Dynamic Skiascopy, or the Action of the Accommodation in Shadow Testing, and other papers. Member of the Optical Society of the City of New York President of the American Association of Opticians, 1900 to 1901 President of the Optical Society of the State of New York 1897 to 1900. Honorary Member of the Central New York Optical Society and of the Rochester N. Y. Optical Club etc. etc. IVith Ninety-Four Illustrations NEW YORK FREDERICK BOGER PUBLISHING CO. 36 Maiden Lane 1903 (f-^oX-thv. r COPYRIGHT. 1903, BY ANDREW JAY CROSS. PRESS OF "THE OPTICAL JOURNAL," NEW YORK. OPTOiNfCTRY LIBRARY To Those Opticians Who Have Shown Themselves Ambitious to Elevate Their Calling This Book is Fraternally Dedicated. MG7514:2 PREFACE. It is with some timidity, both from a literary viewpoint and from tliat of a retailer of spectacles and eyeglasses, and manufacturer of optometrical devices, that this book is offered for the consideration of those who adapt glasses to the eyes of others. If it is true that "necessity is the mother of invention," then perhaps it will be idle to offer further excuses here. The author, however, desires to express his deep appreciation of the work done by the many medical men who have enhanced the value of Sir William Paget Bowman's discovery of the phenomena now known as the "Shadow Test." Especially is the author under obligations to such writers as Dr. Edward Jackson and Dr. James Thorington for aid derived from their able monographs on this subject. But nearly all medical writers having, naturally, written from the standpoint of physicians, that "the use of a reliable cycloplegic is always indicated," it has remained for opticians to devise ways and means for accurately applying the principles of this test with- out resorting to the use of local toxicants of any kind. This small volume has therefore been prepared for the purpose of pointing out new and better paths in practical optometry, as well as the re-blazing of old ones, and the author begs that all optical and medical readers, particularly those who are wedded to old-fashioned methods, will bear in mind that the use of mechanical devices is the order of the day, and that the accuracy of the experiments alluded to in the following pages can be readily proved by any one suf- ficiently interested to attempt their demonstration. 20 East 230 Street, New York. October i, 1(^03. Dbraryof the Alamerfa Counuy. Assoeiation of OpU^metrists CONTENTS CH APT KR I PaRcs 1 1 to 2^. A Descriptive Name. Amount of Optical Knowledge Necessary to Achieve Skiametric Success. The Value of Bowman's Discovery. Difficulties of Ocular Skiametry. CHAPTER II Pages 24 to 49- Adequate and Inado(iuate Examination-Rooms. Illumination: Its Size, Source and Control. The Plane Mirror: Its Construction and How to Handle It. Good and Bad Schematic Eyes, and How to Correct Them. A Short Method for the Reduction and Transposition of Lens Values. CHAPTER III Pages 50 to 69. Optical Principles Involved in Skiametry. The Shadow, What to Look For and How to See It, and How to Imitate Its Action with a Card-board Model. CHAPTER IV Pages 70 to 86. .•\ction of the Shadow and What It Indicates. Appearance of the Sliadow in Regular and Irregular Errors of Refraction and Its Action in Emmetropia, Myopia, Hyper- metropia and Astigmatism. CHAPTER V Pages 87 to 97. Some Theories Regarding the Dulness of the Fundus Reflex in Certain Cases. Also Multiple Methods of Practising Skiametry, Including the Toxic and Non-Toxic Planner of Employing the Static Test by l-5()tli the .Amplifying and Fogging Methods. CHAPTER VI Pages 98 to iii. Dynamic Skiametry and Its Uses in Mastering Tonic and Clonic Spasms. The Importance of Visual Fixation in Obtaining Bright Retinal Reflexes, and the Aid Derived from Using Independent Points, Together with a Few Words .About Ray Values. CONTENTS— Continued CHAPTER VII Pages 112 to 123. Ocular Muscle Action, and the Influence of Habit Upon Accom- modation, Convergence and Innervation, with Special Reference to Spasms, Latent Errors, and the Use of Prisms. CHAPTER Vni Pages 124 to 129. Corroborative IMeasurements in Optometry and the Inter-Depend- ence of Objective and Subjective Methods.— The Value of Mechanical Devices and the Superiority of the Mobile over the Unit Action of Lenses. CHAPTER IX Pages 130 to 146. The Use of Instruments in Skiametric Work, Their Evolution from Single Lenses and the Relative Merits of Some Now Employed, including a Description of the Author's Own Mechanical Contributions in this Line. CHAPTER X Pages 147 to 153. Systematic Ocular Examinations and the Aid Derived from Mak- ing Complete Prime Records. — Resourcefulness in Refraction Work and the Successful Exainination of the Eyes of Children, Mutes and Illiterates. CHAPTER XI Pages 154 to 168. Illustrative Cases, Showing the Comparative Value of Static and Dynamic Skiametry in Various Patients of Different Ages, Occupations and Apparent Physical Condition. CHAPTER XH Pages 169 to 181. Resume of Previous Chapters With a View to Emphasizing the Salient Points of Ocular Skiametry as a System. ILLUSTRATIONS Fig. Page 1 Parallel rays of light being focused 15 2 Fociised rays of light being paralleled 15 3 A "Success" burner oil lamp 27 4 An "Argand" burner gas lamp 28 5 An acetylene gas lamp 29 6 A "Welsbach" gas lamp 30 7 A "Welsbach" gas lamp with a large asbestos-lined "Cross" chimney 31 8 A small asbestos-lined "Cross" chimney for citlier a "Welsbach" or an "Argand" lamp 32 9 A gasolene "Student" lamp x^ 10 Showing spiral filament for electric lamp 34 11 The "Cross" asbestos-covered electric lamp 35 12 The "DeZeng" luminous retinoscope 36 13 Wall bracket for gas or electric lamp 37 14 Section of the "Cross" mirror showing concave l)ack 39 15 The "Cross" Bracket ]\Iirror 40 16 Showing manner of holding mirror 41 17 A "Queen" pasteboard schematic eye 43 18 A home-made "smoke" box 51 19 Showing refraction by lens of proper focus 51 20 Showing refraction by too strong a convex lens 5-2 21 Showing refraction by too weak a convex lens 52 22 Rays of light emerging unrcfracted 54 23 Rays emerging properly refracted 54 24 Emerging rays having a conjugate focus 55 25 Rays emerging divergent 55 26 Rays emerging unrefracted 56 27 Rays emerging insufficiently refracted 56 28 Rays emerging properly refracted 57 29 Rays emerging with divergency increased 57 30 An Emmetropic eye 5^ 31 A Myopic eye 59 32 A Hypermetropic eye 59 33 Illustrating a so-called "shadow" 61 34 Showing now the retma is illuminated l)y reflected light 63 35 Showing the source of the returning light to be the edge of the illumination 63 36 First half of pasteboard model for demonstrating the shadow's action 65 ILLUSTRATIONS— Continued f'^- Page 37 Second half of pasteboard model for demonstrating the shadow's action g^ 38 Showing why the shadow moves with the mirror 71 39 Showing why the shadow moves against the mirror jj 40 Illustrating total refraction in Emmetropia j;^ 41 Ilhistrating a weak degree of Hypermetropia 74 4-' Illustrating a marked degree of Hypermetropia 74 43 Illustrating a weak degree of Myopia 75 44 Illustrating a marked degree of Myopia 75 45 Illustrating Hypermetropic Astigmatism 76 46 Illustrating ^Myopic Astigmatism -(] 47 Illustrating Compound Hypermetropic Astigmatism j'p 48 Illustrating Compound Myopic Astigmatism ^^ 49 Illustrating Mixed Astigmatism yg 50 Illustrating Mixed Astigmatism at oblique axes 78 51 Showing the crescent-like shadow in Spherical Cases 81 52 Showing the straight-edge appearance of the shadow in Astigmatic Cases 81 53 Showing the band observable in high degrees of Astigmatism. . 82 54 Showing an Astigmatic band at oblique axis 82 55 Showing the meridional appearance of the shadow in Compound Astigmatism 83 56 Showing the double band in the "Scissors" movement 84 57 Showing a case of "Irregular" Astigmatism 84 58 Showing a case where "Cortical Cataract" is present 85 59 Showing the shadow as it sometimes appears in "Conical Cornea" 85 60 Showing why the retinal illumination is large in marked errors. 88 61 Showing why the shadow moves slowly in marked errors 89 62 Showing why the shadow is duller in Myopia than in a like degree of Hypermetropia go 63 Showing relative size of retinal illumination in high and low degrees of Myopia 91 64 Showing the optical principles of Penumbra 91 65 Showing optical principles of Penumbra doubled 92 66 Showing the interference of Penumbra in shadow-testing 93 67 Showing emerging rays being bent to a focus by a trial lens. ... 100 68 Shov/ing emerging rays being bent to a focus by accommodation loi 69 Showing how accommodation can be made to absorb a ciliary spasm 103 ILLUSTRATIONS— Continued Fig. Page 70 Showing the assistance offered by niuUiplc fixation points in Dynamic Skiametry 106 71 "The Cross" Fixation Stand 107 ^2 Showing primary position for fixation card 108 73 Showing normal balance between Accommodation and Con- vergence in Emmetropia 113 74 Showing relative Innervation necessary to balance Accommo- dation and Convergence in Emmetropia 114 75 Showing lack of balance between Accommodation and Con- vergence in Hypermetropia 115 76 Showing the unequal Innervation required to balance Accom- dation and Convergence in Hypermetropia 115 '•J Showing lack of balance between Accommodation and Con- vergence in Myopia 1 16 78 Showing the unequal Innervation required to balance Accom- modation and Convergence in Myopia n6 79 Showing the optical principles of mobile lens action produced by changing the relative position of lenses 128 80 The skiascopic lens rack of Wiirdemann 131 Si The disc form of holding lenses, used by Crain, Standart and others 132 82 The constructive principle of the "Jennings" instrument 133 83 The constructive principle of the "Meyrowitz" instrument 134 84 The constructive principle of the "Fay" instrument 134 85 The constructive principle of the instrument designed and used by the author in 1892 135 86 The constructive principle of the "Geneva" instrument pat- ented about 1898 136 87 The constructive principle of the "Cross" Retino-Skiameter 138 88 External appearance of the "Cross" instrument 140 89 Showing how to alter pupillary distance 141 90 Showing manner of using instrument 142 91 Showing instrument in dust-proof case 143 92 Showing comparative size of normal and magnified pupil 146 93 Sample record blank 148 94 Manner of filing records I49 A System of Ocular Skiametry CHAPTER I. A Descriptu'e Name. — Amount of Optical Knowledge Necessary to Achieve Skiamf.tric Success. — The Value of Bowman's Discovery. — Difficulties of Ocu- lar Skiametrv. A DESCRIPTIVE NAME. It is a very doubtful ques- tion whether any method for examining an eye, in any manner whatsoever, can be given as many names, each with as plausi- ble a relevancy as those already given to the phenomena of light and shadow seen in the pupil of an eye. These phe- nomena having been first described by Sir William Paget Bowman some forty years ago, and later known as the ''Sha- dow test." Since the introduction of the blanket word "Optometry." to cover all measurements of an eye whether external, internal or functional, the suffix "metry" seems to have been especially assigned to duty in connection with those ocular conditions wherein it is possible to find a variance from certain fixed standards. Thus the names Ophthalmometry, Dioptometry, Astigmometry, Keratometry, Phorometry, Strabismometry. Pu- pillometry. Perioptometry, and Visuometry are in use. Then if the lines are drawn a little finer there are found such names as Exophthalmometry, Ophthalmotometry, Ophthalmotropo- metry, Chromotometry, and. in the measuring of lenses and prisms, there are also the words Phacometry and Prisopto- metry. The Greek derivative "scope" (sight, or examination), '.eems to have been closely allied in the past with shadow-test- ^^ DESCRIPTIVE NAME ing, and the equivalent of such words as shadow-seeing, retina- seeing, pupil-seeing and others with visual reference have been used. The Greek words for '•seeing" and for "measurin are sometimes construed as opposites, but the English word for "measure" seems to be sharply defined : "To compare with a fixed standard." Now, in shadow-testing this is exacth- what an optometrical examiner really does; he measures an eye by means of reflected light, together with known ray-bending ap- pliances called lenses, and compares his finding's with a cer- tain fixed standard. It is common to speak of measuring time, and it is known that light, shadow and sundials were formerly employed for this purpose. From this to measuring passing shadows is not a long stretch of the imagination, and as an examiner must be informed regarding the direction, speed and volume of all ocular shadows in ascertaining the relation of the ray-bending power of an eye to its retina, it would seem that this part of a refractionist'.s work comes nearer to being cye-shadow- measuring than it docs to anything else. The plea that the word ".Skia."' for shadow, should be omitted from the term used to denote the shadow test does not seem a logical one; for from first to last an examiner is endeavoring to ascertain the action of the shadow, not of the retina, by the changing of lenses in front of an eve for the purpose of measuring the angle of the emergent ra\s. There- fore it follows that the word, "Retinoscopy," though an old one, is not necessarily a good one, because "seeing the retina" in this sense is really not any more descriptive of that which is accomplished than would be the same name if applied to that which is now called "Ophthalmoscopy." The term "Skiascopy" seems a better one than that of "Retinoscopy," although seeing-the-shadow is not verv de- NECESSARY KNOWLEDGE 1 3 scriptive either, unless it be preceded by the general eye term "ocular,'"' when it could be translated "Eye-shadow-seeing." It has been urged as an objection to the use of the word "skia" that it might lead to confusion with the pictures, or skiagraphs, made by the X-rays of Roentgen. In answer to this it may be said that the nomenclature of optical science is already so complex that technical terms must be given sufficient length in order to convey their proper meaning, otherwise misunderstanding and confusion between teacher and student will ever be on the increase. And so it behooves the ocular-shadow-measurer and the shadowgraph expert to each look to his own descriptions in order that they may meet intelligent requirements. The device which reflects the light into an eye and enables an examiner to see the shadow ought logically to be called a shadow-viewer, or skiascope. And the instrument or mechanism which permits of the measuring of the angle under which a shadow is seen might correspondingly be termed a shadow-measure, or skiameter, while the act or process of measuring the behavior of the shadow in an eye could be given the technical name of ocular skiametry. Especially is this term applicable, since, as will be seen later on, it is now quite necessary to qualify and subqualify this term in order to represent gradations of meaning in the exact description of Bowman's great discovery, no longer as a single method, but as a system for examining and measuring the eye in many ways. AMOUNT OF OPTICAL KNOWLEDGE. The general optical principle of the shadow test in its simplest form is not very complicated. Taken, however, in connection with its optometrical associates it represents as a whole a rather high order of knowledge regarding both physical and phvsiological optics. It also implies in its application a cer- 14 NECESSARY KNOWLEDGE tain amount of skill or dexterity in the manipulation of indis- ])ensable mechanical devices, such as the skiascope and ski- ameter, no matter whether the latter be a simple trial frame with test lenses, or a more elaborate and useful apparatus. There are two leading accomplishments in shadow-testing in which an examiner must be proficient before he can achieve success in this work. The first of these is the control of the reflected light and the determination of the direction of the shadow's motion under both favorable and unfavorable con- d ition s. The^ second lies in being able to add and subtract known refractive lens quantities and to tell with precision what their ray-bending value is at all distances from an eye under examination. To express it tersely then, an examiner must be able to detect any action of the shadow and to know exactly what the optical value of this action is when influenced by either lenses or accommodation. The first of the above requirements can be gained by dailv practice, but the secorid requires considerable study and appli- cation, as it involves a knowledge of angles of light, or ray values, as well as of refraction, or lens values. When a patient's eye is considered as an object, instead of as a Subject, then its refractive condition must be determined by noting the behavior of the light reflected from the retina as it leaves the eye, and methods of procedure known as "ob- jective" must therefore be applied. Many students of optics who have confined their efforts to a mastery of "subjective" optometry find themselves quite at sea when they undertake objective methods. And the reason for this usually lies in the fact that they have given attention to the subject of light as it travels in one direction only, namely, as it enters an eye. One of the foundation principles taught in optical text- books is that light returns over the same course which it has NECESSARY K.\()\V[,EDGE I5 traveled. Hence, if parallel rays of lij^^ht are made to pasf through a convex lens they will come to a focus at the so called "strength" of the lens. Invert this order, by placing- a lighted candle at the focus of the lens, and the rays of ligh» will diverge until they pass through the lens, after which they will be parallel again. See Figs, i and 2. Fig. I Sau/(C£ parallel rays of light ueixg focused. Fig. 2. __ ^ FOCUSED KAYS OF [.IGHT DEIXG PARALLELED. In shadow-testing the retina of an eye is the apparent source of light, although in reality the retina is only a noor quality of mirror which reflects the light thrown into an eye by the skiascope. This illumination, or reflection, behaves like a piece of red flannel, or any other visible object which acts as a high or low grade mirror according to its quality, or ability, to reflect light, glass with amalgam backing, and polished metals, being of the highest order, while lampblack and black velvet are of the lowest. The corelation of accommodation and convergence is an- other subject which students of ocular skiametry must under- stand in order to do their work intelligently. Thus it will be seen that skiametric proficiency involves a pretty thorough l6 VALUE OF BOWMAN'S DISCOVERY grounding in something more than rudimentary optics. With the ehmination of the use of the concave mirror, however, and by the aid of modern apparatus it is now possible to dispense with many details which formerly resulted in the confusion of beginners. Still, notwithstanding this, a student will find much that will call forth his best efiforts before he can feel assured of the reliability of his findings. It is one thing to master ocular skiametry under regular conditions and quite another to rightly differentiate the irregu- lar and apply that judgment which secures success. But, as in other studies, the deeper the student delves the more he finds to learn, and the easier do the foundation principles become. The wise searcher after the optical facts which in the aggregate constitute optical knowledge will first learn the^ A, B, C of light and lenses, together with the reduction and transposition of the latter, and next he will master physiological optics, in addition to the art of subjectively correcting with lenses any manifest conditions which may be met. The stu- dent may then be said to possess sufficient optical knowledge to begin the study of ocular skiametry with a fair chance of achieving success. THE VALUE OF BOWMAN'S DISCOVERY. Many inquirers into the merits of shadow-testing seem to ^ possessed with the idea that every case which presents itseK is capable of being both easily and accurately refracted by means of Bowman's discovery. This expectation is as in- consistent with the real facts as it would be to expect like results from trial case tests or any other one optomctrical method. The truth can, perhaps, be fairly expressed by saying that shadow-testing bears to trial case testing much the same rela- tion as the addition of a column of figures from the top bears to its addition from the bottom. VALUE OF BOWMAN S DISCOVERY I7 Skiascopy will uncover at a single sitting optical condi- tions which it would be quite impossible for ordinary trial case tests to do. On the other hand, the latter will show visual conditions of which the former can tell nothing. \'iewed again from a similar position we find that the two general methods for estimating ocular errors of refraction, known by the terms "objective" and "subjective," are like seeing for one's self and taking the testimony of others. Usually either method is fairly reliable in ordinary cases, but in extraordinary ones — the kind that make and break reputa- tions — the evidence can be none too corroborative. So we find skiascopic and trial case-testing to be inter- dependent, both systems having their weak and strong points and one aiding in the judgment requisite for the successful application of the other, skiascopy coming first because it is the great refractive pilot, or pathfinder, and because, too. there are many conditions other than errors of refraction that are shown up by its use, and which, if it were not for this early use, might needlessly prolong an otherwise short examination. It is hoped, therefore, that this point is made clear regard- ing the value of shadow-testing. It is deemed of no more, nor less, value than the trial case test, and that neither one is infallible, and that both are absolutely essential in all prime cases, whether the results obtained by either coincide with those of the other or not, for this very lack of coincidence is often the key which enables a trained judgment to solve a refractive riddle. By basing their judgment upon the principle that the proof of the pudding lies in interviewing him who has chewed the string, some credulous inquirers have been led to estimate the merits of shadow-testing by taking the testimony of those who have falsely pretended to possess a thorough knowledge l8 VALUE OF bowman's DISCOVERY of it, and this unreliable information has led them to believe that if skiametry is faulty in some hands it must be faulty in all. Whereas the reverse reasoning would in all probability be productive of better results, for that which one can achieve by study and practice it is (|uite possible for others to achieve by equal application and efifort, and sometimes by even less where assistance is given by skilled tutors. For nearly four decades the ablest optometrical researchers have striven their utmost to find a better objective means than skiametry for determining the optical condition of eyes, but so far without avail. And judging from the present advanced knowledge regarding optics and optometr_\- it is pretty safe to say that the shadow test is here to stay, for a long time at least, and that those whose duty it is to adapt glasses to the eyes of others will find their work more reliable and much easier if they take the time to thoroughly master this valuable means for ascertaining ocular refractive conditions in a manner independent of the patient's intelligence. Xow this phrase, "independent of the patient's intelligence," may prove somewhat misleading, since even those who are experienced in skiametric work find many cases in which the results obtained are very unsatisfactory indeed. Yet, wlion an examiner measures a case by skiametry and notes an error of refraction which later on is confirmed by the trial case test, he feels that he has received advanced information of a truly "independent" character, upon which he can rely with greater confidence than if this information had been denied him. r)n the other hand, if the trial case test does not con- firm the mirror findings, then the mirror is employed again to confirm the trial case findings. The subjective is used to check the objective, and then the objective again to corroborate the subjective. DIFFICULTI'ES OF SKIAMFTKV I9 DIFFICULTIES. The stumblinc;^ blocks in ocular ski- ametry are not few, and the_\ seem to grow apace as the system becomes perfected in its many details. The first great obstacle which usually presents itself is place, or examination room. The medical refractionist who takes up this work may already be provided with a regulation dark- room for his ophthalmoscopic work, and in this room he attcmi)ts to ])ractice successful skiametry. The conditions being poor, perhaps, he gets poor results and abandons the work in the belief that shadow-testing is sadly overrated. The non-medical refractionist may possibly go to the other extreme. The specious plea that a dark-room is unnecessary is listened to, then, not being able to use a cycloplegic, and having a knowledge only of the static method, as it is now called, he wonders why his skiametric work varies so with his trial case findings. And then, too. because he possesses a sufficient degree of skill to feel comparatively sure of the action of the shadow- in an occasional case, he either blames the system or virtually condemns it by faint praise. Next to a poorly-arranged examination room in point of dis- couragement with shadow-testing comes an inadequate source of illumination. Almost any lamp, whether electric, gas. or oil. looks to a novice as though it ought to prove of sufficient intensity to determine a shadow's action, because when the light is reflected into a naked eye the fundus reflex seems fairly bright, but by placing a lens or two in front of this eye, or by permitting the patient to look in an unfavorable direction, so pronounced a diminution will often be produced in the definition of the shadow as to render accurate work impossible. Even where the source of illumination is up to the standard of forty-candle power, or more, the experienced examiner will meet with cases where the deeply pigmented retina gives 20 DIFFICULTIES OF SKIAMETRY back so poor a reflection that only the greatest care and skill can determine the action of the shadow. There is a vast difference in general illuminating power between a flame that is three-quarters hooded and one that is not, and a dark-room light to be satisfactory must not be too large. Its apparent intensity should resemble the "Glory" 'hole of a furnace, and then, if this should prove too bright for an occasional supersensitive eye, it can always be dimin- ished by moving the patient farther away from it. The law that light decreases in proportion to the square of the distance at which it is used enables a light intensity equal to sixty-candle power at two feet away to be decreased to fifteen-candle power by withdrawing to a distance of four feet. Thus it will be seen that, with a powerful source of illumination, an examiner can readily obtain almost any candle power he desires. Just how great a candle power the human eye can bear without doing it injury varies undoubtedly with individuals and the duration of the exposure. A hundred-candle power lamp, hooded so that only a limited portion of its general radiant energy is available, could probably be comfortably borne by the average eye under ordinary skiametric condi- tions for several minutes at a time, whereas the length of time for a proper measurement is only a matter of seconds. Too bright a light, therefore, need not be feared. In using a bright light, however, there is one thing an examiner should always remember, as it can properly be classi- fied among the stumbling blocks to be avoided, and th at is to never allow himself to look directh'^at t he sour eg of his illumination prior to or during an examinati on. The reason is that the sensitiveness of his own retina is such as to retain the impressions made by a bright light to such a degree that duller objects cannot be clearly seen for several minutes after DimCULTIES OF SKIAMETRY 21 looking at a bright flame. And as a consequence, if an ex- aminer" permits himself to look directly at his lamp for an instant or two and then tries to use his mirror he will find it very difficult to detect the dull outline of the shadow or note its action. If an inspection of the lamp is necessary to deter- mine its condition or distance away it is a wise examiner who will derive his information by looking a few inches to one side of the flame and not directly at it. Another stumbling block in the road to skiametrical suc- cess lies in corroded, soiled and dusty mirrors, especially at or about the so-called "peephole" of the skiascope. Every examiner should possess at least two or more mirrors, so that when one gets out of order it can be sent to the factory for resilvering. Mirrors, to give the best service, must not be of too great a diameter, nor must their peepholes be large or bored through the glass. Consequently these holes, which are really not holes in the strict sense of the word, but round spots of clear glass made by scraping the silver off the mirror, must be kept immaculately clean, so as to prevent particles of dust from interfering with the free passage of light, or else the shadow's action will be perceptibly dimmed. One of the underlying causes of the success achieved in many branches of modern science is undoubtedly due to ultra- cleanliness and attention to details, and so it will be with advanced optometry as regards details. For dealings with imaginary quantities of elastic ether, as light is termed, call for extreme care on the part of the examiner, if he is of the kind that are satisfied with nothing short of the highest attain- ment. A thorough knowledge of lenses is still another important factor. There is probably no one study connected with a skiascopist's educational equipment which demands a more perfect mastery than does that of the reducing, transposing 22 DIFI-ICULTIliS OF SKIA^n-:TRY and combining of lens values. In his desire to attain perfec- tion on the practical side of adapting- glasses to the eyes ,of others the student is apt to pass hurriedly by the Iry under- lying optical principles upon which lenses are based and, as a consequence, after his first few years of "success in every case" his desire to climb higher is interfered with through his lack of knowledge of that which he ought to have learned at the beginning of his optical career. In ocular skiametry a simple stumbling block to many students is their inability to tell what the true refraction of their patient's eye is, when it takes an ordinary lens quantity of, say, one diopter to reverse the shadow in one meridian, while only a half-diopter is required to reverse it in the meridian at right angles to this. Especially is this true where the axes happen to be oblique, or where one lens is plus and the other niiiius.. Now this is all wrong, for if an optometrician ought to know anything he oughtjo know all about lenses — how they are made, what index of refraction and spherical ab^^rration mean, wherein cylindrical, spherical and elipsoidal curv-ntures dififer and, above all, the very best way to combine lens quanti- ties in order to produce the highest degree of central and peripheral vision, together with angle and decentration foi the relief of strain and the production of comfort. A thorough knowledge of light and lenses is the great key to the unlocking of skiametric success. After a student has learned all about the j^hys'ca] side of refraction work the physiological becomes easier, just as a mathematician who has skipped decimal fractions finds his higher work puzzling, but if he has covered his ground carefully then his advancement is less difficult. A very few sittings will give a student the mastery of a skiascopic mirror and enable him to tell the movement of the DIIFRLLTIKS OF SKIAMliTUV 2T, pupillary shadows, but how to control these shadows by the aid of lenses combined with accommodation, and to know the real optical value of the refractive power used, is where one of the greatest of the stumbling blocks in shadow-testing lurks, so that those who become discouraged in their skia- metric efforts must not blame this grand test, but look, rather, to their own weakness in their lack of adequate optical knowl- edge and skill. There is one more point to which attention should be directed, although it can hardly be called a "stumbling block," even though in some cases it seems to act as such. It pertains to an examiner's own vision, which should be such as to enable him to see with definition a moderately pale shadow on a pink background at a distance of at least forty inches. Therefore all examiners having myopia must have their own refractive errors corrected to within less than one diopter before they can do successful skiametric work. All hypermetropes. on the other hand, whose amplitude of accommodation at thirteen inches is less than their error of refraction must also wear their correcting lenses before they can make satisfactory shadow-tests. It would seem that the peephole in a skiascope ought to act as a pinhole-test and give any examiner good vision, no matter what his refractive error might be, but experience, that great teacher, rules it otherwise in those cases where high-class results are sought for. CHAPTER II. Adequate and Inadequate Examination Rooms. — Illu- mination: — Its Size, Source and Control. — The Plane Mirror: — Its Construction and How to Handle It. — Good and Bad Schematic Eyes, and How to Correct Them : — A Short Method for the Reduc- tion AND Transposition of Lens \^'\lues. EXAMINATION ROOMS. To describe an ideal exam- ination room is quite a different matter from attempting the description of an adequate apartment, or compartment, that will serve fairly well as a place in which to practice ocular skiametry. The complaint, or excuse, that is frequently heard from many who attempt to do refraction work without having suit- able place and apparatus is that they lack the requisite space, wh'^reas, an examination of the store or office by one experi- enced as to requirements might lead to the discovery of quite a number of places which could be rendered available by the use of properly strung wires for hanging light-proof curtains, which could also be made decorative in appearance. Then, too, many are persuaded that an examination room must be of Egyptian darkness, and without ventilation, where both exam- iner and patient will be very uncomfortable, especially during warm weather. Now, this is a wrong conception. An adequate examina- tion room needs to be of only semi-darkness. In fact, if it EXAMINATION ROOMS 2$ is light enough to make the head-lines of an ordinary news- paper just discernible at midday, it will be found quite dark enough for all optometrical purposes. The "Mahomet and the mountain'' principle can be made use of by increasing the intensity of the light source, instead of making the room appear as dark and gloomy as the interior of a hearse. The space occupied need not be very wide. The length of room, however, ought to be at least twenty feet, but where this distance cannot be obtained a length of ten feet can be made to appear as twenty by the use of an ordinary wall mirror, test cards with reversed letters being placed over the head of the patient in a position Avhere their reflection can readily be seen in the mirror. The ventilation should be perfect, and the space ought to contain several chairs for the use of those who accompany the patient, while the surroundings should be made as cheerful as they can be made by means of rugs, pictures, etc. Both objective and subjective tests should be used without having to move the patient from the chair in which he is first seated, and, where possible, the better way is to bring all instruments and devices to the patient rather than require the patient to go to them. First impressions are said to be lasting, so that if patients are shown into apartments that look as though they were intended for optometrical purposes their confidence is more than half won. Doing ocular refraction w^ork over a showcase is about as inappropriate and non-professional-looking as it would be to practice dentistry in a like manner. There is a fitness of things, and those whose practice is in accord with this "fitness" gen- erally have occasion to feel satisfied with the results obtained from having given attention to the details of environment. Indeed, it has already been said by many that, next to profes- 26 ILLUMINATION sional knowledge and skill, a well appointed examination room is the very best kind of an advertisement that a refractionist can possibly have. ILLUMINATION. One of the "stumbling blocks" re- ferred to in the preceding chapter, to which the reader's atten- tion has already been called, is the importance of using a proper source of illumination in order to succeed in shadow testing. Therefore it will be to the purpose here to describe, with the assistance of drawings and photo-reproductions, some of the various lamps employed for this purpose and to offer some criticisms regarding the advantages and disadvantages conse- quent upon their use. Practising skiametry by means of a model, or so-called "schematic," eye is very easy in comparison to practising it upon a living eye, for with a model eye almost any kind of light will answer, but not so with the living organ, where, as before stated, the light source should resemble the glow ema- nating from the "Glory" hole of a furnace. Oil lamps, even of the "Rochester" and "Success" burner types, while they are magnificent for general lighting purposes, fall below the standard of efficiency when employed for general skiametric uses. The draught in almost all lamps is an important factor, and as metal chimneys cannot be easily made to take the place of glass ones, on account of the transparent aperture which is required opposite the flame, it has been found necessary to either line or cover all glass chimneys with an opaque substance which extreme heat cannot affect. Fig. 3 illustrates one of the best types, perhaps, of any of the oil lamps, but even where the chimney is lined with a white pig- ment it still falls short of giving satisfaction as an efficient illuminator, although the maker may claim that his lamp has a general efficiency of over one hundred-candle power. ILLUMINATION 27 Ihe reason for the shortcomings of almost all oil lamps using a so-called "flame spreader" is due, no doubt, to the fact that the flame, though large, is apt to be very low, and Fig. 3. A SUCCESS IU:RM:K OIL LAMl' when its general intensity is hooded down to the size of a five cent piece, which is necessary in order to obtain good results with a plane mirror, the flame is found of insufficient bright- ness to meet an examiner's needs. 28 ILLUMINATION This reasoning also applies to nearly all kerosene lamps as now used. Perhaps some day some inventive mind will devise a condensing reflector which will permit of hooding the light down to a small aperture and at the same time obtain the requisite intensity of illumination, but as yet this has not been achieved. Next to oil comes gas. And here the field of illumination broadens, thanks to the inventors of the "Argand" and "Wels- bach" burners, and to the gas-generating qualities of naphtha Fig. 4. AN ARGAND BURNER GAS L.\MP and of acetylene products. Ranking above oil burners comes the Argand gas lamp, which also requires a draught to make it burn properly, hence a glass chimney is necessary for its use. Fig. 4 illustrates this type of lamp. And while this is vastly superior to oil lamps in general, it is still somewhat below the required standard of efficiency, even when working at its very best. The flame has a yellowish white appearance, and seems, like the flame of an oil lamp, to ILLUMINATION 29 lack the illuminating energ-y necessary to meet modern skia- metric needs, although in favorable cases fair work can be done by its aid. Acetylene lamps represent another style of gas burners which in point of intensity of illumination can probably hold their own against all competitors. The style shown in Fig, 5 represents a portable type. Fig. 5. AN ACETYLENE GAS LAMP This lamp is similar in appearance to an ordinary table lamp, with the exception that it has an asbestos-lined metal chimney. It is commercially named an "Electrolite gas lamp," and uses specially pulverized calcium carbide. Its use is endorsed by the board of fire underwriters, and it is easy to care for. The light it gives is over fifty-candle power in 30 ILLUMINATION intensity, while the expense of maintaining it is only about one cent an hour. Skiascopists are certainly to be congratulated on the inven- tion of this lamp, especially those who are compelled to do work away from their properly appointed examination rooms, for as a portable lamp it is very satisfactory. Fig. 6. *5 WELSBACH GAS LAMP In point of brilliancy, reliability, cost of maintenance and ease of adjustment, however, probably no lamp is superior to the Welsbach type, especially with a non-breakable, asbestos- lined chimney, as shown in Fig. 6. ILLUMINATION 31 This burner is of the "Incandescent" kind, wherein chimney draught is not a factor, so that a glass chimney is not really needed, although, owing to its low cost and fine appearance some examiners prefer a glass to a metal one. Figs. 7 and 8 show two patterns of glass chimneys which can be used on the same style of burner. Fig. 7. A "welsbach" gas lamp with large asrestos-lined "cross' CHIMNEY. The Fig. 7 chimney being farther away from the mantle, or flame, the glass does not get as hot as it does in the pattern Fig. 8, but the smaller the chimney the more convenient is the adjustment of the light, the source of illumination beingf 32 ILLUMINATION nearer to tlie surface of the screen ; therefore the Fig. 8 style is to be preferred. Fig. 8. A SMALL ASBESTOS-LINED CROSS CHIMNEY FOR EITHER A "wELSBACH" or an "aRGAND" LAMP. The Welsbach lamp gives an ideal light for shadow-testing, its flame is bluish white, and its intensity is ample when its mantle is properly heated to incandescence. The one unfav- orable criticism which can be made regarding it is on the fragility of its mantles, which are very easily injured or destroyed. But fortunately these mantles are inexpensive and are not difficult of adjustment. In connection with naphtha or gasolene, the Welsbach burner can also be used, and while the light obtained is not ili.i;mination 33 as satisfactory as where so-called "City" gas is employed, yet the results obtained are vastly superior to the use of oil. Fig. 9 shows the well-known student lamp form in which these lamps are usually manufactured. Fig. 9. A GASOLENE "STUDENT They make an excellent portable light when care is used in adjusting them. The general use of gasolene has its disad- vantages, however, resulting from its explosive qualities. This is a question on which the makers of gasolene lamps and the fire insurance companies are not at all in accord. But as efficient illuminators for skiametric uses gasolene lamps, using the Wclsbach type of burner, will be found fairly satisfactory, provided they are always kept in perfect order. 34 ILLUMINATION Electric lamps of the general house-lighting- variety, with long carbon filaments, are, perhaps, among the most unsatis- factory of all illuminators for use in connection with ocular skiametry, no matter whether the glass bulbs are of the "clear" or of the "frosted" variety. An electric lamp, to be of service in this work must have its filament in compact form, similar in shape to the coils of a watch hairspring. Then the light energy of the carbon wires when heated to incandescence can be concentrated, and the results obtained can be made to equal the Welsbach type of lamp. Fig. lo shows an electric lamp filament of the spiral kind referred to. This lamp needs to be handled with great care, since, owing to the size and brittle character of its fila- ment, it becomes easily broken l)y a sudden jar or through rough handling, as in the mail, or when shipped by express. Fig. io. SnOWIXG SPIRAL FILAMENT FOR ELECTRIC LAMP. Fig. II shows this same lamp with its glass bulb coated with a thick asbestos pigment, leaving a one-inch aperture in its side through which the light can emerge. This lamp is rated at fifty-candle power and gives a mag- nificent reddish white light. The white hot filament, or carbon wires, generate considerable heat, which, of course, will melt the rubber, or composition, socket handle if the lamp happens to be used upside down. The heat from one of these lamps ILLU.MIXATIOX 35 has also been known to char th^ curtain hangings of a window with which it came in contact. But used as it is intended to be used, this style of lamp certainly supplies an adequate illumination for any kind of examination room, whether light or dark, or where gas cannot be obtained, such as in modern office buildings, etc. Fi(,. II. THE CROSS ASBESTOS COVERED ELECTRIC LAMP. The cost of maintenance of an electric lamp is somewhat higher than that of a Welsbach gas lamp, and its durability is not as great. It requires careful adaptation to the voltage of local electric currents. Any increase in power even of only a few volts, such as frequently occurs in cities where the current is intensified at sundown, serves to shorten the life of the 36 ILLUMINATION lamp very materially. These uncertainties of current can be controlled by what are called resistance attachments, or rheostats, if an examiner cares to incur the expense. All high candle power electric lamps are sensitive to abuse, but the superior light given by them more than compensates for the trouble and expenditure their use entails. Fig. 12. THE DE 7ENG LUMINOUS RF.TINOSCOPE. Another form of electric illumination, which has the advan- tage of portability, is the combined lamp and mirror known as the "Dc Zcng luminous rclinoscopc." Fig. 12 illustrates its general appearance. ILLUMINATIOX 37 The lamp used is of only one or two-candle power and is located behind, and close to, a strong convex lens, which serves to parallel the light radiation and thereby avoid any waste due to this cause. It obtains its electrical energy by means of either a storage battery or from a general house current passed through a resistance attachment, but like other electric lamps it, too, is sensitive to abuse. The light it gives is a fairly intense one, which, even though the colors of the spectrum are quite pronounced, enables good work to be accomplished in the static method by those who are acquainted with its peculiarities. In the dynamic method, however, it fails to illuminate the examiner's fixation cards, as the regular lamps do, and therefore an additional light for this purpose is required. As a portable light it is compact and easy of transportation and serves the purpose of being useful as an ophthalmoscope as well. Fig. 13 . B B WALL BRACKET FOR GAS OR ELECTRIC LAMP. A point to bear in mind in connection with adequate illumination in skiametric work is to have whatever lamp may be used so arranged that its adjustments, forward and back- 3& MIRRORS ward, to and from an examiner's own eye, is readily obtainable. The adjustment as to heij^ht is not so important as long as the light is about level with the patient's head and is situated from six to twelve inches to the patient's right. The alterable dis- tance forward and backward is. however, quite essential, as it enables the intensity of illumination to be controlled by an examiner as his case demands. A very simf)le way to arrange for this is to use a two or thrcc-arm wall bracket similar to that in Fig. 13. This bracket can also be used to carry an electric bulb, and thus give an examiner a double system of illumination which, in emergency, may prove of very great value in preventing a break-down. In summing up the question of illumination, perhaps the expression from the pen of a western specialist will serve to state the case fairly well. He recently wrote, "I fully realize that proper illumination is the foundation of success in skias- copy." And it may be added that this opinion is shared by many others who have had experience. THE PLAMi MIRROR. The confusion following the use of two forms of skiascopes, such as those having plane and those having concave mirrors, has led to the virtual abandonment of the latter form by many of the ablest skias- copists of the country. There are possibly some conditions under which a concave reflector might give an examiner better service than a plane one would, but these are rare, and for general all-round skiametric i)urposes the plane mirror is to be greatly preferred. All mirrors should be as brilliantly silvererl as possible, and the reflections from them ought to be perfectly round and free from distortion. Regarding the size or working part of a mirror, this can be easily determined by holding it at the maximum distance at which it is to be used, and covering its periphery with washer- MIRRORS 39 like pieces of paper. The size of the reflecting surface neces- sary to produce the best results while in actual service can then be noted. This will usually be found to represent an area of about three-quarters of an inch in diameter. The central aperture, or peep-hole, should be as small as possible and yet permit of acute vision on the part of the examiner. A diameter of one or two millimeters is generally sufficient for the purpose. Having the handle at least six inches long will also be found advantageous. If the metal disc holding the mirror is concaved at its back, as shown in Fig. 14, it will be of assistance in keeping the peep-hole free from dust and dirt. A frequent twist of the skiascope just before it is used, and while its front and back are covered with a handkerchief held between the thumb and forefinger of the operator's hand, is all that is necessary in order to keep it quite clean. Fig. 14. section of the "cross" mirror showing concave b.\ck. As will be seen in subsequent chapters, the use of a card for fixing the vision of the patient at the same distance away as that at which the mirror is operated renders some means for attaching a card to the skiascope almost a necessity. Fig. 15 shows a device to which the name "Bracket Mirror" has been given. The arrangement of the bracket attachment to this mirror is such that the card can be given a number of adjustments to suit possible contingencies. Regarding the proper way tf) handle a skiascope, the various tutors in skiametry dift'er, bat all are agreed that the move- 40 merits of the mirror should be of the slow, steady, straight-line order, and as free from wabbling and semi-circular motions as possible. When movements of the mirror are attempted by the hand-tilting method it takes many years of practice before a positive straight-line motion in all meridians of an eye can be depended upon. But where the movements are made by a body-tilting method the mastery of the mirror is very rapid, some beginners acquiring it almost perfectly after only a few days of practice. Fig. is. THE CROSS hr.xcket mirror. A description of this body method is as follows : The mirror handle is to be grasped near its lower end when the skiascope is held in a vertical jxisition. The elbow and arm of the hand holding the mirror are to be pressed tightly against the side of the body while the upper and inner edge MIRRORS 41 of the metal disc, upon which the mirror is mounted, is to be held firmly against the side of the examiner's nose and resting on the eyebrow in such a manner that the peep-hole of the mirror is exactly in front of the operator's pupil. With the mirror handle held in a rigid manner, almost the entire body is made to assist in giving the proper movements. The torso. Fig. 16. SHOWIXG MANNER OF HOLDING MIRROR. or trunk, acting as though it were pivoted at the waist, while the neck and heaving chest aid in the necessary motions. To say that this action involves a sort of curtsey, or bowmg move- ment, might, perhaps, add to its description. Fig. 16 may also 42 SCHEMATIC EVES serve to give a better idea of how the skiascope is to be held. Where an examiner is compelled to use his left eye, instead of his right one, an excellent way is to cross the right arm over the breast and in this manner operate the mirror as though the right eye were in use. The reverse, of course, applies to those persons who are left-handed, and permits of the graceful handling of the skiascope in place of the awkward non-professional manner sometimes acquired in other methods. It is always better for an examiner to learn to work with both eyes open when locating the reflected light on the face of a patient. After this location the eye not in use at the peep-hole should be closed so as to stimulate concentration, in order to sharpen the brightness of the fundus reflex as well as to define the shadow's edge. The closing of the unused eye serves to obviate any discomfort an examiner may experience, caused by the glaring light from the lamp in use. and especially so if the latter is at close range. The mirror light on the patient's face ought not to move over an inch in any one direction, and the red pupil should hardly ever be allowed to pass entirely from view, after once being found. All movements to be made very slowly, since rapidity of motion often interferes with judgment as to the shadow's direction, just as the spokes in a wheel are found to be more diflficult to count when the wheel is in motion than when it is stationary. SCHEMATIC EVES. The metal and pasteboard model eyes that are on sale in all first-class optical supply houses offer a most excellent means for beginners to familiarize them- selves with the principles of both skiametry and ophthal- moscopy. These eyes, however, are freciuently imperfect in construction, and the printed scales attached to them are often unreliable. As a consequence the student is apt to meet with discouraging results in his initial cfF(irts in using them. SCHEMATIC EYES 43 The cheap pasteboard model, such as shown in Fig. 17, is usually found to be just as trustworthy as the more expensive ones, but all of them require testing" before their findings can be implicity relied upon. Fig. 17. A "queen," PASTKl'.OARD, SCHEMATIC EYE. A good way to determine the accuracy of these models for skiametric purposes is to have an experienced skiascopisl put them to actual test by first setting the scale at "o." and then, if a one-diopter convex spherical lens causes a reversal of the shadow in all meridians at exactly forty inches away, it is quite safe to rely on other findings made by means of the same model. To prove, however, that the scales are properly spaced it is wise to first test a few of the numbers on each side of the "o" before depending upon them for accuracy, for in optometrical work in general it is so easy to be wrong and so difficult to be precisely right. All kinds of ordinary errors of refraction can be artificially created bv means of these model eyes together with a few trial lenses. For instance, if an examiner is operating at a distance of forty inches away, by setting the model so that it shows one diopter of myopia and then by adding a one-diopter concave cylindrical lens, he can create an error of one diopter of 44 REDUCTION OF LENSES hyperopic astigmatism, A one-diopter convex cylinder can be used to produce myopic astigmatism of equal amount. Set- ting the model to show two diopters of myopia and then using a one-diopter convex cylinder lens will create a compound error of minus one-diopter spherical combined with a minus one-diopter cylinder, due allowance of one diopter having been made for the working distance. With the model showing two diopters of hypermetropia, if a two-diopter concave cylindrical lens be added, the exact compound quantity represented by this error would be plus two diopters spherical combined with plus two diopters cylin- drical, and to neutralize it, skiametrically at a distance of forty inches away, it would require an added lens power equal to plus three diopters spherical combined with plus two diopters cylindrical. To illustrate a mixed astigmatic condition the model can be set to show two diopters of myopia, and then by adding a minus two-diopter cylinder at axis 90 an error representing minus one-diopter cylinder axis 180 combined with a plus one- diopter cylinder axis 90 can be obtained, which would require the addition of this lens quantity, or its equivalent, to neutralize it by means of the one-meter shadow test. At whatever axis the cylindrical lens is set the axis of the artificial astigmatism will be in the same meridian. REDUCTION AXD TRANSPOSITIOX OF LENSES. In practical examination-room work with the skiascopic mirror it frequently happens that a saving of time and trouble is effected by making a test right over the patient's own glasses ; this test resulting, perhaps, in the discovery that a compound- lens quantity needs to be either added to or subtracted from the lenses then in use. Upon neutralizing these glasses it is found that they, too. are of the so-called "compound" type, therefore an examiner must be possessed of knowledge that ri:ductu).\ of lenses 45 will enable him to tell the exact ray-bending power of the four lens quantities involved, and to do it also with ease and without waste of time or likelihood of making mistakes. In the consideration of most problems there is the unit or lowest appreciable quantity to be dealt with, so it is with lenses. Speaking macroscopically, the unit of all lenses is a cylinder. Therefore, if it is learned how to combine these cylinders, after having reduced all lens quantities to a cylin- drical basis, the transposition of lenses will be found to be a very easy task, no matter whether the lens quantities dealt with number few or many. This principle is much like the one in the old story related of the quack doctor who had two bottles of medicine with which he could cure all the ills that flesh was heir to. His plan was to give doses out of one bottle which turned every ailment into fits, then the remedy in the other bottle cured the fits, and the patient got well. To carry out a similar procedure it must be considered that two cylindrical lenses of like kind and strength when crossing one another at right angles are equal to a spherical lens. Hence the reverse follows, that a spherical lens is equal to two cylindrical lenses crossing one another at right angles, and whose kind and strength are the same. In the ophthalmic refractionist's consideration of cylinders he will never need them at any other than at right angles to one another, no matter whether they are plus and plus or minus and minus, of the same or unequal strengths, or whether they are plus and minus or minus and plus, equal or unequal, etc., etc. Their axes will always be at right angles, and for the simple reason that if they were crossed at any other than right angles their combined refraction would show a sphero- cylindrical effect, which could be duplicated by right-angle cylinders. 46 REDUCTION OF LENSES Now, in a combination of cylindrical lenses of unequal strength, but of the same kind, it will be seen that when their axes are at right angles to each other their combined refraction will be equal to that of a compound lens whose component parts are of a like nature : as a plus one-diopter cylinder set at right angles to a plus two-diopter cylinder is equal to a plus one- diopter spherical combined with a plus one-diopter cylindrical. The second cylinder in the above case having been robbed of a quantity equal to the strength of the first cylinder in order to convert the first one into a spherical quantity, the robbery is noted and due allowance made therefor. Except for purposes of analysis, the "crossed cylinder" is never to be generally employed; he who prescribes it other- wise only exhibits his ignorance of lenses and their uses, as the function of a lens is to 'bend rays of light, and it matters little whether this bending is done by two cylinders crossing one another at right angles or whether it is accomplished by means of a lens where one surface has a spherical curvature. This rule also applies to so-called "toric" lenses where the curves of one surface of revolution are elipsoidal in character, made so by having one meridian of curvature either greater or less than the one at right angles to it. To crowd an examiner's head with arbitrary rules is likely to lead to confusion, so that in the case of transposing lenses it is well to simplify the process as much as possible. There- fore, imitating the method of the quack doctor with his two medicines, it will be necessary to first reduce all lens quantities to a cylindrical basis and then commit to memory two short rules for the transposition of those cylinders. The following extra long combination may, perhaps, serve to make this reduction principle plainer: -f r. D. S. C -f 2. D. C. 90 C — I. D. C. 180 C + I. D. S. C — 2. D. C. 90 C — 2. D. S. RKDUCTIOX OF LENSES 47 Here, it is seen, are six lens quantities whose chief axes are 90 and 180 degrees. After creating two cohimns all of the lens quantities are written in the cylindrical equivalents whose axes come under these two headings, not forgetting that each spherical lens is equal to two crossed cylindrical ones whose strength and kind are the same. The following is then obtained : Axis 90 + I- + 2. o. -j- I. — 2. Axis 180 + I. 0. + I. 0. — 2. In the axis 90 column the totals are + 4- D. and — 4. D., which, of course, neutralize one another. In the axis 180 column the totals of — 3. D. less + 2. D. leave a remainder of — I. D. axis 180. Take this cxami)lc for instance : + 0.50 D. C. 45 C + 0.25 D. S. Z + 0.25 D. C. 135. Here the two chief axes are 45 and 135 degrees, and, pro- ceeding as before, the results are : Axis 45 + 0.50 + 0.25 o. Axis 135 0. + 0.25 + 0.25 + 0.75 + 0.50 The totals give one cylinder of + 0.75 axis 45 to be crossed 48 TRANSPOSITION OF LENSES by another cylinder of + 0.50 axis 135. the symbols being alike. Now another examjile in reduction : + 1.25 D. S. Z — 1-75 1^- ^- 15 C — 0.75 D. S. C — 0.25 D. C. 105. Being reduced, the results obtained are : Axis 15 Axis 105 + 1.25 + 1-25 — 1-75 o- — 0.75 — 0.75 o. — 0.25 — 1.25 + 0.25 This gives a total of one cylinder of — 1.25 axis 15 being crossed by another cylinder of + 0.25 axis 105, the symbols being unlike. In the three examples shown all lens quantities have been converted into cylindrical equivalents, so that in order to master them the two short rules before mentioned must be used for the transposition of these cylinders, and then the simple lesson will have been acquired. Rule Xo. i. — In a combiuation of cylindrical lenses of a like character, such as f>lus and f^lns. or minus and minus, the stren,(:th of the weakest cylinder should be written as the spherical quantity while the niFi-ERENXE between the tivo lenses should be uritten as a new cylindrical quantity, the axis of the stronger cylinder i^ofer)iin^^ the axis of the cylinder in conu- bination, thus: -\- 0.75 D. C. axis 45 C + 050 D. C. axis 135 should be written as equal to -f- 0.50 D. S. C + 025 D. C.-axis 45. Rule Xo. 2. — In a combination of cylindrical lenses of dif- TRANSPOSITION OF LENSES 49 fcrcnt character, such as f^lits a)ui tiiinus, or )>ilinis and ['his, the strength of cither cylinder can be zcritten as a spherical quantity while the sum of the tico cylinders should be written as a new cylindrical quantity, the axis of the second lens con- sidered governing the axis of the cylinder in combination, thus: — 1.25 D. C. axis 15 C + 0-25 D. C. axis 105. can be written in two ways, the better way being to write it with the minus quantity first, so as to obtain a periscopic effect in the completed lens, this produces : — 1.25 D. S. C + I-50 D. C. axis 105. Or an equal refractive quantity can be obtained by writing it in this way: -f- 0.25 D. S. C — i-50 D. C. axis 15. To re-transpose any of these combinations it is only nec- essary to proceed by the usual reduction to cylindrical form and then apply whichever one of the two simple rules may be called for. The world, metaphorically sj^eaking. takes off its hat to the mathematician. And if the optometrician desires that def- erence be shown him, too, he must acquire enough of mathe- matics to make him proficient in his work. Procrastination and the plea of "no time to take up higher optics" will neither advance the individual nor the calling to which he belongs. Self-education is just as good as any other kind of education, provided it accomplishes its object ; there- fore let him who desires to make substantial skiametric advancement remember that the greatest service he can do himself is to thoroughly master the rudiments of light and lenses and to acquire the ability to juggle with all kinds and quantities of ray and lens values. CHAPTER ITT. Optical Principles Involved in Skiametry. — The "Shadow/' \\'iiat to Look For and How to See It, and How to Imitate Its Action with a Cardboard Model. OPTICAL PRIXCIPLLIS OP SKLIMETKY. The optical principles involved in shadow-testing cover a wide range, when all minor details are followed out to their extremes. This pursuit, of course, is not expected in a book of this kind, and only those optical facts which are essential to a practical understanding of the matter in hand will be attempted here. But -as far as possible provable truths will be made to take the place of mere assertions, and to that end the reader's attention will be called to a series of drawings representing phenomena whose action can be easily shown by experiments made with simple "smoke boxes." These boxes are not difficult of home manufacture. They are to be made of ordinary thin box board in which, with a narrow piece of electrician's tape, one side of the box is to be replaced with a piece of plain glass. Then over a round aper- ture one inch in diameter in front of the box another plain glass is placed, and in one corner a hole is made which dan be corked up. A hole is also to be made in the opposite end of the box, and this, too, is to be fitted with a cork. Several whiflfs of tobacco smoke can then be blown into the box. which is thus made ready to exhil)it the action of a beam of light of an intensity such as might be had from a partially hooded Welsbach gas lamp placed a few feet away. The rays from OPTICAL PRINCIPLES the lamp arc first passed through a properly adjusted convex lens, in order to render them parallel before they enter the box. In measurement the latter is to be four inches high, four inches wide and ten inches long. Fig. 1 8. A IlOME-MAUli ■■-SMOKE BOX. Now, if a beam of light whose source is such that its rays are parallel is permitted to enter the box shown in Fig. i8, an illuminated area will appear at the back end of the inside of this box, the diameter of which area will be the same as that of the one-inch opening in the front end of the box. Fig SHOWING REFRACTIf)N BV LENS OF PROPER FOCUS. Remember that this box is ten inches long, consequently in box (Fig. 19), which is the same in every way, except that a 52 OPTICAL PRINCIPLES trial lens of four diopters convex splicrical has been placed over the front aperture and has thus caused the refraction of the beam as shown in the drawing. Fig. 20. SHO\VING REFRACTION BY TOO STRONG A CONVEX LENS. Now% supi)Ose that in place of the four-diopter convex spherical lens a trial lens of five diopters convex spherical had been placed over the front aperture ; then the refraction would be as shown in Fig. 20. The arrow pointing to the focus, or crossing point of the rays. Fig. 21. SHOWING REFRACTION RV TOO WEAK A CONVEX LENS. In the box (Fig. 21) the trial lens used is a three-diopter convex spherical and as a consecpience its fot should be considered as a source and a note should be made of the rays reflected therefrom. Or, better still, to avoid complicating matters, let a pin-hole opening take the place of the mirror at the back of the box, and then place a lighted candle close to this opening so that its rays may enter through this small hole and be made to behave in about the same manner as though the candle were actually placed right in the very hole. Referring to the box (Fig. i8) again, the emerging rays under the same conditions would leave the one- inch ajjcrture at the front of the box in a divergent manner, as shown by Fig. 22. 54 OPTICAL PRINCIPLES Fig. 22. LIGHT EMKRGING UNREFRACTED. In Fig. 19, owing to the four-diopter convex trial lens over the front aperture having its focus exactly at the source of the light, the rays would emerge parallel, as shown in Fig. 23. Fig. 23. RAY.S EMERGING PROPERLY REFRACTED. In Fig. 20 the emergent rays, owing to the fact that a stronger lens is placed before the one-inch aperture, would con- verge to the conjugate focus of the light source. Fig. 24 shows this convergence to be equal to one diopter. OrTKAI. PKINC Il'I.KS I'^IG. 2JL. EMERGING RAYS HAVING A CONJUGATE FOCUS. In the box shown in Fig. 21 the lens used over the front aperture is too weak to render the emergent rays parallel, and as a consequence they will diverge as shown in Fig. 25, the divergency being equal to one diopter. Fig. 25. RAYS EMERGING DIVERGENT. In Fig 26 the conditions are again slightly changed from those shown in Fig. 22. Here, for the sake of emphasizing another optical fact, one end of the box has been pushed for- ward so as to alter its depth from ten inches to eight mches. 56 OPTICAL PRINCIPLES I^IG. 26. RAYS EMERGING UN REFRACTED. As a consequence, the rays emanating therefrom have a divergency equal to five diopters. The placing of a four-diopter convex trial lens over the front aperture of this box, as was done with the box in Fig. 23, would cause the rays to still emerge with a divergence e(iual to one diopter, as shown in Fig. 27. Fig. 2-]. ^ ^ ^^ c — =^-=1^==^ (^' -^r--rz W^/ ^ ibi)/r/idf /VT -f-i.c. RAV.S EMERGING I N.SUFFICIENTLV REFRACTED. If a fivf-diopter convex trial lens had been used over the front ai)erture, as in Fig. 24, the emergent rays would be parallel, as seen in l''ig. 28. optical principles Fig. 28. 57 RAYS EMERGING PROPERLY REFRACTED. But if a three-diopter convex lens were used, as in Fig. 25, then the emerging rays would have a divergency of two diopters, Fig. 29 showing this principle. Fig. 29. RAYS EMERGING WITH DIVERGENCY INCREASED. These theories of the angle of the emergent rays of light can readily be proved by using a double smoke box, or one placed in front of the other, the front one having a glass end as well as a glass side. To make this test satisfactorily the source of light at the back end of the first box must be greatly 58 OPTICAL PRINXIPLES intensified in order that the hght may penetrate the smoke and enable its direction, or angles, to be seen. It is not a great stretch of the imagination to consider a series of schematic eyes whose various depths are nearly the same as the lengths of the boxes in Figs. i8 to 29. The term "nearly" is used because, instead of changing the lenses, as was done with the boxes, it will now be necessary, in order to obtain similar results, to change the depth of the eyes a little, making them correspond optically with the boxes shown in Figs. 23. 24 and 2'. Fig. 30. AN EMMETROPIC EYE. Comparing with the box in Fig. 23, an eye would have to be ten inches in diameter with its refractive media equal to four diopters. This would be an emmetropic eve. as shown by Fig. 30. The Jk)x in Fig. 23 converted into an eye thirteen inches deep shows it to be myopic, because the emergent ravs are con- vergent one diopter as in Fig. 31. optical principles Fig. 31. 59 a myopic eye. Fig. 32. A HYPERMETROPIC EYE. 6o OPTICAL PRINCIPLES I-'ig. 2/ becomes a hyperopic eye on its conversion, as shown by drawing Fig. 32. This, of course, is pre-supposing that the accommodation of the eye is fully relaxed. In shadow testing it becomes necessary to have all eyes virtually myopic in order to measure the shadows. Before coming to the real consideration of what is meant by the tenn "shadow." it will perhaps be well to first say a few words about this myopia, true and artificial, which has just been mentioned. The myopia requisite for shadow-testing at the usual distance if forty inches is equal to one diopter, to artificially produce which the eye in Fig. 30 would require the addition of a one- iliopter convex lens, as the emergent rays are here parallel. In the eye in Fig. 31 no trial lens would be required, for these rays already emerge convergent equal to one diopter. But if the true myopia were greater or less than this quantity, it would either have to be raised or lowered to this amount, as the case might be. In the eye shown in Fig. 32 the rays diverge one diopter, so that it would take a one-dio])ter convex lens power to alter them to parallelism, hence a single lens of two diopters convex would be necessary to produce the required convergence. Thus it will be seen that if the convergency or divergency of the emergent rays vary more or less in angle, a stronger or weaker lens will be needed to make the rays first parallel, and then after that convergent one diopter, in order to lender them of working value at a distance of forty inches. The point has now possibly been reached where something may be said understandingly alxiut the shadow. It will, there- fore, not be out of place to quote from a little leaflet entitled "The Value of the Shadow Test in the Fitting of Glasses," which was published by the author some years ago. "When rays of light from a lamp, or other illuminator, are reflected into an eye by means of a mirror having a small hole OPTICAL PRINCIPLES 6l in its center, an observer, by i)lacing his eye at this hole, can note that the pupil of the observed eye appears red instead of black. If the mirror is moved from side to side a pale cloud- like shadow will pass or flit across the red pupil. The behavior of this shadow under certain conditions constitutes a test whereby optically defective eyesight can be accurately esti- mated, and from this estimate correcting lenses can be supplied." ^^S- 33 ""la.v give something of an idea as to the appearance of this so-called "shadow." Fig. 33. ILLUSTRATIXG A SO-CALLED SHADOW. To see what causes it, and what it really is, it will be neces- sary to go back to the box in Fig. 18. Through the smoke in that box a spot of light is to be seen at the back end of the box. The area of this spot, as was described, is that of the one-inch aperture in the front end of the box. This spot, or area of light, is surrounded by darkness, or shadow. Now, if the source of light be moved so that the beam enters the box at a different angle, the spot on the back end of the box will, of course, move in the opposite direction to the movement of the light source. As this spot moves so does its edge or "shadow" move, too, and as this edge under certain optical conditions will assume a cloud-like appearance which may cither partially or totally cover the red pupil of an eye. and which seems to behave in a manner cjuitc independent of the movement of the light, which is really responsible for it, the early observers of 62 OPTICAL rUINXIPLES these phenomena gave it the name of "shadow test," whereas "light test" would perhaps be equally applicable, since without a light there can be no edge to a shadow. By reference to Fig. 19 it will be seen that the light area in the box is much smaller than those in Figs. 20 and 21, although in Figs. 20 and 21 the light spots are apparently the same in size even though different lenses are used to produce them. This size of the light area, however, indicates a high or low degree of hyperopic or myopic error, and the smaller the area the less will be the error, besides the action of the shadow will be quicker, for, of course, a large spot takes a longer time to pass a given point than a small one does provided the speed is the same. The edge of the spots, or light areas, claim attention next, and as this edge is practically a curved line, one side of which is dark and the other side light, the action of this line, or edge, is to be observed : the reader is therefore referred to Figs. 23. 24 and 25, as the emergent light will act in a similar manner, the edge of the shadow theoretically taking/ the place of the pin-hole light source in the lx)x. It is but a step from the eyes shown in Figs. 30, 31 and 32 to carry the imagination forward to the real thing, for living eyes are constructed on the same general optical principles as the boxes shown in Figs. 18 to 29. Now, to give another illustration of the principles governing the phenomena discovered by Bowman, Fig. 34 and 35 may serve to emphasize two salient points. In Fig. 34 it will be noted that the light from the candle is reflected into the eye and converges towards an imaginary focus behind the retina, thus producing a much larger area of illumination than if the enter- ing rays of light had been parallel, instead of divergent, and ■were brought to a focus exactly on the back of the eye. It mav also be observed that the iris of the eye serves to EMONSTRATI NG THE shadow's ACTION. THE SHADOW 69 compound. In simple myopia, the shadow moves against the mirror equally in all meridians. In simple myopic astigmatism there is one meridian in which there is no motion ; this is the axis. All of the other meridians show more or less movement. In compound myopic astigmatism the movement of the shadow is against the mirror in all directions, but with greater speed and with less definition in one meridian than in that at right angles to it. In hypermetropia the movements are just the reverse of this ; that is, when the reflected light moves across the pupil of the patient's eye the shadow will be found to move in the same direction as the movement of the mirror. To imitate this with the cardboard models the lamp must remain sta- tionary while the cards are in the same position as before, the card (Fig. 36) is then made to move a few inches, thereby illustrating the movement called "With the mirror." Like myopia, hypermetropia also can be simple or com- pound. In simple hyperopic astigmatism there is one merid- ian in which no motion is observable. This is the axis. In compound hyperopic astigmatism the movement of the shadow is with the mirror in all directions, but unequally so, greater sped and less definition being shown in one meridian than in that at right angles to it. The small slot in the card (Fig. 36) next to the lamp can be used to illustrate high degrees of astigmatism, the long diameter of the slot showing as a band of light on the pink pupil and indicating the axis of the error. Where this error is slight the band will be so wide that only one edge of it will apj)ear in a jnipil at one time, showing merely as a straight edge and dififoring from a spherical error which shows a slight crcscent-likc curve at its edge. CHAPTER IV. Action or riii£ Shadow and What It IxnicATES. — Appear- ance OF THE Shadow in Regular and Irregular Errors of Refraction and Its Action in Emmetropia, Myopia, Hvpermetropia and Astigmatism. THE SHADOW'S ACTIOX. After a student has become thoroughly famiHar with what is meant by the term "shadow." as used here, and is able to observe the shadow in livinj]^ eyes as well as in schematic ones, the next question which naturally arises is as to what its action indicates and why the phenomena are not the same in all eyes under similar conditions of mirror, light and lenses. Attention, therefore, is called to Figs. 38 and 39 for the purpose of making plain the shadow's apparently erratic movements in myopia, hyper- metropia and astigmatism, both simi)le and compound, includ- ing regular and irregular conditions of media and retina. To illustrate an emmetropic condition, as well as a hyper- opic one, the somewhat inaccurate drawing (Fig. 38) is intended to show that the observed eye, P, is of such a depth that the rays of light emerge from it parallel. If this eye were hypermetropic, and there were no accommodation, the rays would emerge divergent : the effect, however, would be the same. Now, it will be perceived that the dart, or arrow, at the back of the eye, P, points downward. Taking a supposed ray at the arrow's point it can be followed. And, as it is well known that all objects appear on the retina of an eye just as THE SHADOW S ACTION 71 they do on the sensitized plate of a camera, namely, upside down, or inverted, it will be seen that the lowest ray which is at the point of the arrow, leaves the patient's eye as the highest, or topmost one, and traveling to the examiner's eye, E, behind the peep-hole of the mirror, it enters this eye as the upper ray still, and then, undergoing refraction, it is photographed upside down and becomes the lower ray, finishing at the point of the arrow on the observer's retina, in the same direction as Fig. ^8. SHOWING WHY THE SHADOW MOVES WITH THE MIRROR. when it started. As a result of this, its position on the retina of an observer's eye will always be relatively the same as its position was at its source, on the retina of the observed eye, no matter how it is moved, provided, of course, that the refrac- tion of the observed eye is not altered by placing any lenses in front of it, or, if it is a living eye, by increasing its accom- modation. In myopia of a conjugate focal length less than the work- ing distance at which an examination is made, when light is reflected into an eye it can be observed that the shadow moves against the movement of the skiascopic mirror. Now, if attention is directed to Fig. 39 (which is also somewhat inac- curate) it will be made plain just why this motion is opposite to the movement of the reflected light from the mirror. •J 2 TMK SHADOW S ACTION The mvopic. or observed eye, I', as will be seen, is of too great a depth, and consec|uently the rays emerging from it cross one another before they reach the examiner's eye. E. Therefore the lower ray in the observed eye, P, at the nrrow's point, emerges as the upper ray, as before, but instead of con- tinuing parallel it converges to the conjugate focus of its -I'urce. and then, after crossing the other rays, it enters the \e, E, as a lower ray, and being refracted again it arrives at lie retina of the examiner's eye as an upper ray. or at the jnoint of the arrow, which, as will be observed, is now pointing Fig. 39. snowiNc, WHY riii£ shahow mo\i:s against tiif. mirror. in the ll.\|)()\\ S ACTION Cross, Fig. 46. illustrates one diopter of myopic astig- matism at axis one hundred and eighty degrees. Each of Fi*^. 45- H51.D. j.D. ILLLSTRATING II VPERMETROPIC ASTIGMATISM. these two optical conditions varies from the fifty-two-diopter standard in one meridian only. I-IG. 46. + D. + 53- D. ILLUSTRATING MVOI'IC ASTIGMATISM. The compound hyi)eropic astigmatism, illustrated by cross, Fig. 47, shows that a lens of + i. D. S. C + i. D- C. axis 90 would be required to bring its refraction up to standard. THE shadow's action Fig. 47- 17 ;o.D. + 51.D. ILLUSTRATING COMPOUND HYPERMETROPIC ASTIGMATISM. Cross Fig. 48, shows compound myopic astigmatism re- 'io„c nf T D S -^ — I. D. C. axis 180 in order to qmrmg a lens ot — i. i->- ^- ^ ^- -^ lower its refraction to standard. Fig. 48. + 53-D- + 53- D- + 54. D. ILLUSTR.XTING COMPOUND MYOPIC ASTIGMATISM. In mixed astigmatism requiring a lens of + i- D. C. axis 00 ^ - I D C. axis 180, the over-refraction in one meridian, and^ihe under- refraction in that at right angles to it arc shown by cross, Fig. 49. the shadow s action Fig. 49. 51- D. + 51.D. + 53- D- ILLUSTRATING MIXED ASTIGMATISM. All these crosses, except that in Fig. 40, are at variance with the fifty-two-diopter standard in one or more meridians. Fig. 50 shows how the same principle can he ilhistrated when -f 55. D. -f 49- D. ILLUSTRATING MIXED A.STIGMATISM AT OBLIQUE AXES. the axes of greatest and least curvature do not lie in either a horizontal or vertical position. As here shown, axis forty- five indicates three diopters of myopia, while axis one hundred and thirty-five shows three dioi)ters of hyperopia. THE SHADOW S ACTION 79 The case, therefore, could be classified as one of mixed astig- matism with oblique axes, the transposed formula calling for a compound lens of - 3- D. S. 3 + ^>- D- C axis 45- or one of + 3. D. S. C — 6. D. C. axis 135. In cross Fi?. 40. if the skiametric conditions were made perfect bv first \ising a so-called "working" lens, or its ac commodative equivalent, there would be no appreciable move- ment of the shadow, no matter in what meridian the mirror were made to travel. In cross. Fig. 41. there would be move- ment with the mirror equally in all meridians. This move- ment would be fairly rapid, but the shadow would not be as dark or as heavv as it would be in cross, Fig. 42. where the error is greater.' In cross. Fig. 42. however, the movemen of the shadow would be more sluggish, and the color would be deeper also. In crosses. Figs. 43 a.-.l 44- tl^e shadow will move aga.nst the movement of the mirror in all n,eri f^). So with hmnan eyes, they become easy to "scope after 80 THE shadow's AIM'KARANCE the student has persevered and not given up with an "I can't," or pronounced the shadow-test a failure after a few unintelHgent efforts to master it. APPEARANCE OP THE SHADOW. The word "regular" is defined by the dictionary as "acting according to rule: following a uniform course," etc. This definition is very fitting when applied to, perhaps, two-thirds of the cases which present themselves for skiascopic examination. A typical "regular" case is one in which the cornea, crys- talline and other media are free from anything which might prevent light from passing from the front to the back of an eye in an unobstructed or undeflected manner, other than that called for by the ordinary refractive condition of eyes in general. There is also a fair-sized ocular pupil and a bright- colored retina in which the light reflex shows a pronounced pink tint. The word "irregular" is defined as "not regular: depart- ing from, or being out of, the usual course." Therefore a thickening of the tissues of the cornea, resulting perhaps from inflammatory processes, might give rise to an irregular re- fractive condition. This irregularity could also result from a disturbance in the transparency of the crystalline, due to cataractous conditions, etc. One of the most frequent causes of unsatisfactory skiametric work lies in the lack of brilliancy of the retina of an eye, this dulness being attributed to an excessive pigmentation at the fundus, giving such a poor reflex that the movement of the shadow is most difiicult of determination with any degree of accuracy. But as the stu- dent advances in the judgment which comes only from ex- perience, these troublesome interferences are minimized. That the reader may more fully understand the appearance of the various shadowv reflexes met with in skiametric work. THE shadow's appearaxce 8 1 attention is called to a few drawings which are intcn.lcd lo aid beginners. Fig. 51 shows a shadow rather too pronounced to be quite natural, but it serves to emphasize the crescent-like Fig. qi. SHOWING THE CRESCEXT-LIKE SHADOW IX SPHERICAL CASES. curve of the advancing edge of the shadow, which indicates thafthe error is of a simple myopic, or hyperopia character, and that it is not astigmatic. For if the error were of the latter kind the shadow would show more of a straight edge, the principle of which Fig. 52 is intended to illustrate. This Fig. 52. SHOWING THE STRAIGHT EDGE APPEARANCE OF THE SH.ADOW IN ASTIGMATIC CASES. Straight edge in pronounced degrees of astigmatism takes on a band-like form as shown in Fig. 53- Injveal^degreethis band is difficult to determine on account of its excessive width, 82 THE SHADOW S APPEARANCE but where the error is two diopters, or more, then tlie axis of the astiginatism is readily cHscerned by noting the direc- FiG. 53. SHOWING THE PAND OPSERVABLE IN HIGH DEGREES OF ASTIGMATISM. tion of the band. Fig. 54 shows a marked error at an axis of 45 d'^'grees. Fig. 54. SHOWING AN ASTIGM \IU I'.AND AT OPLIQUE AXIS. In operating at forty inches distance from an eye of this kind, when a workin g refraction equal to one diopter is in use, if the mirror reflected the light afong the axis of forty- five degrees there would be no apjireciable movement of the shadow either with or against the mirror's movement ; but if the reflected light were mf)vc(l along the meridian represent- ing axis one hundred and thirty-five degrees, then the shadow would show a dark, cloudy edge and would move, very de- cidedly in some cases, cither with or against the movement Tin-: shadow's .\rin:.\K.\NCE 83 of the light which would depend, of course, upon whether the astigmatism was of a myopic or hyperopic character. Fig. 55 is intended to illustrate the difference in curvature between the two edges of the shadow at axes ninety and one hundred and eighty degrees ; also to show a difference in its intensity, which may be seen in cases of compound hyperopia or myopia. Of course, these two shadows do not appear at Fig. 55. SHOWING THE MERIDIONAL APPEARANCE OF THE SH.\DOW IN COMPOUND ASTIGM.VTISM. one and the same time, but if the mirror be rotated in the vertical meridian first and the horizontal next it will be seen that the vertical movement shows a curved edge and a lighter shadow than the horizontal movement. The horizontal move- ment gives a straighter edge and a deeper shadow. A com- pound error of + 2. D. S. Z f 2. D. C. 90 wiU give a good example of this. Figs. 51 to 55, just referred to, are merely intended to give a crude idea of what is meant by the term "regular" conditions, for if an attempt were made to give exact photo- reproductions of the many eyes met with in actual practice the result would be found most confusing and troublesome of comprehension. In Fig. 56 is shown what might be called one of the "irregular" kind. This depicts a double band-hke ap- pearance of a shadowy reflex and is known by the name of the "scissors" movement, for instead of the band passmg cither 84 THF. SHADOW S APPEARANCI£ Fig. 56. SHOWING THE DOUBLE BAND IN THE SCISSORS MOVEMENT. on or off of the pupil in accordance with the motion of the mirror it seems to close in from both sides at one time like the closing of a pair of scissors. The cause of this may lie either outside or inside of an eye. If outside, it indicates that the correcting lens used is considerably tilted in its set- ting or else the eye is viewed too far on one side of its anterior- posterior axis; if inside, that the crystalline is slightly askew in its position, from some cause or other, and that what is called a "cylindrical equivalent" is developed, one side of this so-called "cylinder" quantity being much weaker than the other. Hence the phenomena. Another irregular condition is shown in Fig. 57. This Fig SHOWING A CASE OF IRREGULAR ASTIG.MATISM. illustrates a simple astigmatic case of higji power, where the cornea is so thickened by scar tissue that the ei\ge of the band of light and shadow shows an unevenness which is in marked contrast to Figs. 53 and 34. Of course, it would not be THE shadow's appearance 85 expected that vision in a case like this could be n^ide to equa twentv-twentieths. but the correction of the error might still prove of considerable assistance to some poor unfortunate whose daylight had been somewhat dulled, notwithstandmg the presence of that great benefactor, the sun. In Ficr 58 is shown an irregular condition due to opacities in the crvstalline. resulting in cataractous strict. These cases are frequently very deceptive, for where poor vision is looked Fig. 58. SHOWING A CASE WHERE ^CORTICAL CATARACT" IS PRESENT. for good is often found, and vice versa, so that no matter how many opacities appear it is always wise to determine the refraction skiametrically if possible. Fig. 59. SHOWING THE SHADOW AS IT SOMETIMES APPEARS IN "CONICAL CORNEA." In Fig 59 an attempt has been made to show the appear- ance of a'pupil where the irregularity of the -- -anifes s itself in what is commonly called a 'conical ^^rn . Tl se cases are very difficult to do anything for. either with or with 86 THE shadow's appearance out the aid of skiametry. However, the shadow sometimes serves to enable an examiner to obtain some clue which may possibly be of value in giving slight assistance by correcting a part of the refractive errors present. In such a case the shadow would be very apt to show much heavier on one side of the irregular reflex than on the other. CHAPTER V. Some Theories Regarding the Dullness of the Fundus Reflex in Certain Cases.— Also Multiple Meth- ods FOR Practicing Skiametry, Including the Toxic and Non-Toxic Manner of Employing the Static Test by Both the Amplifying and Fogging Methods. SOME THEORIES REGARDIXG REFLEXES. In the consideration of a subject as broad as the term "oculalr skiametry" would indicate, the temptation of an author to soar his kite in the reahn of speculation and attach to its tail a few theories of his own is very great indeed, even if the theories given should prove "bad pennies" and return to embarrass their giver. In the actual practice of skiametry there arise certain little side details which might be considered as sul>phenomena. and which exercise a more or less important bearing upon the system as a whole. Attention will therefore be invited to a few of these points which perhaps savor more of theory than thev do of practice, but which, nevertheless, seem to answer natural questions likely to arise in the mind of those who desire to know the "why" of everything they are inter- ested in. Ocular pupils of a size not exceeding two millimetres m diameter, notwithstanding an examiner's ability to magnify them, are verv often troublesome, and constitute a part of 88 DULL REFLEXES the cases which it is wise to "refract" by means other than by skiametry. This is especially true if, in addition to the smallness of the pupil, the fundus reflex is of the deeply pig- mented or dull kind. The reason for this is due not so much to a failure on the part of the examiner to see the pupil as it is to the fact that the pupillary aperture prevents a suf- ficient volume of light from entering the eye in order to create a shadow sharp enough in outline to be readily measured. In expecting to occasionally find some dull ocular fundi the examiner must not mistake high degrees of myopia or hyperopia for excessive pigmentation. Especially in myopia, even in an error of only four or six diopters, will the examiner be sometimes puzzled to determine why the reflex is so poor. Fig. 6o. SHOWING WHY THE Ki:ri.\\l. I I.I.I " M I NATION' IS LARGE IN MARKED ERRORS. Adding correcting glasses, however, often brightens the reflex in proportion as the correction ncars the total, although at the exact reversal point of the shadow tiie reflex may again DULL REFLEXES 89 become very unsatisfactory, as may also the movements of the shadow at this time. A slow motion must not be mistaken for no motion. For if Fig. 60 is looked at it will be seen why pronounced degrees of either myopia or hyperopia make large light areas on the retina, thus requiring a longer time for the shadow to come into view than if these illuminated areas were smaller. Fig. 61 also shows this principle emphasized in a myopic eye, the returning rays from the edge of the illumination illustrating the distance the shadow must traverse before reaching the opposite side, this area being much larger than if the eye was an emmetropic one. Fig. 61. SHOWING WHY THE SHADOW MOVES SLOWLY IX M.\RKED ERRORS. The fact that a reflex in a given error of myopia is much duller than in a corresponding one of hypermctropia is due, nodoubt, to the greater distance tTie TetuniTng rays travel in a myopic eye than in an emmetropic or hyperopic one before 90 DULL REFLF.XES reaching^ the pupillary opening, which, consequently, dimin- ishes their number through radiation and absorption. Fig. 62 illustrates a theory of this. The shaded circles show the I'lG. 62. siiowi.vG wii'i rui: shadow is duller i.\ mvoplv tii.\x ix A LIKE DEGREE OF HYPERMETROPL\. relative loss by ciuenching, as the divergent rays from a given illumination on the retina of a myopic, emmetropic, or hyper- opic eye strike the inside of the iris, which serves to diaphragm part of them and prevent their further egress. Fig. 63 shows the comparative loss in intensity of illumina- tion in eyes whose myopic or hyperopic error varies from one to sixteen diopters. Brightness of reflex, however, is not all there is to skiam- etry, for, owing to the optical phenomena called penumbra," it is sometimes i)ossible to obtain a more defined shadow under a moderate illumination than it is under one more intense. A reason for this is suggested in Fig. 64. But just what part the numerous pcnumbrre play in interfering with the sharp- ness of demarcation of the sihadow it is difficult to ascertain, DULL REFLEXES 91 for the emerging light casts its penumbra at the peep-hole of the examiner's mirror just as the entering light iloes at Fig. 63. ->»1 1. ^/\ \ -^ n \A ( • , 1 a\ 1 1 ' — tH-t- w / ^ ^w / showing relative size of retinal illumination in high and low degrees of myopia. \ Fig. 64. SHOWING THE OPTICAL PRINCIPLES OF PENUMBRA. the pupil. And when it is considered that a round aperture is being dealt with it will be seen that the conditions are even 92 DULL REFLEXES more complex than would ordinarily manifest themselves if the shadow cast were a central one instead of being peripheral. Figs. 65 and 66 may serve to make this subject better under- stood, for. as has been said before, a sharp shadow is a great aid to good work. Fig. 6:;. SHOWING OPTICAL PRIXCIPLES OF PEXU.MBRA DOUni.rCD. in I'ig. 65 the same principle can be seen as is shown in Fig. 64, only the conditions are doubled, for here there are three points on the candle flame instead of two, the central point acting in a manner which virtually makes it the same as though four points had been selected. In Fig. 66 this principle has been adapted to an eye where two points of illumination are again shown whose source is located on the mirror-like retina, for it is only the center of the shadow that is involved, this shadow being produced by the iris. If either a plus or minus lens is placed in front of this eye it merely serves to refract all the light in accordance with the lens selected, the penumbra still remaining to add to the indistinctness of the edge of the shadow. MULTIPLE MLTIIODS 93 If this phenomenon could be overcome it would, secmingl>s contribute much toward that skiametrical accuracy for which all skiascopists are striving, but, as has already been shown. Fig. 66. G THE IXTERFEREXCE OF PENUMBRA IN SHADOW SHOWIN TESTING, if a gain be made in one direction a loss .s quite sure o foUow „ another, so that examiners must be content wtl, fuzzy shadows until some master mind solves the problem how to tet rid of them without interfering with the valuable pomts already secured in dealing with other conditions. Operating at a distance, in order to produce parallel, m of the rays, seems at present to be the only means o ove comL this phenomenon, and this, of course, bars out the use "y test's and methods which have been found of grea Lvice in uncovering ciliary spasm, latent errors e^- J " it will be seen that many contributing factors of both success anlailure enter into the problem of accurately es.imatmg "e refractL of an eye independent of a patient's intelligence^ MULTIPLF METHODS. Even in the method to wh.cM 94 MULTIPLE METHODS amincr attempts to induce relaxation of the muscles con- trolling accommodation of an eye by instilling into the cul-de- sac of this organ some one of a series of powerful toxicants, for the purpose of temporarily suspending a part of its func- tional activity, thus, for the time being, partially transforming a living eye into a sort of schematic one. Xon-medical examiners, on the other hand, attempt the suspension or relaxation of this accommodation by having their patients endeavor to look at some distant object in order to thus coax the muscles into a condition of rest, and, in further explanation, it may perhaps be truthfully said that in many cases one overdoes the matter while the* other under- does it. The medical examiner's overdoing consists in forcing the eye into an abnormal condition in which co-ordination be- tween accommodation and convergence is temporarily de- stroyed, this destruction depending, of course, upon the strength of the drug used, and the duration and frequency of its instillation, as well as upon the idiosyncrasies of the patient. The results obtained by measuring the refraction of an eye while it is in a state which may be called "local intoxication" would seem to call for judgment of the very highest order to make the theoretical conform to the practical. Regarding the non-medical examiner's manner of using the static method, especially in those cases where the nuiscle action is liable to be particularly vigorous or active, it can be likened to the old story of the blind leading the blind, for the reason that if the patient fails to maintain the requisite * muscular relaxation the examiner has no means of knowing what action has really taken place, and his findings, therefore, are likely to j)rove very unreliable. ".Spasms of accommodation," as they are termed, are prr)bal)ly responsible for more mistakes being made in the nrtii-toxic manner of emj^loying the static method than can be MULTIPLE METHODS 95 attributed to the carelessness of patients lookino-. or trying to look, at the object or point to which tlu-ir attention has been directed. In dealing with these cases there are two ways in which static skiametry can be used. One consists in beginning an examination with only that lens before the patient's eye which is necessary to produce the required working refraction for the operating distance, whatever that may be, and then, if the case is a hypermetropic one, the convex lenses are to be gradually increased in strength until the reversal point of the shadow is obtained. If the case is myopic, however, then an overcorrection is necessary, and the concave lenses used for this purpose are to be gradually decreased in strength until the reversal point of the shadow is found. This manner of increasing in hyper- metropia and of decreasing in myopia is called the amplifying method. Overcorrecting in hyperopic cases and undercorrecting in myopic ones have been termed the "fogging" method. And where ocular skiametry is performed in a non-toxic manner this method of decreasing lens values in hypermetropia and of increasing them in myopia will often prove of great assist- ance to an examiner, and especially so if applied in a binocu- lar manner, for then the co-ordinate action of accommodation and convergence is such as to give the most reliable results. This, of course, includes more particularly those cases where the age of the patient is such as to lead an examiner to fear spasmodic muscle action. In cases of persons fifty years of age or older, wherein presbyopia has a tendency to overcome spasm of accommoda- tion, then static skiametry will frequently be found quite trustworthy by the non-toxic method, but where the age of the patient is less than fifty years then the toxic method, or 96 -MULTIPLE METHODS one even more reliable than either toxic or non-toxic, must be used to determine true refractive conditions. In the toxic application of static skiametry it, of course, matters little whether the amplifying or fogging method is used, for here the accommodation is supposed to be in abey- ance and the examiner can suit his own convenience in regard to the manner in which he alters his lens quantities. But the toxic method has disadvantages along many lines when it is viewed from both scientific and economic standpoints. In its scientific aspect it fails entirely to tell anything about muscle tension at the reading point, leaving this to be esti- mated, or guessed at, by the examiner, while this reading point, as is well known, constitutes one of the most important ends for which glasses are usually adapted. Frequently, too, for distance purposes a medical examiner is led to advise glasses from a theoretical instead of a practical viewpoint. All cycloplegics are, of necessity, mydriatics as well, and the mydriasis they produce constitutes a disturbing factor in some cases, for, as has been described by others, the pupillary field becomes so enlarged during an examination that the oper- ator is compelled to limit his observations to corneal zones, which in turn add to the skiametric complications and serve to increase the difficulties of the method. Viewed from an economic standpoint the toxic method tends toward the needless distress of patients, causes a quite unnecessary waste of valuable time in waiting for the action of the cycloplegic, and takes a foolish chance, even if only a slight one, in risking a possibility of blindness resulting from glaucoma. Every working distance at which static skiametry is prac- ticed, whether by toxic or non-toxic means, really constitutes a mctho I 25 to an actual pair of spectacles or eye glasses, and because, also, it gives the only means of determining binocular vision with any degree of satisfaction. To ocular skiametry, however, belongs the first place in the refractive scale, not perhaps from its priority of discovery, but rather from a utilitarian standi)oint. It is the great path- finder and points the way for other work. In its most approved application it discloses minute opacities of the cornea and crystalline lens, thereby giving information at once which the ophthalmoscope could not locate except, possibly, after a long time-consuming hunt. It tells of the presence of astigmatism, its character and approximate axis, and also whether it is complicated with any error requiring the correction of spherical lenses. It shows refractive conditions independent of the patient's age, language or answers, and serves to check carelessness in all persons. In children it is of invaluable service, and in those whose hearing is faulty it saves much shouting and misunderstanding. Its use, therefore, comes at the beginning of an examina- tion, and if astigmatism of over one diopter is disclosed it is a source of satisfaction at least, though perhaps not absolutely necessary, to use a keratometer and endeavor to objectively locate the exact axis of the error. Then, following these methods, it is well to ascertain, subjectively, of course, whether vision is in harmony w'ith refraction; if it be found otherwise, then the ophthalmoscope should be employed to ascertain, if possible, why. and thus enable the optometrician to know whether the case is one calling for glasses, for medical treat- ment, or for both. If the vision and refraction agree in a monocular manner, but not in a binocular one, then phorometric devices are to be called into requisition. And. therefore, it will be seen that in 126 CORROnORATUE MEASUREMENTS the order of their use skiametry is first, the keratometer sec- ond, the trial-case third, and then, if needed, the ophthal- moscope fourth, and the phorometer fifth. Three of these methods represent objective means, and two represent subjective ones. It will thus be seen that prac- tically all the five methods are interdependent, the only one which might really be dispensed with being the cornea measure, and this is not advisable. In all-round optometrical work the placing of sole depend- ence upon one method, one device, or one system, for success, is about as foolish as it would be to place like dependence upon one method, one device, or one system in the practice of any other semi-mechanical calling, where the conditions are likely to vary in diflferent cases. Then, too, the use of examination-room ajiparatus whose only value is to mystify patients, and make them believe they are undergoing a thorough scientific examination, is a means hardly calculated to maintain that lasting public confidence which usually contributes to a long and increasing practice. \or is it wise to idle away a patient's time in needless visual tests merely for the purpose of trying to create favorable impressions regarrling professional ability, for there is now enough that is of real value in optometrical work to gain, with intelligent use. the confidence of educated as well as unedu- cated patrons. To attain the very highest order of jiractical scientific results should be the well defined aim of those who devote either all or part of their time and ability to the mastery of physiological optics. And as a means to this end the practice of systematically corroborating all ocular measurements will be found to act as a preventive to the making of those mis- takes which, when discovered by some other examiner, are so difificult of explanation. VALUE OF INSTRUMENTS 1 2/ VALUE OF IXSTRUMENTS. The value of insiru- ments in optometry can hardly be over-estimated, if accuracy, encouragement and speed are to be considered. For, while it is true that all optomctriciins should be so trained as to be able to do their work without apparatus other than crude affairs, it is also true that they should be educated in the expert handlings of such devices as tend to make their work more efficient. All ojitometrical instruments are at best only tools which depend for their usefulness upon the intelligence of those who handle them. The confession, therefore, of inability to use an instrument is tantamount to a confession of incompetency. Some tools, or instruments, have greater scientific and eco- nomic value, of course, than others have, and it frequently happens that the instrument or device whose manipulation is easiest to acquire is not always the best one to use. The inex- perienced should be influenced by the experienced in the selec- tion of their examination-room armament, provided this expe- rience is adequate and its possessor does not belong to that class of examiners who get into ruts and are incapable of extricating themselves, no matter what the value of the opto- mctrical inducement offered to them. In the selection of instruments there is one point which usually commends itself to those who have had opportunities of using various kinds, and that is the superiority of mobile action over the unit action of lenses. This so-called "unit" in the ophthalmic lenses generally used to-day is termed "one dTo])tcr." but in reality the unit comes nearer to being a quarter of a diopter, as lenses are employed, these "quarters'' acting a^ a kind of step whereby the accommodation of an eye is com- pelled to jump from one adjustment to another. To overcome this jumping principle in the measuring of convergence the "Risley" mobile prism was invented. This 128 VALUE UF INSTRUMENTS prism arrangement serves to change these abrupt degree- jumps into a sort of slithng motion, thereby permitting a gradual increase or decrease in the Hght deviation, and resem- bhng the action of a wedge while being made to lift a weight, or split a log. vVith both cylindrical and spherical lenses this sliding i)rin- ciple can be imitated. Thus two convex lenses of four diopters each when placed close together on their optical axes give a combined refraction of eight diopters, with a focal power equal to five inches. Sei)arate these two lenses two inches and their combined action will rc])resent nine dioi)ters of refraction, having a focal distance of four and a half inches. The increasing or decreasing of the distance between Fig. 79. SHOWING THE OPTICAL PRINCIPLES OF MOBILE LENS ACTION PRODUCED BY CHANGING THE RELATI\E POSITION OF LENSES. any series of lenses on their optical axes serves to produce a mobile effect similar to the action of the crystalline lens in a living eye. thus enabling accommodation to be given the same character of assistance as that accorded to convergence by a mobile prism. Fig. jq will perhaps make this point plainer. That this mobile action is a valuable factor in dealing with VALUE OF INSTRUMENTS 129 stubborn cases of spasm, is not difficult to perceive, any more than it is difficult to note the fact that to raise or lower a heavy safe slidii^s: on a plane is easier than it would be by using^ a flight of steps. The evenness of the sliding motion serves to induce an accuracy of action that can hardly be imitated by a motion which may, perhaps, be described as "jumps" or "leaps." An eye might be considered as having an error equal to one and three-quarters diopters, whereas, the true error might really be one and five-eighths diopters or one and seven- eighths, the one and three-quarters merely indicating an approximate correction. In many cases this would undoubt- edly be near enough, but in others a knowledge of the exact refraction is often of importance. Where, therefore, a mobile lens system is employed there is a greater tendency toward precision than where lens units are relied upon. Then, too. the letting down, or relaxation, of accommoda- tion will be found much easier with a mobile lens system than it will be with a unit system, for in reality the mobile prac- tically amounts to the measuring of a living eye by means of an artificial one possessing equal refractive qualities of adjust- ment. The invention of a single mobile lens capable of adapta- tion to the needs of mankind would undoubtedly prove a great boon, but until such an invention appears reliance will have to be placed on a series of lenses arranged for the accomplish- ment of similar purposes. CHAPTER IX. Tin-: UsK of Instkl'.mi:xts i.\ Ski.\.mi:tkic Work. Thf.ir EVOLUTIOX FROM SiXGLE LeXSE.S AXD THE RELATIVE Merits of So.me Now Employed, Ixcludixc. a Descrip- Tiox OF THE Author's Own Mechaxical Coxtribu- Tioxs i.\ This Line. USE OF IXSTRUMEXTS. After the shadow test had been improved and brought to a state approaching prac- tical usefuhiess by such men as Cuignet. Parent and others, it was found that its appHcation entailed such frequent changing of trial lenses as to be very much like hard labor, and for this reason it was therefore neglected by many. Some sage has said that "Genius is lazy."" but this is sup- I^oscd to apply only to the "other fellow." Anyhow, it was found that if this grand test was to be advanced and made I)opular some means would have to be devised in order to encourage its use. The first effort of importance along the line of labor-saving devices for skiametric work appears to have been the lens rack to which the name of Wiirdemann is frc'(|uently attached, although its invention is claimed bv a number of others, l^'ig. 80 depicts the principle upon which this instrument is constructed. As will be seen, it consists of a series of concave and con- vex spherical lenses mounted in such a manner as to permit the patient to hold them before his own eye and move them from one to another, at the request of the examiner. Inas- WUltDKMANN S LKXS RMK I3I much as it proves a saving of time and labor this device is a decided improvement over the use of single lenses placed in a trial frame. Among the drawbacks to its use, however, will be found the stupidity antl carelessness of patients, who fre- quently permit the lenses to rest at angles which interfere with their correct refraction, thus necessitating a constant readjust- FiG. 80. Tllli SKIASCOI'IC LEXS RACK OF WURDEM ANN'. ment of the rack by the examiner. Then, too. owing to this same stupidity and carelessness, a patient will often allow the lenses to come in contact with the skin on his forehead, eyelids or cheeks, and thereby soil the lenses so they become ujifit for use until after they have been cleaned. Quite a number of modifications of the W'iirdemann prin- ciple has been produced, but the drawbacks just mentioned seem common to them all. The Grain and the Standart discs of lenses have been some- what popular: the first named being on a stand for use on a table, while the last named is held by a wall bracket. Fig. 8i shows the large disc of lenses used in both of these devices. The lenses of the disc can be either spherical or cylindrical as the examiner chooses. The disc can also be revolved by the examiner or by the patient at the former's request. These discs, or batteries of lenses, are something of an improvement on the hand rack of Wiirdemann, inasmuch as they hold the lenses in such a position as to permit of no material twisting 132 GRAIN AND STANDART DISC or disturbance to their principal axes. The reUance on the patient to turn the discs and the necessity for troublesome adjustments as to height, together with the annoyances inci- FiG. 8i. THi; DISC FORM OF HOIJJING LENSES USED BY CRAIN, STANDART AND OTHERS. dent to keeping the head of the patient so situated that tiie eyes will always be in proper position, constitute some of the reasons why these devices have not been in greater demand. The instrument of Jennings is based on the principle of a Morton ophthalmoscope in its arrangement for bringing lenses before a patient's eye. It is also a stand or table instrument, its constructive outline being shown in Fig. 82. In [)oint of improvement it seems to have one very decided advantage over the Grain and Standart types, as shown here, and that is in the rod controlling the lenses so that the exam- iner can make changes without depending upon the intelli- gence of his patient. Following this instrument come those of Hamilton and Fav. These cmbodv the disc form of mounting the lenses. JENNINGS' DEVICE 133 with a rotl for their control. The improvement tliey sliow lies principally in having the discs douhlc in nnmber, so that more Fig. 82. THE COXSTRUCTIVE PRlNCirLE OF THE "jEXXIXGs" INSTRUMENT. lens changes are possible. Fig. 83 shows the main points in the Hamilton instrument which is now known as the "Meyrowitz Refractometer." Fig. 84 illustrates the "Fay" instrument. The discs of this device arc arranged for binocular use. while its lenses are mounted at the end of spoke-like rods giving the discs an api)oarance similar to that of a carriage wheel. In its binocular feature the "Fay" instrument mipresses one as being superior to the other devices, for they are. with the exception of the Hamilton form, all monocular m the arrangement of their lenses. 134 REFRACTOMETKRS OF MEVROWITZ AND FAY Fig. 83. THE CONSTRUCTIVE PRINCIPLE OF THE "MEYROWITZ instrument. Fig. 84. THE CONSTRUCTIVE PRI.VCIPLE OF THE "fAV" INSTRLTMENT. author's instrument of 1892 135 Following the instrument of Fay, is the instrument of Dr. Chalmers Prentice, now known as the "Geneva," and patented about 1898. This instrument marks something of a departure from the forms that have heretofore been considered, for in addition to carrying a battery of lenses it also carries its own light and mirror. As an illustration of how it is possible for many minds to be possessed of similar ideas, attention is called to a device which was used in 1892, and which is still in the author's possession. Fig. 85 gives an illustration of its con- struction. As can be seen it is merely a long box containing a stationary lamp, a tilting mirror and two discs of lenses con- trolled by a double rod. Fig. 8s. THE CON.STKUCTINE |-R1\( II'I.H oK TllK INSTRUMENT DESIGNED .\NU USED I!V TIIK AUTHOR IN 1892. The principle of the "Geneva" instrument is shown in Fig. 86, and will be seen to consist of a stationary lamp, a tilt- ing mirror, and two discs of lenses controlled by a double rod. 136 THE GEXE\A KEIIXOSCOI'E The chief difference between the two devices lies in the fact that one is in a box, while the other employs a tube. Fig. 86. THE CONSTRUCTIVE PRINCIPLE OF THE GENEVA INSTRUMENT PATENTED ABOUT 1 898. With the exception of the author's new instrument, which, as will presently be seen, is a radical departure in almost every way from those just referred to, this completes the list, althoujjh there are still quite a number of other instruments which embrace the principles here shown. Jackson in his valuable book on '''Skiascopy, in speaking of the use of disc lenses (page 109) says : "But even in this case, the fact that there is a complete break between the appearances represented by one lens, and the appearances present by the use of the lens next stronger or weaker, makes the information obtained less valuable and satisfactory than that derived from the movement of the surgeon's eye from, one position to another, which allows him to watch the dif- ferent appearances of light and shade as they pass gradually into each other." This "movement of the surgeon's eve" to which he refers has since been practically duplicated by mak- ♦The Edwards & Docker Co., publishers, Pliila., 1895. THE ri:tixo-skiameter 137 ing the lens action mobile instead of fixed, thus securing superior results with less effort. From the instrument of Grain to that of the "Geneva" it may be observed that the proper adjustment of the patient's head by means of chin rests, etc., is an important item, and, with the exception of the "Fay" instrument, all require sep- arate adjustment for each eye. Besides, not one of them is arranged for magnifying the patient's pupil, no matter how small it may be nor how great the difficulty of accurately noting the action of its shadow. The Jennings, Hamilton, Fay and Geneva types are all designed to work at one fixed distance, no latitude being allowed unless the instruments are operated in an awkward manner, and this of course limits their use to the static method only, barring out the dynamic, which, of the two, is really the more valuable. Therefore, all single test instruments are of little service in thorough skiametric work. Wiirdemann's device is, like the use of trial lenses, most excellent in theory, but in practice its shortcomings, seem to be many. In the practical workings of all disc devices their use usually proves of considerable assistance to an examiner, especially when the disc can be controlled by rods, but all disc instruments, it is to be feared, will eventually be relegated to the company of the many other optometrical devices which have been weighed in the balance of actual experience and found wanting. However, it is for others to judge of this after they have informed themselves regarding the advantages gained by the use of the next instrument to which attention is here called. THE "CROSS" RETIXO-SKIAMETER. This little device, among its other features, was designed to accomplish a purpose similar to that of the various disc contrivances in overcoming the necessity for having examiners change posi- 138 THE RETIXO-SKIAMETER tion every time a lens needed changing. Like Wiirdemann's device, too, it was designed for a liand instrument, so as to make the adjustments rapid and easy, but unHke Wiirde- mann's rack, it was arranged to be so placed that its position should be as secure as that of a trial frame. Enlargement of ocular pupils was sought for and achieved, together with mobile lens action. Variety in methods of oper- ating was not so much a first consideration as it was an after- thought. The construction of the instrument, however, needed Fig. 87. d o o o 3H. f:qz_k 0. THE COXSTKUC ri\E I'KIXCII'LE OF THE "CROSs" RETINO- SKLV METER, very little alteration after dynamic skiametry was developed, although the addition of this method to the value of the instru- ment has been found too great to be estimated by figures. Binocular action was also an original feature sought for; then simplicity ; not only simplicity of construction and action, but simplicity of operation. Fig. 87 shows the mechanical arrangement of the instrument and the manner of the adjust- ment of its lens system. Tiiii ki:ti.\()-ski.\.mi:ti:r 139 Two convex ami two concave cylindrical lenses of seven diopters each are mounted in cells A and B, and A and B prime, with their axes at rig^ht ang^lcs to each other, each lens being slightly inclined from the perpendicular on its meri- dional axis. The two concave lenses are stationary while the two convex ones are made movable. The cells of the latter slide on rods H, being controlled by a doubled cord C, D. thirty-six inches in length, this cord passing over pulley F. The cord's length being always the same, an operator has merely to turn his hand at the wrist in order to obtain full control of the refractive changes of the instrument. The tilting of the lenses is to obviate the multiplicity of light reflec- tions which usually hover about them when they are used in an upright position in connection with skiametry and ophthal- moscopy. It will be seen, therefore, that cylindrical lenses had to be used ; for the tilting of a spherical lens at any angle produces a disturbance in its refraction. In the use of convex cylindrical lenses it was found that by placing each cell on a sliding block, such as shown by K and E, and by then using a hook the operator could unfasten them while working at a distance by merely moving his hand a few inches to one side. The cylindrical lenses used in this way could also be made to serve a double purpose : for when used together they acted as a single spherical lens does, but when used singly, or unlocked, they acted as simple cylinders. Another valuable principle was also discovered and made use of. It was found that as the plus cylinders, A and B prime, were moved away from the minus cylinders. A and B, any object, such as a patient's eye, when placed close up to the concave lenses would be magnified by the convex ones, just as though a plain seven-diopter convex spherical lens had been used, this magnification taking place without causing the least interference in the refraction of the four lenses while they were 140 THE RETIXO-SKIAMETER being used as a lens series to produce mobility of action. Other attachments comprise a self-adjusting brow rest to give the instrument stability, a handle, a base and a means for separating the two tubes. The large holes now in the top of the tubes of the latest model instrument facilitate the cleaning of the lenses, whose cells are all attached to blocks on the slide Fig. E.XTERXAL APPEAR.ANCE OF THE CROSS INSTRUMENT. rods. The minus cylindrical lenses are secured in position by a set screw in the block holding cell B. Fig. 88 shows the instrument com])letc, with its auxiliary lens disc containing three sphcricals. i. e, — i. — 3. and — 6. D,. THE RETINO-SKIAMETER 141 to be used in converting the instrument from the "static" to the "dynamic" method, and also for changing the total refrac- tion from plus to minus, as occasion demands. The pupillary distance is made easy of alteration by partially unscrewing the handle. This is accomplished by holding the instrument in the manner shown in Fig. 89. The index finger of the left hand is used to support the connecting bars between the two tubes, or sides, and enables their distance to be altered by merely giving the handle a half turn or so with the right Fig. 89. SHOWING HOW TO ALTER PUPILLARY DISTANCE. hand. It also prevents awkwardness and keeps the two tubes always parallel. The six-pound base is used chiefly as a holder for the instru- ment when not in use, although it can be employed on a table for children if the examiner prefers. The light weight of the device, however, which is only nineteen ounces, enables chil- dren even as young as four years of age to hold it very satis- factorily by using both hands. 142 THE RETIXO-SKIAMETEK The manner of operation is not difficult. After showing the patient how to hold the instrument by first holding it be- fore his own eyes, the examiner seats himself and directs the patient to look at the largest letter on the card of the fixation stand, fifty-three inches distant, if the examination is to be made by the dynamic method, or over his shoulder at some object twenty feet away, if the static method is to be employed. The examiner then takes one of the looped cords in his hand and pulls it just sufficiently to keei) it taut all the time, he is also to hold it in such a manner that one turn of tlie Fig. 90. SHOWING MANNER OF USING INSTRUMENT. wrist will give him the full refractive power of the instrument without re-adjustment. This permits of a range from zero up to six diopters of plus or minus spherical or cylindrical lens quantity. A slight pressure of the little finger on the cord serves to keep it from slip])ing. And if the little finger on the mirror hand is placed on the thumb of the cord-hand, a distance THE RETIXO-SKIA.MliTliU M3 equal to exactly forty inches can be readily secured. The double cord being just thirty-six inches long, the arrangement of the hands can be made to supply the other four inches. Fig. 90 illustrates these points. F[G. 91. SIIOWIXG IXSTRUMKXT IX DUST-rROOF CASE. With one hand controlling the skiascope and the other hav- ing "a trial case on a string," as the instrument has been called, it is easy to see that shadow and refraction can be made to bear a relation to one another not unlike that of "bowing" and "fingering" in the playing of a violin. 144 THE RETINO-SKIAMETER The wooden case for holding the instrument when not in use is intended to keep it free from dust and dirt. This case can be made in a variety of woods and can be finished in either a plain or elaborate manner. The cover is double-hinged in such a way that the removal of the instrument from its base is made very easy. Fig. 91 will give an idea of the construction and appearance of the case, which makes a handsome ofifice ornament. As a very liberal criticism of other instruments and de- vices has been made in these pages, it would seem to be no more than fair that the shortcomings of this retinal-shadow measure be held up for inspection also. It is an instrument that perhaps requires more skill to use than do any of the other de- vices to which attention has here been called. It needs to be kept in excellent order to obtain good' service from it. It is somewhat easy to get out of adjustment, and is very sensitive to abuse. It gets dirty readily, and needs more care than ordi- nary instruments do. It is troublesome of manufacture, be- cause it takes so much skilled labor and attention to detail in the selection and mounting of its lenses in order to secure the accuracy essential to the proper performance of its work. And when it does get out of order it generally has to be sent to the factory for readjustment. There is one disputed point of excellence in the operation of the instrument to which especial attention is invited. This refers to the apparent production of mydriasis by magnifying the pupil. Enlargement by magnification would seemingly mean an increase in size at the expense of definition. Let it be considered, then, whether this is really so or not. The word "definition," as used in optics, means the po^ver of a lens to give an image of anything, or part, so as to clearly dis- tinguish it from its surroundings. But this is perhaps mislead- ing, as experienced microscopists say that a lens of low power THE RETINO-SKIAMFITKR I45 often works better than one of high power, because with a low- power lens a better general idea may be had of the object, even at the expense of size, than if it were viewed through a high- power lens. It might be reasoned from this, therefore, that it applied to the magnifying of the retinal shadow. But here comes the examiner's own vision and the law of a five-minute angle gov- erning its acuity. If he operated at a much nearer point than forty inches, any increase in size of the shadow without cor- respondmg increase in the substance of which it is composed might interfere with the sharpness of demarcation caused by magnifying. But many ocular pupils are so small that the retinal shadow gives a visual angle of less than one-half, per- haps, of what it should at forty inches away, and so, when the pupil is magnified to several times its original size, the increase in visual angle more than compensates for the decrease in sharpness of outline. Hence this explanation to theoretically account for that which those who use this method learn to be a fact from actual experience. A simple experiment can also be tried which will serve to emphasize the superiority of this magnification principle as here used. If a patient will hold a convex seven-diopter spherical lens two inches in front of his eye and let an examiner compare the sharpness of outline of the union between the iris and the sclerotic, from a distance of forty inches away, and then determine which one of the patient's two eyes is the easiest for him to see, the magnified or the unmagnified one. the differ- ence will be very apparent, and the larger iris will seem to lose little if any of its color or intensity on account of magnifi- cation. Fig. 92 illustrates the relative size between an average ocular pupil and one enlarged three diameters, this being the maximum magnification by means of the author's skiameter. The working distance at which an examiner operates will 146 THE RETIXO-SKIA.METER of course affect the magnifying power of whatever lenses may be used, but as a rule this magnifying principle will be seen to Fig. 9: o SHOWING CO.Ml'ARAin'E SIZE OF NORMAL A.NJ) .UAGMl-lED PUPIL. take care of itself, for in high degrees of hypermetropia, where small pupils are apt to be found, the cnlargmcnt caused by the lenses in the instrument will usually be anii)lc for all practical purposes. CHAPTER X. Systematic Ocular Examinations an iTit Aid Dkrivhd FROM ]\Iaking Primi-: Ri:coiiL)s. — Ri:sourcefulnes: in Refraction Work and the Slxcessful Examinatkj.v OF THE Eyes of Children, Mutes and Illiterates. SYSTEMATIC EXAMIXATIOXS. As one of the pro- nounced aids to successful examination-room work, a brief reference will here be made to systematic examinations and the practical assistance to be derived from carefully recording them. The great value of this troublesome detail can not be em- phasized too frequently, for, as has been remarked before, ocu- lar skiametry is the great refractive pilot or pathfinder, and, therefore, when the path has once been found it is wise to keep it, and also to keep track of its various windings. A blank form should be used, containing properly named spaces wherein entries can be systematically made, so that nothing of importance may be overlooked in the hurry of busy days. This blank should be large enough to contain on one sheet a complete record of everything pertaining to a case. This is an age of card indexes, but the makers of these valuable time-savers do not seem to appreciate the nccrls of those who are engaged in optometrical work, for they make their cards much ioo small for all that ought to be entered on them. In the author's own examination-room work he has adopted 148 SYSTEMATIC EXAMINATIONS Fig. 93. Res.d Occu palion Slale of OD PHYSICAL EXAMINATION. CO... s Total > D VUlon Befora uomeuon. D Test Case: FUNCTIONAL I 0. D XAMINATIO^ Cri. .... 1 .n.r^cX;?So..oo. OD S s ;o. s. Esophona= E.ophoria = Hyperphona= Presbyop.a Adduclion= Abduclion= Sursjmdn= 1 8pb., D Pratsnt Clatses: Cyl.. Ai* 1 I Note: OS 1 1 p_|0. 1 1 10 s 1 1 Praicrlbed ay FACIAL MEASUREMENT; Temples ■ No. of Mounting For D O.S S'le ol Len». Oelivet Retetred by . s.« o( Lens Pnce % FftP % For No. Of Mounting O.S Price. $ ArT]ount Paid, $_ Tolal. $_ _ Due. $. £» mr. SAMPLE RECORD BLANK. SYSTEMATIC liXAMIXATIONS 149 a plan which, so far, seems to answer all purposes very well indeed. The regulation blank used is shown by the illustra- tion Fig. 93. The back of it is plain. This sheet is of very heavy linen paper, and is about six inches wide by nine inches long. The records, of course, are kept in a fireproof safe, and to have them occupy as little room as possible a number of heavy tin boxes, or drawers, are provided for them, these boxes being kept in compartments so as to avoid confusion. They are also designed in such a manner as to facilitate the addition or removal of a single sheet. Fig. 94 illustrates the appearance of one of the boxes when filled with blanks. Each sheet carries an index Fig. 94. MANNER OF FILING RECORDS. number, and a very large, three-letter, index book enables the name and record number of each patient to be easily and quickly found The blanks, both new and old. which are made use of during one week, serve as a kind of day-book, and thus. 150 SVSTliMATlC EXAM IXATIOXS in the end, really save more labor from a bookkeeping stand- point than their use entails. ' To describe more completely the uses for which the blank is intended it can be stated that the age of the patient may be marked in cipher so that inquisitive persons cannot gratify idle curiosity, if the record happens to be left carelessly ex- posed. The skiametric examination is made first, then follows the keratometric one, that is, if astigmatism of over one diopter is present. Vision before correction is then recorded, after which corroboration with trial lenses is made, and vision after correction is noted. If, in view of the refractive condition, satisfactory vision is not obtained when trial lenses are used, then the ophthalmoscope is resorted to for any possible clue which it might be able to give as to the reason therefor. Under the term "Fundus," a pale or choked disc, or any other abnor- mality may be entered. Under "Lens" the various kinds of cataracts can be abbreviated "Cort. Cata.," signifying the cor- tical kind, while a rough pencil sketch marking the inside of a circle will often serve to note, at a future exammation, whether any increase or descrease in number and density of striae, or spicula. has occurred. Under "Cornea" the word "slight" or "dense"' as pertain- ing to opacity, frequently makes plain the reason for the low percentage of acuity which the figures under "V^ision after correction" indicate. Presbyopia is always recorded, when present, and should be made to harmonize as far as possible with Bonder's rule, approximating one diopter for ages forty- five to forty-eight, one and a half for forty-eight to fifty, two diopters for fifty to fifty-three, two and a half for fifty-three to fifty-eight, and three for sixty and upwards. T'^is of course is in addition to any so-called "errors" wh'ch may ai?o be present. If standard visual acuity is obtained, and the examiner feels SYSTEMATIC EXAMIXATIONS I5I certain that a "latent" error is not present, his patient having persevered long enough with glasses to have pretty well broken up any old ocular muscle habits, and yet there still remains an unsatisfactory condition, then it is the part of good judg- ment to record the action of the extrinsic muscles in the space devoted to this purpose, and resort to the use of prisms. All of these failing, the examiner should then do as many other professional men have done before him and appeal to another "court." or advise a consultation. "Present glasses," and by whom prescribed, is always a satisfactory minute, for it enables an examiner to keep track of the mistakes of others, even if he cannot of his own. "Facial ^Measurements" end the findings, and then comes the judgment requisite to insure success. The style of mount- ings, prices, time of delivery, by whom the case was referred, and. lastly, the examiner's initials or name. For the purpose of explanation, let it be supposed that the examiner is fallible and docs err in judgment; the patient re- turns and has a slight change made. This is duly recorded on the back of the blank, as well as the fact that no charge wasr made for this change. In the course of a few months, perhaps, a new "O. D." is supplied, which, with its price, is also recorded, and thus the blank will last for years, for whenever the patient calls the blank can be taken into the examination-room and made to serve as a complete "book of the play." When skiametric examinations are recorded they seldom have to be repented in return cases, for only subjective tests and variations in judgment being all that is ordinarily required. Therefore, when it is stated that systematic ocular examina- tion records are very valuable, from a practical standpoint, this statement might go further and class them along with examina- tion rooms and claim both as absolutely necessary in this day 152 RESOURCEFULNESS and age, when a pronounced success in optometry is only achieved by paying strict attention to every detail of method, place and device. RESOURCEFULNESS. Resourcefulness is another im- portant quality for optometricians to possess. For when to rely on a patient's "Yes" or "No," and when not to, requires no small amount of knowledge of human nature as well as ability as a cross-questioner. Of course ocular skiametry and other objective means place an examiner in a position largely independent of a patient's answers or intelligence, yet it is always a source of satisfaction to have one method corroborate another, since there are many ways in which to be wrong, and, as a rule, only one way in which to be exactly right. With so-called "regular" conditions skiascopists are likely to have little trouble, but the "irregular" kind frequently call for con- siderable versatility on the part of an examiner in order to ex- tricate himself from a refractive corner, so to speak. To illustrate this, a case of nystagmus once presented itself which had been seen by a half-dozen able specialists. The age of the patient was twenty years, and the glasses in use were one diopter concave sphericals for both eyes, which gave an acuity of vision equal to about ten two-hundredths. The use of the skiascope showed the presence of myopic astigmatism, linth the rule, but the spasmodic action of the mus- cles precluded the ascertainment of the amount. By recourse to the keratometer and engaging the patient in a long conver- sation regarding Fchool work, in order to quiet the so^sm. it became possible to locate a corneal mal-curvature of about five diopters. The use of the mirror again showed the error to be free from hypermetropia, and glasses of four diopters concave cylindrical at axis 180, after thirty days' use, increased vision up tf) a little better than twenty one-hundredths. When it is borne in mind that this case had been under care- RESOURCEFULNESS 1 53 ful observation for twelve years, and that no expense had been spared in consulting the most prominent specialists, the joy of the patient over the results achieved can be readily imagined. A hap-hazard trial of test lenses might have resulted, in this case, in a clue, which could have been followed up satis- factorily, perhaps, but this less certain method frequently leads an examiner astray through the patient's failure to appreciate and give an encouraging answer to a partial correction. In the refractive examination of the eyes of children, mutes ^and illiterates, ocular skiametry offers about the only truly reliable means for independently determining the kind and strength of proper glasses. In this same category might be included those persons who are only partially deaf and who fail to respond to all questions asked them. Also those persons who do not speak the same language the examiner does, and careless persons who some- times prefer to joke and unconsciously cause an examiner to become careless himself. Then there are the ultra-careful per- sons, whose answers are about as misleading as if they were careless. All of these classes, and many others, tend to show the value of ocular skiametry, for success must be achieved no matter what the excuse or obstacle may be, as any examiner, no mat- ter where located, can ill afford to endanger his reputation by doing poor work. CHAPTER XI. Illustrative Cases, Showing the Comparative A'alue of Static and Dynamic Skiametrv ix \'arious Patients OF Different Ages, Occupations and Apparent Phys- ical Condition. ILLUSTRATIVE CASES. The expression "Figures talk" is especially applicable in describing the relative merits of static and dynamic methods in practising ocular skiametry. This chapter, therefore, will be devoted largely to descriptions of various cases for the purpose of emphasizing points already alluded to, and of incidentally calling attention to the influence of occupation and the importance which attaches to the condi- tion of the patient's general health. All the examinations referred to here were made without the aid of cycloplegics, con- sequently the static test mentioned is the non-toxic one. It will be observed in viewing these cases that the measure- ments founfl and the formulas given are not always in accord. This is due to the fact that optometric findings, as a rule, must be tempered with judgment, for old examiners know from experience that their "come-back" cases show more faults in this respect than to mistakes due to variations in subsequent measurements. ILLUSTRATIVE CASES l.SS CASE A. :Master S., age 7. In school. Health, seemingly good; O. S. shows slight convergent squint. Vision = O. D. 20/20. O. S. 20/100. Static test =: O. D. + 2.50 D. S. O. S. + 3- D- S. Dynamic test at forty inches = O. U. 4" 3-50 D. S. Dynamic test at twenty inches = O. U. + 4. D. S. Dynamic test at thirteen inches = Unsatisfactory. Trial case test = O. D. + 2. D. S. O. S. + 3. D. S. Vision = O. D. 20/20. O. D. 20/80. Formula given = O. U. -f 3. D. S. For constant use, with instructions to return in one year. Two years later. O. U. + 3.50 D. S. was readily accepted. Xo squint. \'ision r= O. D. 20/20. O. S. 20/40. CASE B. Master W., age ir. In school. Health not rugged. In- ability to see blackboard. Vision = O. U. 20/100. Static test = O. U. — i. D. S. Dynamic test at 13 inches = about, O. I'. — 0.50 D. S. Trial case test = O. U. — T.25 D. S. Vision = 20/20 in both eyes. Formula given = O. U. — 0.50 D. S. For constant use. Instructed to return in three months. Six months later vision O. U. = 20/20 with O.D. — 0.75— D. S. O. S. — 0.75 — D. S. ^5^ ILLUSTRATIVE CASES CASE C. Miss N., age i6. In school. Health fair. Headaches. Vision = O. U. 20/30. Static test = O. U. + 0.50 D. C. 90°. Dynamic test at 20 inches, same. Trial case test = O. U. — 0.50 D. C. 180°. Vision = 20/20 in both eyes. Formula given = 0.11.4- 0.50 D. C. 90°. Vision = "misty." To use at study, and oftener if more comfortable. Later on she reported •'no headaches," and vision was found to be 20/20. CASE D. Mr. G., age 20. In college. Reports his health good. No discomfort, but "can't see at a distance." Vision = O. U. 20/200. Static test = O. U. — 2.50 D. S. Dynamic test at 40 inches = O. U. — 2. D. S. At 20 inches, about the same. Trial case test = O. U. — 2.75 D. S. Vision =r 20/20 in both eyes. Formula given = O. U. — 2. D. S. For constant use. Instructed to return if he had any further trouble. No report. CASE E. Mr. S., age 24. Mechanic. Something of an athlete; com- plains of headache. Vision O. U. = 20/20. Static test O. U. = + 1.25 D. S. Z + 0.25 D. C. 90°. ILLUSTRATIVE CASES I57 Dynamic test at 13 inches = O. U. = + 2. D. S. C + 0.25 D. C. 90°. Trial case test O. U. = + i. D. S. 3 + 0.37 D. C. 90°. Vision = 20/20. Formula given O. U. = + i. D. S. ~ + 0.25 D. C. 90'. Advised to return in one year, which he did, and was given O. U. = + 1.50 D. S. C + 0.25 D. C. 90°. Was then advised to return again in two years more. CASE F. Miss F., age 26. Seamstress. General health not good. "Weak eyes." Vision = O. D. 20/40. O. S. 20/80. . Has been using O. U.-|- i D. S. Static Test = O. D. 4- 0.50 D. S. C + 2. D. C. 90°. O. S. + 2. D. S. C + 2. D. C. 105°. Dynamic test at 40 inches = O. D. + I. D. S. C + 2. D. C. 90°. O. D. 4- 3. D. S. C + 2. D. C. 105°. Dynamic test at 20 inches = O. D. + 1.50 D. S. C + 2. D. C. 90°. O. S. + 3- D. S. C + 2. D. C. 105°. Dynamic test at 13 inches ^ Not satisfactory. Keratometer = O. D. 2. D. 90°. O. S. 2. D. 1 10°. (Note change in axis of O. S.) Trial case test = O. D. + 0.25 D. S. C + 2. D. C. 90^ O. S. + 1.50 D. S. C + 2. D. C. 105". A'ision =: O. D. 20/20. O. S. 20/40. Formula given = O. D. + 1. D. S. C + 2. D. C. 90°. O. S. + 2.50 D. S. C + 2. D. C. 105°. 158 ILLUSTRATIVE CASES For constant use, with instructions to "never mind if distant objects are a trifle blurred for a few weeks." Returned in a week with a history of occasional discomfort. Gave advice to persevere. Returned in four months with a broken lens and wanted a new one "immediately." Glasses were very satis- factory. \'ision = O. D. 20/20. O. S. 20/40. CASE G. Mr. C, ai::::e 29. Bookkeeper. Reports health good when not working too hard. Eyes and head "feel bad" afternoons. Has been wearing glasses = O. D. — I. D. S. Z + 2. D. C. 75°. O. S. — I. D. S. C + 150 D. C. 105°. For three years. Vision with present glasses = O. U. 20/30. Fundus reflex is very poor. Keratomcter shows = O. D. 2. D. axis 75°. O. S. 2. D. axis 105°. Dynamic test at thirteen inches, with trial lenses, show the myopic quantity to be only 0.50 D. in both eyes. Trial case test = O. D. — I. D. S. C + 2. D. C. 75°. O. S. — I. D. S. C + 2. D. C. 105°. Vision = 20/30. Formula given = O. D. — 0.50 D. S. C + 2. D. C. 75°. O. S. — 0.50 D. S. ~ 4- 2. D. C. I05^ Repr)rt received in two weeks was "All r'ght n av." CA.SE TT. Mr. R.. age 34. Grf)cer. History of health unsatisfactory. Vision poor for past few months. ILLUSTRATIVE CASES 159 Present vision = 20/80 in both eyes. Static test = O. D. + I. D. S. I + 0.50 D. C. 180°. O. S. 4- I. D. S. Z + 0.50 D. C. 180°. Dynamic test, about the same. Trial case, about the same. Optical correction no material aid to vision. Ophthalmo- scope shows pale discs. Close questioning leads to conclusion that it is a probable case of nicotine poisoning due to im- moderate smoking and moderate use of alcohol. Gave no glasses. Advised to consult an oculist first. CASE J. Miss B.. age 38. Stenographer. Says health is good ex- cept for headaches. \-ision = O. U. 20/20. Static test = O. U. + 0.50 D. S. Dynamic test at 40 inches = O. U. + 0.75 D. S. Trial case test = O. U. + 0.25 D. S. \lsion = 20/20 trifle "hazy." Formula given = O. V. + 0.50 D. S. For reading and near work. Good report. CASE K. Mrs. A., age 41. Has household cares only. General health none too good. Complains of inability to see to thread her needle and do fancy work. No headaches. Vision = O. U. 20/20. Dynamic test at 40 inches r= O. U -+- i.D.S. Trial cast test = O. U. + 0.75 D- -^- Vision = 20/20. Formula given = + i. D. S. for both eyes, with instructions to use for near work. No report. l6o ILLUSTRATIVE CASES CASE L. Mrs. L., age 46. Housekeeper. Health appears good. Dif- ficulty in reading. Xo headache. Vision = O. U. 20/20. Static test r= O. U. + 0.25 D. C. 90°. Dynamic test, about the same. Trial case, about the same. Presbyopia = i. D. Formula given = C). L'. + i. D. S. Z + 0.25 D. C. for reading etc. Xo report. CASE M. Mr. D., age 52. Court stenographer. Health seemingly good. X'ever has had any glasses that proved quite satis- factory. Vision = O. U. 20/80. Dynamic test at 50 inches = O. D. + 1.25 D. S. C + 0.25 D. C. 135°. O. S. + 1.25 D. S. C + 0.25 D. C. 90°. Trial case test, the same. \'ision = O. U. 20/30. Presbyopia = 2.25 D. Gave bi-focals. Reported in sixty days that vision was good but glasses did not seem quite right. "Guessed" he was working too hard. Re-cxamination by dynamic test at 30 inches = O. D. -f 1.50 D. S. C + 0.25 D. C. 120°. O. S. + 1.75 D. S. C -f 0.25 D. C. 80°. Trial case, the same. \'ision := O. U. 20/20. Presbyopia = 2. D. Reported in six months "O. K. now. 'twas the glasses after all." ILLUSTRATIVE CASES l6l CASE X. Mr. O'B., age 55. Driver. Health good. "Can't see." Vision = O. U. 20/80 . Static test = O. U. + 1.50 D. S. Presbyopia =: 2.50 D. Trial case test = O. U. + 1.50 D. S. Vision = 20/20. Formula given = O. U. -f 4- D. S. for reading. No report. CASE O. Mr. E.. age 59. Tailor and cutter. Health good. Working distance is about twenty inches away. Present glasses are -J- 3. D. S for both eyes, and are not very satisfactory. Vision = O. D. 20/30. O. S. 20/100. Static test = O.D. + 0.50D. S. O. S. + 1. D. S. + i.D.C. 180°. Keratometer shows no corneal nial-curvature in either eye. Trial case test, same as static test. X'ision = O. D. 20/20. O. S. 20/40. Presbyopia at working distance = 2. D. Presbyopia at reading di.stance = 2.75 D. Formula for working glasses = O.D. + 2.50 D.S. O.S. +3. D. S. + I.D.C.I8o^ Formula for reading glasses = O. D. + 3.25 D- ^'^• O. S. + 3.50 D. S. + I. D. C. 180°. Instructed to return if not satisfactory. No report. I 62 ILLUSTRATIVE CASES CASE P. Mrs. M.. age 62. (Occupation (?). Health (?). Looks well. \'ision less than 20/200 in both eyes. Static test : First attempt, no retinal reflex. Without ski- ameter the mirror shows small pupils and slow plus movement. With skiameter, lenses being set to register + 3., enlarged pupil shows better movement and reveals long, narrow, spike- like patches. Error about -|- 4. D. S. in both eyes. Trial case test = ( ). L". + 3.50 D. S. Vision = ( ). D. 20/40. O. S. 20/60. Presbyopia = 3. D. Ophthalmoscope shows slight cortical cataracts. Gave formulas : Distance = ( ). L'. + 3.50 D. S. Reading = ( ). U. + 6.50 D. S. With instructions to be sure and have a strong light coming over shoulder wilien reading or sewing. Sent letter to family physician. CASE Q. Mr. McE., age 67. Health fair. Retired. Now using glasses + 4 D. S. for reading; wonders if they can be inn proved. Vision = O. U. 20/80, which is improved by partially closing the eyelids. Static test = O. U. -f i. D. S. Trial case test the same. Vision = O. U. 20/30. Presbyopia = -f- 3. D. S. Formula for distance = + i. D. S. Advised to continue with present reading glasses and to increase his illumination when using his eyes for near purposes. No report. JLLISIRAT1\1-: CASIiS 163 Note. — According to Donders the near point of distinct vision in an emmetropic eye is as follows : At 10 years of age it is 2^4 inches away. " 20 4 " 30 5'^ *■ " 40 ' " " 9 " 50 ' " 16 " 60 40 " " To illustrate in fuller detail the workings of dynamic ski- ametry, let the following case be considered: Mr. H., age 25. Contractor's timekeeper. Leads outdoor life. General health excellent. Complains of occasional headache. Vision = O. U. 20/20. Static test = O. L'. + 1.25 D. $. Dynamic test at 13 inches =: O. U. + 2. D. S. Trial case test = O. U. + i. D. S. \'ision = 20/20. "By reference to Fig. 75, it will be seen that when his accom- modation and convergence each receives the standard amount of innervation, as shown in Fig. 74. the convergence will be greater than the accommodation, and binocular confusion will result, thus giving rise to esophoria unless the innervation is altered in some way so as to produce the condition called for by Fig. 76, where the innervation for accommodation is in excess of that for convergence. .\ test of his extrinsic mus- cles, however, shows a manifest orthophoria without glasses. Now what are the deductions that may be drawn from this case? Twenty-five years of daily use of the eyes without glasses has established a habit of adjustment whereby the stand- ard relation between accommodation and convergence has been replaced by a condition in which convergence has given way a little, otherwise esophoria or a mild form of convergent strabis- mus would have manifested itself. 164 ILLUSTRATIVE CASES The non-toxico-static test shows a reversal of the shadow when one and a quarter diopters of convex lens power are added. This is in addition, of course, to the quantity necessary to create the skiametric, or working, refraction when trial-case lenses are used. Thus proving that habit has not mastered all of the error, as the accommodation readily accepts partial as- sistance and relaxes its miuscle tension as much as five-eighths of the full ametropia present. The remaining three-eighths of the total error can be called latent, but in reality it represents a tonic spasm, a knowledge of the presence of which materially aids an examiner in the formation of his judgment and in the advice and prognosis he gives a patient. To determine the amount of tonic spasm present in a case such as the one under consideration, it will be necessary to resort to either the toxico-static test or else to the dynamic one, and as it is usually wise to go in the direction of least trouble, especially when this direction is the best one, recourse, there- fore, is to be had to the test that calls for a pronounced exer- tion of the patient's accommodation. An emmetrope 25 years of age is supposed to have about eight diopters of amplitude of accommodation. The nearest point of distinct vision is then five inches away from the eyes. A dynamic test made at thirteen inches calls for an accommo- dation equal to three diopters. The i)atient's error being two diopters, it follows that a total ocular muscle exertion equal to five diopters is necessary in order to enable the patient to dis- tinctly read small letters on a card whose distance away is the same as that of the examiner's mirror. More than five diopters of accommodative effort can. of course, be exerted by the patient in this case. Yet this amount will generally be found, in like cases, quite sufficient to break up any tonic spasm, or habit of muscle exertion, that mav have been formed. If the examiner is supplied with a device which ILMSlKAl 1\ 1-. CASES 1^5 permits of mobile lens action, he has only to slide in, so to speak, the required refractive assistance necessary to cause the accommodation to relax until it has assumed its standard rela- tionship with convergence. When this test is made at thirteen inches, as before stated, the accommodation equals three diop- ters. Therefore, five diopters less three diopters leaves two diopters as the lens quantity that will reverse the shadow by the dynamic test under these conditions. If the test had been made at twenty inches, then four diop- ters would represent the total muscle effort called for. If at ten inches, then six diopters would be the full accommodation needed. The difference between these amounts and that required to maintain normal relationship between accommoda- tion and convergence at whatever distance the test is made will show at once in the lens quantity required to reverse the shadow, provided an adequate lens system is used, and pro- vided, also, that the eyes are examined in a semi-binocular man- ner—namely, first one eye and then the other, alternating fre- quentlv— so as to insure an equality of visual fixation. One point which seems to puzzle many examiners who take an interest in making theory substantiate practice is to under- stand why an emmetropic eye when under an accommodative tension of three diopters at thirteen inches, will not relax to two diopters when one diopter of assistance is offered it. The answer to this query probably lies in a better understanding of muscular co-ordination and innervation, for. as stated in earlier chapters, the eyes of a healthy person, free from intoxi- cation cannot converge without accommodating, nor can they accommodate without converging. And this co-ordinate relationship will respond to approximate standards unless long- standing abnormal requirements have induced irregular habits. In this latter case refractive measurements must be taken m such a manner as to estimate the real influence of these habits by l66 II.I.ISTKATUK CASES making the eyes work, for the time being-, in a manner as far removed from old beaten paths as possible. Now another case will be cited in order that the details of skiametric procedure may be accentuated. Mr. ( ?), age thirty-five, occupation watchmaker. Has been studying optics for two years. States that he has fitted himself with O. U. — 0.50 D. S. — 0.75 D. C. axis 180°. that his vision without glasses is O. D. = 15/30 O. S. = 15/20. and that he has four degrees of esophoria. As the above information, excepting the age, is supplied after the examination is finished, the examiner, of course, pro- ceeds in the usual manner to seat the patient and place a skiam- eter in his hand. He then directs the patient to look at the let- ters on a fixation card situated fifty-three inches distant. Be- ginning the examination at forty inches, the examiner finds that in the right eye there is a fairly distinct edge to the shadow and that it points a little to the left of the vertical meridian. Adding convex lens quantity, it is found that one diopter is needed to reverse the shadow in the horizontal meridian, and that in the vertical, with no lens power added, the motion is a trifle against the mirror. With the patient still looking at the fixation card, fifty-three inches away, the examiner finds that he must advance his mirror ten or twelve inches nearer to his patient before he obtains a reversal of the shadow in this meri- dian. So he notes on his examination blank "( ). D. — 0.25 D. S. I + I. D. C. ax. 105."' In the left eye the horizontal motion is reversed with a half- diopter convex lens quantity. In the vertical meridian there is just a suggestion of a motion icith the mirror, when the ex- aminer is forty inches away. Adding even a slight convex lens power stops it. The axis seems to be about fifteen degrees to the temporal side of the head. The examiner notes "O. S. -|- 0.50 D. C. ax. y=,." With the skiameter removed the exam- ILLUSTRATIVE CASES IbJ iner see a brighter reflex and a more pronounced astigmatic straight edge, and with the patient looking over the examiner's shoulder at test types twenty feet away the shadow moves tit7/i the mirror in all meridians in both eyes. With the patient look- ing at the fixation card again the motion in the vertical meridian of the right eye can not be reversed until the fixation stand has been moved eighty inches away. Corroborating with the trial case, it is found that vision O. U. 20/20, a trifle "misty." can be secured with O. D. — 0.50 D. S. ~ + 0.75 D. C. axis 105 and O. S. + 0.25 D. C. axis 75. Patient reads well with this correction, and " — 0.50 D. S. or + 0.50 D. S. added in a binocular way ofifers no aid. Corroborating skiametrically again with the full correction on, it is found that a cpiarter- diopter convex lens quantity reverses the shadow in all meri- dians when the patient looks at the brow card on the examiner's mirror, no matter whether its distance be twenty or sixty inches away. With the quarter-diopter convex lens power removed, the shadow sho\ys a suggestion of a movement icith the mirror at the same distances of twenty and sixty inches away. The above formula is then subjectively confirmed and the patient is instructed to wear the glasses as much as possible and to report in a month. All the tests taken together occupy not over ten or twelve minutes of time. In analyzing this case the occupation of the patient is l)orne in mind as one calling for considerable accommodative adjust- ment. Then the previous wearing of concave lenses is perhaps partly responsible for the four degrees of esophoria complained of. for, with these glasses on, one end of the astigmatic interval in the left eye calls for one and a half diopters of accommoda- tion, whidi in turn calls for two and a quarter degrees of con- vergence in order to maintain standard co-ordination. And this for one eye only. The age of the patient, the habit of excessive convergence l68 ILLUSTRATUE CASES due to occupation, also the habit of accommodation aggravated by the occasional use of glasses calling for increased ciliary effort, are all factors to be considered by an examiner, espe- cially if his patient returns in a day or two and complains of a "thin fog," etc. The temptation for a rcfractionist to advise the immediate use of lenses which he feels sure represent the full correction of his patient's ocular error is very strong indeed, and if he has an intelligent patient to reason with this judgment is often correct. But if his patient happens to be of the timid kind, or one who thinks the acuity of vision to be had after one day's use of glasses is the only thing to judge their merits by, then it is wise to "make two bites of a cherry," and indulge the patient's own notions by giving a temporary correction slightly over or under that which is really indicated, and which will eventually have to be given. It is cases such as these that render the science of physio- logical optics inexact, for an examiner must always remember that attached to every pair of eyes is a different individual with a different body, a different occupation, different habits and different ideas as to different things, and so each patient requires different judgment and different explanations and encourage- ments. And it is for these differences that in optometry, as in other specialties, "Many are called and few are chosen." CHAPTER XII. Resume of Previous Chai'Ters With a \ik\v to Emphasiz- ing THE Salient Points of Ocular Skiametry as a System. The definition of the verb "name," is, to fix a tliought in a word, and a definition of "description" is the act of depicting by words or signs so that another may form a correct mental image or idea. This is the reason why the name "ocular ski- ametrv" has been given to eye-shadow--mcasuring. as set forth in Chapter I. If any refractionist can find a better name for the test, or series of' tests, to which this term so aptly applies, then this name can be easily relegated to the company of the French word "Fantoscopie-retinienne," as suggested by Chibret. or ta the manv others that have been proposed for this purpose. In Chapter I. the amount of optical knowledge necessary to achieve skiametric success was called attention to. Perhaps it would have been wise to have also included a knowledge of higher mathematics, mechanics and medicine, in the require- ments set forth, for knowledge of almost any character is ot undoubted value, if for no other purpose than that of mental g>mnastics. But in work of the kind referred to here the prac- tical must be kept sight of. and then. too. experience counts for much, so that the inexperienced theorist has to step aside for the one with practical knowle.lge. even though this knowledge may be gained in obscure channels and unaccompanied by bnl- liancv in other directions. Regarding the true value of ocular skiametrv. in order to be correct, it must be based upon recent app.u.o.tment. lor every few years some enterprising intellect designs a device or ofifers a suggestion that betters the whole system. Bowman's discovery of the shadow's action under reflected light was similar to Babbage's discovery of the ophthalmoscope; inasmuch as neither Bowman, the physician, nor Babbage, the optician, realized the full or even partial value of their find until masterminds like Cuignet. Parent, Helmholtz and others came forward and advanced the work which has already bene- fited humanity so much. To be able to even approximately estimate the refractive con- dition of another person's eye without asking a question is in- deed marvelous, but to make this estimation with exactitude, as can now be done in some cases, entitles the system by which this is accomplished to all the deference due to "one of high de- gree." Every refractionist, therefore, should give such a systep" more than casual consideration, nor should he be satisfied until after he has thoroughly investigated its merits, consulting only those who arc its masters, for it logically follows that those who are not masters of it are hardly qualified to render intel- ligent opinions. .As to the stumbling blocks in skiametrv. they are so many and varied that reference to tluni will he by chapters, as thev ai)i)ear in this resume. In Chapter TT., attention was called to adequate and inade- quate examination rooms. Surely work that requires such attention to details as does successful optometry also requires attention to the surroimdings which contribute to success. Practicing refraction work in an inappropriate place is like an itinerant watchmaker erecting his bench on a street curb, for even if he could manage to clean a watch fairly well, the public RESUMK 171 is not to be blamed for lacking confidence in the conditions under which this work is attempted. Regarding illumination, to which the second chapter also refers, it can perhaps be truthfully said that poor light is the cause of more failures in ocular skiametric work than any other one defect. Good work usually requires good tools, and a gas or electric lamp of not less than a forty-candle power illumination is about the poorest light that can really be called good. And then, too, this light needs to be properly hooded, so that only a round aperture of about three-quarters of an inch in diameter is avail- able. As to the kind of lamps to be used : the gas calls for the "Welsbach," or incandescent variety, whose mantle should al- ways be in perfect condition, while the electric calls for a car- bon filament that is closely formed in spiral shape, so as to pre- sent the appearance of a small solid light in place of the usual one which has its filament wires arranged to form a large double ring of light. Those who have no "city" gas or electricity will find an excellent substitute in acetylene. The one point to be remembered, then, is to be sure to have a good powerful source of illumination, and if it is desired to weaken, or lessen, the intensity of the light all that an examiner has to do is to operate it farther away from his patient. In plane mirrors, that which is needed is a small one with a two-millimeter peep-hole and a six-inch handle, the silver- ing on the mirror to be perfect and the peep-hole to be kept scrupulously clean and free from dust. The correct handling of the mirror is essential to skiametric accuracy. The bodv movement far surpasses a hand motion, because it permits of straight line action in place of curves. and thereby enables an examiner to make closer corrections than where the mirror is allowed to wabble, thus causing a vertical motion to resemble a horizontal one. Schematic eyes are of value only to beginners, or for experi- mental work. However, a correct one is necessary to obtain reliable results, and reliability of findings is a prerequisite to the encouragement of students, for if a novice fails to obtain accu- rate measurements he soon tires of practice and fears that the system is at fault. Transposition of lenses — "There's the rub," as the Bard of Avon puts it. To be sure the reduction and transposition of lens values is not confined to ocular skiametry alone, but in this work it is found of especial value. In the subjective use of trial case lenses transpositions can be proved by making second tests, but in skiametric work this is too troublesome and also wastes too much valuable time. The successful examiner, therefore, must be able to either take his pencil and figure out his results in a careful manner, or else be able to arrive at like ends through mental computations. The method for reducing and transposing lens values, as set forth in the second chapter of this book, is believed to be the shortest and easiest of all the methods that have been devised for this purpose. Its salient points are that all lens quantities should be reduced to cylinders, and then the memorizing of two short rules governing plus and minus symbols, which show that when they are alike the cylinders are to be subtracted, and when they are unlike the cylinders are to be added, the cvlinders al- ways being considered at right angles to each other, for under no other condition rlncs a rcfractionist require them. A knowledge of tlic rules involved in this system enables an examiner to juggle with all kinds of lens values, no matter whether the quantities number one or one hundred. In Chapter TIT. the reader is asked to familiarize himself with the optical principles involved in skiametry. A boiling- down of these f)rinciples, so to speak, shows that the real object RESUME 173 of employing the shadow is for the purpose of measuring' the exact relation of the lens system of an eye to its retina. If an eye is seven-eighths of an inch in depth then of course it requires a lens system to have a seven-eighths-inch focus. But if an eye has a seven-eighths-inch focus and only six-eighths of an inch depth, then the lens system must be arti- ficially assisted by adapting spectacles or eye glasses. In trial case-testing the optical condition is determined by cross-questioning a patient as to vision, while rays of light are being bent before they enter the eye. In examining an eye by skiametry its optical condition is determined in a manner in- dependent of a patient's answers by noting the l)ehavior of the pupillary shadow, which, in turn, represents the action of the rays of light as they emerge from the eye under extmination. In subjective tests with trial-case lenses the operator relies on a general standard of measurement when the object looked at is situated twenty feet away, in order to render the rays of light practically parallel. In objective examinations by ski- ametry the examiner deals with a standard in which conver- gence of the rays takes the place of parallelism. This conver- gence, varying as the distance at which examinations are made, is changed to meet certain other requirements. To express it tersely then: subjective work requires the following of a ray of light to its retinal focus, while objective work requires the following of a ray of light from its retinal focus. In subjective tests the source of the ray is the test type on the distant card. In objective examination the source of the ray lies at the retina of the patient's eye, and at a point which is represented by the line of demarcation which separates the illuminated spot from its surrounding shadow. Chapter III. also describes a practical and easily made model which will undoubtedly do more in the way of impressing upon a beginner the true meaning of the term "shadow," as used in 174 RESUME connection with skiametry, than many pages of carefully written discussion could do, no matter how intelligently it might be expressed. For it is said that pictures tell a better story than letters, but that working models surjiass both letters and pic- tures. In Chapter I\'. the shadow's behavior under certain optical conditions is touched upon, and a description of regular and irregular refractive errors is attempted. Such descriptions are always more or less difficult owing to the fact that eyes differ in appearance almost as much as faces do. For this reason only the more pronounced features are emphasized here, leaving it to each examiner to differentiate for himself as his experience increases. It is a well-known fact that those who are interested in sci- entific pursuits often become very dogmatic when it comes to promulgating theories for the causation of certain phenomena. Then, too, many a long controversy has ensued between two theorists which, when fully understood by both, was found to prove that each one was in the right, the arguments turning out to be the same mental picture, only taken from different points of view. The theories expressed in Chapter \'.. while having no prac- tical value, except to account for disturbing phenomena, will perhaps interest those who like to delve beneath the surface. Multiple methods for using skiametry, however, savor very much of the practical, and the reader is here introduced to the various methods for applying skiamctric principles which, upon being enlarged, have served to lift Bowman's discovery to the dignity of a system. It is to be regretted that no better term than "non-toxico- static-method" can be suggested for the non-medical rcfraction- ists' manner of practising static skiametry without the aid of cycloplegics, but, to repeat that which has been said before, as the nomenclature of optics increases {greater care must l)c ex- ercised in selecting words accurately descriptive. In the word "fogging" an old ac(|uaintance is met. The word amplifying, however, as used in connection with other portions of optometry besides accommodation, is somewhat new and emphasizes the value of a mobile lens system over that of a battery of single lenses, the action of a living eye itself resem- bling the former rather than the latter. Too much stress can not be laid upon the usefulness of lens mobility in connection with ocular skiametry, owing to the ease with which the emerging rays of light can be bent, and this without having to re-adjust or search for a favorable position every time a lens is changed, as frequently occurs when the bat- tery, or "unit," system of lenses is employed. In Chapter \T. the author's new, or "dynamic." method is exploited. And, as the novelists say. "the plot thickens," for while the physical and physiological principles involved in ocu- lar skiametry in general are very simple indeefl. when once un- derstood, yet many experienced examiners find it difficult to grasp this new method at a glance, and are therefore prone to think that the system is at fault rather than their own under- standing. The old puzzle picture of the wild cat in the tree will i>os- sibly serve to illustrate this point. The guesser looks at the picture intently, but can see only an ordinary tree having leaves, twigs and branches. He keeps looking and turning the picture in all directions, when lo! there is the wild cat outlined by the leaves, twigs and branches, and as plain as day. only a little dif- ferent in appearatKe from what he expected. .\fter once being found this wild cat never can he lost sight of again, and the beholder wonders how he ever failed to solve the puzzle at the first glance. So it is with dvnamic skiametry, after the principle is made 176 RESUME plain, that the patient's own accommodation is to be substituted for that of the working-distance lenses used in the static method, then all is comparatively easy. It must be borne in mind, how- ever, that this accommodation is not to be depended upon where it does not exist in sufficient strength to make it available. For that reason patients above forty-five or fifty years of age are to be measured by the static method, while those younger are to be measured by the dynamic. The question is frequently asked by students of the dynamic method whether an eye is emmetropic or not when an examina- tion is made at forty inches away and there is no motion to the shadow, the patient in the meantime looking at a fixation card attached to the examiner's mirror. Now this tells only one thing, and that is that the eye is not myopic to any considerable extent. To determine whether the eye is hypermetropic, artificial convex lens power must be added to see whether it causes the shadow to move against the mir- ror's motion, and the strength of the lens necessary to cause the reverse movement represents the amount of the error. In myopic cases the myopia must be over-corrected, which really makes the eye falsely hypermetropic and causes the ac- commodation to be exerted. This over-correction is then meas- ured and subtracted from the concave lens quantity that has been used to create the false hypermetropia. The remainder ob- tained represents, of course, the true myopia. Astigmatism has been described as a half error and the re- fractive diflfercnce between its weakest and strongest points, or limitations, can be called the astigmatic interval. In cases of so-called "mixed astigmatism" dynamic skiametry will be found of especial value in determining whether the nearest end of this interval is truly myopic or is only a spasm, as frequently occurs. Once more let it be said that dynamic skiametry is not an infallible method, because it depends for its accuracy upon the RKSL'MI-: 177 relation existing between the two forces represented by accom- mo(iation and convergence, and they, in turn, depend upon bodily conditions involving nervous impulses. Hut all experi- enced refractionists know that nine prcsbyopes out of every ten require about a two-diopter convex spherical lens to restore the harmony between this same accommodation an^ Chromotometry 11 Compound error "/j Conjugate focus 72 Convergence, influence of. . 120 Corroborative measure- ments 124 Crain disc 132 Cross retino-skiamcter 137 Cuignet 170 Cylindrical equivalent 84 Dark-room 24 Difficulties of skiametry. ... 19 Dioptometry 11 Discovery, Bowman's 11 Donders 163 Dull reflexes 87 Dynamic skiametry 98 Electric lamp 35 Emmetropia -j}^ Examination-rooms 24 " systematic ... 147 Fantoscopie-retinienne .... i6f) Fay instrument 134 Fixation points 106 Fixation stand 107 Focused light 15 Fogging method 95 Fundus reflex 87 Gas lamp 28 Gasolene lamp Z}, Page Geneva retinoscope 136 Habit, influence of 120 Hamilton refractometer. . . . 134 Hartridge 99 Helmholtz 170 Holding the mirror 41 Hypermetropia 74 Illumination 26 Illustrative cases 154 Influence of accommodation 120 " convergence... . 120 " habit 112 Instruments of Crain 13-2 Cross 137 DeZeng 36 Fay 134 Geneva ' 136 Hamilton 134 Jennings 133 Meyrowitz 134 Prentice 136 Standart 132 Wiirdemann 131 Irregular errors 84 Jackson 5, 136 Jennings' Device. 133 Keratometry 11 Lamps 26 Acetylene 29 .•\rgand 28 DeZeng 28 Electric 35 (iasolcne 2,2, Oil 26 Rochester 26 Success 27 Welsbach 30 Lenses, reduction of 44 " transposition of... . 44 mobile action 128 Light 26 reflections 139 Magnified punil 144 Meyrowitz refractometer.. . 134 Mirror, plane 38 " bracket 39 Mixed astigmatism "j^ Mobile lens action 128 INDEX Continued Page 05 4^ 93 112 75 II 94 152 M Mudel for shadow schematic eye Multiple methods Muscle action and habit... Myopia Name descriptive Non-toxico-static method. Nystagmus Objective method Ocular skiametry 1.3 Discovery of 16 Principles of 50 The shadow 64 Amplifying 95 Fogging 95 Dynamic 98 Static 93. 100 Ophthalmometry 11 Ophthaimotometry 11 Ophthalmotropometry Optometry Parent Penumbra, single " double " in shadow-test- ing Plane mirror Position of light " " mirror Perioptometry Phacometry Phorometry Prentice retinoscope Prisoptometry Prisms, use of Pupillary distance Pupillomctry Ray values Records Reduction of lenses Regular errors Resourcefulness Resume Retinal illumination " reflex Retinoscopy Retino-skiameter Risley prism Roentgen Schematic eye Shadow Action of Models for demonstrat- 11 II 170 91 92 93 40 II II II 136 II 121 141 II no 148 44 81 152 169 63 63 12 1.17 127 ing Movements of. Appearance in spher- ical cases Appearance in astig- matic cases Appearance in com- pound cases Appearance in scissors movement Appearance in irregular astigmatism Appearance in cortical cataract Appearance in conical cornea Shadow test Skiametry Skiameters Skiascopy Skiascope Spasms Standart disc Static skiametry Strabismometry Subjective method.. Systematic examinations... Theories Thorington Toxico-static method Transposition of lenses Using skiameter Value of Bowman's discov- ery Value of instruments Vision of examiner Visual fixation Visuometrv Wall bracket Welsbach "^V'ith" the mirror Wisdom Wiirdcmann's lens rack Pace '3 42 64 70 6S 70 81 81,82 83 8U &4 85 85 13 13 12 38 "7 132 93. 100 II 14 147 f^7 94 48 142 16 1^7 23 105 7! 113 131 14 DAY USE RETURN TO DESK FROM WHICH BORROWED OPTC^TF".TRY LIBRARY This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. 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