Dynamic skiametry in theory and practice
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'^ •' ' V V •;. : : T^TfFjr %*^~V V;^'V %'^-V' .** ** •-?»?.• >" & *^T* A <, '••»• A0 v.** y^i£% V,/ :£fa\ %^ Sj$M£\ V,/ c\ av *JUo»v* ^ ** "(2 Si* ^ *& *J«fo*L* 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 1911 0j£ v<^ <&* \* i\1 COPYRIGHT, 191 1, BY ANDREW JAY CROSS Saft. N..Y. State Qptomstric issoc frial* ■»** Press of S. L. Parsons & Co. 45 Rose Street, New York 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 Cyto- 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 others prior to 1902, when the dynamic method was originated and made public by the author. New York, October, ign. PREFACE TO SECOND EDITION From one viewpoint it might be deemed unfortunate that success in the practice of dynamic skiametry calls for the high class skill it does in order to correctly read the shadow's action, but from another angle this may be a means of separating the competent from the incompetent, for only through consci- entious and painstaking effort can adeptness in this work be achieved. No startling developments have taken place in either the theory or the practice of dynamic skiametry since the publica- tion of the previous edition of this book. The author, how- ever, has changed his views somewhat regarding the correct- ness of classifying the difference between the findings by the static method and those by the dynamic as being entirely com- posed of latent errors, for he now has reason to believe that the visual organs in their effort to meet individual require- ments adjust accommodation and convergence in a manner to give the fusion sense the greatest possible aid. Thus exophoria might cause some hyperopia to still remain latent, or it might explain why presbyopia is often less than that indi- cated by the Donder's table. As harmony between accommodation and convergence is an essential, it logically follows that the method which objectively determines the exact amount of refractive assistance required to produce this harmony must be of infinitely greater value than those methods where an estimate of the dynamic condition is necessarily based upon the uncertainty of static findings. Dynamic skiametry is perhaps not all it will be some day, but as at present understood it gives a most valuable datum to the examiner, and one that is unobtainable by any other known, dependable means. It is therefore very gratifying to note the increasing numbers of those who are now recognizing this merit. New York, January, 1915. 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 Orthophoric 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 i 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.. 75 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 magnified 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 Wurdemann 158 65 Lens disc used by Crain and others 159 66 Standards "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 hypermetrope of low. Long standing habits of suppression of accommodation 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 applica- 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 itr 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- case 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 two leading accomplishments in shadow testing in which an examiner must be proficient before he can achieve OPTICAL KNOWLEDGE NECESSARY 19 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. 1 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 light 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. Focus. Lens PARALLEL RAYS OF LIGHT CONVERGED. Fig. 2. U^J Lens DIVERGENT RAYS 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 light is reflected into a naked eye the fundus reflex seems fairly bright, but by placing a lens or two in front of this eye, or by permitting the patient to look in an unfavorable direction, 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 23 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 inches 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 2$ 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 2Q, 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 efficiency when employed for general skiametric uses. The draught in almost all lamps is an important factor, and as metal chimneys cannot be easily made to take the place of glass ones, on account of the transparent aperture which is required opposite the flame, it has been found necessary to either line or cover all glass chimneys with an opaque substance which extreme heat cannot affect. 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 Fig. 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. 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, therefore 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 fortunately 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. 