THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES LOGIN BROi MEDICAL BOO Dynamic Skiametry in Theory and Practice Embracing Its Association with Static Skiametry and with Those Optometric Methods Wherein the Correlation of Accommodation and Convergence Must Be Considered BY ANDREW JAY CROSS, D.O.S. Author of "A System of Ocular Skiametry." Lecturer on Theoretic and Practical Optometry, Columbia University. President American Optical Association 1900-01, and Honorary Life Member of Its Scientific Section. President Optical Society State of New York 1897-98-99-1900. Dean of The New York Institute of Optometry 1907 08-09. Honorary Member Rochester, N. Y., and Syracuse, N. Y., Optometric Societies, etc., etc. IVith Seventy-One Illustrations (Sixty-one of which are Original Drawings) PUBLISHED BY A. JAY CROSS OPTICAL COMPANY 20 East Twenty-third Street New York COPYRIGHT, 191 1, BY ANDREW JAY CROSS Press of S. L. Parsons & Co. 45 Rose Stueet, New York O\on(r0 VJVJ nil To My Colleague, CHARLES F. PRENTICE, M E., One of the First to Recognize the Scientific Value of Dynamic Skiametry, and With Whom I Have Been Associated in Opto- METRic Organization and Education Work Since 1895, This Little Volume is Affectionately Dedicated. PREFACE The progress made in optometry during the past decade has perhaps been influenced by no one division more than it has by that of Dynamic Skiametry, for the revolutionary character of this new objective method has of necessity developed many minor factors which were unknown when the former book, "A System of Ocular Skiametry," was issued. For this reason it is now deemed wise to present new expla- nations of the theory and practice of dynamic shadow testing and, at the same time, retain all of the old that has been found good. Static skiametry, or that method known by the blanket term "retinoscopy," has been explained so often by other writers that it is thought quite unnecessary to add to the length of this little volume further than by a brief description of the cardinal points involved, hence it will be taken for granted that the reader is more or less familiar with the optical principles under- lying Bowman's great discovery, and with the added improve- ments made by Cuignet and athers prior to 1902, when the dynamic method was originated and made public by the author. New York, October, 1911. CONTENTS CHAPTER I Pages 15 to 26 Ocular Skiametry as a System, Its Value in Optometry and the Optical Knowledge Necessary to Master It, Including Difficulties to be Overcome. CHAPTER II Pages 27 to 46 Proper and Improper Examination Rooms, Size Intensity and Control of Illumination, the Plane Skiascope, How to Handle It, and Some Novel Inventions. CHAPTER III Pages 47 to 59 Schematic Eye Practice and Its Importance to Students, Model Eyes and the Exercise of Care in Their Adjustment. Reduction and Transposition of Lens Values, and the Necessity for the Complete Mastery of This Work in Successful Skiametry. CHAPTER IV Pages 60 to 79 Why Ocular Pupils Appear Red When Viewed Through a Skia- scope, With a Brief Description of the Cardinal Points Involved in Static Skiametry as Practiced With the Plane Mirror, Includ- ing Some Theories Regarding Fundus Reflex. CHAPTER V Pages 80 to 99 Theory of Dynamic Skiametry, and the Importance of Reliable Fixation in Co-ordinate and Independent Observation, With a Reference to Three Essential Myopias, and an Explanation of "Ray Values". CHAPTER VI Pages 100 to 114 Orthophoria and Heterophoric Conditions, and the Influence of Habit Upon Accommodation and Convergence, With Special Consideration of Spasms and the Use of Prisms. CHAPTER VII Pages 115 to 123 Practice of Dynamic Skiametry, Its Use in Measuring Regular and Irregular Astigmia, and Its Special Value in the Objective Esti- mation of Presbyopia and Sub-Normal Accommodation, To- gether With Its Relationship to Other Methods and Tests. CONTENTS— Continued CHAPTER VIII Pages 124 to 137 Illustrative Cases, Showing the Comparative Value of Static and Dynamic Skiametry in Patients of Different Ages, Occupation and General Physical Condition. CHAPTER IX Pages 138 to 152 Multiple Methods in Optometry and Their Value in Corroborative Measurements, the Systematic Keeping of Records and the Im- portance of "Case History", Including Resourcefulness and Mechanical Mydriasis. CHAPTER X Pages 153 to 172 Value of Instruments in Practising Optometry. Mobile and Unit Lens Systems, Various Instruments Used in Skiametry, With Description of Their Mechanical Construction. CHAPTER XI Pages 173 to 196 Questions and Answers Pertaining to Static and Dynamic Ski- ametry and Correlated Subjects. With Pertinent Remarks Em- phasizing the Salient Points Involved. CHAPTER XII Pages 197 to 218 Opinions of Others Regarding the Value of Skiametry in General and the Dynamic Method in Particular, With Comments on Objective Versus Subjective Optometry, and the Relationship of Accommodation and Convergence, Including Quotations on Mental Perception, and an Epilogue. ILLUSTRATIONS Fig. Page 1 Parallel rays of light converged 20 2 Divergent rays of light paralleled 20 3 Acetylene gas lamp with metal chimney 30 4 Asbestos lined metal chimney 31 5 Asbestos lined glass chimney 32 6 Asbestos-paper chimney-cover with iris diaphragm opening.. 33 7 Spiral filament in electric lamp 34 8 Author's asbestos covered electric lamp 36 9 DeZeng electric retinoscope 37 10 The "Hardy" wall bracket for gas or electric lamp 38 11 A simple skiascope 40 12 Author's double bracket skiascope 41 13 Skiametric fixation card 42 14 Skiametric fixation card 42 15 Manner of holding author's skiascope 43 16 Reisner's retinoscope 45 17 Klein's retinoscope 46 18 DeZeng-Standard schematic eye 48 19 Queen's pasteboard schematic eye 49 20 Two cylindric lenses of unequal focus and axis 52 21 A crossed cylinder-lens of unequal meridional focus 53 22 Three cylindric lenses of equal focus, one at axis 90, and two at axis 180 53 23 One crossed cylinder of equal meridional focus and one simple cylinder at axis 180 54 24 Crossed cylindric lens of plus and minus curvatures 57 25 Three cylindric lenses, two plus and one minus 58 26 Illumination of second card through hole in first one 61 27 Return rays from second card entering eye through tube in candle 61 28 Substituting a skiascope for candle tube 62 29 Illuminating the ocular fundus 64 30 The illuminated area on the fundus 65 ILLUSTRATIONS— Continued. Fig. Page 31 Rays returning from edge of illuminated area on the fundus. . . 65 32 Returning rays influenced by a convex lens 66 33 Why the shadow moves "with" the mirror 67 34 Why the shadow moves "against" the mirror 68 35 Pupillary appearance of a so-called "shadow" 69 36 Why the retinal illumination is larger in ametropia than in emmetropia 74 37 Why shadows move slower in ametropia than in emmetropia.. 7$ 38 Why a shadow is duller in myopia than in a like degree of hypermetropia 76 39 Relative size of retinal illumination in high and low degrees of myopia 77 40 The optical principles of penumbra 77 41 The optical principles of penumbra doubled 78 42 Interference of penumbra in shadow testing 79 43 True myopia ( Static Method) 84 44 Artificial myopia ( Static Method) 85 45 Accommodative myopia (Dynamic Method) 86 46 How the accommodation can absorb a ciliary spasm 88 47 Multiple fixation and observation points 91 48 Author's fixation stand 92 49 Fixation stand target card 93 50 Reverse side of fixation card 93 51 Position for initial examination 94 52 Balancing the accommodation and convergence in emmetropia. 104 53 Equal innervation necessary to balance accommodation and convergence in emmetropia 105 54 Imbalance of accommodation and convergence in hyperme- tropia 106 55 Unequal innervation required to balance accommodation and convergence in hypermetropia 106 56 Imbalance of accommodation and convergence in myopia 107 57 Unequal innervation required to balance accommodation and convergence in myopia 107 ILLUSTRATIONS— Continued. Fig. Page 58 Author's record blank 145 59 Regular size pupil 151 60 Area of mag^nified pupil 151 61 Refraction by lenses placed close together 154 62 Refraction by lenses separated 155 63 King's binocular hand trial set 156 64 Skiametric lens rack of Wiirdemann 158 65 Lens disc used by Grain and others 159 66 Standart's "Umbrameter" 160 67 The Meriden "Oculometroscope" 162 68 The "Geneva" retinoscope 164 69 DeZeng's Optometer, Phorometer and Skiameter 167 70 Constructive principle of the author's skiameter 170 71 Author's skiameter without base 171 Dynamic Skiametry in Theory and Practice CHAPTER I. Ocular Skiametry as a System. — Its Value in Optometry AND THE Optical Knowledge Necessary to Master It, Including Difficulties to be Overcome. SKIAMETRY AS A SYSTEM. Formerly there was only one method employed in measuring eyes by the so-called shadow test, and this one method was made to do service for the six-year-old child and the sixty-year-old adult. This one method was also employed for measuring the myope of high degree and the hypermctrope of low. Long standing habits of suppression of accoinmodation or conver- gence, due to impaired co-ordination, were entirely 'ignored and presbyopia was never a factor to be considered. Indeed, all eyes, like all Chinamen, were said to "look alike." It is not many years since eminent authorities claimed that the recognition and correction, with lenses, of hyperopic errors of less than one diopter was equivalent to taking an un- fair advantage of a patient, and therefore savored of charletan- ism. What is now known as eye strain was classified as latent hypermetropia and of trifling importance unless of high degree. But the foreword in optometry, particularly during the past few years, has been to pay more attention to minor details for, as in other walks, both service and comfort are often secured by giving heed to the little things. This care and attention has developed the fact that greater accuracy in shadow testing can only be obtained by applying methods which take into consid- eration the patient's age, and other previously overlooked factors requiring varying procedure in varying cases. The word "system" is defined as "a series of methods," so 1 6 SKIAMETRY AS A SYSTEM it is easy to understand why ocular skiametry or, literally, eye- shadow-measuring, came to be known as a system of ocular skiametry. VALUE OF SKIAMETRY. In the general practice of his profession the most difficult problem that confronts the optometrist is the fitting of his patient's preconceived notions as to the kind of glasses needed, and the manner in which they are to be worn, so in attempting to make plain the true value of static and dynamic skiametry the problem which confronts the writer is the overcoming of the previously formed opinion that most readers have, for many inquirers into the merits of shadow testing seem to be possessed with the belief that every case which presents itself is capable of being both easily and accurately refracted by means of Bowman's great discovery. This expectation being as inconsistent with the real facts as it would be to expect like results from trial case tests or by any other one optometric method. The truth can perhaps be fairly expressed by saying that shadow testing bears to trial case testing much the same rela- tion that the addition of a column of figures from the top bears to its addition from the bottom. Skiametry will uncover at a single sitting optical conditions 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. Viewed again from a similar position we find that the two general methods for esti- mating 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 reputations — the evidence can be none too corroborative. So we find skiametric and trial case testing to be inter-de- pendent, both systems having their weak and strong points, and VALUE OF SKIAMETRY 1 7 one aiding in the judgment requisite for the successful appUca- tion of the other, skiametry 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 the one 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 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 pro- ductive of better results, for that which one can achieve by study and practice it is quite possible for others to achieve by equal application and effort, and sometimes by even less where assistance is given by skilled instructors. For nearly four decades the ablest optometric 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 knowl- edge regarding optics and optometry 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 will take l8 OPTICAL KNOWLEDGE NECESSARY the time to thoroughly master this valuable means for ascertain- ing ocular refractive conditions in a manner independent of the patient's intelligence. Now 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, when 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 advance information of a truly "inde- pendent" character, and upon which he can rely with greater confidence than if this information had been denied him. On the other hand, if the trial case test does not confirm the mirror findings, then the mirror is employed again to confirm the trial- ca've findings. The subjective is used to check the objective, and then the objective again to corroborate the subjective. OPTICAL KNOWLEDGE NECESSARY. Expressed in a broad way, the optical knowledge necessary to achieve skia- metric success is all the optical knowledge a student can obtain. But keeping in mind the practical side of the work there will be found a few essentials which are clearly indicated before intelligent progress can be reasonably expected. The general optical principles of the shadow test in its simplest form are not very complicated. Taken, however, in connection with its optometric associates skiametry represents, as a whole, a rather high order of knowledge regarding both physical and physiologic optics. It also requires in its applica- tion a certain amount of skill or dexterity in the manipulation of indispensable 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 tyvo leading accomplishments in shadow testing in which an examiner must be proficient before he can achieve OPTICAL KNOWLEDGE NECESSARY I9 success. 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 conditions. 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 fixation. The first of the above requirements can be gained by daily practice, but the second requires considerable study and applica- tion, 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 objective must therefore be applied. Many students of optics who have confined their efforts to a mastery of subjective optometry alone often 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 but one direction, 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 traveled, hence if parallel rays of light are made to pass 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 light will diverge until they pass through the lens, after which they will be parallel. See Figs. I and 2. In shadow testing the retina of an eye is the apparent source of light, although in reality the retina is only a poor quality of 20 OPTICAL KNOWLEDGE NECESSARY mirror which reflects the Hght thrown into the eye by the skia- scope. 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 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. Fig. I. Focu3 PARALLEL RAYS OF LIGHT CONVERGED. Fig. 2. DIVERGENT R.\YS OF LIGHT PARALLELED. The correlation 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 grounding in something more than rudimentary optics. With the elimination of the use of the concave mirror, however, and DIFFICULTIES TO OVERCOME 21 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 efforts 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 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 physiologic optics, in addi- tion to the art of subjectively correcting with lenses any mani- fest conditions which may be met. The student may then be said to possess sufficient optical knowledge to begin the study of ocular skiametry with a fair chance of achieving success. DIFFICULTIES TO OVERCOME. The stumbling blocks in ocular skiametry are not few, and they 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 at- tempts to practice successful skiametry. The conditions being poor, 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 of the static method only, he wonders why his skiametric work varies so with his trial case findings. And 22 DIFFICULTIES TO OVERCOME 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 in- tensity to determine a shadow's action, because when the Hght 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, such a pronounced diminution will often be produced, in the definition of the shadow, as to render accurate work impossible. Even where the source of illumination is fifty-candle power, or more, the experienced examiner will meet with cases where the deeply pigmented retina gives 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 hooded so as to show only a small aperture and one that is not, for a dark-room light to be satis- factory 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 diminished 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 reduce it to almost any candle power he desires. DIFFICULTIES TO OVERCOME 2^ Just how high a candle power the human eye can bear with- out 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 conditions for several minutes, whereas the length of time for a proper measurement is only a matter of seconds. Too bright a light therefore need not be feared when the patient does not look directly at the reflection in the mirror. 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 that is to never allow himself to look directly at the source of his illumina- .tion prior to or during an examination. The reason is that the sensitiveness of his own retina is such as to retain the im- pressions made by a bright light to such a degree that duller objects cannot be clearly seen for several minutes thereafter, and, as a consequence, if an examiner 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 a lamp is necessary to determine its condition, or distance away, it is a wise examiner who will derive his information by looking a few jnch es to one side of the flame and not directly at it. Another stumbling block in the road to skiametric success lies in corroded, soiled and dusty mirrors, especially at the edge of the so-called "peep-hole" of the skiascope. Every examiner should possess at least two or more of these instruments, so when one gets out of order it can be sent to the factory for re- silvering. Mirrors, to give the best service, must not be of too great a diameter, nor must their peep-holes be large or bored through the glass. Consequently these peep-holes, which are really not holes in the strict sense of the word, but round spots 24 DIFFICULTIES TO OVERCOME of clear glass, made by scraping off the silver of 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 imag- inary 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 will be satisfied with nothing short of the highest attain- ment. A thorough knowledge of lenses is still another important factor, as there is probably no one study connected with a skia- metrist's educational equipment which demands a more perfect mastery than does that of the reducing, transposing and com- bining lens values. In his desire to attain perfection on the practical side of adapting glasses to the eyes of others, the stud- ent is apt to pass hurriedly by the dry underlying optical prin- ciples 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 minus. Now this is all wrong, for if an optometrist ought to know anything he ought to know all about lenses — how they are made, what index of refraction and spherical aberration mean, wherein DIFFICULTIES TO OVERCOME 25 cylindrical, spherical and ellipsoidal curvatures differ and, above all, the very best way to combine lens quantities in order to produce the highest degree of central and peripheral vision, together with angle and decentration for the relief of strain and the production of comfort. _A thorough knowledge of light and lenses is the great key to the achievement of skiametric success. After a student has learned all about the physical side of refractive work the phys- iologic becomes easier, just as a mathematician who has skipped some fundamental principle finds his higher work puzzling, but where he has covered his ground carefully then his advance- ment 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 pupillary shadows, but how to control these shadows by the aid of lenses combined with fixation, 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 skiametric efforts must not blame this grand test, but look, rather, to their own weakness in their lack of adequate optical knowledge and skill. There is one more point to which attention should be di- rected, 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, should have their own refractive errors corrected to within less than one diopter before they attempt 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, and this applies, too, to presbyopes who should wear their reading correction 26 DIFFICULTIES TO OVERCOME when using the mirror at near points. It would seem, however, that the peep-hole in a skiascope ought to act as a pinhole test and give an 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. Some examiners have found difficulty in keeping the peep- hole of a skiascope exactly in front of their fixing eye, and thus obtaining a good reflex. This is usually due to their inability to close the unused eye, hence practice in this respect is important, for if one eye is in use the other should be occasionally closed in order to make sure that the fixing eye is properly located. CHAPTER II. Proper and Improper Examination Rooms. — Size^ Inten- sity AND Control of Illumination. — The Plane Ski- ascope, How to Handle It, and Some Novel Inven- tions. EXAMINATION ROOMS. Since the fitting of glasses "over the counter" has practically passed away, the considera- tion of where the fitting should be done is now in order. To describe an ideal examination room is quite a different matter than it is to attempt the description of an adequate apart- ment that might serve fairly well as a place in which to prac- tice ocular skiametry. The complaint which is frequently heard from many who attempt to do refraction work without having suitable place and apparatus is that they lack the requisite space, whereas an examination of the store or office by one experienced as to re- quirements 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, and which .could also be made decorative in appearance. Then, too, many are persuaded that an examination room must be of Egyptian dark- ness, without ventilation, where both examiner 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 is just light enough to make the headlines of an ordinary newspaper discernible at midday it will be found dark enough for all opto- metric purposes. The "Mahomet and the mountan" principle can be made use of by increasing the intensity of the light 28 EXAMINATION ROOMS 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 where 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 possible 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, it being better 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 in- tended for optometric purposes their confidence is more than half won. There is a fitness of things, and those whose practice is in accord with this "fitness" generally have occasion to feel satis- fied with the results obtained from having given attention to the details of environment. Indeed, it has already been said by many that next to professional knowledge and skill a well appointed examination room is the very best kind of an adver- tisement an optometrist can have. SOURCES OF ILLUMINATION. The reader's atten- tion is called to the importance of using a proper source of illumination in order to succeed in shadow testing. Therefore it will be the purpose here to describe, with the assistance of drawings and photo-reproductions, some of the various lamps SOURCES OF ILLUMINATION 29 employed for this purpose, and to offer criticisms regarding the advantages and disadvantages consequent upon their use. Practising skiametry by means of a 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 the light source should resemble the glow emanating from the "glory-hole" of a furnace. Oil lamps of all types, while many are magnificent for gen- eral lighting purposes, fall away below the standard of efificiency 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. But even where the chimney is lined with a white pigment 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. The reason for the shortcomings of oil lamps, even those 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 when its general intensity is hooded down to the size of a two or three centimeter aperture, which is necessary in order to obtain good results with a plane mirror, the flame is found of insuf- ficient brightness to meet an examiner's needs. Perhaps some day some inventive mind will devise a con- densing reflector which will permit of hooding the light of oil lamps 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 30 SOURCES OF ILLUMINATION and of acetylene. Ranking above oil burners comes the Argand gas lamp, which also requires a draught to make it burn prop- erly, hence a glass chimney is necessary for its use. But while the Argand gas lamp is vastly superior to oil lamps in general, it is still somewhat below the required stand- ard of efficiency, even when working at its best. The flame has a yellowish white appearance and seems, like the flame of an oil Fig. 3. ACETYLENE GAS LAMP WITH METAL CHIMNEY. lamp, to lack the illuminating energy necessary to meet modern skiametric needs, although in favorable cases fair work can sometimes 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. 3 represents a portable type. SOURCES OF ILLUMINATION 31 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 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, while the expense of maintaining it is only about one cent an hour. Fig. 4. ASBESTOS LINED METAL CHIMNEY. Skiametrists 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. In point of brilliancy, reliability, cost of maintenance and ease of adjustment, however, no gas lamp is superior to the 32 SOURCES OF ILLUMINATION Welsbach type, especially with a metal asbestos lined chimney, as shown in Fi^. 4. This burner is of the "Incandescent" kind wherein chimney draught, as with oil burners, is not so much of a factor. A glass chimney, however, gives better results than a metal one does, due to the fact that the light aperture is covered, thus preventing an inrush of air which can interfere with the white heat of the Fig. 5. O ASBESTOS LINED GLASS CHIMNEY. mantle. Figs. 4 and 5 show two patterns of chimneys, one metal and the other glass, which can be used on the same style of burner. The Fig. 5 chimney being further away from the mantle, the sides do not get as hot as in the pattern shown in Fig. 4, which is made in both glass and metal. But the smaller the chimney the more convenient is the adjustment of the light, the source of SOURCES OF ILLUMINATION 33 illumination being nearer to the surface, tiierefore Fig. 4 is to be preferred. In Fig. 6 is shown an asbestos paper chimney cover, with iris diaphragm opening. This cover can be used outside of an ordinary glass chimney, and serves the purpose of a light screen very well, although it has the faults of the chimney shown in Fig. 6. ASBESTOS PAPER CHIMNEY COVER WITH IRIS DIAPHRAGM OPENING. Fig. 5 as to size. The iris diaphragm, also, seems to be an unnecessary arrangement, for it is the size of the skiascopic mirror that regulates the facial area of illumination and not the size of the chimney aperture, unless a condensing lens is used, which would indeed be troublesome. The Welsbach lamp gives an ideal light for shadow testing, 34 SOURCES OF ILLUMINATION as its flame is bluish-white, while its intensity is ample, espe- cially when its mantle is evenly heated to incandescence. The one unfavorable criticism which can be made regarding it is the fragile character of its mantles, which are easily injured or de- stroyed. But fortuna-tely these mantles are inexpensive and are not difficult of adjustment. In connection with naphtha, or gasolene, the Welsbach burner can also be used, but while the light obtained is not as satisfactory as where so-called "City" gas is employed, yet the results secured are vastly superior to the use of oil. The general use of gasolene has its disadvantages, resulting from its explosive qualities. This is a question, however, on which the Fig. 7. SPIRAL FILAMENT IN ELECTRIC LAMP. 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 Welsbach type of burners, will be found more satisfactory than oil, provided they are kept in perfect order. Wood alcohol is now being extensively used for lighting and heating, so there is every reason to believe that lamps suitable for optometric purposes will soon be devised where this illumi- nant can be employed. Electric lamps of the general house-lighting variety, with long carbon filaments, are perhaps among the most unsatisfac- tory of all illuminators for use in connection with ocular ski- SOURCES OF ILLUMINATION 35 ametry, no matter whether the glass bulbs are of the clear or 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 made superior to the mantle type of gas lamp. Fig. 7 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 filament it becomes easily broken by a sudden jar, or through rough handling in the mail, or when shipped by express. Fig. 8 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 has been known to char the 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, but once used, an examiner is rarely ever satisfied with any other form of lamp. Most high candle-power lamps, however, are somewhat expensive and burn out easily, they should therefore never be burned for long periods of time but be turned off and allowed to cool as frequently as possible. A snap switch in place of the ordinary socket key should be used, and a lamp ought never to be burned on a current that is of a higher voltage than that for which the lamp is made. Attention to these details will often prolong the life and serv- 36 SOURCES OF ILLUMINATION ice of a lamj) two or three- fold, for when the inside of the lamp bulb shows a reddish deposit on the glass it indicates de- composition of the carbon filament, due to overuse, or abuse of some kind. Fic. 8. AUTHOR S ASBESTOS COVERED ELECTRIC LAMP. The cost of maintenance of an electric lamp, too, is some- what higher than the mantle-type of gas lamp, and its durability is not as great. Any increase in current, even of only a few volts, such as frequently occurs in cities where it is intensified at sundown, serves to shorten the life of a lamp very mate- rially. These uncertainties of current can be controlled by what are called resistance attachments, or rheostats, if an examiner cares to incur the expense. The superior light given by these high candle-power electric lamps, however, more than com- pensates for the trouble and expenditure their use entails. Another form of electric illumination, and one which has the added advantage of portability, is the combined lamp and mir- ror, known as the luminous type of retinoscope. Fig. 9 illus- trates the appearance of a late model of one of these instru- ments, for which the makers put forth the following claims : SOURCES OF ILLUMINATION Z7 "This self-contained Electric Retinoscope, combining as it does the battery, lamp, mir- ror and illuminated fixation letters, leaves nothing to be desired in an instrument of this kind. It offers tremendous advantages in every branch of Retinoscopy. It can be used in any place or position irrespective of circumstances. "The Jumbo Handle Battery shown is designed to meet the demand for a very compact and portable instrument of maxi- mum endurance. It is made of aluminum and is designed to hold a two-cell battery of regular stock size. This handle is well pro- portioned, is convenient to hold and has a compressible circuit breaker conveniently located for the thumb of the operator. It is arranged for either a touch or a fixed contact, the lamp being lighted either by pressing the extending arm against the handle or swinging it around in contact with the clip on the top of the cap as desired. "The Illuminated Fixation Letters, ap- pearing to the left of the mirror, are invalu- able in the Dynamic System of Skiascopy. "They are clearly cut in white against a black background, and being located in close proximity to the mirror, the refraction can- be estimated almost at the macula. These letters are rendered visible in the dark room by a divergent shaft of light which passes through an opening in the tube containing the lamp, which illuminates the mirror. This coincident illumination of the mirror and Fig. 9. DE ZENG electric RETINOSCOPE. 38 SOURCES OF ILLUMINATION fixation letters obviates the necessity of any extraneous light, thereby affording the operator the advantage of a perfectly darkened room when so desired." 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, but like other electric lamps it, too, is sensitive to abuse. The light it gives is a fairly intense one and enables good work to be accomplished by those who are acquainted with its Fig. id. THE "hardy" W^ALL BRACKET FOR GAS OR ELECTRIC LAMP. peculiarities. For dynamic work its fixation letters are limited in both numbers and position. As a portable light it is compact and easy of transportation and, in an emergency, serves the purpose of being useful as an ophthalmoscope as well. A point to bear in mind in connection with adequate illumin- ation in skiametric work is to have whatever lamp may be used so arranged that its adjustments, forward and backward to and from an examiner's own eye, are readily obtainable. The adjust- ment as to height is not so important as long as the light is VARIOUS SKIASCOPES 39 about level with the patient's head and is situated from six to twelve inches to the patient's right. The alterable distance for- ward and backward is, however, quite essential, as it enables the intensity of illlimination to be controlled by an examiner as his case demands. A very simple way to arrange for this is to use a wall bracket similar to the one shown in Fig. lo. This bracket is arranged for gas and electricity, and thus gives an examiner a double system of illumination which, in case of necessity, 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 wrote, "I fully realize that proper illumination is the foundation of success in skiametry." And it may be added that this opinion is shared by many others who have had experience. VARIOUS SKIASCOPES. 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 abandon- ment of the latter by most of the skiametrists of the country. There are possibly some conditions under which a concave re- flector might give an examiner better service than a plane one would, but these are rare, and for general all-round skia- metric purposes the plane mirror is to be greatly preferred. All mirrors should be as brilliantly silvered 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- like pieces of paper. The size of the reflecting surface neces- sary to produce the best results while in actual service can then 40 VARIOUS SKIASCOPES 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 be found especially advantageous in using body move- ments. Keeping the peep-hole free from dust and dirt is also im- portant. A frequent twist of the skiascope just before it is used, and while its front and back are covered with a handker- FlG. II. A SIMPLE SKIASCOPE. chief held between the thumb and forefinger of the operator's hand, is usually all that is necessary in order to keep it quite clean. Fig. II represents the skiascope generally employed in the practice of what is usually called "retinoscopy," where accom- modation is supposed to be relaxed by having the patient look over the examiner's shoulder, or where "a reliable cycloplegic" has been used. As will be seen in subsequent chapters, the use of cards for fixing the vision of the patient at the same distance away as that VARIOUS SKIASCOPES 41 at which the mirror is operated renders some means for attach- ing cards to the skiascope almost a necessity. Fig. 12 shows a device to which the name "Double Bracket Skiascope" has been Fig. 12. AUTHOR S DOUBLE BRACKET SKIASCOPE. given. The lenses in the disc at the back of this skiascope contain plus spherics of i. D, 1.50 D. 2. D. 2.50 D. 3.D. 4.D. and 5. D. for the purpose of correcting any presbyopia that an 42 VARIOUS SKIASCOPES optometrist may have, himself, while working at any distance less than forty inches, so as to obtain a clear view of the action of the shadow. When an examiner is under forty-five years of age the use of this lens disc is seldom required. The arrangement of the brackets attached to the mirror frame are such that the cards can be given a number of adjust- ments to suit possible contingencies. Also for the purpose of maintaining fixation so it can be relied upon, for variation in fixation means, of course, an alteration in the patient's refrac- tion. The usual procedure is for the examiner to request that the irregularly placed letters on the fixation card. Fig. 13, be Fig. 13. Fig. 14. SKIAMETRIC FIXATION CARDS. counted by the patient. Then, if this does not prolong the fixa- tion period sufficiently, the request is made to state what letters of the alphabet are missing, or what letters appear more than once. If the visual angle needs changing, or the focal fixation re- quires slight alteration, then the other fixation card, Fig. 14, can be made use of, reliable fixation being maintained by dis- puting the patient's count of the dots. The position of the Fig. 13 card also has an advantage in its being placed so as to corre- spond with the examiner's own nodal point. When the light source is found unpleasant to the unused eye of an examiner it may be obviated by having card. Fig. 14, made longer and adjusted so as to act as a screen. HANDLING THE PLANE SKIASCOPE 43 HANDLING THE PLANE SKIASCOPE. Regarding the proper way to handle a skiascope, the various tutors in skiametry differ, but all agree that the movements 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- FiG. 15. ^^^^^HHHKi^^^^^^^^^H ^^IL&JB \m ■'■ M ^^^^ '■' ^!, ^^^B ^ i' ^"-^ - MANNER OF HOLDING AUTHOR S SKIASCOPE. 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. A description of this body method is as follows : The mirror handle is to be grasped near its lower end, when the skiascope is 44 HANDLING THE PLANE SKIASCOPE held in a vertical position. 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 of the metal disc, upon which the mirror is mounted, is to be held firmly against the side of the examiner's nose or 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 examiner's upper torso, or trunk, acts as though it was pivoted at the waist, while the neck and heaving chest aid in the necessary motions. To say that this action involves a sort of courtesy, or bowing movement, might perhaps add to its description. Fig. 15 may also serve to give a better idea of how the skiascope should be held. 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 and 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 pink pupil should hardly ever be allowed to pass entirely from view after once being found. The examiner should direct his attention to one edge of the patient's pupil only, as in this way he can quickly determine whether the shadow is with or against the mirror's movement in any one meridian. All movements should also be made very slowly, since rapidity of motion often interferes with judgment as to the shadow's action, just as the spokes in a wheel are found to be more difficult to count when the wheel revolves rapidly than when it goes slowly. NOVEL SKIASCOPES 45 NOVEL SKIASCOPES. Among the novel devices for shadow seeing can be mentioned the Reisner "Retinoscope," shown in Fig. i6. The claims for this instrument are set forth by the manu- facturers as follows : Fig. 1 6. REISNER S RETINOSCOPE. "The Reisner Retinoscope has a small lever for tilting the mirror mechanically with self-recording axis on back. Tilting a mirror mechanically instead of the old way gives a positive movement straight across the meridian being neutralized, thereby avoiding the circular motion common with the ordinary mirror, enabling an operator to determine definitely the differ- ence between a case of conical cornea and mixed or irregular astigmatism. Referring to the back of the instrument, the 46 NOVEL SKIASCOPES number indicated by pointer represents the axis at which lens should be placed in frame." Another departure in retinoscopes is the Klein "Non-Irritat- ing Retinoscope," shown in Fig. 17. The mirror of this instrument is made of amber glass sil- vered and mounted in the usual way. The makers ask the fol- lowing pertinent question : Fig KLEIN S RETINOSCOPE. "Why continue to use the ordinary white glass retinoscope, and cause needless annoyance to your patients, when, for a little more, you can obtain Klein's Non-Irritating amber glass retinoscope? This instrument is monochromatic, reduces the actinic rays, insures from ten to tzventy per cent, greater dila- tion of the pupil and minimizes the diffused rays, which are annoying in the reflex of many eyes." CHAPTER III. Schematic Eye Practice and Its Importance to Students. — Model Eyes and the Exercise of Care in Their Adjustment. — Reduction and Transposition of Lens Values, and the Necessity for the Complete Mas- tery OF This Work in Successful Skiametry. SCHEMATIC EYE PRACTICE. Skill is called "famil- iarity with and dexterity in the execution of any science, art or handicraft," and no division of optometry calls for more prac- tice work, in order to be skillful, than does skiametry. It is exceedingly tiresome to a patient to sit quietly and have some inexperienced student gaze for minutes through the peep-hole of a mirror because he is unable to correctly determine the shadow's action. But with an experienced examiner it is different, all he needs is one or two flashes, or excursions of the reflected light, and the behavior of the shadow, under usual conditions, is made plain. Skill in this work can only come through experience, and experience gained through the use of a metal or paste-board model eye is just as valuable to the student as though it was gained through looking into a living organ of vision. Students can begin with large ocular pupils, and then, as their skill increases, these pupils can be made smaller and smaller. The expert in any field where skill is a factor is the one who practises morning, noon and night, or at every oppor- tunity that presents itself. Human eyes, owing to variations in the pigmentation of the fundus and to the uneven thickness of the corneal tissue giving rise to what is called "irregular" astigmia, are factors 48 SCHEMATIC EYE PRACTICE that make some living eyes most difficult to measure by means of the shadow test, so if an examiner makes himself a thorough master of the model eye, where conditions can be made ideal, he will have far less trouble with living eyes than if this pre- liminary practice on the model eye has been neglected. Fig. i8. THE DE ZEXG-STANDARD SCHEMATIC EYE. SCHEMATIC EYES. For research work, as well as for practise, a good schematic eye requires careful selection and adjustment. The metal and pasteboard models that are on sale in all first-class optical supply houses offer a most excellent means for beginners to familiarize themselves with the prin- ciples of both skiametry and opthalmoscopy. These eyes, how- ever, are frequently imperfect in construction, and the printed SCHEMATIC EYES 49 scales attached to them are often unreHable. As a consequence^ the student is apt to meet with discouraging resuhs in his initial efforts to use them. Fig. i8 shows one of the newer makes of the all-metal kind, which has five sizes of pupil. The backs of these eyes are arranged for the insertion of colored miniature plates, which serve to illustrate the various pathologic conditions of the retina, and are to be used in con- nection with the study of ophthalmoscopy and the use of the ophthalmoscope. The double cell in front of the model eye makes it of excellent service in skiametric practice ; as the well Fig. 19. QUEEN S PASTEBOARD SCHEMATIC EYE. marked axis scale also aids a beginner in obtaining the merid- ional accuracy which is so necessary to good work. The cheap pasteboard model, such as shown in Fig. 19, is usually found to be almost as trustworthy as the more expen- sive ones, but all of them require testing before their findings can be implicitly relied upon. A good way to determine the accuracy of these models for skiametric purposes is to have an experienced skiametrist 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 50 SCHEMATIC EYES 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 optometric 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 by 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 cylindric lens, he can create an error of one diopter of hyperopic astigmia. A one-diopter convex cylinder can be used to produce myopic astigmia of equal amount. Setting the model to show two diopters of m^yopia and then using a one-diopter convex cylinder lens will create a compound error of minus one-diopter spheric combined with a minus one-diopter cylindric, due allow- ance of one diopter having been made for the working distance. With the model showing two diopters of hypermetropia, if a two-diopter concave cylindric lens be added, the exact com- pound quantity represented by this error would be plus two diopters of spheric combined with plus two diopters of cylindric. And to neutralize it skiametrically at a distance of forty inches away would require an added lens power equal to plus three diopters spheric combined with plus two diopters cylindric. The added diopter of plus spheric representing the false myopia, often called the "working quantity." 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. And it will be noted REDUCTION OF LENSES 5 1 that at whatever axis the cyhndric lens is set the axis of the artificial astigmia will be in the same meridian. Thus it will be seen that a schematic eye can be made a very useful and patient patient. REDUCTION OF LENSES. In practical examination- room work with the skiascopic mirror it frequently happens that a saving of time and trouble is efifected 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 examlHer must be possessed of knowledge that will enable him to tell the exact ray-bending power of the four lens quantities involved and to do it with ease and without waste of time or likelihood of mak- ing mistakes. Now while this subject is not classified as be- longing to theoretic skiametry, yet it is, nevertheless, a very im- portant factor in contributing toward successful shadow work, and especially so in connection with the dynamic method, for ski- ametry is sometimes considered as neutralisation at long range. Therefore a few pages will here be given to what is thought to be a very simple solution of a so-called complex subject. In the consideration of most problems there is the unit or lowest appreciable quantity to be dealt with, so it is with lenses. Speaking microscopically the basis of all lenses can be said to be prisms, but speaking macroscopically, the unit of all lenses is a cylinder. Therefore, if it is learned how to combine these cylin- ders, after having reduced all lens quantities to a cylindric 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 52 REDUCTION OF LENSES 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 cylindric lenses of like kind and strength when crossing one another at right angles are equal to a spheric lens. Hence the reverse follows, that a spheric lens is equal to two cylindric lenses crossing one another at right angles, and whose kind and strength are the same. In the optometrist's consideration of cylinders he will never need them at any other than at right angles to one another, no Fig. 20. ^/n^ -h^D TWO CYLINDRIC LENSES OF UNEQUAL FOCUS AND AXIS. matter whether they are plus and plus or minus and minus, of the same or unequal strengths, or whether they are plus and mi}ius 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-cylindric effect, which could be duplicated by right-angle cylinders. Now, in a combination of cylindric 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 REDUCTION OF LENSES 53 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 spheric combined with a plus one-diopter cylindric. The second cylinder in the above case having been robbed of a quantity equal to the strength of the first cylinder, in order to,. ■/■^I}' A CROSSED-CYLINDER LENS OF UNEQUAL MERIDIONAL FOCUS: convert the first one into a spheric quantity. The robbery is noted and due allowance made therefor. By referring to Fig. 20 it will be seen that the cylinder lens "a" has a ray bending power of plus i. D., while cylinder lens Fig. 22. //-^ //Z7 THREE CYLINDRIC LENSES OF EQUAL FOCUS. ONE AT AXIS 9O AND TWO AT AXIS 180. //i^ "b"' has a power of plus 2. D. Their axes, of course, are at right angles to each other, also to their ray bending power. If these two lenses were merged back to back their appearance would be illustrated by Fig. 21. 54 REDUCTION OF LENSES Instead of two cylinders being used, one plus i. D. and the other plus 2. D., it is shown in Fig. 22 that three plus i. D. cylin- ders can be employed to accomplish the same purpose as those in Fig. 20. In Fig. 23 it is shown that lens "a" has been converted into a I. D. spheric quantity by the use of the borrowed lens "c" of Fig. 22, and as it takes "c" and "d" to equal the plus 2. D, of "b" in Fig. 20 it is plain that "d" is the remaining quantity. Thus the formula: + I. D. C. axis 90 3 + 2. D. C. axis 180 equals -f- i- D- S. C + I- D. C. axis 180, Fig. 22,. -f-lD ONE CROSSED-CVLINDER OF EQUAL MERIDIONAL FOCUS AND ONE SIMPLE CYLINDER AT AXIS iSo. 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 spheric curvature. This rule also applies to "toric" lenses where the curves of one surface of revolution are ellipsoidal in character, made so by having one meridian of curvature either greater or less than the one at right angles to it. REDUCTION OF LENSES 55 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, in imitating the method of the quack doctor with his two medicines, it will be necessary to first reduce all lens quantities to a cylindric basis and then commit to memory two short rules for the transposition of cylinders. The following extra long combination may, perhaps, serve to make this reduction principle plainer: + I. D. S. C + 2. D. C. 90 C — I. D. C. 180 C + I. D. S. C — 2. D. C. 90 C — 2. D. S. Here, it is seen, there are six lens quantities whose chief axes are 90 and 180 degrees. After creating two columns, all of the lens quantities are written in their cylindric equivalents whose axes come under these two headings, not forgetting that each spheric lens is to be written twice as it is equal to two crossed cylindric ones whose strength and kind are the same. The following is then obtained : Axis go Axis 180 + 1. +1. + 2. — I. + 1. +1. — 2. — 2. — 2. — I. o 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. and -|- 2. D. leave a remainder of — ,1. D. axis 180. Take this example for instance: + 0.50 D. C. 45 C + 0.25 D. S. C + 0.25 D. C. 135. 56 REDUCTION OF LENSES Here the two chief axes are 45 and 135 degrees, and pro- ceeding as before the results are : Axis 43 Axis I S3 + 0.50 + 0.25 + 0.25 + 0.25 + 0.75 + 0.50 The totals give one cylinder of + 0.75 axis 45 to be crossed by another cylinder of + 0.50 D. axis 135, the symbols being alike. Now another example in reduction: 4- 1.25 D. S. C — 1-75 D- C. 15 C — 0.75 D. S. O — 0.25 D. C. 105. Being reduced, the results obtained are : Axis 13 Axis 103 + 1.25 + 1-25 — 175 — 075 — 0.75 — 0.25 — 1-25 + 0.25 This gives a total of one cylinder of — 1.25 D. axis 15 being crossed by another cylinder of -J- 0-25 D. axis 105, the symbols being unlike. In the three examples shown all lens quantities have been converted into cylindric 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 les- son will have been acquired. TRANSPOSITION OF LENSES. Rule No. i.—In a combination of cylindric lenses of a like character, such as plus TRANSPOSITION OF LENSES 57 and plus, or minus and minus, the strength of the weakest cylinder should be written as the spheric quantity while the DIFFERENCE between the tivo lenses should be written as a new cylindric quantity, the axis of the stronger cylinder governing the axis of the cylinder in combination, thus: -j- 0.75 D. C. axis 45 C + 0.50 D. C. axis 135 should be written as equal to -(- 0.50 D. S. C + 0.25 D. C. axis 45. Rule No. 2. — In a combination of cylindric lenses of dif- ferent character, such as plus and minus, or minus and plus, the strength of either cylinder can be zvritten as a spheric quan- tity while the (arithmetical) sum of the two cylinders should ^/.£SD. CROSSED-CYLINDRIC LENS OF PLUS AND MINUS CURVATURES. be written as a new cylindric quantity, the axis of the second lens 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 + 1-50 D. C. axis 105. Or an equal refractive quantity can be obtained by writing it in this way: + 0.25 D. S. C — I-50 D. C. axis 15, Fig. 24 shows a plus 0.25 D. C. lens "a" at axis 90, being 58 TRANSPOSITION OF LENSES crossed at right angles by a minus 1.25 D. C. lens "b." Fig. 25 shows that the plus 0.25 D. cylinder "a" must be crossed by a plus 0.25 D. cylinder "c" in order to convert the "a" lens into a spheric quantity, and that the "b" cylinder must then be in- creased to minus 1.50 D. in order to produce a ray bending power of minus 1.25 D. after the strength of the borrowed lens "c" has been neutralized. Fig. 25. -^ffjsn. y-O.SSD THREE CYLINDRIC LENSES, TWO PLUS AND ONE MINUS. -^501) It will be noted in Rule No. 2 that the word " arith'^etical" is parenthesized, this is done to indicate that for purposes of common understanding liberties have been taken with an algebraic problem. To re-transpose any of these combinations it is only neces- sary to proceed by the usual reduction to cylindric form and then apply whichever one of the two simple rules that may be called for. There is one other form of transposition which it might be well to mention here, and that is in the changing of plus and plus and minus and minus compounds into plus and minus or minus and plus equivalents for the purpose of producing meniscus forms of lenses. For instance, the following two formulas "a" and "b" have like ray bending powers, also "c" and "d." TRANSPOSITION OF LENSIiS 59 a. + I. D. S. C + I- D. C. axis 90, or b. + 2. D. S. C — I- D- C. axis 180 and c. — I. D. S. 3 — I- D. C. axis 180, or d. — 2. D. S. C + I. D. C. axis 90. The rule governing this transposition is as follows: Rule No. 3. — Add the two lens quantities together for a new sphere, change the symbol of the cylinder and alter its axis ninety degrees. The world, metaphorically speaking, takes off its hat to the mathematician, so if the optometrist desires that deference be shown him, too, he must acquire enough of mathematics to make himself proficient in his work. Procrastination and the plea of "no time to take up higher optics" will neither advance the individual nor the profession to which he belongs. Self-education is just as good as any other kind, provided it accomplishes its object; therefore 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 IV. Why Ocular Pupils Appear Red When Viewed Through A Skiascope With a Brief Description of the Card- inal Points Involved in Static Skiametry, as Prac- tised WITH the Plane Mirror, Including Some Theories Regaiuding Fundus Reflex. WHY PUPILS APPEAR RED. Before considering retinal illumination it will be well to again call attention to the fact that all visible objects that do not generate light must, of course, reflect it, hence every non-luminous object that can be seen may be considered as being some kind of a mirror. Glass with amalgam backing, as before stated, makes the best reflector, while a dark uneven surface, like black velvet, makes the poorest. With a so-called "looking glass" the rays of light are reflected in an almost unbroken manner, whereas with most visible objects their surfaces are such as to break up the light into irregular rays, or angles of reflection. This breaking up being well illustrated by a lamp having a clear glass chimney, and one whose chimney is of the ground or "frosted" kind. In the clear glass the flame is the true source of light. In the ground glass the surface of the chimney is the apparent source. The inner back part o r, so-called^ fundus of an eye is about the same sort of a mirror as rough red tissue paper makes, and as it is necessary to illuminate this fundus in order to obtain an apparent source of light, or target, from which to measure emergent rays, it will be seen that consideration of why the pupil of an eye appears red through the peep-hole of a skia- scope will be well for all students to understand, reference is therefore to be had to Figs. 26, 2y and 28. WHY PUPILS APPEAR RED 6i In Fig. 26 the candle "a" radiates light rays which fall on card "b" and pass through the aperture "c" thus illuminating a portion of card "d." Fig. 26. d L-^ b a ILLUMINATION OF SECOND CARD THROUGH HOLE IN FIRST ONE. Card "d" being something of a mirror, reflects the light from the illuminated spot, or area, "e" in all directions, some of which, of course, passes back again through the aperture "c" in card "b," as shown in Fig. 27. An eye, therefore, at "f" in Fig. 27. RETURN RAYS FROM SECOND CARD ENTERING EYE THROUGH TUBE IN CANDLE. order to see the edge, or any portion, of the illuminated area "e" on card "d," must be placed behind a tube "h," which passes through the flame of the candle "a," where it will then be en- abled to intercept the returning rays from "e." 62 WHY PUPILS APPEAR RED In Fig. 28, the skiascope "g," having a peep-hole in the center of its mirror, practically replaces the tube "h," shown in candle "a," of Fig. 27. The eye "f" in Fig. 28 is therefore enabled to see the illuminated area "e" on card "d." Now, by enclosing cards "b" and "d" in the rim of a hollow sphere and placing a lens over the aperture "c" in card "b," there would be Fig. 28. SUBSTITUTING A SKIASCOPE FOR CANDLE TUBE. created the optical principles of a real eye whose pupil would appear red, because the fundus of the eye would be visible to an observer. This observer could then readily see the posterior inner surface of the eye where the mesh work of blood vessels gives it the usual pink-like appearance so familiar to the expe- rienced examiner. It follows, therefore, that the reason why the pupil of an eye ordinarily appears black is because returning rays can not be intercepted without using an instrument. RETINAL ILLUMINATION 63 RETINAL ILLUMINATION. Subjective optometry can be said to be the treating, or bending, of rays of light before they enter an eye, whereas, objective optometry can be called the bending of the rays after they leave the eye. Rays entering an eye can be of two kinds, those that proceed direct from some light source, such as a lamp flame, and those that are reflected from some kind of a mirror. In either case the fundus of the eye is illuminated, and the intensity of the illumination is dependent upon the distance that the rays have traveled, and to the quality of the mirror that reflects them. Rays traveling in the form of a beam of light can be made to enter the eye parallel, convergent or divergent, and the size of the retinal illumination is dependent upon this parallelism, convergency or divergency. Retinal illumination is a necessity in skiametry, because some kind of a target, or apparent light source, must be ob- tained from which the emergent rays can emanate, and the means for obtaining this target is to be had by reflecting light from a lamp flame into a patient's eye, and then by intercepting the returning or reflected rays, at the peep-hole of a skiascope, an examiner is in a position to measure these rays and thus to determine the state of the patient's refraction. Two factors in skiametric work must be kept in mind, namely, that the entering rays are solely for the purpose of creating a source from which the emerging rays can start, while it is the angle of the emerging rays, alone, which denotes the refractive condition of the eye, and it is the measurement of these emergent rays that gives the name of shadow measuring to skiametry. The so-called "shadow" being created by the inner edge of the iris, which limits the size of the illuminated area on the retina, made by the entering rays of light. Errors of refraction, and the distance from the external light source, serve to influence the sise of the retinal light spot, and this, in turn, influences the speed and brightness of the 64 RETINAL ILLUMINATION shadow in its skiascopic behavior, butj^t has no direct bearing upon the refraction of the eye under measurement, as the emergent rays originate from the internal light source created by the illumination on the retina, and it is the movement of the edge of this light spot that gives the with and against motions of the shadow, made mention of later on. In Fig. 29, "a." is the external light source, "b" is the skia- scope, "c" the lens system of the eye, "d" the retinal illumina- tion, or light spot, and "f" the focus of the entering rays, which the retina stops at "d." Fig. 29. ILLUMINATING THE OCULAR FUNDUS. The size of "d," it will be easily seen, is regulated by the positions of "a" and "b," for if the light source, or the mirror, is near to the eye then "d" will be larger than if either one were farther away. Fig. 30 shows the fundus of an eye with an illuminated area of light, "e," the shadow being represented by "d, d, d, d," while "g" calls attention to the demarcation between "d, d, d, d," and SHADOW PHENOMENA 65 "e." This demarcation creating the edge of a target, or curved line of light, acting as a source from which measurements can be calculated. Fig. 30. THE ILLUMINATED AREA ON THE FUNDUS. SHADOW PHENOMENA. In Fig. 31 the marginal line "g," between the light spot "e" and the non-illuminated area "d" "d" constitutes the starting point for the emerging rays, and Fig. 31. RAYS RETURNING FROM EDGE OF ILLUMINATED AREA ON THE FUNDUS. if the lens system "c" is in proper relation to the target "g," producing what is known as emmetropia, the rays will emerge parallel. 66 SHADOW PHENOMENA On the other hand, if the emerging rays diverge or converge in one or more meridians of the eye it denotes an error of re- fraction of some kind. When a convex lens of known power is placed in front of an emmetropic eye, as shown by "k" in Fig. 32, the emerging parallel rays will be brought to a focus, or crossing point, at the focal length of the lens used, and if at this focal point an exam- iner places his own eye behind the peep-hole of a skiascope, at "h," he will be unable to note any motion of "g," or of any appearance of a shadow in the pupil of the patient's eye. On the contrary, if he advances toward the patient so that the point of crossing of the emergent rays is behind his own eye,. Fig. 32. g \ J/ k h RETURNING RAYS INFLUENCED BY A CONVEX LENS. then he can note, if he is skillful, that "g" now appears as the edge of a pupillary shadow and moves with the movement of the light on the patient's face as he rotates his skiascopic mirror from side to side or top to bottom across the eye. And if the eye is emmetropic this movement of the shadow will be alike in all meridians of rotation. The reverse order prevails if the examiner recedes from his patient, so that the focus, or crossing point of the emergent rays, is in front of his own eye, then all rotations of his skiascopic mirror cause the shadow to move against the light that is re- tlectcd on the patient's face. This crossing point of the emergent rays then becomes known as the "point of reversal" of the shadow, and this reversal point enables an examiner to THE shadow's action 6/ determine the refractive status of his patient's eye, either by altering the distance of his view-point, or by changing the strength of lens "k" so the rays v^ill focus, or cross, at any given point selected. The difference between the lens an eye ougJit to take to produce the shadow's action at a given point, and the lens it actually does talze, constitutes the measurement of the error. THE SHADOW'S ACTION. Just why the shadow be- haves in the with and against manner here described can be- seen by reference to Figs. 33 and 34. In Fig. 33, if a single ray is followed from the point of the Fig. 33. PaT/enfj eye, Cxamineris eye WHY THE SHADOW MOVES ''wITh" THE MIRROR. arrow, "e," on the fundus of the patient's eye, to "e"' on the fundus of the examiner's eye, the two arrows will be seen to point in the same downward direction. All motion can then be said to be syncJironous, or coincident, as the upper emergent ray from the patient's eye becomes the upper entering ray of the examiner's eye. In Fig. 34, owing to the refraction by lens "k," the upper emergent ray from the arrow's point "e" in the patient's eye be- comes the lozver entering ray of the examiner's eye, and the arrow "e" undergoes a reversal of position at "e"' by pointing upward. Motion can then be said to be opposite, or against the skiascopic movement of the facial light. 68 THE SHADOW S ACTION It is this motion of the shadow zvith and against the skia- scope's movement that enables an examiner to locate the cross- ing "x" of the emergent rays, and the distance from the patient's eye at which this crossing occurs, together with the lenses neces- sary to cause it, constitute the means for determining the meas- urement of the error. Thus if a plus 1.50 D. lens, "k," is required to create a crossing of the rays at forty inches from an emmetropic eye, whose accommodation is relaxed, then the error would be plus 0.50 D., because plus i. D. is the correct strength for a lens necessary to cause a crossing at this distance. If a lens of only plus 0.25 D. caused a crossing of the rays at forty inches from the patient's eye, then the error would be Fig. 34. Patlenfi eye * f'xaminci'^ eye WHY THE SHADOW MOVES '"aGAINST" THE MIRROR. minus 0.75 D. And if twenty inches was the distance selected for the crossing, then plus 2. D. would be the working lens "k," from which calculations should be made, instead of plus i. D. Skiametry has been ably described by Burnett as being con- trolled by the law of conjugate foci, therefore if an emmetropic or hyperopic eye is made artificially myopic, the strength of the lens required to produce this artificial myopia must enter into the final calculations. And if, on the other hand, the eye has true myopia, then the amount of the error can be easily deter- mined by finding the crossing point of the emerging rays. Thus if the rays crossed at fifty-three inches from the patient's eye the error would be neutralized by a lens of minus 0.75 D If THE shadow's imitation 69 the crossing was at twenty-two inches distance, then the lenses needed would be minus 1.75 D. THE SHADOW'S IMITATION. Fig. 35 will convey a slight idea of the appearance of a so-called "shadow" in passing across the pupil of an eye. A much better illustration of this shadow phenomenon can be had by taking a piece of cardboard, say six inches square, and cutting a hole in it one inch in diameter, then by pasting over this aperture a piece of pink, translucent paper, and by holding the card before a lamp, a fair idea of the reddish appearance of the pupil of an eye when looked at through a skiascope may be Fig. 35. PUPILLARY APPEARANCE OF A SO-CALLED SHADOW. had. The edge of another piece of cardboard passed between the first card and the lamp will, by varying the distance, give a very good representation of the shadow as seen in an eye by means of a skiascope. SHADOW MEASURING. All eyes must be measured in two or more meridians, so as to determine the greatest and least refraction. If these two are vertical and horizontal, and a plus I. D. S. is placed before a patient's eye and the shadow is found to reverse at forty inches in the horizontal meridian, and at twenty inches in the vertical, then the error could be neu- tralized by a lens of minus i. D. C. axis 180, but if it took plus I. D. in the vertical and plus 2. D. in the horizontal, then yo SHADOW MEASURING the lens required would be plus i. D. C. axis 90. Or if it took plus 2. D. in the vertical and plus 3. D. in the horizontal, then plus I. D. S. combined with plus i, D. C. 90 would be the lens indicated. It will be noted that i. D. of artificial myopia was first created for the forty inches observation point, then in the final deductions this created myopia was allowed for. If true myopia was present then no artificial myopia would be needed, unless the true was weaker, and had a point of crossing farther away than the point of observation. To illus- trate this, suppose the true myopia was a half-diopter, the emergent rays would then cross at eighty inches, and if forty inches was selected as the point of observation the crossing of the rays would occur back of the examiner's skiascope, and the motion of the shadow would be zvith the mirror, the same as in emmetropia without lenses. In emmetropia, however, the plac- ing of a plus I, D. lens ^before the patient's eye would cause crossing at forty inches, but in a true myopia of a half-diopter the plus I. D, lens serves to overdo the matter, therefore, a plus 0.50 D. would be the needed lens, or, if the observation was changed to twenty-six inches then the crossing could be found without changing the i. D. lens. So the rule, before mentioned, that the difference between that which an eye does take and that which it ouglit to take, to cause reversal at a given point, represents the error of refraction in the meridian measured. PRACTICE OF STATIC SKIAMETRY. In the practical application of static skiametry, a fifty-candle power light source should be situated at the patient's right, on a level with, and from six to twelve inches from, his ear. After placing a trial frame before the patient's eyes, two plus 4. D. S. lenses are to be inserted in each cell of the frame and the patient directed to look over the examiner's shoulder at some object placed twenty or more feet away, the examiner to be seated squarely PRACTICE OF STATIC SKIAMETRY 7I in front of the patient and operating his piano skiascope exactly forty inches from the patient's eyes. In emmetropia, under these conditions, the shadow will move against the mirror in all meridians, and the four diopters of lens power will have to be reduced to one diopter before reversal, or the neutralizing point of the emerging rays, occurs, the plus one-diopter lens being the so-called "working quantity" for an examination that is made at forty inches. In hypermetropia of, say, one diopter the reduction of the four-diopter lens would be two diopters. And of these two diopters one diopter would represent the error, and the other the working quantity or artificial myopia necessary to create a standard condition, variations from which are to be measured by adding or subtracting lenses, or by altering the distances at which observations are made. This can be illustrated as fol- lows : In a true myopia of, say, two diopters the movement of the shadow would still be against the mirror, even after the four- diopter lens had been removed, the examiner could then add minus spheric lenses, or he could advance his skiascope up to within twenty inches of the patient's eyes, as the emergent rays from the retina of an eye having two diopters of true myopia would have a conjugate focus at this distance. Had the eye been myopic one diopter, the conjugate focus would have been exactly at forty inches without lenses of any kind, hence the rule in calculating all errors of refraction in static skiametry, where the working, or observation, distance is forty inches, is to add one diopter of minus quantity to the total findings, and this sum will then represent the error. Astigmia being truly a half-spherical error the examiner is to proceed as in simple hypermetropia or myopia, with the excep- tion that the error is to be measured only in the meridian at right angles to the axis, and the axis is to be determined by noting the meridian in which there is no motion of the shadow when the zvorking lens is in position. 'J2 PRACTICE OF STATIC SKIAMETRY Take as an example a patient with two diopters of hyperopic astigmia in the horizontal meridian ; if the examiner operates at forty inches with a plus one-diopter spherical lens before the patient's eye he will note a marked movement with the mirror in the horizontal meridian, but no movement in the vertical, this being the meridian of no error it is of course the axis. So, after noting this axis, by the general band-like appearance of the shadow, and that the error at right angles to this axis is hyperopic, all that remains for the examiner to do is to de- termine how great the error really is, and this is ascertained by adding plus lenses until reversal occurs. Lenses used for neutralizing purposes can be either spheric or cylindric, for it will be remembered that two cylinders of equal strength and kind with their axes crossing one another at right angles are equal to a sphere. Therefore any astigmia where the axis is vertical can be measured by the aid of a spheric lens used in its horizontal meridian, the vertical meridian being ig- nored. Where cylindric lenses are used great care must be exer- cised in placing the axis of the cylinder so it exactly coincides with the astigmatic axis of the eye, otherwise confuson of shadow-action and inaccuracies will occur. So-called "compound" and "mixed" errors can be considered as being astigmatic in the two meridians of greatest and least refraction that are at right angles to each other. After a working lens is in position, suppose it takes a plus one-diopter to arrest the motion in the vertical meridian, and a plus two-diopter to do the same in the horizontal, then the case could be likened to a one-diopter cylinder being crossed by a two-diopter cylin- der, which, if transposed, would resolve itself into the following formula: plus i. D. S. combined with a plus i. D. C. axis 90. Looking at it another way, if the vertical meridian had taken a minus instead of a plus lens to neutralize its error the formula would then read minus i. D. C. axis 180 crossed by a plus 2. D. C. axis 90, or transposed it would equal a minus i. D. S. THEORIES REGARDING DULL REFLEXES 'J'^ combined with a plus 3. D. C. axis 90, so that all cases of com- pound and mixed errors are merely cases of crossed cylinders, and are capable of neutralization by either spheric or cylindric lenses. THEORIES REGARDING DULL REFLEXES. In the consideration of a subject as broad as the term ocular skiametry would indicate, the temptation of an author to soar his kite in the realm of speculation and attach to its tail a few theories of his own is very great indeed, even if some of the theories should prove "bad pennies" and return to embarrass their giver. In the actual practice of skiametry there arise certain little details which might be considered as sub-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 may perhaps savor more of theory than they do of practice but which, nevertheless, seem to answer natural ques- tions likely to arise in the mind of those who desire to know the why of everything they are interested in. Ocular pupils of a size not exceeding two millimeters in diameter, notwithstanding an examiner's ability to magnify them, are often very troublesome and constitute a part of the cases which it is wise to determine by means other than by skiametry. This is especially true if, in addition to the small- ness of the pupil, the fundus reflex is of the deeply pigmented 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 sufficient 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, nor hyperopia, for excessive pigmentation. Especially in myopia, 74 THEORIES REGARDING DULL REFLEXES even in an error of only four or six diopters, will the examiner sometimes be puzzled to determine why the reflex is so poor. Adding correcting glasses, however, often brightens this reflex in proportion as the correction nears the total, although at the exact reversal point of the shadow the reflex may again 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 Fig. 36. WHY THE RETINAL ILLUMINATION IS LARGER IN AMETROPIA THAN IN EMMETROPIA. if Fig. 36 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 surround- ing the light circles "b" and "c" to come into view than if these illuminated areas were smaller. Fig. 37 also shows this principle emphasized in a myopic eye, the returning rays from the edge "d" of the illuminated area "b" illustrating the distance the shadow must traverse before reaching the opposite side, this area being much larger than THEORIES REGARDING DULL REFLEXES 75 if the eye was an emmetropic one, where the proportion would be similar to "a" in Fig. 36. The fact that a reflex in a given error of myopia is much duller than in a corresponding one of hypermetropia is due, no doubt, to the greater distance the returning rays travel in a myopic eye than in an emmetropic or hyperopic one before reaching the pupillary opening which, consequently, diminishes Fig. 37. WHY SHADOWS MOVE SLOWER IN AMETROPIA THAN IN EMMETROPIA. their number through radiation and absorption. Fig. 38 illus- trates a theory for this. The lines "h' e' m'" show the relative loss by quenching as the divergent rays from a given illumina- tion on the retina of a myopic, emmetropic, or hyperopic eye strike the inside of the iris, which serves to diaphragm part of them and prevent their further egress. The hyperopic eye, owing to its shorter depth, permits of the least interference with its radiation, but its retinal illumination, see "c" in Fig. 36, shows why its reflex is dull, though not as dull as that of the myopic eye "b," of Fig. 36. Fig. 39 shows the comparative loss in intensity of illumina- 7^ THEORIES REGARDING DULL REFLEXES tion in eyes whose myopic or hyperopic error varies from one to sixteen diopters, "a" to "f" showing the relative size of the retinal area, illuminated, and thus accounting for the especially poor reflex in marked degrees of myopia. Brightness of reflex, however, is not all there is to skiam- etry, for, owing to the optical phenomena called "penumbra," it is sometimes possible to obtain a more defined shadow under Fig. 38. WHY A SHADOW IS DULLER IN MYOPIA THAN IN A LIKE DEGREE OF HYPERMETROPIA. a moderate illumination than it is under one more intense. A reason for this is suggested in Fig. 40. But just what part the numerous pennmhrce play in interfering with the sharp- ness of demarcation of the shadow it is difficult to ascertain, for the emerging light casts its penumbra at the peep-hole of the examiner's mirror just as the entering light does at the pupil. And when it is considered that a round aperture is being dealt with it will be seen that the conditions are even more complex than would ordinarily manifest themselves if the shadow cast was a central one instead of being peripheral. THEORIES REGARDING DULL REFLEXES 17 Figs. 40 and 41 may serve to make this subject better under- stcx)d for, as has been said before, a sharp shadow is a great aid to good work. Fig. 39. RELATIVE SIZE OF RETINAL ILLUMINATION IN HIGH AND LOW DEGREES OF MYOPIA. Fig. 40. THE OPTICAL PRINCIPLES OF PENUMBRA. In Fig. 41 the same principle can be seen as is shown in Fig. 40, 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. The opaque object 78 THEORIES REGARDING DULL REFLEXES "a" interferes with the passage of the horizontal rays of light from the candle "c." The oblique rays, however, pass at dif- ferent angles and give rise to confused shadows as illustrated by "d^" and "d^" In Fig. 42 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 Fig. 41. THE OPTICAL PRINCIPLES OF PENUMBRA DOUBLED. the iris acting as an opaque object. If either a plus or minus lens is placed in front of this eye it merely serves to refract all the Hght in accordance with the lens selected, the penumbra still remaining to add to the indistinctness of the edge of the shadow. If this phenomenon could be overcome it would, seemingly, contribute much toward that skiametric accuracy for which all skiametrists are striving but, as heretofore shown, if a gain be made in one direction a loss is quite sure to follow in another, so that examiners must be content with fuzzy shadows until THEORIES REGARDING DULL REFLEXES 79 some one solves the problem of how to get rid of them with>^ out interfering with the valuable points already secured in dealing with other conditions. Operating at a distance, in order to produce parallelism of the rays, seems at present to be the only means of over- coming this phenomenon, but this, of course, bars out the use Fig. 42. INTERFERENCE OF PENUMBRA IN SHADOW TESTING. of many tests and methods which have been found of great service in uncovering ciliary spasm and latent errors, etc. Thus it will be seen that many contributing factors toward both success and failure enter into the problem of accurately estimat- ing the refraction of an eye "independent of a patient's intelligence." CHAPTER V. Theory of Dynamic Skiametry, and the Importance of Reliable Fixation in Co-ordinate and Independent Observation, with a Reference to Three Essential Myopias, and an Explanation of "Ray Values." THEORY OF DYNAMIC SKIAMETRY. The optics of dynamic skiametry can be considered as being the same as the optics of static skiametry, one being fixation within infinity while the other is fixation at infinity. The word "dynamic" is derived from the Greek word dynamis, and signifies forces not in equiUbrium, or motion as the result of force, being the opposite of "static." Dynamic skiametry then, as might be readily inferred, is an application of Bowman's discovery made under some kind of force. This force is the muscular one which is familiarly known as accommodation. While the static method of practising skiametry is one where the ciliary muscle of an eye is at rest, the dynamic method is the exact reverse of this, and is made Avhile the accommodation is exerting itself sufficiently to readily accept refractive assistance up to a point where its relation with an- other muscular force called "convergence" is interfered with. The principle involved is a simple one. It is well known that a pound weight placed upon the shoulder of a sturdy man creates no appreciable burden or discomfort. But load this same man down to almost the limit of his endurance and then add this pound of additional weight and its presence will be noticed at once. So it is in dynamic skiametry, a call is made for a pronounced increase in tension of the accommodation THEORY OF DYNAMIC SKIAMETRY 8l by having the patient read a series of test letters placed either on an examiner's brow, attached to his skiascope, or to a fixa- tion stand, then, by varying this tension as judgment teaches and by being able to easily supply required artificial lens power, the accommodation is reduced to its normal relationship with convergence. And most eyes, no matter what the age of the patient may be, will only surrender the accommodative excess which has been required to maintain near-vision. This excess will therefore be composed of what has formerly been called by the name of "spasm," or "latent" hypermetropia. The relationship between accommodation and convergence, ■ if roughly stated, is found to be in about the proportion of one to three for the two eyes ; that is* when the accommodation is exerted to the extent of one diopter, the convergence required, according to Hartridge, is a little over one and one-half de- grees for each eye. This, of course, is subject to variation, for when the accommodation equals ten diopters the conver- gence is estimated to be slightly over eighteen degrees for each eye. Another way of viewing this is to consider that for every diopter of accommodation exerted there is a corresponding meter angle of convergence called for, and a meter angle is described as the angle created by an imaginary line drawn from an eye to a point forty inches away, where it crosses another line called the "median," which is drawn from midway be- tween the two eyes. Two meter angles converging at twenty inches away would then be in harmony with two diopters of accommodation, and so on. To prove that the relationship between the two factors governing ocular adjustment is reliable, and that eyes under normal conditions do not converge without accommodating, nor accommodate without converging, the fol- lowing simple experiment can be made: Take a person under forty years of age with known emmetropia and place him so that his eyes are exactly forty inches away from the smallest 82 THEORY OF DYNAMIC SKIAMETRY test type which he can see to read at this distance, then while he is engaged in the act of reading add a half-diopter convex lens quantity to both eyes, and it will be noted that the print will be perceptibly blurred or dimmed. In making this experi- ment be absolutely sure that there is no latent error present, or the test will prove unreliable. Now this seems to show that the relationship between these two forces is a quantity adjusted finely enough to be relied upon, for without the control of the convergence the accom- modation would have immediately relaxed when the lens was added which took away the necessity for the amount of mus- cular exertion represented by the fraction of one diopter which the lens called for. An understanding of this point is very essential, as it repre- sents the basic principle upon which the dynamic method is founded. It is well known that both accommodation and con- vergence are elastic quantities, as an emmetrope with normal accommodation can wear minus lenses up to the limit of his amplitude without producing diplopia, also that the adduction and abduction of orthophoric eyes run into large figures before the doubling of an object occurs. Hence it will be seen that the relationship between these two forces is hable to vary in accordance with the training received by the muscles controlling them. And these, in turn, by the innervation of the nerves that transmit the vitalizing impulses. Two elastic bands linked together, in which the strength of one may be double that of the other, will find a point of balance, or equality of resistance, if stretched to any degree whatsoever, therefore when it is borne in mind that accommodation and convergence always have an individual balance for every visual point for which they co-operate it can be understood why axed rules for adapting prisms must prove unreliable. A case in point may emphasize this : A person born with two diopters of hypermetropia is compelled to suppress six THEORY OF DYNAMIC SKIAMETRY 83 degrees of convergence while looking at a distance, this becomes a sort of habit which, owing to the greater development of the ciliary muscles through more than ordinary use, requires a new standard of relationship to be maintained that is adapted to this special condition. It might be reasoned that there are two ways of correcting such a case as this, one by relieving the tension on the intrinsic muscles with lenses, and the other by adapting prisms for the aid of the extrinsic ones. In some cases perhaps both forms of relief are indicated, but in most cases the wise procedure is to correct the refractive defect and then, by time and develop- ment, train the weakened extrinsic muscles up to an approx- imate standard of "i to 3" relationship with the intrinsic ones. Most eyes have some kind of a refractive error, or else they are so placed in the head that their relative position as to pupillary distance is at fault, and for this reason there is a sort of individual equation in almost every case which calls for special judgment. And it is this individuality that causes one patient to fix upon sixteen inches as a comfortable reading point, while another person, of the same age, will select a twelve-inch point. It has been stated that accommodation is captain of the visual ship, but captains have other persons to boss them, the same as fleas are said to have "other fleas to bite them," etc. So convergence might be called first mate, as its influence is undoubtedly felt in the compromise that must occur for all binocular visual fixation points. If this was not the case then emmeiropes would read with comfort with spheric lenses of plus three diopters at thirteen inches, no matter how young or old the person was. The facts are, however, that an cmmetrope with abundant amplitude of accommodation resents even a quarter of a diopter of artificial lens aid when reading at any near point, and the reason therefor is because the natural locking point, if it can be called such, between 84 THEORY OF DYNAMIC SKIAMETRY accommodation and convergence has been interfered with. This natural locking point, or relationship, constitutes the foundation upon which dynamic skiametry rests, and it ca n be. relied upon to automatically adjust itself to the proper refractive quantity for any given distance — habit, general health and muscle inner- vation considered. In the optical principles of static skiametry, as given in the previous chapter, it was required that the rays of light ema- nating from the retina of a patient's eye have a conjugate focus or, in other words, that true or false myopia be present in Fig. 43. Myofiic AD Skiascope TRUE MYOPIA (STATIC METHOD). order to measure it, and thus to secure data upon which other calculations can be based. Fig. 43 shows an eye with true myopia of one diopter, hence emerging rays of light come to a natural focus forty inches away. False myopia can be of two kinds, artificial and accommo-^ dative. Artificial myopia is produced by using a plus spheric trial lens, and accommodative myopia is temporarily acquired by having the patient look at a fixation object. By the static method it is required that a one-diopter trial lens be employed when the working distance is forty inches away, so that artificial myopia may be produced and the THEORY OF DYNAMIC SKIAMETRY 85 emerging rays from an eye made to focus, or cross one another, at the focal length of the lens used. This is correct for this method, but for the dynamic method no trial lens is required to produce this false myopia^ as the crystalline lens of the eye under examination takes its place. Fig, 44 will perhaps make this plainer, for in the drawing the false myopia under the static method is created by means of a trial lens, the eye being emmetropic and the rays emerging parallel before being refracted by the plus i. D. trial lens. Fig. 44. i <5hias'cop& ARTIFICIAL MYOPIA (STATIC METHOD). Under the dynamic method, Fig. 45, the false myopia is created by means of an increase in the convexity of the crystalline lens, better known by the term accommodation, or "punctum accommo datum," as Burnett calls it, the vision being fixed on some nearby object. This eye, the same as in Fig. 44, is emmetropic, thus making the two working conditions practically equal. By the static method, if the test is to be made at ten inches, a four-diopter convex lens must be used. By the dynamic method this four-diopter convex lens power can virtually be added to the crystalline lens of the eye under examination by advancing the fixation card of the examiner up to a distance 86 THEORY OF DYNAMIC SKIAMETRY of ten inches from the eyes of the patient, thereby causing the accommodation to be exerted to this degree in order that the letters on the card may be distinctly read ; this effort, or the result of it, is why it is termed accommodative myopia, as dis- tinguished from the lenticular or artificial kind produced by the trial lens. To illustrate the value of this method, and also to show its practical adaptation, a case will be considered whose error of refraction is two diopters of hypermetropia, one diopter of which is manifest, and one diopter latent, or in a condition Fig. 45. /Accommodation / D. <3kia3cofte. ACCOMMODATIVE MYOPIA (DYNAMIC METHOD). somewhat spasmodic. In examining this eye at a distance of forty inches, the patient looking at some object twenty or more feet away, it is found that the static method shows one diopter of hyperopia, as it takes a two-diopter convex lens to produce a reversal of the shadow at this distance, one diopter of which represents the artificial myopia, or the working refraction re- quired. The dynamic method being used in this case, it is dis- covered that when the patient looks at the fixation card, forty inches away, a convex lens of a diopter and a half can be added before a reversal of the shadow is obtained, the examiner then advances so as to make the test at a distance of thirteen inches and finds that two diopters can be added before reversal takes THEORY OF DYNAMIC SKIAMETRY 8/ place. Advancing to within ten inches of the patient's eyes makes no change. Withdrawing to forty inches again, it is found that very Httle aheration in appearance of the shadow has occurred unless the patient has looked away in the mean- time, when the spasm will most likely reassert itself. Now what has taken place? The accommodation called for by the dynamic method at forty inches was one diopter. The patient having two diopters of hypermetropia had, there- fore, to make a total accommodative effort of three diopters, in order to see the letters on the fixation card. The examiner supplies refractive assistance until one diopter and a half of convex lens quantity has been added, the accommodation relax- ing to this degree and the shadow showing a reverse movement. Perhaps this case is one where the age of the patient is less than twenty years, general health considered good, and muscle tension, or unconscious habit of exertion, is suspected of being vigorous. A new test is made at a distance of thirteen inches where the total accommodation called for is five diopters, of which two represent the hyperopia and three the amount called for in emmetropia at this distance. Under this burden the eye will be found to accept a two-diopter convex lens quantity be- fore reversal occurs. Repeating the test again at ten inches no more relaxation is found, thereby proving the second finding to be correct. To analyze still further, it may be stated that at thirteen inches, where an emmetrope uses three diopters of accommo- dation, nine degrees of convergence are called for. A patient, therefore, who is making five diopters of accommodative effort, ought, correspondingly, to make fifteen degrees of convergence, thus calling for a distance of eight inches. So, as before stated, while both accommodation and con- vergence seem somewhat elastic they, nevertheless, appear to have a tendency to attain a standard co-ordination when dis- turbing factors are removed. 88 THEORY OF DYNAMIC SKIAMETRY Fig. 46 is intended to illustrate the mechanical action of a spasm of accommodation equal to a half-diopter. The line "a" shows the position of the lens when the accommodation is at rest. The line "b" shows its position when the accommoda- tion is equal to a half-diopter of involuntary effort, called "spasm," and the dotted line "c" shows when the accommodation is equal to one diopter. Fig. 46. HOW THE ACCOMMODATION CAN ABSORB A CILIARY SPASM. If the Spasm, or unconscious muscle effort, equals a half- diopter, and the patient be required to look at an object which calls for one diopter of accommodation, then the conscious effort will, of course, be equal to the difference, or one half-diopter more. It can thus be seen that a spasm may be gotten rid of by what might be fittingly termed absorption. In applying this method to cases where the spasm is likely to be greater than one diopter, such as in children, or in those cases when heterophoria is present, the test should be made at a shorter distance than forty inches. Twenty inches, for instance, calling for two diopters of accommodation, ten inches for four diopters, and so on. THEORY OF DYNAMIC SKIAMETRY 89 Another view for the elimination of spasm by the dynamic method can be taken : A man is told that be owes one dol- lar which he is desired to pay. He consults his books and finds that he has already paid fifty cents of this amount. Then all that can be righteously collected of him is the difference, or the remaining fifty cents. Now this same man consults an optome- trist, and is required to exert his accommodation equal to one diopter, but having a spasm of a half-diopter — and ocular spasms, as it is known, are really unconscious muscular efforts • — he has, therefore, unknowingly contributed a half-diopter of accommodation which constitutes one-half of the amount re- quested of him, so he thus needs to consciously add a half- diopter more and the request is fully complied with. If his obligation, or error, had been higher the request would have had to be greater, but the principle is the same, and so when it comes to actual tests made by this method it will be found that the theories here expounded will be borne out in practice. Spasms of accommodation are now generally classified under two heads — the toniCj or persistent muscle effort, which is often called latent hypermctropia, and the clonic, or inter- mittent action of the ciliary, which is frequently met with in ^O the objective estimation of both hypermetropia and myopia, f particularly the latter. Both kinds, however, are factors in ocular skiametry of which cognizance must be taken and methods used whereby their disturbing' influence can be overcome. It is a rule that in persons over forty years of age, ski- ametric findings made by the static method are fairly reliable, but the drawback to the universal use of this one method lies in the inability of an examiner to differentiate his cases and to tell in advance whether he is dealing with a case wherein spasm of accommodation is a factor until after he has used the dynamic method and compared results. Therefore, to save time and avoid error, it is wise for an operator to use the dynamic method 90 RELIABLE FIXATION in^ll cases under fifty years of age; and where the patient is under thirty it is best to use this method at as close a point as from ten to thirteen inches away. RELIABLE FIXATION. This subject, which has a direct bearing upon the question of brightness of the fundus reflex and corresponding definition of the pupillary shadow, relates to the visual angle under which examinations should be made, also to the point at which the patient's accommodation is to be adjusted during the observations. The point of reversal of a shadow is a very difficult one to decide upon with any great degree of exactness, for the reason that at the precise crossing point of the returning rays the shadow is most erratic and very difficult of determination as to its action. And as no lens power can be added to or subtracted from a patient's accommo- dation without interference with this action, it will readily be seen that the examiner's mirror must be operated either inside or outside of a patient's exact point of fixation if the behavior of the shadow is to be accentuated. In other words, if a patient is looking at a point exactly fifty-three inches away, and an examiner makes his observa- tion from a point forty inches distant, the shadow will show a hyperopic movement equal to a quarter of a diopter. On the other hand, if the examiner views this same eye from this same distance of forty inches, the patient, however, being re- quired to look at a card situated thirty-two inches away, the shadow's action will be a myopic one of a quarter of a diopter. Fig. 47 will perhaps make this plainer, for here it will be seen that the adjustment of the patient's accommodation was first made for a distance equal to three-quarters of a diopter, and then for a distance equal to a diopter and a quarter, the observer being at a distance of one diopter in both cases, the action of the shadow was, therefore, first with the mirror and then against it, whereas if a test had been made at one diopter the shadow. RELIABLE FIXATION 9 1 of course, would not have shown any motion. Thus it will be seen that the method is not unlike solving the question of the exact middle of a piece of string by simply doubling it, for this is what it practically amounts to. In estimating the behavior of the shadow the advantage gained by an examiner in making his estimate from two or three focal adjustments of his patient's accommodation is, of course, just double or treble that of making it from a single point. Besides, it frequently occurs that an examiner desires to approximately ascertain the amount of any myopia present before he changes his lenses. All he has to do, then, is to Fig. 47. MULTIPLE FIXATION AND OBSERVATION POINTS. advance his mirror nearer to his patient while the latter is still looking at the forty-inch fixation point. If he finds that the shadow does not reverse until the mirror is advanced to within twenty inches of the eye under examination, then he knows that the myopia present is about two diopters, one of which represents the error and the other the accommodation. And if he has to advance up to within ten inches before obtaining a reversal he then knows that the error is nearer to three diopters, and so on. The means for accomplishing these results are very simple indeed. Fig. 48 shows a fixation stand, ana the manner of its construction which, as can be seen, is merely a light-weight nickel-plated brass stand carrying a card having dots upon it 92 INITIAL EXAMINATIONS which the patient is asked to count. The stand being movable, and adjustable as to height, permits of its usefulness in many ways. Fig. 49 shows the style of card this stand can carry, on the reverse side of which is printed a small astigmatic dial, Fig. Fig. 48. AUTHORS FIXATION STAND. 50, which is merely a convenience to be used in other tests not concerned with skiametry. This stand, however, can be made to do excellent duty as a dynamic astigmometer, enabling sub- jective measurements to be taken at any near point desired. INITIAL EXAMINATIONS. In initial skiametric tests the card. Fig. 49, is usually placed about a foot behind an ex- aminer's head, several inches to his right, and high enough for INITIAL EXAMINATIONS 93 its dots to be illuminated by the light source used, Fig. 51, showing the relative position of light, examiner, patient and fixation card. In operation the examiner requests his patient to first look at the dots on the fixation card, fifty-three inches away. In Ftg. 4Q. Count the Dots. FIXATION-STAND TARGET CARD. Fig. 50. 10 80 90 100 u^ REVERSE SIDE OF FIXATION CARD. making his lens changes he stops with the shadow moving slightly with the mirror, then he requests the patient to look at fixation target cards on his skiascope. If he gets a reversal of the shadow he knows that he is very close to a correction for this distance. He then proceeds to finish the test and eliminate 94 INITIAL EXAMINATIONS the presence of any ciliary spasm by moving the mirror and its targets nearer to the patient, and at the same time crowding on all the convex lens quantity possible without producing an against motion. The card on the fixation stand may be used in place of the skiascope card for the shorter focal tests, but it will not be found as convenient. The greatest advantage to be derived from the employment of the fixation stand, however, lies in the angle at which an eye can be viewed, also in the apparent brilliancy and enlargement of the patient's pupil, which is due, no doubt, to a Fig. 51. POSITION FOR INITIAL EXAMINATION. corresponding decrease in light stimulation. But, be that as it may, the use of the fixation stand in most cases is productive of much better initiaj results than accrue when its use is dispensed with. Especially is this true where the examiner is somewhat lacking in the experience which comes only with years of practice. The system of multiple cards, or fixation points, has an added value to an examiner from the fact that it enables him to make the first observation of a patient's eye under the most favorable conditions possible for obtaining a bright fundus reflex. He can then govern himself accordingly in further examination of the case. RAY VALUES 95 If the reiiex is a bright one, the change of vision from stand to skiascope card will not materially affect its brilliancy. On the other hand, if the reflex is poor then further tests should be continued by aid of the stand, which, being light in weight, its position can be easily altered by the examiner. There is another very valuable feature connected with the use of independent -fixation points, and that is in the estimation of weak errors of refraction by instructing a patient to look back and forth from skiascope to fixation stand while the dis- tance of these two from the patient's eye differs slightly. RAY VALUES. The term "ray" is used in preference to the term "wave," simply for purposes of easy explanation and ready comprehension; therefore, from the foregoing, it will be understood how important it is for an examiner to have a thorough knowledge of ray values as well as of lens values. And by "ray values" is meant the strength of the lens that would be required to parallel a ray of light when intercepted at any distance from its source. Fig. 2 in the first chapter emphasizes this principle. The value of a ray ten inches from its source is 4. D. ; at thirteen inches, 3. D. ; at sixteen inches, 2.50 D. ; at twenty inches, 2. D. ; at twenty-six inches, 1.50 D. ; at forty inches, i. D., and so on. In using both static and dynamic skiametry the position of an examiner and the dioptric values of his lenses in their relation to his patient's visual fixation are all factors in the correct estimation of ocular errors. Thus, if a patient is look- ing at an object situated eighty inches distant, and a skiametric examination is made at forty inches away, the ray value at eighty inches may be said to equal a half-diopter, while its value at forty inches is one diopter ; one less a half leaves a half, and the shadow under these conditions will behave as though the error were a half-diopter of hypermetropia. If the patient was looking at an object forty inches away, 96 RAY VALUES and the examiner viewed the eye through the skiascope at a distance of eighty inches, then the reverse would of course be the case, and the error indicated would be a half-diopter of myopia. All of this, however, presupposes that the examiner knows just what his patient's accommodation is doing, and so by having fixation cards situated at forty, fifty-three, eighty, one hundred and sixty, and three hundred and twenty inches away, respectively, it would be possible for an examiner working at one meter to observe the shadow's action in the four quar- ters of a diopter of hypermetropia. This would necessitate the securing of an especially intelligent and obedient patient, to- gether with expert skill on the part of an examiner. Later on it will be seen that other methods can be used, where the intelligence of the patient is not so important a factor. The same principle governing ray values, however, can be applied to shorter distances. Thus, a patient with a one-diopter convex trial lens before his eye, when looking at a fixation card forty inches away, can be measured for a quarter, a half, or three-quarters of a diopter of hypermetropia, if the exam- iner will carefully measure the distances from his patient's eye so as to hold his mirror exactly thirty-two inches away when he is measuring for three-quarters of a diopter, twenty- six inches away when he is measuring for a half, twenty-two inches away when he is measuring for a quarter, and so on, the ray value modifying the lens value and the two combined influencing the total. In fact, in skiametry the determination of ray values can perhaps be summed up by deducting the value of what the ray is actually doing from what it really ought to do under the circumstances. The possibilities in the way of juggling with accomm,oda- tion, lens values, ray values and the shadoiv, are almost limit- less, and it is to be expected that as ocular skiametry and its methods become better known there will be found many more RAY VALUES 97 ways of applying its principles which may not be all fully understood at this time. As the student acquaints himself with the principles involved in ray and lens values, however, he will find that the practical side of his work can be made much easier. The following examples, too, may show the applica- tion of some of the more important points : When an eye looks at an object situated forty inches away it must exert its accom- modation at least one diopter. Place a plus one-diopter spherical lens over this eye and if it is emmetropic the emergent rays will converge at a point twenty inches away. One diopter will then represent the accommodation, and one diopter the trial lens, or artificial myopia, making a total of two. If a plus two- diopter lens is used, the point of convergence will be at thirteen inches; if a plus three-diopter is employed, it will be at ten inches ; the accommodative myopia increasing the total myopia. Now suppose a patient has an error of two diopters of hypermetropia, then what occurs when a three-diopter lens is added? Why, the accommodation under the stress of carrying a burden immediately surrenders its error, read- justs its accommodation and convergence to a relationship of least resistance, and there is left only one diopter of what we call artificial myopia, and one of accommodative myopia. The point of convergence of the emergent rays would then be at twenty inches, instead of at ten inches, which Avould be the point of crossing of the rays from an emmetropic eye under the same conditions of lens and accommodation. Therefore it will be seen that where the rays ought to cross is at ten inches, and where they do cross is at twenty. The difference in ray value being two diopters — the amount of the error. In cases of this kind it is only natural for a student to ask why an eye under these conditions does not surrender more than two diopters, especially when it is exerting its accommo- dation one diopter for fixation. In reply it can be said that, without compulsion, an eye which is making three degrees of 98 RAY VALUES convergence will naturally try to make one diopter of accommo- dation in order to maintain the harmony of the theoretic stand- ard of one to three relationship. This explanation can also be given to account for the discomfort an emmetrope of, say, twenty years of age, experiences when attempting to read at thirteen inches distance with a pair of half-diopter plus spheric lenses on, his accommodation and convergence will not be in accord. The convergence required at thirteen inches is nine degrees, which calls for a co-ordination of three diopters of accommodation, but try to reduce this accommodation by even a quarter-diopter and the harmony will be disturbed, causing discomfort to manifest itself. In true myopia, of one diopter, similar conditions of ray and fixation values are present, one diopter for accommodation at forty inches, and one for the true myopia, make two, the ray value of which is twenty inches. In all pronounced errors of refraction an examiner must ever bear in mind that the relationship of one: to three between accommodation and convergence may have been upset, and a different co-ordi- nation established. Measurements in myopic cases frequently vary under ray and lens value procedure, but while skiametry always gives the refraction exactly as it is under the existing conditions, these conditions may be such as to trouble an examiner in the formation of his judgment. Hence measure- ments taken in different ways are productive of better results. In measuring cases by ray values a student can work out any combination if he is well grounded in these principles. Simple, compound or mixed cases of astigmia are merely to be solved by measurement of meridians, as for instance : An eye having one diopter of hyperopic astigmia, axis vertical, in fixing at forty inches with a plus one-diopter spheric lens before it would show reversal of the shadow in the horizontal meridian at forty inches, and at twenty in the vertical. RAY VALUES 99 In a myopic eye represented by the correcting formula of minus i. D. S. combined with minus i. D. C. axis i8o and fixing at, say, twenty inches, reversal would occur in the vertical meridian at ten inches and in the horizontal at thirteen. The student can therefore see that the whole subject is practi- cally one of figures, where axis and error are always to be con- sidered as at right angles to one another, and that myopia may be classified as of three kinds : true, accommodative and artiUciaL CHAPTER VL Orthophoric and Heterophoric Conditions, and the In- fluence OF Habit Upon Accommodation and Con- vergence, With Special Consideration of Spasms and THE Use of Prisms. ORTHOPHORIA AND HETEROPHORIA. In order to again emphasize the strong points of dynamic skiametry it will be wise to further consider ocular muscle action and note its relation to other factors that are involved in the production of binocular vision, both with and without so-called strain. The perfect balance of the extrinsic muscles, termed ortlw- phoria, is a rare condition, because with orthophoria there must, theoretically, be emmctropia, and experienced examiners know that scarcely one eye in a hundred is truly emmetropic. There- fore the concomitant of ametropia, as will be seen later on, is hetcrophoria, for, owing to the distribution of the ocular nerves, a single impulse would seem to affect both the intrinsic and extrinsic muscles in a co-ordinate manner, unless unconscious habits have been formed whereby one muscle action can be suppressed while another is exerted, even though the nerve supply is the same. Now this involves the question : What is a muscle and why does it act? So, without making the subject too lengthy, the dictionary may be briefly quoted as stating that a muscle is an organ composed of contractile fibres, through the contraction of ■ivJiich bodily movement is effected. These muscles are classified as voluntary, involuntary and mixed, the voluntary being subject to the will, the involuntary acting inde- pendent of the will, and the mixed combining in some degree the functions of both the others. INFLUENCE OF HABIT lOI All muscles are made to contract by means of nerves, and a nerve is described as a cord-like structure composed of delicate filaments through which sensations, or stimulatioti im- pulses, are transmitted to and from the brain. These impulses being an expression of what is called nervous energy, which represents the active principle of organic life, this nerve action being dependent for its existence upon warmth, food and the orderly performance of the functions of the body. It will be seen, then, that this question is much like the ancient's descrip- tion of wisdom, which was represented as a serpent with its tail in its mouth, for the reasoning resembles a circle, no be- ginning or ending, since the nervous energy seems to reproduce itself. In other words, it takes health to make nervous energy, and nervous energy to make health. INFLUENCE OF HABIT. As before referred to, ocular habits of adjustment are factors to be reckoned with in the adaptation of glasses, for it is not difficult to understand that if an eye has been exerting its accommodation for many years in overcoming an error, like hypermetropia, the convergence has had to form special habits, too, in order to harmonize with the accommodation. Then when glasses are given for the cor- rection of the hypermetropia it follows that convergence must be re-educated and adapted to the new relationship. Prof. William James says that "habit has a physical basis" and, that "the moment that one tries to define what habit is, one is led to the fundamental properties of matter." He further says that the phenomena of habit in living beings are due to the plasticity of the organic materials of which their bodies are composed, and that the philosophy of habit at bottom is a physical principle and therefore belongs to physics rather than to physiology or psychology. Leon Dumont says : "Every one knows how a garment, after having been worn a certain time, clings to the shape of I02 INFLUENCE OF HABIT the body better than when it was new, and so in the nervous system the impressions fashion for themselves appropriate brain paths." Muscle habits are really brain habits, and of habits in gen- eral Professor James can be quoted still further, he says : "Habit is what keeps us all within the bounds of ordinance, and saves the children of fortune from the envious uprisings of the poor. It alone prevents the hardest and most repulsive walks of life from being deserted by those brought up to tread therein. It keeps the fisherman and the deck-hand at sea through the winter; it holds the miner in his darkness, and nails the country- man to his log-cabin and his lonely farm through all the months of snow ; it protects us from invasion by the natives of the desert and the frozen zone. It dooms us all to fight out the battle of life upon the lines of our nurture or our early choice, and to make the best of a pursuit that disagrees, because there is no other for which we are fitted, and it is too late to begin again. It keeps different social strata from mixing. Already at the age of twenty-five you see the professional mannerism settling down on the young commercial traveler, on the young doctor, on the young minister, on the young counsellor-at-law. You see the little lines of cleavage running through the char- acter, the tricks of thought, the prejudices, the ways of the 'shop,' in a word, from which the man can by-and-by no more escape than his coat sleeve can suddenly fall into a new set of folds. On the whole, it is best he should not escape. It is well for the world that in most of us, by the age of thirty, the character has set like plaster, and will never soften again. "If the period between twenty and thirty is the critical one in the formation of intellectual and professional habits, the period below twenty is more important still for the fixing of personal habits, properly so called, such as vocalization and pronuncia- tion, gesture, motion, and address. Hardly ever is a language learned after twenty spoken without a foreign accent; hardly INFLUENCE OF HABIT IO3 ever can a youth transferred to the society of his betters un- learn the nasality and other vices of speech bred in him by the associations of his growing years. Hardly ever, indeed, no matter how much money there be in his pocket, can he even learn to dress like a gentleman-born. The merchants offer their wares as eagerly to him as to the veriest 'swell,' but he simply cannot buy the right things. An invisible law, as strong as gravitation, keeps him within his orbit, arrayed this year as he was the last; and how his better-clad acquaintances contrive to get the things they wear will be for him a mystery till his dying day. "The great thing, then, in all education, is to make our nervous system our ally instead of our enemy. It is to fund and capitalize our acquisitions, and live at ease upon the inter- est of the fund. For this we must make automatic and ha- bitual, as early as possible, as many useful actions as we can, and guard against the growing into ways that are Ukely to be disadvantageous to us, as we should guard against the plague. The more of the details of our daily life we can hand over to the effordess custody of automatism, the more our higher powers of mind will be set free for their own proper work." The influence of habit upon our ocular functions is very strong indeed, as in most cases we are able through muscular effort to adapt ourselves to our visual necessities, even though we unconsciously pay a price in what is called "nervous strain." A hypermetrope in accommodating for his error is not unlike a man in walking with his feet turned outward at too great an angle, for, while both are possible for a period to time, yet con- tinued effort will eventually be productive of bad results. Habits of seeing under proper refractive and muscular adjust- ments should be acquired by all at as early an age as will prove practicable for a person to wear glasses. Figs. 52, 54 and 56 are designed to diagrammatically illus- trate the relationship between accommodation and convergence 104 INFLUENCE OF HABIT in emmetropia, hypermetropia and myopia, while Figs. 53, 55 and 57 may give some idea of the nervous adjustment required to meet the ocular needs, from the standpoint of innervation and enervation. Fig. 52 is intended to show, by means of converging lines, the normal balance in an emmetropic eye between the muscles governing accommodation and those controlling convergence. Here it will be seen that the harmony between these two factors necessary to binocular vision is in accord, and that the Fig. 52. £MMET RO PI/\ o.s. ■ ^" ■ BALANCING THE ACCOMMODATION AND CONVERGENCE IN EMMETROPIA. lines representing accommodation and convergence meet at a common point on another imaginary line called the median, which extends forward from midway between the two eyes, the innervation of course being directed toward the internal rectus and the ciliary muscles of each eye. ^^S- 53 is designed to show a pair of thermometer-like tubes which, for convenience, might be styled a neuromcter, these tubes being filled with an imaginary vital fluid repre- senting the nervous energy required to maintain the action of accommodation and convergence. The darts are to call atten- INFLUENCE OF HABIT 105 tion to the height of the fluid in the two columns, and to show the relative amount of nervous expenditure necessary to main- tain requisite muscle effort. Thus in emmetrppia the darts seem to be equal and indicate that whatever the proportionate relationship really is, it remains a quantity that can be con- sidered as constant. In Fig. 54 may be seen the disturbed relation between accommodation and convergence which would take place in hypermetropia if the innervation was normal, as shown in Fig. Fig. 53. U EQUAL INNERVATION NECESSARY TO BALANCE ACCOMMODATION AND CONVERGENCE IN EMMETROPIA. 53, Thus esophoria and the origin of convergent strabismus can be surmised. In Fig. 53 the darts, "A" and "C," show that binocular vis- ion would be impossible under these conditions, and so individ- uals having eyes of this kind must learn to either decrease convergence or increase accommodation in an independent manner. This relative effort is shown by the position of the darts in the ncuronietcr, Fig. 55, the nervous impulse here being much greater for accommodation. In Fig. 56 the lack of harmony between darts "A" and "C" indicates that in myopia accommodation must be decreased or convergence increased before the two can be brought together. io6 INFLUENCE OF HABIT Fig. 57 shows a relative adjustment of innervation to which nature undoubtedly resorts in cases of this kind. With an understanding of the disturbance in innervation which a hypermetrope or myope has to make in order to con- FiG. 54. _ HY P£RM£TROP/A o.s. . O.D.[ IMBALANCE OF ACCOMMODATION AND CONVERGENCE IN HYPER- METROPIA. Fig. 55. UNEQUAL INNERVATION REQUIRED TO BALANCE ACCOMMODATION AND CONVERGENCE IN HYFERMETROPIA. stantly maintain binocular vision, it is quite easy to see how readily an individual can form habits of muscular action con- trolling accommodation and convergence which would prove most difficult of reformation. And this leads the reasoner to INFLUENCE OF HABIT 107 a general consideration of so-called "spasms," "latent errors," "muscular insuMciencies," etc., etc. It also serves to emphasize the importance of keeping errors of refraction under constant correction, thereby enabling indi- FiG. 56. o.s." ^^^ 0.0 imbalance of accommodation and convergence in myopia. Fig. 57. UNEQUAL INNERVATION REQUIRED TO BALANCE ACCOMMODATION AND CONVERGENCE IN MYOPIA. viduals to form new ocular muscle habits which will be in accord with standard visual requirements. Then, too, it will show the wisdom of postponing the adaptation of prisms until after old habits have been broken up by altering the conditions responsible for them. I08 CILIARY SPASMS CILIARY SPASMS, ETC. In arguing against the use of cycloplegics some optometrists are inclined to question the frequency of spasms and latent errors and to contend that their importance is overestimated, but familiarity with static and dynamic skiametry will soon prove the contrary. Spasms are defined as : "Inv oluntary convulsive contraction of mu scles^ and "convulsion," in turn, is described as '^irregular and violent commotion." Now, this latter definition might perhaps be modi- fied a little and thereby convey a somewhat better understand- ing of the term spasm, as it is used here. Owing to the contractile quality of muscles, they can only do one thing, and that is to pull, not push. Therefore, when the term "violent commotion" is used, it is immediately asso- ciated with the idea of a rapid and intense muscular activity, which resembles the seizures of epilepsy, and wrong impressions are apt to be formed. As before stated, spasms can be of two kinds — the steady and the vacillating. So, where the word tonic is used in con- nection with them, they are to be associated with the idea of an unconscious, involuntary, regular, steady and persistent muscle tension. Where the word clonic is used, however, it should be under- stood to mean unconscious, involuntary, irregular, contracting and relaxing muscle action, which, when taken in connection with the so-called accommodation of an eye, may be either rapid or slow in its operation. The term tonic spasm of accommodation, as understood to-day, bears a close resemblance to what many readers of Bonders are led to infer from his use of the term "latent" in connection with hypermetropia, for the word "tonus," from which the word "tonic" is derived, signifies an involuntary condition which might be likened to increased vigor or tone in a muscle whereby it may insist on doing more work than is really intended for it. This, of course, is alike applicable ^ CILIARY SPASMS IO9 to myopic and emmetropic eyes, though not occurring with the same frequency as in hypermetropic ones. The cause of these spasms is also a varying one. In hyper- metropia the constant muscle tension required in order to maintain vision is no doubt a pronounced factor. In myopia and in emmetropia an uncontrollable supply of nervous energy actuating the muscles is probably one reason for their invol- untary contraction. Occupation, too, which leads to the for- mation of muscle habits, is undoubtedly another cause, and so quite a number of reasons might be suggested. The work of the optometrist, however, deals only with the determination of their presence and the estimation of their strength or influ- ence on the refraction of an eye. Reference has been made to the use of a local toxicant,' technically termed a "cycloplegic," which is employed to par- alyze the muscle action of an eye, but the drawbacks attendant upon the use of this means are so many, both from a practical as well as a theoretical standpoint, that some method had to be devised in order to eliminate the guess work which must necessarily enter into all cases where the exact refraction of an eye at its reading or working point is not known. And by "exact refraction" is meant the muscle tonus, or muscle inner- vation, which constitutes an individual factor in each case. By subjective methods, if the patient is of sufficient intelli- gence and shows the proper amount of interest in the case, a cross-examination in connection with other tests will enable an examiner to form a fairly satisfactory idea of the range of accommodation, and from this known range he can make a close estimate as to the strength of the glasses required. But by the method known as dynamic skiametry a more certain estimate can frequently be arrived at^ for by this examination the burden of judgment is shifted from the shoulders of the no CILIARY SPASMS patient to those of the examiner who, as his experience in- creases, should be a better judge of his patient's needs than the patient himself. In the dynamic method attention has been called to the fact that where the muscles controlling accommodation were heavily taxed it was easy to see how readily they might be induced to surrender any excess of energy which they had hitherto concealed, no matter whether this concealment was total, in the form of a tonic muscle effort, or intermittent in the form of a clonic one. Attention will be invited in another chapter to the advan- tages to be derived from the use of mobile, rather than of fixed, lens values, and this may serve to make this point plainer, for spasms of accommodation, especially of the clonic variety, require a lens action which will in a measure imitate the in- crease and decrease in convexity of the crystalline lens of an eye itself. To deal successfully with spasms, an examiner must, therefore, use some means which will enable him to make his lens changes in a gradual, rapid and accurate manner, and at the same time operate at any desired distance from his patient. x^ (Then, too, if habits are to be broken up, the patient must ^ be required to make some refractive adjustment that is unusual, J and differing from the manner in which his eyes have been >/ generally employed. X Many persons with uncorrected hypermetropia use their eyes as little as possible for reading or near work. Their muscle adjustment for distance, however, is almost a constant quantity during their waking hours. If such a person is young, and the ocular muscle action is vigorous, a static test, without cycloplegia, made at twenty feet is very likely to un- cover only a portion of the error, for the reason that the CILIARY SPASMS III accommodation muscles have been so in the habit of exerting themselves at this distance that when lenses are supplied which remove the necessity for this exertion these muscles seem to refuse much of the assistance offered them. On the other hand, if this case is examined skiascopically at a distance nearer than that of the usual reading point, it will be observed that the eye will now accept lenses very much stronger, for here habit is not such a factor, and under the burden of increased muscle effort the accommodation will readily give up the excess tension which it was so accustomed to exert for distant uses. Thus, habit being temporarily broken, the muscles governing accommodation and convergence are more likely to assume standard relations, and especially so if a lens system is used which permits of binocular mobile action. In myopia, particularly in those cases which have remained uncorrected for a number of years, the accommodation is likely to prove disappointing in its action by behaving either spasmodically or quite the reverse. For this reason greater care must be exercised, when measuring these cases, to employ corroborative methods. A myopic case can seemingly be readily changed into a hypermetropic one by over-correction of the error, but muscle habits formed through ordinary occu- pations cannot be changed in a minute, and this fact must ever control the judgment of an examiner and cause him to adopt such methods and measures as will give him the fullest informa- tion regarding the requirements of each and every case that presents itself. Of the two forces, accommodation and convergence, the former is probably the controlling one. So-called "muscular insufficiencies" are, therefore, doubtless the result of inco- ordination between the ciliary and the recti groups. If the theories as illustrated by Figs. 52 to 57, inclusive, are correct ones, then the fallacy of adapting prisms for the purpose of permanently assisting the muscles controlling convergence is 112 CILIARY SPASMS made plain for many cases, for where the error of refraction is not fully corrected it is logical to expect that the muscles controlling convergence will not behave in accordance with standards which are applicable only in emmetropia. Where the recti muscles are mal-attached to the eyeball, or where there is a permanent deficiency in the amount of their innervation, then prisms would seem to be indicated, but experience teaches that these cases are rare. It is wise, then, for an examiner to correct his patient's refraction first and wait for the muscles governing accommodation and convergence to adapt themselves to the new order of things. If they fail to do this, and the examiner is sure that the error of refraction is fully corrected, then, and only then, is he warranted in the use of prismas, unless for the accomplishment of some tempo- rary result. While adduction, abduction and sursumduction offer import- ant data by which to aid an examiner's judgment, they are usually of little avail until after emmetropia has been estab- lished and maintained long enough for accommodation, con- vergence and innervation to have ordinarily acquired a habit indicative of normal muscle balance. To make these points plain it can be seen that if the rela- tive proportion of three degrees of convergence, or one meter angle, to one diopter of accommodation is taken as a standard of estimation, then eyes having one diopter of hypermetropia in reading at a distance of sixteen inches away, which calls for two and a half diopters more, must make an accommodative effort equal to three and one-half diopters in all. This ciliary action would call for a harmonious conver- gence equal to about ten degrees, and ten degrees, in turn, would call for a reading distance of eleven inches away, thus showing that while the eyes were accommodating for sixteen inches they would really have to converge to a point five inches CILIARY SPASMS II3 nearer in order to use a standard or proportionate amount of nervous energy ; and as this would be quite impossible without disturbing binocular vision, hence the only other course left is for the innervation of the convergent muscles to be correspond- ingly decreased. As emmetropic eyes cannot alter their co-ordinate muscle action without previous training, it would seem to be reason- able to assume that ametropic eyes ought also to require an equal time and training in order to enable them to convert their latent errors, or those due to habits of suppression, into manifest ones, or those within the individual's own control. To what extent habit interferes with the accuracy of static skiametry must be left to the imagination of those examiners who have not yet had opportunities of personal observation. And while dynamic skiametry offers a marvelous improvement in estimating the true refractive errors of young persons, it, too, is frequently interfered with by this same disturbing factor of habit, or "individual equation," as it is sometimes called. It must not be overlooked, however, that, even after latent errors have been determined, and spasms unlocked, it is not always wise to endeavor to force eye muscles to accept standard relationship in too short a period of time, for while it is true that usually the best results are obtained after emmetropia and orthophoria have been established, yet it is equally true that, in some cases, it is better to ignore a small spasm and permit the muscles to work, for a while, in excess of normal re- quirements. This, therefore, indicates that optometric findings are one thing and the judgment used in adapting glasses and prisms is quite another. For with the experienced examiner it often happens that in youthful cases he may find an error of perhaps four diopters of hypermetropia by the dynamic method, and yet correct only two diopters of this in the first glasses given, taking several months of time in order to educate the eye 114 CILIARY SPASMS muscles up to a normal standard where they can bear a full correction of their refractive errors. The making of the original examination in a thorough and reliable manner will thus be seen to greatly assist an examiner in prognosticating his cases, as well as in giving advice the correctness of which will be borne out in due course of time. There is an old adage which says that "it takes a long time to teach old dogs neiv tricks," and the reason for this lies, perhaps, in the fact that before new habits, or ^'tricks," can be formed the old ones must be broken up, and this in some cases may be more of a task than even the formation of the new. Spasmodic muscle action, while unimportant in some cases, is quite the contrary in others, so that the wise examiner will take no chances, but proceed to thoroughly master all of his cases. Any unsatisfactory results that may occur can then be at- tributed more to faulty judgment than to a lack of knowledge of the optical and correlated intrinsic and extrinsic muscle action involved. CHAPTER VII. Practice of Dynamic Skiametry, Its Use in Measuring Regular and Irregular Astigmia, and Its Special Value in the Objective Estimation of Presbyopia AND Sub-Normal Accommodation, Together With Its Relationship to Other Methods and Tests. PRACTICE OF DYNAMIC SKIAMETRY. It is un- questionably true that no other one instrument in the arma- mentarium of an optometrist requires more practice to success- fully master than does the skiascope, and, it might be added that the dynamic method requires much greater skill in order to read the shadow's action than does that of the static. The reason for this is to be found in a consideration of the subject of retinal illumination, for it is well known that the shorter the distance between the light source and the patient's eye the larger and brighter will be the area of illumination on the fundus of the eye under examination. This, taken in connection with the optical laws regulating penumbra, and the variation in retinal pigmentation, together with size of ocular pupils, added to light stimulation, will show why the nearer a shadow test is made the more difficult becomes the reading. If it was not for the erratic relationship that frequently exists between accommodation and convergence, and for the irregularities existing in corneal curvatures, then the static method would be the best to use, and the greater the distance it was employed the better the results would be. But ski- atnetry, like many other systems, has to be taken as it is, and not as the operator or patient would like to have it. Practice, plenty of practice, the same as one becomes expert Il6 TRACTICE OF DYNAMIC SKIAMETRY in shooting a gun, or in making examinations by indirect ophthalmoscopy is the only way in which an examiner can become skillful in skiametry. The general variation in appear- ance of eyes is such that a comprehensive description of all kinds is almost impossible, the student must, therefore, see for himself. But to put into words a description of the procedure necessary in making an examination by the dynamic method the following may give a better understanding. In order to secure as favorable a fundus reflex as possible, place the Hxatioti-stand target card forty inches from the pa- tient's eyes and then make the first skiascopic observation from a point about two or three inches to the left of this card and an inch or so nearer the patient. If motion against is not present then add plus spheric lenses until motion against occurs in some meridian. The strength of the lens next zveaker than the one that just causes the against motion is the one that represents the error for this distance in the meridian measured. If, on the other hand, at the first observation an against motion is noted, then, with the patient still looking at the fixation-stand card, the examiner is to advance toward the patient until a zvith motion is obtained in some one meridian. This distance, estimated in diopters, less the i, D. of accommo- dative myopia, due to the forty-inch fixation, will represent the true myopia for the meridian measured, and if astigmia is present all the meridians will not measure alike. After this first estimation of the error, or errors, the exam- iner is to place the estimated correcting lenses in position on the patient's face and make a subjective test, after which he is to return to skiametry again and make another examination at a reading or much nearer point than the first one was made, using the fixation cards on his bracket skiascope. The changing back and forth from objective to subjective and from subjective to objective will be seen to give all the refractive data that it is possible to obtain, after which the use of other instruments and MEASURING ASTIGMIA BY SKIAMETRY II7 methods may, or may not, be indicated, as their necessity is usually determined by the previous findings. Two typical cases may possibly make these points plainer: Case i. Age of patient, thirty years. Fixation, forty inches. Observation, thirty-nine inches. Shadow ivitli the mirror. Can add plus I. D. before reversal occurs. Subjective test also shows I. D. of hyperopia. But with fixation and observation at sixteen inches, a total of plus 1.50 D. can be added before reversal takes place. Same result is obtained when fixation and observation are at thirteen inches. Case 2. Age of patient, thirty years. Fixation, forty inches. Observation, thirty-nine inches. Shadow shows against the mirror. Reversal takes place at twenty inches which, after deducting i. D. for the accommodative myopia at forty inches, leaves i. D. of true myopia. Subjective test calls for a minus 1.50 D. S. lens. Fixation and observation at sixteen inches show a zvith mo- tion with a minus 0.75 D. S. lens, and at thirteen inches a minus 0.50 D. S. lens is the weakest one a ■with motion can be obtained with. The student must not forget, however, that data and prescription are not always one and the same. MEASURING REGULAR AND IRREGULAR AS- TIGMIA BY SKIAMETRY. " The variation in the appear- ance of astigmatic eyes is often baffling to the skiametrist. Some shadows show an even edge that makes their measure- ment most easy, while others scatter and re-form several times in crossing over a pupil. The cause for this can perhaps be found in a consideration of scar tissue, for where reliable his- tory of many cases of this kind has been obtained it is found that in childhood the patient suffered from inflamed eyes, and the sequela was the irregularly thickened cortical tissue which manifests itself in unequal refraction. A study of these cases with the skiascope will often enable an examiner to cope with those which at first seem almost hope- Il8 MEASURING PRESBYOPIA BY SKIAMETRY less, and this rule holds good with all classes of skiametric cases, no matter what method is used. The skiametrist must simply perfect his skill by ample practice. When the conditions are ideal an examiner will be able to note a sort of straight edge action to the shadow. If the error is marked then this edge will be well defined, but if the error is slight then the edge will be difficult to note. In those cases where what is called the "scissors" movement appears an examiner will observe two shadows in one meridian, one with the mirror and the other against. This is frequently caused by making the observation too far to one side of the line of fixation, especially where the correcting lenses used are of high power. The generally accepted theory of the cause of the "scis- sors" movement attributes it to the crystalline lens being slightly askew in its capsule, but if an examiner will keep the lines of observation and fixation as near together as it is possible and obtain a good fundus reflex he will find that true crystalline displacements are quite rare. In partially opaque corneas, and in cases of cortical cataract where transparency is interfered with, an examiner will often be rewarded through results obtained by careful work with his skiascope. Cataracts in process of development can often be detected, too, long before the vision of the patient is per- ceptibly impaired. The spicula in the crystalline showing trans- lucent upon the red background of the pupil, and especially do they appear plain when mechanical mydriasis is made use of. MEASURING PRESBYOPIA BY SKIAMETRY. Presbyopia is usually regarded as an inilrmity, rather than as an error of refraction, but it can also be considered in the latter light, too, for research proves that the crystalline changes begin almost at birth and continue all through life. Loss of elasticity due to changes in density and to production of an altered MEASURING PRESBYOPIA BY SKIAMETRY 1 19 index surely indicate refractive variations, and as the only dif- ference between lenses of like kind is variation in ray-bending power, therefore presbyopia would seem to rightfully belong under the classification of refractive anomalies, just as much as under accommodative anomalies. Presbyopia is the one, so-called, "easy" ocular condition that is often the most difficult of satisfactory correction, for the reason that occupation, illumination, habit, pupillary distance and innervation, or bodily vigor, are all factors to be reckoned with. Then, if combined with this the ignorance and stupidity of many patients in answering questions is taken into consid- eration, it is easy to see why changes in reading glasses are so frequent when "there's nothing the matter with the eyes." Up to the time of the development of the dynamic method of practising skiametry there was no known method of estimat- ing presbyopia in an objective manner. All static methods, whether with or without cycloplegia, are solely for determining the refractive condition of an eye while its muscles are in a state of complete relaxation, therefore the static method gives no definite aid in presbyopia whatsoever. Dynamic skiametry supplies the refractionist with a method that often proves of the very greatest aid in mastering a troublesome case, as it enables the eyes to be studied at all points, near as well as distant, this study being directed toward steadiness of convergence and accommodation, both of which can be detected through the use of a skiascope and lenses. As illustrative of this, suppose a case presents itself having a history of discomfort in reading, etc., the patient is directed to look at the skiascope Hxation-card fourteen inches away while plus one-diopter spheric lenses are before each eye. If right eye shows a with motion while left eye is against, it indicates either an error of refraction or an unequal innervation of the muscles controlling accommodation, notwithstanding that at infinity both error and vision seemed alike in the two eyes. The 120 SUB-NORMAL ACCOMMODATION use of the mirror at this distance also enables the detection of any deviation in convergence of either eye when fixation for this point is maintained for a considerable time. It is conceded, of course, that it takes more skiametric skill to measure an eye at fourteen inches than it does at forty, but that which is possible for one examiner is also possible for another with equal intelligence and perseverance. Dynamic ski- ametry being particularly adapted to the needs of those who are advanced in optometric knowledge and skill. SUB-NORMAL ACCOMMODATION. In comparatively recent years that which was formerly called "latent" hyperme- tropia has been classed as an early or sub-normal accommoda- tion, and practitioners of repute are now adapting bi-focal lenses to young persons with great success in some cases. To the optometrist it matters little whether the patient's inability to see nearby objects is caused by latent hypermetropia or by an early change in the index of refraction of the crys- talline, or by a weakened muscle action, what he needs to know is the true condition of the refraction at all points, near and distant, and then by keeping the patient under observation it can be determined whether the case is one requiring the aid of the family physician or of the ophthalmic specialist. As in presbyopia, dynamic skiametry instantly detects a lag in accommodation, and if this "lag" is present it needs looking after. A case is reported of a young Miss, fifteen years old, who was behind in her school work. She had been atropinised and fitted with O. U. plus i. D. S. lenses for constant use. So- called "retinoscopy," in the hands of an optometrist who thought well of his own ability, confirmed the findings of the oculist who prescribed the glasses. Static skiametry also con- firmed the prescription given. Dynamic skiametry, however, showed that a plus 3. D. S. lens was indicated at thirteen OTHER TESTS 121 inches. Plus 2. D. S. lenses, O. U. worn for a month did not succeed in relaxing any more than the original correction for infinity. Bi-focals of plus i. D. S. upper and plus 3, D. S. lower gave almost perfect results after this form of glass had been worn a few weeks. Adduction and abduction were both poor, but the eyes were orthophoric. If dynamic skiametry had not been used in this case the chances are that it would have gone the way of many others, and the patient allowed to suffer on. Perhaps a stronger cycloplegic, or a long period of wearing fogging lenses would have revealed more latent error, but the indications were that this was a case of what is termed "pre- mature presbyopia" or "sub-normal accommodation," and dynamic skiametry was the only method by which it could be intelligently refracted. OTHER TESTS. It may do no harm to repeat once more that no one optometric test is the "whole thing," as the urchins say. The definition of the word optometry, it must not be forgotten, is eye-measuring. That of skiametry is shadow- measuring, and as the word ocular means eye, it Avill therefore be seen that ocular-skiametry is only one division of optometry, and that the dynamic method constitutes merely a sub-division. If all divisions of a subject bear an integral relation to the whole, then dynamic skiametry is but one means for obtaining data which, together with that secured by other methods, con- tributes to the formulation of the proper prescription, and "proper prescription" means the glasses that are best for the patient to use. Thus, if by subjective measurement the patient says that his vision is improved by the use of a convex spheric lens of two diopters, but by keratometry a mal-curvature of a half-diopter of the cornea is shown, and by dynamic skiametry a three-diopter convex spheric is called for, then it may be wise to prescribe a two and one-half sphere, especially if the patient 122 OTHER TESTS has been wearing a partial correction and gives a history of eye-strain. It is the combination of important factors in a given case that often makes success possible. Blind adherence to any one test, or method, is no doubt responsible for many optometric failures. Duction tests of the muscles give data that have great influence in determining the strength of lenses to be prescribed, even though prisms are not used, and where heterophoria is present the data secured by means of dynamic skiametry may often save the prescribing of prisms. The aim of the thorough-going optometrist should be to determine as carefully as possible the true condition of his patient's eyes. And this involves many tests in some cases and few in others, for a case of simple presbyopia, with standard vision and no history of discomfort, does not need the time nor the many tests that are indicated where the ametropia is complex and the history of general health and nervousness poor. Of course the treatment of impaired health of a patient is foreign to the service that an optometrist is generally consulted for, but a "poor history" forms data that has an indirect bear- ing upon all optometric cases, so the wise examiner acquaints himself with all tests and methods that will aid him in the for- mation of correct judgment, for, after all, it is this "judgment" which makes one succeed where another fails. Perhaps one of the hardest strains a new method has to stand is the extravagant claims made for it by enthusiasts. Take the keratometer for instance, a very valuable instrument but often most unreliable where its findings are blindly adhered to. One great mistake made by many practitioners of op- tometry is that they do not take their profession seriously enough. It seems so easy to use a few instruments, ask a ques- tion or two and then form snap judgment as to a patient's OTHER TESTS 1 23 requirements, therefore it is little wonder that this field attracts to it many incompetents. As optometry develops, however, it is hoped that the general public will grow wiser and learn to reward those who give the time and make the effort to fully master each and every detail that has direct and indirect bearing upon the practice of optometry in its larger sense. CHAPTER VIII. Illustrative Cases^ Showing the Comparative Value of Static and Dynamic Skiametry in Patients of Dif- ferent Ages, Occupation and General Physical Condition. ILLUSTRATIVE CASES. The expression "Figures talk" is especially applicable in describing the relative merits of static and dynamic methods in practising ocular skiametry. Space, therefore, will here be devoted 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 were made without the aid of cycloplegics, conse- quently the static test mentioned is the non-toxic kind. 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. -f 2.50 D. S. O. S. -|- 3. D. S. Dynamic test at forty inches = O. U. -f 3.50 D. S. Dynamic test at twenty inches r= O. U. -f- 4, D. S. Dynamic test at thirteen inches = Unsatisfactory. Trial case test = O. D. -f 2. D. S. O. S. + 3. D. S. Vision = O. D. 20/20. O. D. 20/80. Formula given = 0.17.-}- 3. D. S. for constant use, with instructions to return in one year. ILLUSTRATIVE CASES I25 Two years later O. U. -f 3.50 D. S. was readily accepted. No squint. Vision =r O. D, 20/20. O. S. 20/40. CASE B. Master W., age 11. Health not rugged. Inability to see blackboard. Vision = O. U. 20/100. Static test = O. U. — i. D. S. Dynamic test at 13 inches = O. U. — 0.50 D. S. Trial case test := O. U. — 1.25 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. CASE C. Miss N., age 16. In school. Health fair. Headaches. Vision = O. U. 20/30. Static test = O. U. + 0.50 D. C. 90°. Dynamic test at 16 inches, same. Trial case test = O. U. — 0.50 D. C. 180°. Vision = 20/20 in both eyes. Formula given = O. U. + 0.50 D. C. 90°. Vision = "misty." Instructed to use at study, and oftener if more comfortable. Later on she reported "no headaches," and vision was found to be 20/20 with glasses. 126 ILLUSTRATIVE CASES CASE D. Mr. G., age 20. In college. Reports his health good. No discomfort, but "can't see at a distance." Vision =z O. U. 20/200. Static test = O. U. — 2.50 D. S. Dynamic test at 40 inches = O. U. — 2, D. S. At 16 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. C + 0.25 D. C. 90°. Dynamic test at 13 inches == O. U. = -I- 2. D. S. C + 0.25 D. C. 90°. Trial case test O. U. = + i. D. S. C + 0.37 D. C. 90°. Vision = 20/20. Formula given O. U. = + i. D. S. C + 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°. Advised to return again within two years. 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. ILLUSTRATIVE CASES 12/ 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. -f I. D. S. C + 2. D. C. 90°. O. S. + 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. -f 3. D. S. C + 2. D. C. 105°. Dynamic test at 13 inches = Sam' Kcratometer = O.D. 2. D.go°. O. S. 2. D. 110°. (Note difference 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°. Vision = O. D. 20/20. O. S. 20/40. Formula given = O. D. + I, D. S. C + 2. D. C. 90°. O. S. + 2.50 D. S. C + 2. D. C. 105°. Instructed to wear constantly and 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. Vision = O. D. 20/20. O. S. 20/40. CASE G. Mr. C., age 29. Bookkeeper. Reports health good when not working too hard. Eyes and head "feel bad" afternoons. Has been wearing glasses for three years of the following formula : O. D. — I. D. S. C + 2. D. C. 75°. O. S. — I. D. S. C + 1-50 D. C. 105°. 128 ILLUSTRATIVE CASES Vision with present glasses = O. U. 20/30. Fundus reflex very poor. Keratometer shows = O. D. 2. D. axis 75°. O. S. 2. D. axis 105°. Dynamic test at thirteen inches, with trial lenses, shows 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. C + 2. D. C. 105°. Vision = "Foggy." Report received in two weeks: "All right now." CASE H. Mr. R., age 34. Grocer. History of health unsatisfactory. Vision poor for past few months. Present vision = 20/80 in both eyes. Static test = O. D. 4- I. D. S. C + 0.50 D. C. 180°. O. S. + I. D. S. C + 0.50 D. C. 180°. Dynamic tests, at 40 and 16 inches, 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 immod- erate smoking aggravated by the moderate use of alcohol. Gave no glasses. Advised to consult an oculist first. CASE J. Miss B., age 38. Stenographer. Says health is good except for headaches. ILLUSTRATIVE CASES 1 29 Vision = O. U. 20/20. Static test = O. U. + 0.50 D. S. Dynamic tests at 40 and 16 inches = O. U. -|- 0.75 D. S. Trial case test = O. U. + 0.25 D. S. Vision = 20/20 trifle "hazy." Formula given =1 O. U. + O-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 tests at 40 and 16 inches = O. U. + i. D. S. Trial case test = O. U. + 0.75 D. S. Vision = 20/20. Formula given = -{- i. D. S. for both eyes. Instructed to use for near work. No report. CASE L. Mrs. L., age 46. Housekeeper. Health appears good. Difiiculty in reading. No headache. Vision r= O. U. 20/20. Static test = O. U. + 0.25 D. C. 90°. Dynamic test, at 40 inches, about the same. Trial case, about the same. Dynamic test at 15 inches = -{- i. D. Formula given = O, U. + i. D. S. C + 0.25 D. C. 90° for reading, etc. No report. CASE M. Mr. D., age 52, Court stenographer. Health seemingly good. Never has had any glasses that proved quite satis- factory. 130 ILLUSTRATIVE CASES Vision r= 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. Vision = O. U. 20/30. Presbyopia r= 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-examination by dynamic test at 30 inches = O. D. + 1.50 D. S. C + 0.25 D. C. 120°. O. S. + 1.75 D. S. C + 0.25 D. C. 80°. Vision = O. U. 20/20. Presbyopia = 2, D. Reported in six months "O. K. now, 'twas the glasses after all." CASE N. 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. + i-50 D. S. Vision =z 20/20. Formula given = O. U. + 4- D. S. for reading. Would not wear distance correction. No report. CASE O. Mr. E., age 59. Tailor and cutter. Health good. Working distance about twenty inches away. Present glasses are -|- 3* D. S. for both eyes, and are not very satisfactory. Vision = O. D. 20/30. O. S. 20/100. Static test = O. D. 4- 0.50 D. S. O. S. 4- I. D. S. + I. D. C. 180°. ILLUSTRATIVE CASES I3I Keratomctcr shows no corneal mal-curvature in either eye. Trial case test, same as static test. Vision = O. D. 20/20. O. S. 20/40. Presbyopia at working distance = 2. D. Presbyopia at reading distance = 2.75 D. Formula for working glasses = O. D. + 2.50 D. S. O. S. + 3. D. S. + I. D. C. 180°. Formula for reading glasses = O. D. + 3.25 D. S. O. S. + 3.75 D. S. + I. D. C. 180°. Instructed to return if not satisfactory. Xo report. CASE P. Mrs. M., age 62. Occupation (?). Health (?). Looks well. Vision less than 20/200 in both eyes. Static test : First attempt, no retinal reflex. Without skiameter the mirror shows small pupils and slow plus movement. With skiameter, lenses being set to enlarge the pupils, better move- ment is obtained and long, narrow, spike-like patches show. Error about -|- 4. D. S. in both eyes. Trial case test = O. U. -j- 3.50 D. S. Vision = O. D. 20/40. O. S. 20/60. Presbyopia = 3. D. Ophthalmoscope shows slight cortical cataracts. Gave formulas : Distance = O. U. -|- 3.50 D. S. Reading = O. U. + 6.50 D. S. With instructions to be sure and have a strong light coming over shoulder when reading or sewing. Sent letter to family physician. 132 ILLUSTRATIVE CASES CASE Q. Mr. McE., age 67. Health fair. Retired. Now using glasses -\- 4. D. S. for reading; wonders if they can be im- proved. Vision = O. U. 20/80, which is improved by partially closing the eyelids. Static test = O. U. -|- i. D. S. Trial case test the same. Vision = O. U. 20/30. Presbyopia =: -j- 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. Note. — According to Bonders the near point of distinct vision in an emmetropic eye is as follows : At 10 years of age it is 2^ inches away. 20 " ' " " - 4 30 " . . . . ^y 40 " • " " " 9 50 " ' " " " 16 60 " ' - " " 40 To illustrate in fuller detail the workings of dynamic ski- ametry, let the following case be analyzed. 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. U. + 1.25 D. S. Dynamic test at 13 inches = 0.11.4- 2. D. S. Trial case test ^ O. U. + i. D. S. Vision = 20/20. By reference to Fig. 54, it will be seen that when his accom- modation and convergence each receive an equal amount of innervation, the convergence will be greater than the accommo- dation, and binocular confusion will result, thus giving rise to esophoria unless the innervation is altered in some way so as ILLUSTRATIVE CASES 133 to produce the condition called forJiby Fig. 55, where the innervation for accommodation is in excess of that for con- vergence. A test of his extrinsic muscles, however, shows a manifest orthophoria without glasses. Now what are the deductions to be drawn from this case? Twenty-five years of daily use of the eyes without glasses has established a habit of adjustment whereby the standard rela- tionship between accommodation and convergence has been re- placed by a condition in which convergence has given way a little, otherwise esophoria would have manifested itself. The static test shows a reversal of the shadow when over one and a quarter diopters of convex lens power are added. This is in addition, of course, to the quantity necessary to create the artificial myopia. Thus proving that hahit has not mastered quite all of the error, as the accommodation readily accepts partial assistance and relaxes its muscle 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 is enabled to give 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 the dynamic method which calls for a pronounced exertion of the patient's accommodation. An emmctrope twenty-five years of age is supposed to have about eight diopters of amplitude of accommodation. The near- est point of distinct vision is then five inches away from the eyes. A dynamic test made at thirteen inches calls for an ac- commodation equal to three diopters. The patient'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. 134 ILLUSTRATIVE CASES 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 quite sufficient to break up any tonic spasm, or habit of muscle exertion, that may have been formed. Five diopters less three diopters leaves two diopters, and a lens quantity of this strength should 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 re- quired to maintain normal relationship between accommodation and convergence at whatever distance the test is made will show at once in the lens quantity required to reverse the shadow, provided the eyes are examined in a semi-binocular manner, namely, first one eye and then the other, alternating fre- quently 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 thirteeen 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 coercion, cannot converge without accommodating, nor can they accom- modate 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 in such a manner as to estimate the real influence of these habits by mak- ing the eyes work, for the time being, in a manner as far removed from old beaten paths as possible. ILLUSTRATIVE CASES 135 Now another case will be cited in order that the details of skiametric procedure may be accentuated. Mr. Z., 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. C — 0.75 D. C. axis i8o°, 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 and directs the patient to look at the letters on a fixation stand card situated fifty-three inches dis- tant. In an observation made 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 to fourteen inches nearer to his patient before he obtains a reversal of the shadow in this me- ridian. So he notes on his examination blank "O. D. — 0.25 D. S. C -f I. D. C. axis 105." In the left eye the horizontal motion is reversed with a half- diopter convex lens quantity. In the vertical meridian there is a motion zvitli the mirror, when the examiner 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. -f 0.50 D. C. axis 75." Corroborating subjectively, it is found that vision O. U. 20/20, a trifle "misty," can be secured with O. D. — 0.50 D. S. C + 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 -f- 0.50 D. S. added in a binocular way offers no aid. Corroborating 136 ILLUSTRATIVE CASES skiametricaUy again with the full correction on, it is found that a quarter-diopter convex lens reverses the shadow in all merid- ians when the patient looks at the brow card on the examiner's mirror, no matter whether its distance be twenty or forty inches away. With the quarter-diopter convex lens power removed, the shadow shows a suggestion of a movement with the mirror, at the same distances of twenty and forty inches away. The above formula is then ordered and the patient is instructed to wear the glasses as much as possible and to report in a month. In analyzing this case the occupation of the patient is borne 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 which, 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 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 to advise the immediate use of lenses which the optometrist feels sure represent the full correction of his patient's ametropia 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 tzvo bites of a cherry" and indulge the patient's own notions by giving a temporary correction slightly over or ILLUSTRATIVE CASES 1 37 under that which is really indicated, and which will eventually Iiave to be given. It is cases such as these that render the science of optometry 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 encouragements. And it is for these differences that in optometry, as in other specialties, "many are called and few are chosen." CHAPTER IX. Multiple Methods in Optometry and Their Value in ■Corroborative Measurements. — The Systematic Keeping of Records and the Importance of "Case History", Including Resourcefulness, and Mechani- cal Mydriasis. MULTIPLE METHODS. Eye-measuring embraces many "metrys" and the able optometrist must be master of them all. Even in the method to which the name static skiametry has been logically given there are many ways of applying its optical principles. The word "static," as is well known, is used to designate bodies at rest, or forces in equilibrium. The medical examiner attempts to induce this rest of the muscles, controlling the accommodation of an eye, by instilling into the cul-de-sac of this organ some one of a series of powerful toxicants, and thus, for the time being, practically transform- ing a living eye into a sort of schematic one. Non-medical examiners, on the other hand, attempt the relaxation of this accommodation by having their patients look toward some distant object in order to thus coax the muscles into a condition of inactivity, and in further explanation it may be truthfully said that in many cases one overdoes the matter while the other underdoes it. The medical examiner's over- doing consists in forcing the eye into an abnormal condition in which the co-ordination of accommodation and convergence is temporarily destroyed, this destruction depending of course upon the strength of the drug used, and the duration and fre- quency of its instillation, as well as upon the idiosyncrasies of the patient. The results obtained by measuring the refraction MULTfPLE METHODS 1 39 of an eye while it is in a state of what might be called "local intoxication" would seem to call for judgment of the very- highest type in 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 muscle action is liable to be particularly vigorous, 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 prove very unreliable. Spasms of accommodation, as they are termed, are probably responsible for more mistakes being made in the non-toxic manner of employing the static method than can be attributed to the carelessness of patients in looking, or in trying to look, at the object to which their 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 artificial myopia required 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 one of true myopia, however, then an over- correction 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 increas- ing in hypermetropia 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 140 MULTIPLE METHODS method of decreasing lens values in hypermetropia and of in- creasing them in myopia will often prove of great assistance to an examiner, and especially so if applied in a binocular manner, for then the co-ordinate action of accommodation and conver- gence 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 trust- worthy, but where the age of the patient is less than fifty years then a method more reliable 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 abeyance 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 estimated and guessed at by the examiner, while this reading point, as is well known, constitutes one of the most important ends for which glasses are adapted. Frequently, too, for distance purposes a medical examiner is led to advise glasses from a theoretical instead of a practical knowledge of the true conditions present. All cycloplegics, as pointed out before, are of necessity mydriatics, and the mydriasis they produce constitutes a dis- turbing factor, causing the pupillary field to become so enlarged as to add to skiametric complications and to increase the diffi- culties of the method. Viewed from an economic standpoint the toxic method tends CORROBORATIVE MEASUREMENTS I4I toward the needless distress of patients, causes a quite unneces- sary waste of valuable time, in waiting for the action of the cycloplegic, and takes a foolish chance, even if only a slight one, of 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 method in itself, and for the reason that the nearer a patient's eye an examination is made the more carefully must the appear- ance of the shadow, as well as other features of the test, be studied. For instance, in an examination made at eighty inches a half-diopter convex working lens quantity would have to be placed before the patient's eyes in order to produce artificial myopia and focus the parallel rays of light emanating from the retina of an emmetrope. Here the behavior of the shadow would be much quicker, while its color and intensity would be more pronounced than if the test were made at forty inches, where a one-diopter convex working lens was used. A test made at forty inches might also prove very satis- factory, while one made at ten inches, using a four-diopter lens, might be anything but satisfactory, even in the same eye. Thus it will be seen that as a student delves deeper into the intricacies of skiametry the more complicated does the system seem and the more manifold do its methods appear. Experience, however, does wonders in developing skill and judgment, so that old examiners, as well as students, profit by constant every-day work, just as old users of the ophthalmoscope improve by daily practice with this valuable little instrument for scanning the ocular fundus. The truly wise, therefore, will never miss an opportunity to examine a case. CORROBORATIVE MEASUREMENTS. It is not so very long ago when to possess an optometer of simple make, or a modest trial case, seemed to be all that an examiner needed in order to cope with the requirements of his cases. But, thanks 142 CORROBORATIVE MEASUREMENTS to the progressives, which include patients as well as examiners, that time has gone by and accuracy and attention to detail are now the order of the day in optometric practice. To do high-class work at the present time (and he who does not do it is pretty sure to be left in the race), an optometrist must be thoroughly familiar with the various methods and devices which have received the stamp of approval of those of recognized ability in this field. At first thought, it would seem as though the trial case ought to be given primary attention, on account of its age, but, logically, it should come last because it offers the nearest approach to an actual pair of glasses, and because, too, it practically 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 from its priority of discovery, but rather from a utilitarian standpoint. It is not only the great pathfinder that points the way for other work, but it is also the great verifier that tells whether the other work is correct or not. 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 astigmia, its character and approxi- mate 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 both the beginning of an exam- ination and at its end, and if astigmia of considerable amount CORROBORATIVE MEASUREMENTS 143 is disclosed it is a source of satisfaction, though perhaps not absolutely necessary, to use a keratometer and endeavor to ob- jectively locate the exact axis of the error. Then following these methods it is well to ascertain, subjectively of course, whether vision is in harmony with refraction, if it be found otherwise then the ophthalmoscope should be employed to ascer- tain, if possible, why, and thus enable the optometrist to know whether the case is one calling for glasses, for medical treat- ment, or for both. If the vision and the refraction agree in a monocular manner, but not in a binocular one, then phorometric devices are to be called into requisition. Thus it will be seen that in the order of their use skiametry is first, keratometry second, trial-case lenses third, and then, if needed, ophthalmoscopy fourth, phorometry fifth and perimetry sixth. Three of these methods represent the objective, and three the subjective, so that practically all of the six methods are inter- dependent, the only one which might really be dispensed with being the cornea measure, and this it not advisable. In all-round optometric 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 professional calling, where the conditions are likely to vary in different cases. Then, too, the use of examination room apparatus, 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. Nor is it wise to idle away a patient's time in needless visual tests merely for the purpose of trying to create favorable impressions regarding professional ability, for there is now enough that is of real value in optometric work to gain, with 144 SYSTEMATIC CASE RECORDS intelligent use, the confidence of educated as well as unedu- cated patrons. To attain the very highest order of practical 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 physiologic 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 difficult of explanation. SYSTEMATIC CASE RECORDS. 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 pathfinder, and tlierefore when the path has once been found it is wise to keep it, and 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, and the makers of these valuable time-savers seem, at last, to appreciate the needs of those who are engaged in optometric work, for they now make their cards large enough to meet the optometrist's purposes, as shown by Fig. 58. This card is five inches wide by eight inches long, and is plain on the back so as to permit of space for entries covering repairing and changes. It is designed in such a manner as to systematic case records Fig. 58. 145 Date., Name, __ Street. City Occupatic Ophthalmo- scopic Skia- 0. D 6 M 6pb. 1 M 8pb. 33 C 8pta. cn. AllB Kerato- mettic Amt. Am metnc 0. s. SUB-4CCTIVC Pheco- O.D Sph. Cyl. Alls U. V. C. V. Amp. Presby. metric as. Prlsmo- ao. 0. s. Xi. Ab. Sur. Ei. Es. Hyp. metric f»HEVIOU» FORMUI_A O.D. Order In use ijffars p n H'ght._ FACI "] Page Binocular fixation 140 " trial set 156 " vision 104 Black velvet 60 Blind leading the blind.... 139 Body tilting method 43 Bowman, Sir. William Pa- get 3 Bracket skiascope 41 Breaking up of habits no Bright reflex 94 Brow cards 42 Burnett, Swan M 68 Calcium carbide 31 Candle power 23 Captain of the visual ship... 83 Card-board model 69 Card illumination 61 Careless examiners 149 Carelessness of patients.... 218 Case records 146 Cases illustrative 124 Chimney covers Z2> Ciliary spasms 108 Cleaning the skiascope 40 Clonic spasms 108 Co-incident motion 67 Color of shadow 25 Combining of lenses 52 Cook, H. J 206 Compound errors . , 70 Concave mirrors 39 Conjugate foci 68 Contents 7 Contraction of muscles 108 Cortical cataract 131 Convergence interfered with 194 Corroborative measure- ments 141 Cost of maintenance of lamp 31 Grain's disc 159 Created myopia , 85 Crossing point * of emer- gent says : 66 Crowding on plus lens quantity 94 Crystalline lens ' 88 INDEX— Continued Page Cuignet 5 Cul-de-sac 138 C\'cloplegics 140 Cyliiidric equivalents 61 Deaf persons I49 Decomposition of carbon filament 36 Demarcation of light and shadow 64 Development of skill 116 DeZeng's electric retino- scope^ 2)7 DeZeng's optometer, phoro- meter and skiameter.... 167 DeZeng's standard's Sche- matic eye 48 Difference in "does" and "ought to" 67 Different measurements.... 142 Difficulties to be overcome. 21 " of skiametry 21 Disputing the count 42 Donder's rules 132 Double bracket skiascope... 41 Duction tests 122 Dull reflexes 76 Dynamic skiametry in the- ory 80 Dynamic skiametry in prac- tice 134 Economic stand-point 140 Elastic bands 82 Electric retinoscope yj Emergent rays 68 Enlargement by magnifica- tion 150 Epilepsy 108 Epilogue 216 Equal innervation 105 Examination rooms 27 Examiner's nodal point.... 42 " own vision .... 25 Experience 115 Extrinsic muscles 114 Facial light 66 False myopia 85 Fifty candle power lamps.. 35 Page Filament in lamps 34 Final calculations 137 First mate convergence.... 83 Fixation 90 " cards 93 " stand 92 " position 94 Fixed rules unreliable 217 Formulas 54 Forty-inch crossing point. . 84 Fundus reflex 88 Gas lamps 29 General health 124 Geneva retinoscope 164 Glass chimneys 32 Glory-hole 22 Guessing 109 Habit in convergence iii " influence of loi Handle of skiascope 40 Handling the skiascope.... 43 Hardy's wall bracket 42 Haskins, A. S 213 Hartridge 81 Harmonious convergence.. 104 Head of author's skiameter 171 Heat of lamps 35 Heterophoric condition.... 100 History 146 Holding the skiascope 43 Ideal conditions 27 Illiterates 149 Illumination 28 Illuminated fundus 65 Illuminating the fundus.... 64 Illustrations, list of 9 Illustrative cases 124 Imbalance of accommoda- tion 106 Imitation of shadow 69 Improper examination rooms 27 Increase in electric current 36 Increased convexity of the crystalline 88 Infinitj' 194 INDEX— Continued Page Influence of brightness .... 22 " " habit loi Innervation 105 " in emmetropia 105 " " hyperopia.. 106 " " moypia ... 107 Initial examination 92 Instruments as tools I53 Instrument of Standart 160 " " Meriden ... 162 " " Geneva .... 164 " " DeZeng . . . 167 " " Author .... 171 Intensity of illumination... 22 Involuntary contraction of muscles 108 Irregular astigmia 117 Iris diaphragm 33 Jackson, Edward 168 Jarvis, Chas. A 212 Judgment in examination.. 218 Keratometric 174 Klein's retinoscope 46 King's binocular trial set. . 156 Knowledge necessary 18 Kratometric 147 Lamps Acetylene 30 " Argand 30 Gas 30 " Electric 34 " DeZeng's 37 Welsbach 32 Latent errors loS Latent hyperopia 108 Law of conjugate foci 68 Law of light 22 Lens values SI reduction 55 " transposition 56 Lenticular myopia 86 Lockwood, R. M 204 Luminous retinoscope 37 Macroscopically 51 Magnification of pupil .... 150 Mal-attached muscles 112 Measuring astigmia 115 " presbyopia — 118 Page Mechanical mydriasis 149 Median line 104 Mental perception 213 Meriden oculometroscope.. 162 Metal chimneys 31 Metre angle 81 Microscopically 51 Mirrors 23 Mixed astigmatic condition 50 Mixed errors 50 Mixed muscle action 100 Mobile lens action 154 Model eyes of metal 48 " " " pasteboard . 54 Moore, H. B 200 Multiple cards 42 " fixation 91 " methods 138 Muscle balance in emme- tropia 104 Muscle balance in hypero- pia 106 Muscle balance in myopia.. 107 " innervation 105 Muscular insufficiencies . . . 105 Mydriatics 149 Myopia, true and artificial. 84 " accommodative . . 86 Nervous energy loi " impulses loi Neutralization at long range 51 Neurometer 104 Nodal point 42 Non-luminous objects 60 Non-toxic skiametry 139 Normal relationship of ac- commodation and con- vergence 104 Novel skiascopes 45 Objective vs. subjective op- tometry 213 Ocular fundus 65 " skiametry as a sys- tem 15 Oculometroscope 162 Ocular pupils 151 Oil lamps 29 One to three relationship.. 81 INDEX— Continued Page Opinions of others 197 Optical knowledge neces- sary 18 Optometer of DeZeng 167 Orthophoria and Hetero- phoria 100 Other tests 121 Parallelism of rays 66 Paralleled rays of light converged 20 ■ Parent 157 Partially deaf persons .... 149 Peep holes 23 Penumbra double 78 " in skiametry ... 79 " . single 77 Perception 213 Phacometric 146 Ph9rometer of DeZeng. . . . 167 Phorometric 147 Pigmentation ~;i Pink translucent paper.... 69 Piano skiascope 39 Point of reversal 66 Position of light and mir- ror in skiametry 94 Pound weight 80 Practice " of dynamic ski- ametry 115 Preface 5 Prentice, Charles F 3 Presbymetric 147 Presbyopia by skiametry... 119 Prisms 133 Principles of the author's skiameter 170 Prismometric 147 Proper rooms 27 Pupils, large and small... 150 " why they are red. . 60 Quack doctor 51 Queen's schematic eye 49 Question and answers 173 Quickness of shadow 74 Ray bending power S3 Rays direct 61 " rellected 62 Page Ray values 95 Recognition of ability 59 Record blank 145 Records 144 Red pupils 60 Reduction of lenses 51 Re-education of- converg- ence and accommodation 107 Regular astigmia 117 Eeisner's retinoscope 45 Relation of accommodation and convergence 81 Relationship of extrinsic and intrinsic muscles.... 81 Relaxed accommodation . . . 216 Reliable fixation 90 Research work 48 Resourcefulness 148 Retinal illumination 63 target 64 Retinoscope of Geneva 164 Retinoscopy S Reversal of shadow 69 Rheostats 36 Risley's moble prism 154 Scar tissue 117 Schematic eyes 48 " ej-e adjustment. 50 " eye practice.. 47 Scissors movement 118 Segements 147 Shadow actions 67 " measuring 69 Shadow's imitation 69 Shadow phenomena 65 Size of retinal shadow.... 77 " " skiascopic mirror. 39 Skiascopes 43 Skiameter of the author. . . . 171 Skiametry as a system 15 Skiametry, its value in op- tometry 16 Skill 47 Slader, A. R 209 Sliding motion 155 Slow shadows 75 Snap switch 35 Space for examination .... 27 Sources of illumination .... 28 INDEX— Continued Page Spasmodic muscle action . . io8 Spasm of accommodation. . 89 Spiral filament 34 Stammer, A .W 197 Standart's umbrameter. . . . 160 Static method 70 Straight line mirror move- ment 43 Stumbling blocks 2t, Subjective optometry 109 Sub-normal accommodation 120 Sub-phenomena JT) Synchronous 76 Systematic case records.... 144 System of ocular skiametry 16 Theoretic skiametry 69 Theories regarding dull re- flexes 73 Theory of dynamic skiam- etry 80 Tension of accommodation in " on extrinsic and in- trinsic muscles 106 Tonic spasms 108 Toxic skiametry 138 Transposition of lenses.... 56 Trial case last 142 Tropometric 147 True myopia 84 Page Umbrameter of Standart. . 160 Unconscious habits loi " muscle effort.. 108 Unequal innervation 106 Unit lens action 156 Units 51 Use of instruments 157 Value of instruments 153 " " skiametry 17 Various instruments used in skiametry 157 Various skiascopes 39 Visible objects 60 Visibility of fundus 65 Visual 90 Visual fixation 106 Voltage 35 Voluntary muscle action... .104 Wall bracket 38 Wambold, F. A 198 Welsbach lamps :i2 Wh}' the pupil appears red 60 "With the mirror" 67 Wood alcohol 34 Working lens ". . . 71 Wiirdenmann's lens rack. . 158 Form L9-Series 4939