HOW TO SEE WITH THE MICROSCOPE: BEING USEFUL HINTS CONNECTED WITH THE SELECTION AND USE OF THE INSTRUMENT; ALSO SOME DISCUSSION OF THE CLAIMS AND CAPACITY OF THE MODERN HIGH- ANGLED OBJECTIVES, AS COMPARED WITH THOSE OF MEDIUM APERTURE; WITH INSTRUCTIONS AS TO THE SELEC- TION AND USE OF AMERI- CAN 'OBJECT - GLASSES OF WIDE APER- TURES. BT J. EDWARDS SMITH, M. D. ( PROFESSOR OF HISTOLOGY AND MICROSCOPY IN THE CLEVELAND (O.), HOM- CEOPATHIC HOSPITAL COLLEGE; CORRESPONDING MEMBER OF THE 6A FRANCISCO, THE DUNKIRK. AND ILLINOIS STATE MICROSCOPICAL SOCIETIES; MEMBER OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, ETC. SECOND EDITION ILLUTRATED. DUNCAN BROTHERS. 1885. Afrit, P*** Copyrighted by DUNCAN BROTHERS, Id3 and 135 Wabash Ave. CHICAGO. DEDICATION: TO THE HON. P. H. WATSON, WHOSE FRIENDSHIP HAS ENCOURAGED AND WHOSE PABTICIPATION HAS GIVEN DOUBLE PLEASURE TO THE INVESTIGATIONS RECORDED IN THESE PAGES ; THE TRUE FRIEND, AND THE LIB- ERAL PATRON OF SCIENTIFIC EXPERI- MENT, THIS LITTLE BOOK IS DEDICATES BY THE AUTHOK. 256409 PREFACE TO THE SECOND EDITION. It is gratifying to the author that a second edition is demanded, and especially that the views advanced in this work are being very generally accepted by micro- scopists throughout the world. The first edition of this work having been exhausted some time has given occasion for such corrections as crept into the former edition in spite of the utmost care. As a hand book for beginners it is a pleasure to know that it has prevented unlearning many now obsolete notions. The wonderful advances recently shown especially by makers of stands does not now perhaps warrant the strong bias made in the text. The best stand to-day may not be to-morrow. The general rules for selection, however, hold good. If it continues as " a help to see" none will rejoice more than THE AUTHOR. PREFACE TO THE FIRST EDITION. INTRODUCTORY AND APOLOGETICAL. In the spring of 1874 I received from R. B. Tolles, Esq., (the well-known optician of Boston, Mass.,) a one-sixth immersion object glass, which he requested me to study carefully, and to report the results to him. The angular aperture of this new one-sixth was said to be 180, and the objective was one of the first " du- plex " or four-system glasses devised by Mr. Tolles, and destined, in a few short months, to create quite a stir in microscope circles. Having, at that date, leisure at my command, and being very much interested in the performance of ob- ject glasses, I was very glad to give this objective careful and prolonged attention. The result was, that after thirty days' experience I had arrived at a settled con- viction that Mr. Tolles had made a decided advance in the construction and performance of microscope object glasses. Believing this experience of mine to be of value to microscopists, I wrote a short account of the perform- ance of this one-sixth, which was published in the Cin- cinnati Medical News. Shortly afterwards Mr. Tolles kindly sent me another of the " duplex " or four-system objectives, this time a 10 PREFACE. one-tenth. This glass also occupied my leisure time for a considerable interval. The performance of the one-tenth fully sustained the high appreciation I had formed of the performance of the duplex glasses. In some respects the work of the one-tenth excelled that of the one-sixth. The article published in the Cincinnati Medical News was therefore supplemented by several other communi- cations written by me, giving my further experience with the duplex objectives. Among the conclusions I had arrived at from the study of the new four-system glasses were to be found em- bodied ideas radically opposed to popular dogmas which were at that date fully received and accepted by inicro- scopists generally as settled matters of fact. For in- stance, it was claimed for the duplex one-sixth and one- tenth that either of these objectives would not only do the work of, but excel the performance (under amplifi- cations from 275 to 4,000 diameters) of any one-fiftieth extant. Furthermore, it was claimed that the central light work of the new wide apertured duplex objectives would surpass that of the low angles. Referring to the resolution of the severest test objects, it was the express purpose of the said contributions to the Cincinnati Medical News to claim and insist on, as a stubborn fact, that the work of the duplex one-sixth or one-tenth excelled that of any glass extant, " be it a one-fifth or a oue-fiftreth." These, as well as other statements so thoroughly hete- rodox to established belief, met, as a matter of course, PREFACE. 11 with earnest opposition from my readers. A large cor- respondence ensued, while many microscopists visited me for the purpose of witnessing, by way of ocular demonstrations, the performance of the duplex glasses. Meanwhile, in London, England, a lively controversy, known in microscope circles as " the war of the aper- tures," appeared in the columns of the London Monthly Journal of Microscopy, in which controversy the posi- tions assumed by Mr. Tolles were assailed by Mr. F. A. Wenham. The issues involved, however, appertained only to optical possibilities, the performance of the du- plex objectives being an entirely outside matter. During the London battle Mr. Tolles was ably assisted by Dr. J. J. Woodward, of Washington, D. C., and by Prof. Keith of Georgetown. Suffice it here to say, that frorr the standpoint of optical science Mr. Tolles maintainet his positions, a fact, I believe, now generally admitted, And now, after a period of nearly six years, it is to me a matter of pride to here record that of the several 11 heterodox" positions I had assumed in public print, there has occurred, neither in the interim, or at this present writing, no occasion to retract one word of what was claimed by me in my former contributions to the Cincinnati Medical Ifeivs. The duplex objectives have steadily, and perforce of intrinsic worth, forced their way into general use, while the leading opticians are exerting themselves to still further improve their performance. Having, as I have above briefly set forth, had a very large experience with the four-system objectives, it was 12 PREFACE. suggested to the author that a small work, giving in detail the manipulations of the new objectives, with, perhaps, some other items resulting from an experience of over fifteen years as a microscopist, would not only be acceptable, but would fill a place at present unoccu- pied; and it is in response to this suggestion this little book is presented, with the hope that it may prove of some service to some of my brother microscopists. The book is entirely innocent of literary pretensions. It has been my aim to express myself in the simplest possible manner. The diction is at times redundant, gossipy and commonplace, the dominant idea through- out being to hold a good natured chat with my readers. During the past eight years the author has received thousands of letters asking for information in the mat- ter of microscope object glasses, and especially has his experience been called on as to methods of manipulation involved in the use of wide apertured objectives. In the following pages he has endeavored to supply this in- formation, introducing, it is believed, the first attempt to teach the difficult art of collar adjustment, as will be seen in the graduated series of lessons, which, imperfect as they are, it is hoped will nevertheless be of value to those commencing to use the four-system objectives. A.nd now, by way of apology, it remains to state that circumstances entirely unlocked for, and entirely beyond the author's control, have caused a delay in the publica- tion, and many have suffered disappointment therefrom. This delay, however, has furnished me with the oppor- tunity to revise the MS., rendering it, I trust, more PREFACE. 13 acceptable. It has also given me the opportunity to introduce new matter not originally contemplated. During the period of delay above mentioned the writer was in receipt of hundreds of letters of enquiry from as many kind friends. It was impossible to reply to all of these. He,' however, improves the present as a fit- ting occasion to return his grateful thanks to those of his correspondents for the tangible evidences of confi- dence and esteem thus manifested. And to the Messrs. Duncan Bros., too, is he under obligations for the hearty manner in which they took the little brochure under their wing, pushing the same through the press with their accustomed energy. 323 Euclid Avenue, Cleveland, O., September 1880. CONTENTS. PAGE. Introduction and Apologetical 5 CHAPTER I. Something about American Stands, etc 17 Zentmayer's American Centennial Stands. ... 34 The American Histological Stand . . . . . 39 Tolles' large " B B " Microscope ...... 47 Tolles' Largest " A " Microscope 48 Tolles' Student's Microscope 49 The Professional Microscope 53 Large Student's Microscope 46 Family Microscope . . . . . ^ . . . 57 Bulloch's First-Class Microscope 59 Small best Stand " A B " ....... 67 Mr. Bulloch's " D " Stand 67 Mr. Bulloch's New Biological Stand .... 68 R. &. J. Beck's Microscope . . . ... . . 71 Beck's Popular Microscope, Minocular or Binocular . 72 Beck's Economic Microscope . . . . . . 72 Beck's New Histological Dissecting Microscope . . 79 The New National Microscope 80 The New Acme Stand 84 13 14 CONTENTS, CHAPTER II. What is AuguJar Aperture . ..... 93 Angular Aperture ... .... 93 How Shall we Measure Angular Aperture ... 95 Object Glasses . 97 Something Further about Objectives .... 108 Balsam Apertures 123 Flatness of Field . , . . . . . . .126 Mounting of Objectives . . . . . . . 129 Nomenclature of Objectives 130 Table of the Magnifying Power of Single Convex Lenses 134 CHAPTER III. Objectives Continued . 137 Objectives of Lower Balsam Angle 144 Adjustable Objectives 148 Eye-Pieces 156 CHAPTER IV. Manipulations Wenham's Reflex Illuminator . . 157 The Woodward Illuminator ...... 163 Tolles' Traverse Lens 179 CONTENTS. 15 CHAPTER V. Illumination 183 Sunlight 186 . Artificial Light ' 187 CHAPTER VI. Choice of Objectives for Regular Work . . . 202 Selection of Covering Glass 213 Bull's Eye Condenser 225 Working with Low Powers 225 The Spencer one-inch of 50 Broad Guage Objectives, etc. 230 CHAPTER VII. Work with the Higher " Powers " 235 Position of Observer 246 Mean of ten Measurement of Moller Test Plates . .253 Position of Observer ....... 289 Work over dry Mounts with high Aperture Objectives . 298 Oil Immersion Object Glasses ....' 309 New Oil Immersion Objectives 310 16 CONTENTS. CHAPTER VIII. A Word or two on Volumetric Analysis . . . 318 Apparatus necessary . . . , . .. . . ,320 Analysis of Urea, etc 325 Analysis of Sugar, etc 332 Preparation of Urinary Constituents 339 APPENDIX. Names and Address of Dealers in Microscopes, Object- ives, etc., Alphabetically Arranged .... 341 The Investigator Microscope ....... 345 International Microscope Stand, 348 Prices of Accessories . . . . . . . . 352 SUPPLEMENT. Contributions to the Cincinnati Medical News . . 358 High Angles 359 Angular Aperture and Central Illumination . ... 361 On the Performance of Objections ..... 368 Angular Aperture once More .' 370 The use and abuse of the Microscope .... 375 A Chapter on Elementary Physics ., ... 393 Choice of Objectives .,,..... 397 Microscopical Examinations of Oleomargarine ... 400 HOW TO SEE WITH THE MICROSCOPE. CHAPTER I. SOMETHING ABOUT AMERICAN STANDS, ETC. The choice of a stand is a matter of interest to every working microscopist, and to most of us it has been a source of much annoyance and needless expense. It may be safely affirmed that most workers have wasted more money in previous purchases of unsatisfactory stands than would suffice to pay for the one at present in use ; very many indeed would feel grateful to be let off with this record. I therefore propose to assist the novice in the important matter of the selection of a stand, and shall, with this end purely in view, give him fearlessly what, in my opinion, will be sound advice. And be it known again that there will be some adver- tising done in this department. As 1 have before inti- mated, it is quite impossible to say what I shall have to say, of real value to the reader, without giving promi- nence to one or more of the several makers. The re- sponsibility is mine, and I accept it. It has been my province, in times past, to select many stands for my friends and acquaintances ; those who have visited me with the special object to ascertain my methods of using and working objectives have, in many instances, fol- 18 HOW TO SEE WITH THE MICROSCOPE. lowed my recommendations as to stands, and in every case, I believe, have expressed to me their satisfaction. The author has not in the past stood so much alone as to his ideas of a stand as was the case with his opinion of objectives. Now, in what follows, it is to be assumed that the reader desires one stand, and one only, and that he wishes to invest his money to the best advantage, i. e. 9 make the same go as far as possible ; that he desires a really good and reliable instrument, and one that will last for a lifetime. During the past three or four years, the microscope stand has been greatly improved, both in Europe and America, and as a rule, really serviceable instruments can now be obtained either of American or London workmanship, and at a moderate cost. It is to be re- gretted that our German friends have not followed suit, but, on the contrary, have been content with the old form of stands which were patent ten years ago. In the late improvements by the American and Lon- don makers, it is first noticealle that the weight of the stand has been considerably reduced ; the old idea, that to secure sufficient solidity it was necessary to employ stands weighing from twenty to forty pounds, being now practically abandoned. Again, it was formerly considered a sine qua non that the microscope stage be thick enough, heavy enough, and solid enough to bear the whacks from a sledge- hammer. It must be here kept in inird that years ago there SOMETHING ABOUT AMERICAN STANDS, ETC. 19 were no such things as wide-angled objectives, and, as a matter of course, the work was principally done with central or centrally disposed light. To this the old and heavy stage offered no objection, for it was quite possible, with the aid of achromatic condensers, prisms, etc., to work with all the obliquity the objective would respond to, using a stage two or three inches in thickness. Luckily, there were those who would at times "fight objectives," play with diatoms, etc., and in response to their demands the optician increased his angles and working force of the object-glasses. To meet this in turn called for the construction of condensers of greater angle; until finally it occurred that the aper- ture of the objective had arrived at proportions to which the condensers did not satisfactorily respond. Something had to be done, and something was done, for necessity is the " mother of invention." The simplest way is the best, and this was to reduce the thickness of the stage. The "fighter" of objec- tives had discovered the fact that a stage, one-fifth of an inch in thickness, was solid enough for any and all of the delicate work required by the.microscopist, while at the same time he derived a great advantage in thus pro- viding play for the aperture of his objectives. The writer remembers with pride that he " took a hand" at this; he remembers, too, the unalloyed satis- faction he experienced in seeing two heavy, lumbering, and expensive stages, alone costing several hundred dol- lars, removed from imported stands, and their place substituted by plain, thin plates made by the local 20 HOW TO SEE WITH THE MICROSCOPE. watchmaker; he recollects, also, the exclamation of one of his friends, after looking at an object as dis- played by the improvised stage : " Well, I declare ! This instrument was made for any thing in creation but to see through." Hence it was, responding to the increasing call, our American and London makers decreased the thickness of their stages. While it is yet true that many of the present stages are unnecessarily thick, the reduction is still palpably manifest. It may be remarked, then, that the real improvement of late years in the construction of American and Lon- don stands may, as a rule, be manifested in these two items, viz., reduction of weight and thickness of stage. We have learned something from the Germans, too, within the past few years. It has been well known that they favored the vertical stand with its short tube. Our experience has taught us that both of these have their advantages, and our later instruments are so con- trived as to be used with short, and also with standard tube, and in a vertical or inclined position. I desire in this place to record the fact that the stands made in the United States are not excelled in any qual- ity or condition going to make a number one, reliable instrument. The stands produced by our home makers are quite equal in every respect to those of any other countries, while their cost is not one whit higher. Those contemplating the purchase of a stand will, as a matter of course, consult their individual taste and inclinations to a considerable extent : thus, A may select SOMETHING ABOUT AMERICAN STANDS, ETC. 21 a large and heavy stand, while B would prefer a smaller and lighter one. The party, too, will naturally take into consideration the particular field of work he may have in contemplation. It is quite possible to give the latter consideration too much weight. It will be found, as a rule, better policy in the long-run to purchase a stand capable of doing almost any work, and thus avoid the possibility of being compelled to sell at some future date, at a heavy discount, and purchase another and more capable stand. Fortunate it is that one can now purchase, at moderate figures, reliable and well-made stands, suitable for almost any purpose of tl e micro- scopist. In the reference that has been made to American stands, it is proper to state that the Messrs. Beck, of London, have a regular agency here for the sale of their wares, and that this agency is in charge of an American gentleman. There can be no good reason to regard them as other than home folks. At all events, the author is of this opinion, and will act accordingly. Among the essentials that deserve attention in the selection of a reliable stand may be mentioned : FIRST. See that the stand is well balanced in every position that it manifests no disposition to topple either one way or the other; that it stands tolerably firm (for its weight) on its legs. SECOND. If it has coarse adjustment by rack and pinion, see that the movement is as smooth as oil; reverse the milled head between the thumb and finger promptly, and notice if there be any lost motion; try 22 HOW TO SEE WITH THE MICROSCOPE. this test all the way of the "run" of the rack, and if there be " lost motion" detected discard the stand. THIRD. Examine carefully the bearings on which the body slides ; these should be broad, and the fitting of the body to the limb accurate; test by placing a small object on the stage a circular diatom will be the thing; now examine with a half-inch glass; rack the tube up and down a little, and see if the object keeps its centre ; seize the body-tube near the eye-piece, and twist it a bit from right to left, and vice versa, noticing whether the object "travels" or not; ^epeat this ex- periment by punching holes in a card-box, say three- quarters of an inch thick, thus forming a supplemental stage; focus again and try the twist once more. The instrument that will stand this test is all right as to its bearings. FOURTH. Place the diatom on the stage, under the half -inch, as before, and if the instrument has fine ad- justment by nose-piece, test by moving the fine wheel quickly either way, and see if the object " travels." If satisfactory with the half-inch, try an eighth, or tenth ; test also by taking hold of the objective as one would in the adjustment for cover, by giving it a little twist right and left, and see if the object changes its position. Finally, examine the run of the fine adjustment; see if it is quick and sensitive to the touch, without "jump;" if satisfactory so far, so good. FIFTH. Should the stand have concentric stage, test the accuracy of its fittings with diatom and half-inch glass. If the stand has suffered transportation, or has SOMETHING ABOUT AMERICAN STANDS, ETC. 23 in any way been submitted to fatigue, the probability will be that the stage has become somewhat decentred, and any effort of your own to re-adjust will be likely to make things worse. The very best stages will not remain centred long if roughly used. Still, with your half-inch and the diatom you can manage to get some idea, which will be better than none at all. Centre your diatom, and focus; now seize opposite edges of the stage with both hands, hold it firmly, giving it "jerks" either way, right and left, but not sufficiently to absolutely move the stage on its centre. If you suc- ceed in moving your object, with a corresponding move- ment of the stage, then the latter is not accurately fitted, and cannot be accurately centred. This "slip" can be detected with the hands alone. SIXTH. It the stand be furnished with mirror fitted to radial arm, see that all these fittings are strong and likely to last. This portion of the stand meets with more fatigue than any other. See that the friction sur- faces are not both the same metal; this will apply, too, to any part of the stand. Notice particularly whether the universal motions of the mirror are properly con- structed, and likely to last for years ; this is a most im- portant point. FINALLY. It will be well to see that the joint for inclining the instrument at various angles is strong and well made, and that it have compensation for wear. Notice the general " get up" the general finish of its various parts. And now, having received a lesson on the use of diatoms , you may put the little fellow away until again wanted. 24 HOW TO SEE WITH THE MICROSCOPE. Aside from these matters involving sound mechanism, there remain other points connected with the choice of a stand, which may, to some extent, be regarded as a matter of election. For instance, some of the most costly stands are furnished with mechanical stages whereby motions are given to the object-carrier by various milled heads. I have used these stages in times past, anc 1 have to record my disapprobation of them, and for the following reasons : FIRST. They are an impediment to quick work. It is much quicker to run from one end of an object to another by one single movement given by hand than to wait the slow motions of the screws. There are, how- ever, some advantages arising at times from the use of the mechanical sta fact, one of my friends made one for himself. Provide a sheet of well-hammered brass, heavy enough,, so that when planed or turned down the stage shall be one-sixteenth of an inch in thickness, with both faces truly parallel. Cut the circle which is to form your stage as large as your instrument will permit, and in accordance with the above directions. Cut the well-hole one-sixteenth of an inch larger than the w r ell-hole of your stage ; make a collar, or short tube, out of the same material used for the stage; turn the outside to proper dimensions, so as to fit the- well-hole of the new stage, the upper edges of both being " flush," and solder in position. Next: Turn accurately the under and projecting part of the short collar, or tube, so that it will exactly fit the well-hole of your main stage ; place it thereon, and 28 HOW TO SEE WITH THE MICROSCOPE. cut off any portion of the collar that may project beneath said stage. In the stage thus far towards completion, it so be that the collar projects one-sixteenth of an inch, this will be found ample for its support ; thus you will be enabled with some stands to steer clear of sub-stage appliances, etc. All that remains to be done is to fit your new stage with plain spring-clips, which can be done in a few moments out of a piece of watch-spring; or, if there is room enough, you can provide an object-carrier, made on Mr. Zentmayer's principle; all of which is plain work, and easily accomplished by any machinist of toler- able skill. Keep in mind that this stage adds somewhat to the thickness, and govern yourself accordingly. " But," says one, " I have no assurance that this stage will rotate in the optical axis." I grant it, with the remark that if it did so rotate with one objective, it would be pretty sure to fail with another. The com- pensation must be supplied by finger manipulations, easily acquired, and as easily practiced. As I have previously hinted, the best rotating stages remain con- centric but fora short time, especially if much used; while to the real worker, the very bother of adjusting the most expensive stage extant would be an intolerable annoyance, and a willful waste of time. A grand good thing about this improvised stage is, that it can be placed in position or removed therefrom in a moment's time. This, to the author, is a real boon, SOMETHING ABOUT AMERICAN STANDS, ETC. 29 for he has often to remove the supplemental stage, and to work for hours with the mounts placed on the main plate, using not even the spring-clip or any other attachment, the microscope being the while in the ver- tical position. Says another: Why not have the maker furnish some such stage at the date of purchase ? He can do- these things better than any one else." I respond, Yea, verily. Thousands of times the question has been asked r 44 Which do you prefer the binocular or the monocu- lar? " and as it is more than probable that this question will arise in the minds of some of those who read this- book, perhaps a word or two on the subject may not be amiss. Mr. Henry Crouch, F. R. M. S., a well known maker of microscopes, visited this country during the Centen- nial Exhibition, and on his return complained bitterly of " an eminent German microscopist, who assisted in examing the microscopes on exhibition at Philadelphia , and who from the first loudly proclaimed the useless- ness of binoculars, . . . but whom he afterwards found out had never used one." 'The author is pretty much in the same boat with the eminent German. Since the introduction of the binocular, the writer has made several downright square and honest attempts to use the binocular long enough to be able to express an opinion worth consideration, but in each and every case the double barrelled machine proved too much for his patience. With the low powers the instrument is 30 HOW TO SEE WITH THE MICROSCOPE. capable of giving very pretty shows, particularly where a certain stereoscopic effect is supposed to add force. Hence the binocular is eminently fitted for such dis- plays, and is eminently adapted, too, for the entertain- ment of those of our lady friends who visit the soirees of microscopical societies, without feeling specially interested in microscopy as a science. If so be that there can be structure displayed by the binocular that cannot be seen equally well by the mon- ocular, be it so, and the author will joyfully add his testimony to the same, after having arrived at the fact. Many of my friends have binocular instruments, but I can not recall the name of any person who uses one ; that is, when I am present, for inevitably the first thing done by the owner is to displace the prism, and to use the stand as a monocular. I have never found a soli- tary exception to this rule. But some one may say: "What possible objection can there be to purchasing a binocular, since, as you admit, the instrument can be changed instantly to a monocu- lar, and the purchaser has his choice. He certainly has all that you have got, and perhaps (allowing force to the opinion of others) more too." Well, that looks lucid enough; and in truth that argument has sold many a binocular. Nevertheless, " all is not gold that glitters." Let us take a look from another stand point, thus : First. The binocular involves greater weight ; as a rule, they are unwieldly, lumbering things, destitute of grace, symmetry, or aught that goes to make a clean, well proportioned stand. SOMETHING ABOUT AMERICAN STANDS, ETC. 31 Second. The binocular instrument is much more complicated, and hence more expensive, the purchaser being required to pay, without getting in return value received. Third. This form does not admit of the instrument being used with short, as well as standard tube, and to the " worker " this item has particular force. Fourth. The extra expense attending the purchase of a binocular can be better applied in other directions. Those who value dollars and cents will find force in this objection. Fifth. I oppose the binocular, because the monocu- lar is good enough, and because the real work of the microscope has been and will continue to be done with the monocular. /Sixth. The binocular is to a certain extent impract- icable, because the two eyes of the observer are not alike. There are exceptions, but not to an extent suffi- cient to invalidate the rule. Seventh. I oppose the usual form of binocular instruments, believing that the binocular eyepiece invented and made by Mr. R. B. Tolles is a preferable way of obtaining binocular vision. Having thus presented my objections, let the reader elect for himself, with this assurance oru my part, viz., better get a good binocular than a poor monocular. All stands should be furnished with plane and con- cave mirrors. If the mirrors are attached to radial arm, hinged at a greater or less distance from the under sur- face of the stage, then it is apparent that when the 32 HOW TO SEE WITH THE MICROSCOPE. radial arm is placed in position for oblique light the mirror will be nearer the well hole than when the arm is centrally disposed ; therefore, the mirror should be arranged to slide on the arm, so that the proper com- pensation may be effected. The mirror should, of course, be well mounted in its own semicircular arm, so that universal motions may be obtained, and the semicircle should be closely attached to the slide moving on the radial arm; that is to say,. there should be no intermediate, short, and jointed arm& or elbows such as are found on many so-called first class instruments, and are a most intolerable nuisance. It is quite possible at times to remove these intermedi- ate arms, and attach the mirrors directly to the radial arm, and at a trifling expense, thus transforming a faulty stand into a really serviceable one. Examine the concave mirror carefully, as to its tocal length. To do this, place a piece of white letter paper on the stage, and using a common candle reflect the light on the paper ; now move the mirror up and down on the radial arm, and see if you can get a tolerably well defined image of the flame. It may occur the radial arm is too short for the focal length of the mirror, and that it would be impossible to lengthen it sufficiently ; this being the case, reject the stand. One would naturally suppose that an item like the last would surely be attended to on the part of the maker; nevertheless, many stands are made and sold with glaring defects of this character. At the risk of being tiresome, let me especially insist SOMETHING ABOUT AMERICAN STANDS, ETC. 33 on the absolute importance of looking carefully to the entire apparatus connected with the hanging of the mirrors. This part of the mechanism is called to endure more fatigue, and is ottener thus called on than any other portion of the stand. For instance : In the act ot observing, one often has occasion to seize the mirror, and to move it and the radial arm by a single impulse, simultaneously as it were ; and here let me say that it will be well for the observer to acquire this habit; nevertheless, it is terribly straining on the joints thus called on, and they should be strong enough to endure the fatigue. See also that all the eye pieces slip in and out of the tube easily; this too without decentring the object in the field. Any difficulty in putting eye pieces in place or removing therefrom is a first class botheration. We now proceed to the description of the stands made and sold by American makers. In placing Mr. Joseph Zentmayer at the head of this list, the author feels assured that he does no violence to the feelings of others. Mr. Zentmayer was one of our earliest and most energetic makers, and his work has, as a rule, proved honest and reliable. In his expenditure of capital, his facilities for execution, or his pride in presenting first- class work, he stands second to none, as his competitors will generally attest. Mr. Zentmayer's largest and most costly stand has, I may say, but just been placed on the market. It was especially designed for exhibition at the Centennial Exposition. The description which follows is to a con- 34 HOW TO SEE WITH THE MICROSCOPE. siderable extent the same as that printed in Mr. Zent- mayer's catalogue some portions being suppressed, and the wording occasionally changed at the election of the author.* ZENTMAYER'S AMERICAN CENTENNIAL STAND. Constructed especially for the Centennial Exhibition. It is mounted on a tripod, with revolving graduated platform ; the bar and trunnions are in one piece, and swing between two pillars for inclining the instrument to any angle. The coarse adjustment is accomplished by rack and pinion. Thus far it is similar to the " Grand American Stand" by the same maker. The swinging sub-stage, which carries the condenser or other illuminating apparatus, including the mirror, swings around a pivot, the axis ot which passes through the object observed, so that this object is in every posi- tion in the focus of illumination. The stage may be detached with facility, and replaced by one constructed for oblique illumination; the swinging illuminator may then (i. e., with the last-named stage) be used for illumination from above. The sub-stage is provided with a graduated circle for indicating the degree of obliquity. An object placed on the stage being in a plane with the axis of the trunnions, it is obvious that if the in- strument is placed in a horizontal position, the object *In the several descriptions of stands, that of the respective makers will be given as far as possible. The author will, however, at his election, sup- *>ress certain portions or change the diction as to him may appear desirable. ZENTMAYER'S AMERICAN CENTENNIAL STAND. 37 is in the axis of revolution of the graduated platform, and the angular aperture of an objective focused on this object can easily be measured. It is equally obvious that in this position the object is in the centre of all the revolving parts of the instrument, to wit, the re- volving stage, swinging sub-stage, and the platform. The principal stage is similar to the circular one pre- viously, used on the " Grand American;" it is provided with adjusting screws for accurate centring, and re- volves in a large outside ring, giving facilities for oblique illumination up to 70 degrees from axis (140 degrees aperture), while the graduations serve as a goniometer for the measurement of crystals, etc. The sub-stage is divided into two cylindrical receivers, to facilitate the adaptation of several accessories at one and the same time ; the lower cylinder can be moved up and down or entirely removed. The fine adjustment (in all other instruments of the Jackson model being in front of the body) is removed to the more stable part of the stand. The bar is pro- vided with two slides, one for the rack and pinion movement, and close to it another one of nearly the same length for the fine adjustment, moved by a lever concealed in the bent arm of the bar, and acted on by a micrometer screw. Thus the body is not touched when using the fine adjustment, and the relative distances of objective, binocular prism and eye-piece remain un- changed. The smaller stage of the American Centennial Stand is also provided with screws for accurate centring ; this 38 HOW TO SEE WITH THE MICROSCOPE. stage is three inches in diameter and extremely thin, allowing, in connection with the swinging sub-stage and mirror, not only the greatest obliquity of illumina- tion, but the mirror and achromatic condenser will rise above the stage when required, as in the case of sun- light illumination, that of opaque objects, etc. The diameter of the sub-stage is the same as that of the " Grand American;" the accessories of that stand are therefore interchangeable. As to the general character of Mr. Zentmayer's work, the author can affirm with confidence that it is not ex- celled in any particular. The stand just described is beautiful in design, is nicely proportioned, and in every repect reliable and durable. It will stand all of the tests named in the preceding pages. Those wishing a first-class stand cannot fail to be satisfied with the Cen- tennial. The swinging sub-stage carrying the mirror, etc., is a most valuable improvement, and one that the observer can hardly afford to be without; the mechanism, too, by which this end is accomplished is of the strongest and most workman-like order. Attention also is invited to the method of attachment of the stage. The latter is solidly held in position, or can in a moment be detached, and another stage substi- tuted. It has occurred to the writer that the " principal stage" mentioned might very well be dispensed with the smaller stage being quite sufficient. Possibly a large and plain stage might at times be found a convenience, THE AMERICAN HISTOLOGICAL STAND. 39 this could easily be substituted for the more expensive one furnished with the instrument. As to the smaller stage referred to, the author can "speak by the card." He had used .one of Mr. Zent- mayer's army hospital stands for years, and the instru- ment gave him good satisfaction. Two years ago, how- ever, desiring a thinner and a revolving stage, he begged Mr. Zentmayer to devise one and fit the same to his stand. Quite a correspondence ensued, and the army stand was thus equipped, the new stage being practic- ally the same as the small one furnished with the Cen- tennial. It worked nicely, and was, in truth, all that the author desired. In the change of stages proposed, attention is called to the fact that the smaller stage has not the graduated edge. This to the majority of users would not be a serious objection, while, on the contrary, many would gladly avoid the cost of the graduated circle. Mr. Zentmayer makes some eight or nine different forms of stands. We have room to describe only one other, viz. : THE AMERICAN HISTOLOGICAL STAND. The requirements held in view, in the construction of this little instrument, were the combining of the facilities of a first-class stand with moderate cost. The entire instrument is made of brass, the base and uprights are one piece, of a peculiar shape, and of great rigidity, to which the bell-metal bar is attached by a 42 HOW TO SEE WITH THE MICROSCOPE. joint, allowing the use of the instrument at any of inclination; perpendicular and horizontal positions are indicated by stops ; the coarse adjustment is accom- plished by a sliding tube, the tube being but five and one-half inches long, but capable of elongation to the standard length. The fine adjustment is similar to that of the Centen- nial a concealed lever moving the entire body ; this adjustment is reliable and very delicate. The sub-stage, plane and concave mirrors, swing in the same manner as do those of the Centennial, having the object in its centre, even when swung over the stage. The sub-stage carries the diaphragms, of which three are furnished with the instrument. Any piece of sub- stage apparatus, such as condensers, paraboloids, prisms, in fact anything from the lists, can legitimately be adapted to the sub-stage, or the same can be instantly removed, with the mirror also, if desired, thus leaving the stand free from any obstruction below the stage. The sub-stage slides up and down in strong dovetailed grooves, and has centering adjustments by hand. The weight of this little stand I judge to be about three or four pounds; it can, on a pinch, be carried in one's great-coat pocket. It is worthy of mention in connection with these two stands of Mr. Zentmayer's, that the fine adjustment has been removed from the front to the rear, or, as Mr. Zentmayer says, " to the more stable part of the instrument." The author, when his attention was first called to these stands, regarded this change of the fine THE AMERICAN HISTOLOGICAL STAND. 43 adjustment as one of doubtful utility; and he once made the remark to a friend, that should he purchase a Centennial, he should insist that the maker furnish a fine adjustment, both front and rear. Certain it is that, in the process of correcting a first- class wide-angled objective, it is a convenience to have the fine adjustment at the front. It is, however, ex- ceedingly difficult to so accurately fit the sliding nose- piece that it shall move up and down with perfect free^ dom when acted upon by the fine screw, and at the same time to be free from lateral motion, and this lateral motion constitutes a first-class fault to which the attention of the reader has been called on another page. The very best makers have found it difficult to steer clear of this. The writer once met with one of Mr. Zentmayer's larger stands exhibiting this defect in a marked degree. His own " Grand American Stand/' however, in this respect, is faultless. Thus it will be seen that, in the nature of things, a really fine adjustment acting on the nose-piece involves skilled labor, and this, in turn, involves cost. Again, the movable nose-piece necessarily changes the length of the body-tube, and this, in turn, again, changes the amplifying power of the objectives, and to get rid of this is indeed " a consummation devoutly to be wished." Having had the little Histological in constant use t. e. 9 from six to ten hours daily I arn now prepared, from my own experience, to state that I am quite well satisfied with the fine adjustment as placed by the 44 HOW TO SEE WITH THE MICROSCOPE. maker. It would probably take me a trifle longer to adjust nicely an objective on the Histological, and but a trifle ; while, on the other hand, all the objections con- nected with the adjustment at the nose-piece are avoided. The Histological, as furnished by the maker, has simply a plain stage with spring-clips. This defect did not pass unnoticed by the writer, Mr. Zentmayer re- sponding promptly to his request with the improvised stage described on a preceding page. It remains to be noticed that the Histological has neither rack nor pinion,* and that the coarse adjustment is effected by sliding the body within an adjustable "jacket." There is no novelty in this, for the sliding tube is " as old as the hills," and has been extensively adopted in the construction of cheap stands. The author felt very much like kicking at this feature of the Histological. In a little time, however, experi- ence taught him that the sacrifice of the rack and pinion was not such a serious matter as might be supposed. The sliding movement of the 1 body tube within its jacket in the little stand is very smooth, regular, and reliable, while, on the other hand, there are some ad- vantages accruing to the slide that are not to be ob- tained by the use of the rack and pinion. For example: suppose we are working over wet preparations, and unfortunately the front of the objec- tive becomes immersed in the liquid a misfortune *Mr. Zentmayer now furnishes the Histological, with or without the rack and pinion coarse adjustment. AUTHOR. THE AMERICAN HISTOLOGICAL STAND. 45 liable to occur daily. It is then, in such cases, a posi- tive convenience to be able to pull the body- tube out of the jacket, cleanse the objective, and return to its place. All this can be done in much less time than would be required, were the instrument furnished with rack and pinion, to unscrew the lens, cleanse, and screw in place again. The greatest objection that the writer has to urge against the Histological stand is this: When it is placed in a vertical position, there is a liability of its tipping forward. This can be prevented by clamping the instrument to the table with a small iron clamp, such as are used by carriage-builders and are sold at the hardware shops for a dime. In this simple way the stand is rendered as solid, more stable indeed than those of the heaviest build. One of my correspondents, to whom I had stated the above-mentioned objection, informs me that Mr. Zent- mayer has made some changes in the foot, securing thereby greater steadiness. Previous to the introduction of the Histological, it was generally taken and accepted that the purchaser of a cheap stand ought not, in the nature of things, to expect an instrument capable of all work. To a slight extent, and to a slight extent only, does the remark hold good to-day, for the Histological has not the revolving platform, for the measurement ot angular apertures (apertures can, however, be measured on the stand), nor has it the circular graduated stage; nevertheless, the Histological will accomplish a larger variety of 46 HOW TO SEE WITH THE MICROSCOPE. work than can be performed on the " Grand American Stand" of the same maker, and this, too, be the char- acter of the work what it may, be it the study of a Histological object or the display of the No. 20 of the Moller plate, or the 19th band of Nobert. As may be arrived at by the tenor of the preceding- remarks, the author regards the introduction of the histological as marking an era in the progress of micro- scope stands. The long-sought problem has been solved, and in the Histological we have a cheap, reliable, and universal stand, suitable for almost any work which may be required, and capable of carrying any and all of the various accessories which in the past have been sup- posed to pertain only to the heavier and (so-called) first-class instruments. Furthermore, the author desires in this place to put on record his unfaltering opinion that, in the devising, construction, and introduction of the Histological stand, the maker has bestowed a greater boon on the " body corporate " of microscopists than has been accomplished by others, either at home or abroad. If there be an error in this statement, " time, with its revenges, will set it forth." TOLLE'S LAKGE " BB" MICROSCOPE. This instrument was designed to meet the require- ments of the scientific investigator. The instrument, constructed on the Jackson model, is eighteen inches high, when placed in a vertical position, and weighs about fourteen pounds. The curved arm is supported on a steel trunnion between two strong brass pillars 48 HOW TO SEE WITH THE MICROSCOPE. made for durability, and not liable to get out of order, and is provided with means ot compensation for wear. It has rack and pinion for coarse and micrometer screw for fine adjustment, the latter being placed in front of the body, as has been usual in first-class instru- ments, on the Jackson model. It is furnished with graduated draw-tube ; sub-stage with rack and pinion, and centring screws for accessory apparatus ; plane and concave mirrors on double-jointed arm; Tolles' thin stage, admitting light of great obliquity ; with rectan- gular movements by screw and rack and pinion, and rotation on the optical axis of about 270. Mr. Tolles makes a modification of this sized stand, the stage being carried by friction rollers, and having entire rotation on the optical axis. The cost of the in- strument is thereby somewhat enhanced. TOLLES' LARGEST "A" MICROSCOPE weighs twenty pounds, and is one of the largest and most solid instruments extant. The stage is six inches in diameter, and makes a complete revolution on the optical axis. The whole instrument rotates on a stout plate graduated to degrees, and is similar in all respects of style and construction to the " B" stand. Either of the two stands named, made by Mr. Tolles, may be unhesitatingly pronounced first-class. The work- manship is of the very highest order ; the circular stage can be so nicely adjusted as to allow of an entire revo- lution, under a one-twenty-fifth objective, without the TOLLE'S STUDENT'S MICROSCOPE. 49 object being sensibly displaced. Either form of stand can be fitted with radial arm to carry accessory appar- atus at any angle. Any thing that Mr. Tolles makes is sure to be made well. TOLLES' STUDENT'S MICROSCOPE* This stand is fifteen inches high, weight six pounds; the base, uprights, and curved arm are of iron, hand- somely japanned. On a trunnion joint, made on a plan to wear well, the instrument can be placed in any posi- tion, from vertical to horizontal, and has a stop to pre- vent movement in either direction beyond these points. The stage is plain, with spring-clips for holding the ob- 50 HOW TO SEE WITH THE MICROSCOPE. ject slides ; revolving 1 diaphragm ; concave mirror, with movement to give oblique light, and for the illumina- tion of opaque objects the mirror is removed to an up- right stand. The coarse adjustment for focus is effected by sliding the compound body which is held in its place by a spring; fine adjustment by a movable plate and screw on the stage, which is efficient with high powers. The stand is made with all the care bestowed on his first-class instruments. The form is that of the Jackson pattern. To this instrument Mr. Tolles supplies several variations and additions, as a matter of course increas- ing the cost as well as its capacity. Among these sev- eral extras may be mentioned sliding stage, giving ver- tical and horizontal motions by hand, and adapted to the use of the " Maltwood Finder;" sub-stage for acces- sory apparatus; fine adjustment by lever and microm- eter screw ; rack and pinion for coarse adjustment ; thin glass stage to rotate on the 6ptical axis; the stand en- tirely of brass, etc. For a " student's stand " this is an instrument of good round proportions. It stands firmly on its legs, and the stage is remarkably roomy. The body-tube is nickel-plated, and the entire instrument symmetrical in its proportions, and not without pre- tensions to style. Like all of Mr. Tolles' work, it is made " for keeps." There are many of them in use and doubtless giving satisfaction. The Bausch & Lomb Optical Company manufacture several excellent stands, which were designed under the immediate superintendence of Mr. Earnest Gundlach. The firm furnish some seven or eight models, from which we select the following three : THE PROFESSIONAL MICROSCOPE. 53 THE PROFESSIONAL MICROSCOPE. This is the largest and most expensive instrument made by the company, and may be described as follows : Heavy brass foot and pillars, both highly finished, carrying the axis for inclination of the body, which movement can be easily tightened or loosened by two strong milled-head screws. Coarse adjustment by rack and pinion, moving along prismatic slide, of first-class workmanship, attached to the body; fine adjustment by a new and patented frictionless motion. The object slide rests upon a newly devised carrier ; the body tube has an inner draw tube, with society screw to which ob- jectives of very long focal distance can be attached ; large plane and concave mirrors ; sub-stage for receiving acces- sories of standard size, and two revolving diaphragms, one of the latter belonging to the condenser ; all attached to the swinging mirror bar, the axis of which is placed on the level of the object so that the diaphragm and mirror swing concentrically around it. The mirror as well as the sub-stage can be moved on the iniiror-bar to and from the object, and both can be removed, the latter by a hor- izontal prismatic slide. The sub-stage ring is provided with internal " society screw" for objectives, condenser, etc. There are also two slot diaphragms of different widths, covering the whole surface of the mirror, and only allowing light to pass through the slot in such a direction that very sharp shadows by oblique light will be produced. 56 HOW TO SEE WITH THE MICROSCOPE. LARGE STUDENT'S MICROSCOPE. Heavy japanned cast-iron foot, with highly finished brass pillars, carrying 1 the axis for inclination of the body ; brass arm ; coarse adjustment by rack and pinion ; fine adjustment by a new and patented motion. The special advantages claimed for this new adjustment are : (1) exceedingly easy and smooth movement ot the fine screw both ways; (2) perfect freedom from all lost motion; (3) perfect freedom from any side motion of the image; and (4) extraordinary durability. The microscope is provided with a movable slide holder, serving as a substitute for a mechanical stage. This slide holder consists of a German-silver plate of very light weight, moving on a strong glass plate which forms the immovable stage ; only four small points of the German-silver plate touch the top of this glass stage, while two prolongations of the former, bent downward and backward, and acting as springs, press against the underside of the glass plate with just suffi- cient force to keep the slide holder in position, and to prevent it from slipping off when the instrument is inclined. Two small knobs facilitate the handling of this slide holder, and it is claimed that this arrange- ment exceeds in smoothness and evenness of motion the movable glass stages commonly used, while the mova- ble part has less weight, and allows the glass plate to be of sufficient strength to guard against easy breaking. The instrument has also plane and concave mirrors ; sub-stage of the extra size required to receive standard FAMILY MICROSCOPE. 57 sized English accessories; revolving diaphragms, etc. These are all attached to the swinging mirror bar, the axis of which is placed at the level of the object, so that the diaphragm and mirror swing concentrically around the object. The mirror can also be moved on the mir- ror bar to and from the object, and the distance between the latter and the sub-stage can be varied by reversing it. Both sub-stage and mirror can also be removed. It is to be observed that in these two instruments the importance of the swinging bar, before described in connection with the Centennial and Histological stands of Mr. Zentmayer, has been recognized by Mr. Gruncl- lach. The mechanism, how r ever, by which these makers accomplish the swing in the plane of the object, is by no means the same in their respective stands. Thus the peculiar method of securing the stage to the limb, em- ployed by Mr. Zentmayer, allows that the sub-stage with its accessories, and the mirror, be brought entirely above the stage, the only thing preventing &full revo- lution being the body tube, while in the instruments of Mr. Gundlach the swinging-bar plays in a slot cut in the rear of the main stage plate, and the swinging motion is thus circumscribed by the stage slot. FAMILY MICROSCOPE. This has japanned cast-iron foot and pillars support- ing the axis which carries the body, so that it may be inclined to any angle revolving diaphragm below the stage; rack and pinion for adjustment of focus; con- 58 HOW TO SEE WITH THE MICROSCOPE. cave mirror, adjustable for oblique light; plain stage with spring clips. This, although the least expensive instrument made by Mr. Grimdlach, is nevertheless well put together, strong and really serviceable, and, with proper care, ought to last a lifetime. It can be readily made capa- NOTE. Since the above was written, we learn that Mr. Gundlach has separated from the B. & L. optical company, and that he now devotes his entire attention to the manufacture of object glasses, the Messrs. Bausch & Lomb, however, continued to furnish both stands and objectives, and are by purchase, the proprietors of many of Mr. Gundlach's patents. A 1. 59 ble of doing most ol the work required by the physician and to those who can afford the luxury of having two stands something of this kind will be found a real con- venience. Mr. W. H. Bulloch, of Chicago, manufactures seven different forms of stands, from which we present the following : BULLOCK'S FIEST-CLASS MICROSCOPE A 1. It has the concentric, rotating, and mechanical stage, with graduations for measuring angles ; is also adjusta- ble, so that it can be accurately and perfectly centered. There are also graduations connected with the horizon- tal and vertical movements of the stage, by which the exact position of an object can be noted and found with more certainty than with the " Maltwood Finder." The whole stage is sufficiently thin to admit an angle of oblique light as high as 134 degrees, and, if required, the stage can be made reversible. The sub-stage is fitted with the most complete move- ments for centring or for oblique light with the achro- matic condenser. It has one-fourth inch movement each way ; rack and pinion ; divided circle for polariz- ing; is so arranged that the sub-stage can be used either above or below the main stage, and can be oper- ated by hand or by tangent screws ; it is entirely separ- ate from the mirror, but if desired can quickly be connected, so that the mirror and sub-stage turn 60 HOW TO SEE WITH THE MICROSCOPE. together around the same centre, which is the thickness of an average slide over the stage. The entire sub-stage with its milled heads can be taken off, so that there shall be nothing in the way when using direct light. The mirror is arranged so that it can be used in any direction, backward, forward, over or under the stage. The mirror and also the sub- stage have graduated circles, so that the obliquity can be noted. The movement of the body is effected by rack and pinion connected with two milled hea^s, which connect with the lever of the slow motion, thus preserving the distance between the objective and the eye-piece. The slide on the body tube is long and broad, thus prevent- ing vibration or lateral motion when using the milled heads or the micrometer screw ; the latter is grooved so that it can be used for photographic purposes. The instrument is mounted on tripod base, with re- volving platform. The platform is graduated, and upon it two standards are fixed, between which the instru- ment turns to the angle when so used, or turning hori- zontally for drawing, or for measuring angular aperture. The centre on which the instrument turns, when placed horizontally, is in a direct line with the object on the stage. The binocular model is arranged with rack and pinion for different width of eyes ; the prism is so fixed that the distance remains the same. The society screw at the end of the body is arranged as a safety nose-piece, with spring, so that the danger of breaking slides is BULLOCK'S FIRST-CLASS MICROSCOPE A 1. 63 avoided. The iris diaphragm has the society screw so that any objective can be used for a condenser, or it 'can be used above the objective as an adapter, to reduce the light in the instrument. The stand is nineteen Inches high when arranged for use. This stand and the " Centennial " of Mr. Zentmayer, may be considered as rival instruments, and neither of them enters the list for a slow race ; both are respec- tively the masterpieces of their makers, and are claimed to be the very embodiment of all and singular that can be desired by the microscopist. Purely in the interest of the reader, we proceed to compare these stands with each other ; the final verdict shall be left a matter of individual election. First. The price of both instruments is precisely the -same, while the Bulloch has additional the iris dia- phragm, the mechanical stage with its adaptations for 64 HOW TO SEE WITH THE MICROSCOPE. use as an object finder, the safety nose-piece, the duplex arrangement of sub-stage and mirror arm, whereby these move in unison, or independently of each other r and the centring and rotating sub-stage. To meet this, it might be claimed, on the part of the- Centennial, that the above-named additional contri- vances have been purposely discarded by the maker of the Centennial ; that really there is no particular advan- tages to be derived from the iris diaphragm, the mechan- ical stage, the centring sub-stage, or the duplex arrangement above referred to, and that all these con- trivances serve unduly to complicate the instrument. As to the centring sub-stage, it might be also said that with the introduction of the " swing" there is no longer need of wide-angled achromatic condensers. Furthermore the secondary body of the Centennial being itself accurately centred, there is no occasion to introduce especial appliances for this purpose. It will be noticed that in the Centennial the mirror and sub-stage rise without hindrance above the stage, the movement being only stopped by contact with the body tube (supposing the smaller stage to be employed,) all this time the mirror remaining in fixed focal posi- tion. To accomplish the rise of the mirror above the stage in Mr. Bulloch's stand, the jointed arms connected with the mirror are brought into play, and indeed can- not be dispensed with. A close comparison of the mechanism will reveal the fact that in respect to the swinging arrangement, there is a wide difference in the construction of the two instruments. The angle of ME. BULLOCK'S "D" STAND. 67 obliquity obtained with the Centennial stage, is greater than that obtainable in Mr. Bulloch's stand. The principal points are thus presented ; it remains for the reader to use his own election. Either of the two stands will bear comparison with any foreign stand extant. Mr. Bulloch also supplies another stand, which he calls his SMALL BEST STAND " A-B." This instrument is similar in construction to the large stand, but smaller, excepting the body tube, which is of the standard length in all his instruments. MR. BULLOCH'S "D" STAND. This has been selected by the author for description principally because it is a medium-sized " student's stand," and furnished with a concentric adjustable stage, it being desirable to present as much variety as possible within our circumscribed limits. The adjustable concentric stage can be centred to any objective ; the edge of stage is bevelled and graduated, so that angles of crystals can be measured ; it has a complete revolution. Its glass stage is perforated in the centre, has brass fittings, so that the "Maltwood Finder " can be used ; and its motion is perfectly smooth under the highest power. To the underside of stage is fitted a tube for accessories; this can be removed so that the utmost angle of oblique light can be obtained. Plane and concave mirrors and lateral motion for ob- 68 HOW TO SEE WITH THE MICROSCOPE. lique light, coarse adjustment by rack and pinion, fine ditto, by delicate screw, diaphragm; draw tube gradu- ated, with society screw at end. The instrument can be inclined to any angle, has English horseshoe base, and is sixteen inches high when arranged for use. BULLOCK'S NEW BIOLOGICAL STAND. The cut shows the instrument about two-fifths the real size, it stands twelve and a half inches high, and the stage three and a half from the table when in an upright position. Body tube five inches long, draw tube five inches long; with marked ring when drawn to the standard length. Standard size and sub-stage fitting, adapted in sub-stage with the society screw for the use of objectives as condenser, and the diaphragm fits in the same screw and can be used close up to the object slide. Plain and concave mirrors each, swing over the stage can be used together, or separate, can be clamped in any position, by milled screws shown behind the limb. Spring stop to mirror and sub-stage when in line axis. Concentric revolving stage, can be adjusted concentric to the axis by means of capstan- head screws, the sub-stage also can be adjusted con- centric by means of capstan screws. Spring clips to stage or the sliding glass stage as desired. When the stage is not required to revolve, it can be clamped in any position by the milled screw shown in front; and when there is any danger of injuring the stage by the use of acids, the stage lifts out of the ring in which it revolves, and any* ordinary piece of glass E. & J. BECK'S MICROSCOPES. 71 can be used on the ring by placing the instrument in an upright position. Eack and pinion quick motion, lever fine motion placed behind the limb moving the whole body tube; the body tule is fitted with broad gua^e screw one and a quarter inch diameter, and in which fits adapted with regular society screw, tripod ba'se with single pillar, the axis on which the instrument turns is placed in such position that the instrument is perfectly balanced when placed in horizontal position for drawing. When so placed the centre of eye-piece is seven and a half inches from table, the standard is furnished with a B eye-piece. The mirror and sub- stage can be fitted with divided arch for measuring the obliquity of light, the sta^e can also have divided circle for measuring the angle of crystals. Stand all made of polished brass. K. & j. BECK'S MICROSCOPE. K. & J. Beck, of London, the old and well-known firm, have established an American agency in the United States. In their facilities for the production of first-class workmanship they are not excelled. As makers of London instruments, they have been long and favorably known, and the Londoner points with commendable pride to the names of Ross, Powel, Lea- land, and R. & J. Beck. No attempt can be made in these pages to describe a tenth part of the stands manufactured by the Messrs. Beck. Their catalogue will be found to include all the various grades, where may be found elegant and expen- 72 HOW TO SEE WITH THE MICROSCOPE. sive stands, which, with the accompanying accessories, etc., cost $1,600 and upwards, down to the more simple forms, within the reach of all. It is from the latter that we make the following selections : BECK'S POPULAR MICROSCOPE, MONOCULAR OR BINOC- ULAR. In this stand the arrangement for changing the angle of inclination of the body is new and durable. It has coarse adjustment by rack and pinion, fine adjustment by micrometer screw ; the stage is fitted with improved object-holder and concentric revolving fittings; the mirror slides on the main stem, and has its own semi- circle for universal motions. A diaphragm is provided with perforated revolving disk. The instrument is on the "transverse arm" model, which has been so ex- tensively used by Ross, of London, as well as* others. The instrument can be furnished with all the usual accessory apparatus as supplied by the makers; a mechanical stage, giving horizontal and vertical motions by screws, is also furnished if desired. BECK'S ECONOMIC MICROSCOPE. The makers say in their catalogue that " the micro- scope is now such an absolute necessity for the student, to enable him satisfactorily to carry on his investiga- tion, that it is more than ever incumbent on the optician to construct a sound economic instrument adapted to the special requirements of this large and increasing class." To this proposition let this little book say, Amen. FLQJL BECK'S ECONOMIC MICROSCOPE. 77 " For ordinary pathological, physiological, and botan- ical investigations, many of the delicate adjuncts applied to the higher-priced instruments are unnecessary, and tend rather to confuse than to assist the beginner." To this let these pages say again, Amen. ' A firm stand and well-corrected object-glasses are, however, indispensable." In response to this we not only say amen, but shall, in the proper place, have something further to add. "The stand of the "Economic" is made in two forms : the one with a sliding adjustment for focussing the object, and the other where the quick movement is produced by rack and pinion. On both stands the fine adjustment is given by means of a milled head at the top of the stem. The stand is fitted with the society screw. The foundation of the stand is a heavy horseshoe base A, at the head of which is a firm pillar B, having at its top a hinge joint C, which allows the body D to be inclined to any angle, and is sufficiently firm to per- mit of its beino* placed horizontal for use with the camera lucida. The body tube is short, but by means of the length- ening draw tube Y can be made of the standard length. In the more expensive stand the coarse adjustment is by rack and pinion; in the cheaper instruments the quick movement is produced by sliding the body D up and down the tube H, sliding over the inner stem with a spring inside, the fine adjustment being accomplished by the milled head I. 78 HOW TO SEE WITH THE MICKOSCOPE. The stage K, upon which the object is placed, has two springs LL, the pins attached to which may be in- serted in any of the four holes on the stage, and by their pressure, which can be varied, they will hold the object under them, or allow it to be moved about with the greatest accuracy. The mirror M, besides swinging in the rotating semi- circle N, attached to a bar O, with a joint at each end, allowing a lateral movement, so as to throw oblique light on the object, and for this purpose the tube beneath the stage, carrying the diaphragms, has semi- circular openings cut on either side, leaving a clear and thin stage, allowing the utmost obliquity of illumina- tion. This tube also carries the polariscope, etc. The diaphragm P slides in the sub-stage fitting, and consists of a tube containir.g two caps furnishing two sizes of openings, immediately in contact with the under surface of the slide to be examined, and also completely cutting off all light from the mirror, when opaque objects are to be viewed. The instrument packs, without being taken to pieces, in a small and neat case, is very convenient for travel- ing purposes, and entirely adequate for very many pur- poses. The general workmanship seems in keeping with the reputation of its makers, while its cost places it within the reach of all. At an advanced cost, it can be supplied with all the usual accessories. Last, but not least, I now have the pleasure of pre- senting to my readers BECK'S NEW HISTOLOGICAL DISSECTING MICROSCOPE. 79 BECK S NEW HISTOLOGICAL DISSECTING MICROSCOPE. This instrument combines a compound microscope with a single and dissecting one in a very compact and practical form. The stout immovable arm carrying the lens when used as a single microscope, is so arranged that a compound body with eye-piece and draw lube may be attached to its upper surface, whilst beneath it is 80 HOW TO SEE WITH THE MICKOSCOPE. fitted with the society screw, whereby an objective may be used with it. The rack-and-piuion adjustment work so smoothly that a one-fourth inch objective may be focussed with exactness. The mirror beneath the stage is so adjusted upon a swinging arm that it may be turned up over the stage for the illumination of an opaque object. A revolving diaphragm, with various sized openings, is attached to the under side of the stage. The outfit consists of a single lens of one-inch focus for dissecting and botanical work, and an achro- matic objective of one-fourth inch focus, the same as furnished with the economic microscopes, and one eye- piece, giving a range of powers, with the draw tube, of between 200 and 300 diameters, a pair of brass pliers, two dissecting needles in ebony handles and a glass plate with ledge. The whole packed in a neat mahog- any case with lock. Several accessories are applicable to this instrument. THE NEW NATIONAL MICROSCOPE. The stand which is fifteen inches in height, is con- structed entirely of brass, of the highest finish and best workmanship, having a broad, heavy tripod base. From the centre of this base rises a stout column, to the top of which is attached, by a firm joint, the Jackson model arm, carrying the compound body, by which the inclin- ation can be varied to any degree, from vertical to hori- zontal, the whole instrument being perfectly steady and free from tremor in any position. The very highest powers may be used with it, as the body, being sup- THE NEW NATIONAL MICROSCOPE. 83 ported by the arm throughout its entire length, cannot have any unsteadiness or motion of its own. The quick adjustment of focus is effected by means of rack and pinion, with ]arge milled heads, which works so smoothly that there is no need to use the fine adjust- ment for any power lower than one-quarter of an inch. The latter adjustment is by means of a delicate micro- meter screw and lever attachment, working with abso- lute freedom from all motion, and by which the very highest powers may be focused with the greatest exact- ness. The stage is of glass, with a complete rotation in the optic axis, upon the top of which is a sliding object- holder, very thin, and with a spring clip for holding the object in place during rotation. This clip is removable, in an instant, and the stage forceps can be inserted in its place, thus allowing the latter to be moved about with the object-carrier. Beneath the stage is a tube carrying all the sub-stage apparatus, as the achromatic conden- ser, Wenham's parabola, polarizing apparatus, etc., etc. This is securely attached to the stage by a bayonet- catch, and can be instantly detached, leaving a very thin and unobstructed stage for oblique illumination. The shutter diaphragm is of novel construction, with the various-sized openings almost in contact with the underside of the object under examination, a great improvement upon the old revolving disk diaphragm. A double mirror concave and plane is hung upon a swinging bar, and with every possible motion for di- rect and oblique illumination. 84 HOW TO SEE WITH THE MICROSCOPE. THE NEW ACME STAND. During the session of the " congress" of microscop- ists at Indianapolis in 1878, Mr. John W. Sidle, of Philadelphia, and myself formed ourselves into a com- mittee of two for the purpose of devising a new micro- scope stand. Mr. Sidle and myself were agreed in our opinions that notwithstanding the recent improvements which of late years had obtained, that there was still room for further effort in the construction of microscopes. Is was then and there proposed, deliberated on, and agreed, that we would unite our energies in the en- deavor to construct a microscope stand, which should combine every possible good, be equal to any and all work, exhibiting all the latest appliances, and withal, to combine really reliable workmanship, at the lowest possible cost, in fact at figures no higher than those commanded by inferior instruments. This arrangement contemplated that the author should furnish all the suggestions which his long ex- perience with the microscope might afford, while Mr. Sidle was charged with the mechanical part in the con- struction of the new stand. This agreement has been carried out to the letter, arid from the date named, until August last, Mr. Sidle and myself have been in close correspondence, and for the purpose named. The first stand built under our compact was received by me from Mr. Sidle early in August last, and as a THE NEW ACME STAND. 85 matter of course, it got a pretty severe overhauling, suffice it to say that I became so much pleased with our new bantem, that I gave it the name it bears, and have no hesitancy in recommending this joint production of Mr. Sidle and myself to my friends. With this preliminary statement, I proceed to give the reader a brief description of our new stand. The Acme with closed tube, when in the vertical posi- tion, is about fourteen inches in heighth, and its weight about five pounds. The foot is of cast-iron; " horseshoe" shaped and similar to the Hartnack patterns, combining in the smallest compass the necessary weight and accompany- ing solidity. For real work in the laboratory this form of foot is believed to be superior to all others. For those who prefer the tripod model, Mr. Sidle will fur- nish this form at the same cost, or in finished brass at an additional cost of $3.50. The entire instrument is supported by a heavy single pillar of solid brass, the lower portion of which passes through the cast-iron foot, the two parts being held firmly together by a clamp-screw underneath the foot. By virtue of this arrangement, the foot can be detached almost instantly, or, by a half-turn of the lower clamp, the foot can be reversed on the axis of the pillar, thus ensuring the greatest stability when the stand is in the horizontal position. The lower shoulder of the brass pillar passing through the foot is accurately turned and fitted, and when desired the moulding at the part of the brass pillar immediately adjacent to the top of the 86 HOW TO SEE WITH THE MICROSCOPE. foot can be divided to degrees for the measurement of angular aperture, my own stand is thus arranged. Besides those already mentioned other advantages occur, from having the foot of the stand removable, some of which may be enumerated as follows : First, by detaching the base, the instrument can easily be carried in the pocket; it is also much easier to pack, and occupies much less space in the packing. It can thus be carried from place to place even in the nar- row accommodations afforded by a small valise, the bother of lugging about the usual microscope case being no longer necessary. Second, Mr. Sidle provides at a small extra cost a neat black walnut base board, furnished with small lamp fitted with universal movements. This board has also the necessary brass fittings to receive the pillar of the microscope, which can be clamped to the board. By this arrangement all that is necessary to convert the instrument into a first-class hand microscope is to bring the body to the horizontal position and to adjust the object and the illumination. With the stand as thus arranged I have repeatedly exhibited objects illustrative of my lectures to the col- lege class of 150 students, the instrument being passed from hand to hand throughout the entire class, and re- turned to me everything remaining in perfect order. To teachers this feature of the stand will be of value. It will also be found a great convenience at times when the microscope is called on to furnish entertainment in THE NEW ACME STAND. 87 the family circle, and objects can thus be exhibited with great rapidity. By means of a strong trunnion joint the body of the instrument can be inclined at any desired position between the vertical and horizontal, the requisite stability of motion being secured by heavy " cheek- blocks;" the joint has also compensation for wear. All the working part of the Acme above the base are of solid brass, bright finished, and nicely lacquered. The main body-tube is one and five-sixteenth inches in diameter. This tube articulates with the limb by means of heavy "T" guides or angle pieces, thus secur- ing broad bearing surfaces and also perfect freedom from lateral displacement. Mr. Sidle has put himself to much trouble to perfect this portion of the stand. The main-tube is five inches in length. This is sup- plemented with a draw-tube, which can be drawn out to the standard length of ten inches when desired. The coarse adjustment is by rack and pinion. The rack is well cut and durable, and the movement of the tube by means of operating the large milled heads is exceedingly smooth and entirely without "lost motion." The fine adjustment is by a large milled head place at the rear of the limb, and operating the main tube. This milled head is one and one-fourth inches in diam- eter, and is divided into twenty divisions. It acts directly, i. e. 9 without lever, on a micrometer screw cut fifty threads to the inch, each division of the milled head representing one one-thousandth of an inch. The fine adjustment can therefore be easily made to answer 88 HOW TO SEE WITH THE MICROSCOPE. the purpose of a micrometer for measuring small inter- vals, or for the measurement of cover glasses, etc. In this adjustment the same security against lateral dis- placement is provided for by means of the " T" angle- pieces, as applied to the rack and pinion movement. The lower end of main tube carries a nose-piece fitted with the " society screw." This piece can, however, be detached so that the broad-guage objectives now in pro- cess of construction can be used on this instrument. The main stage consists of a circular metal plate, three and one-quarter inches in diameter, firmly bolted to the heel of the limb, and in such a manner as to be isolated from the movements of the sub-stage apparatus; four holes are drilled through the main stage plate, and so arranged that the spring clips may be adjusted to hold the object-slide in either a vertical or horizon- tal position. The spring clips may also be tiansferred to the under surface of the stage, holding the object- slide in contact therewith, when very oblique illumina- tion is to be employed. The well-hole is of the usual size and is provided with a standard screw-thread, by means of which the Woodward prism and other acces- sory pieces can be readily placed in position. The stand is thus fitted for such emergencies as required a fixed and central sub-stage, separate from the movements of the mirror ; the polariscope is also nicely provided for. (NOTE. It has lately come to the surface that the diameter of the * society screw" Is not sufficiently large to meet the requirements of the optician when low powers of the widest apertures are demanded. Mr. Sidle is now making for the author a one -inch glass having a diameter of over one inch, and the Acme has been designed to meet such requirements). THE NEW ACME STAND. 89 The main stage has als*o conveniences for centering. Furthermore, a solid plug is furnished which screws into the well-hole, and forming when desired, a solid stage. This plug has an "X" engraved thereon for centering purposes. The sub-stage proper, as well as the concave mirror, are attached to a swinging bar by dove-tailed blocks, The slides having compensation for wear, the sub-stage can be centered. It is one and one-half inches in diam- eter and is fitted to carry any of the accessory pieces usually accompanying a first-class outfit. The sub- stage and the mirror both slide with easy friction on the swinging bar. The swing-bar traverses the face of a circle of brass placed at the rear of the stage, the centre of this circle being in the plane of an object placed on the stage; the centre of this circular plate is solid, that is to say, it presents a solid core of about one inch diameter; outside of this an angular groove is cut therein, in which swings the heel of the swing bar, the method of at- tachment of the swing bar to the circular plate giving great solidity as well as firmness in motion. Thus it will be seen that the " swinger" swings not on a centre but around a centre. This part of the mechanism needs only to be seen to be appreciated. The mirror is of course removable, and a toy candle- holder is provided to take its place for the measure- ment of apertures. A supplemental circular and revolving stage plate is also furnished, which "slips" on and off instantly as 90 HOW TO SEE WITH THE MICROSCOPE. may be required. And by virtue of a nice little con- trivance of Mr. Sidle's, the rotatory movement is very smooth and nearly as central as would be expected from stands of much higher cost. When desired, Messrs. Sidle and Poulk furnish a mechanical stage which slips on and oft* in place of the rotary plate above mentioned; this mechanical stage has vertical or horizontal motions to the extent ot three-fourths of an inch. This, however, involves an extra cost of $14. To exhaust the list of American makers would be- quite beyond the province of this book. There remain the names of Grunow, McAllister, Pike, Queen, Schraueiv Wales, George Wale, and others. Descriptions of the stands supplied by these various makers have been omitted, partially because the author unfortunately has- little acquaintance with their work, while that of oth- ers which at times he has seen, seem practically to be much the same as those already described, and the- reader would gain nothing by the repetition. By way of concluding this chapter, and as supple- mental to what has already been said as to the choice of a stand, the writer would especially insist on the im- portance of a stage thin enough to admit a beam of light at 70, from axis, not that the observer will always work with oblique light, but, when occasion calls for it, the stand should be capable of responding, and, as has been before hinted, in these latter days these kind of calls occur more frequently than was the case in the days of yore. Let the novice especially, then, who THE NEW ACME STAND. 91 proposes to purchase a stand that he will not feel com- pelled to sell again in a few years, in order to procure a better one ; keep this point well in mind. The author, too, unhesitatingly gives his support to those stands having the swing stage and mirror; the advantages accruing from this late improvement are very valuable, of which we make particular mention of one, to wit: In former times, when sub-stages were practically a fixture, it became necessary, in order to secure oblique light, to employ acchromatic condensers of wide aper- tures and short focal distance ; the instrument was costly, and necessitated the employment of accurate and expen- sive sub-stage fittings, and, in general, could only be em- ployed at a great sacrifice of pains, time and attention. With the introduction of the swinging sub-stage and mirror, all this is a thing of the past; the wide-angled condenser is no longer necessary for the display of diffi- cult structures; on the other hand, it now seems desir- able to employ condensers of the lowest angles, the required obliquity being obtained by swinging the sub- stage, mirror and condensers to the proper angle. The expensive centring apparatus not being required with the low-angled condenser, their cost and bother are thus avoided; at the same time better results are obtained, and in a simpler and more convenient way. There are other advantages, too, pertaining to the use of the swinging stage and mirror, of which mention will be made in the succeeding pages. Finally, let me impress on the mind of the reader 92 HOW TO SEE WITH THE MICROSCOPE. that the large and expensive stands are not at all neces- sary; the smaller and cheaper stands, if due care be taken in the selection, will do any practically all work required in the progress of scientific investigation. Furthermore, keep constantly in mind the fact that a great deal of work, legitimately in the line of investi- gation, can only be conveniently accomplished with the small stands. Where expense is no object, or even when one can afford two stands without feeling thua crippled in other directions, it will be found a luxury to have the two. In the very nature of things, it is impossible to obtain the firmness and solidity of a large and heavy instru- ment through the medium of the smaller stands, and occasionally this stabilit}' is a real convenience, resulting in a saving of time and labor, as, for instance, in draw- ing with the camera lucida. The graduated revolving platform for the measurement of angles of aperture, and similarly the circular graduated stage, with its centring adjustments, are handy things to have in the house, but cost money, and, as a rule, can be dispensed with. And last in this matter, far from being least should the res angusta domi pinch so bitterly as to compel you for the time being to omit some necessary investment, then I say, by all means make sure of the necessary ob- ject glasses, quantity and quality included, and let the stand " sweat" until the "good time coming" shall enable you to provide a new and superior instrument. Remember, too, that the finest and most expensive stand extant, fitted to a poor objective, becomes not only a dangerous tool, but also a positive nuisance. CHAPTER II. WHAT IS ANGULAR APERTURE. During the lengthy controversy which occur ed be- tween Mr. F. A. Wenham. optician to the firm of Ross and Co., of London, and Mr. R. B. Tolles, of Boston, Mass., the originator of the celebrated duplex object glasses, the former presented ideas as to the functions- and nature of angular aperture which were not in strict accordance with the popular views. Since then others have done likewise, and it may be well enough to leave the question open, without attempting to answer the above interrogatory at all. For the information of those, however, who are just entering the study of microscopy, we w T ill undertake to- tell what angular aperture was, or to be more definite, what it was in 1856; and for this purpose, the author selects the definition found in the Micrographic Diction- ary by Messrs. Griffith and Henfrey, a work generally acknowledged as authority in matters microscopical. The angular aperture of an object-glass is the angle measured by the arc of a circle, the centre of which is formed by the focal point of the object-glass, the radii being formed by the most extreme lateral rays which the object-glass will admit. Thus let L, in the left-hand figure 1 below, repre- sent the lower portion of a microscope, objective, 93 JJ4 HOW TO SEE WITH THE MICROSCOPE. & perpendicular section of the lowest combination of an object glass of small aperture. "a is the angle of aperture, and/ e the most oblique rays which the object-glass will admit. The angle is measured by the dotted arc b in the object-glass of larger aperture (Fig. to the right) ; the arc 6, which measures the angle, is much larger, and the radii repre- senting the extreme lateral rays are much more oblique. Hence it is evident that the object-glass of larger aper- ture admits all those rays admitted by that of less aperture, and a certain number of other rays, these being more oblique." The above definition has been and is now generally accepted. Messrs. Griffiths & Henfrey proceed to give their demonstrations of the effect of aperture, as thus defined, in an interesting and lengthy article, one that is well worth perusal by those desiring to know how things stood twenty-two years ago. Our limited space forbids its reproduction here. HOW SHALL WE MEASURE ANGULAR APERTURE. 95 HOW SHALL WE MEASURE ANGULAR APERTURE. While the nature and functions of angular aperture remain a matter of controversy, this too may be allowed to remain an open question. Recognizing, however, the fact that object-glasses continue to be made and sold, the makers claiming for them specific apertures, we proceed to give the plan usually adopted by the optician, and at the same time adapted to the use of those working with the smaller class of stands. It will be advantageous to the beginner to operate either in a dark room or in the evening. Select a good sized table, and on this, at the end nearest the operator, pin down a large sheet of paper, say twenty inches square. On this paper place the microscope, bring the tube to a horizontal position, and screw on the object- ive to be tested. Mark with a pencil certain points along the base of the stand, so as to enable you to describe a circle with pencil and dividers, in which the base will just revolve without lateral play. Next, provide a common candle, and, if necessary, cut this down so that the height of the flame shall be level with the objective in its horizontal position ; light the candle, point the objective directly at the light, having pre- viously removed all sub-stage incumbrances, crouching down, and with the eye at the eye-piece, adjust the direction of the instrument as nearly as possi ble to the candle ; the field of view will of course be illuminated from the radiant. Now revolve the entire instrument, being careful to 96 HOW TO SEE WITH THE MICROSCOPE. keep the base always within the circle drawn on the paper, and continue the movement (either to the right or left, as the case may be), until one-half of the field shall be bisected i. e., one-half bright and one-half in the shade. Now select some straight portion of the base, marking a straight line coincident therewith with a pencil, or, placing a rule in contact with any two salient angles of the base, draw a line. Next, revolve the entire stand in the contrary direc- tion, passing the caudle, and until you again get a field half bisected similarly, as before mentioned. Now selecting the same side, or the same two salient corners of the stand, mark another line. These two lines will be found divergent with each other, and with a parallel rule it will be found easy to produce parallel lines which can be continued until they meet; this done, it only remains to measure the angle obtained with a common protractor, or other in- strument designed for the measurement of angles. Where tolerable accuracy is important it will be well to cross-question the result by repeating the operation. There ought not to be a discrepancy of more than one- half of one degree. Those who possess the large stands fitted with gradu- ated and revolving platform will of course avail them- selves of the convenience; the operation is practically the same; thus, having bisected the field (say to the right), read the angle, which will be indicated by a mark on the platform adjacent to the graduations ; then bisect the field to the left, read the angle again, sub- OBJECT GLASSES. 97 tract one reading- from the other, and the difference is the angle of aperture. There being no means for the actual determination of the bisection of the fields, this having to be judged of by estimation, follows that the plan is not rigidly accurate ; the author has, however, measured repeatedly the same object-glass, following as above described, and without a variation in the results of more than a half degree; many of his pupils, too, on their very first attempts, are quite as successful. The bisection of the fields is much more easily arrived at in the evening. OBJECT-GLASSES. Years ago wrote Dr. Carpenter thus : " It may be safely affirmed that the most perfect object-glass is that which combines all the preceding attributes (viz., defining power, penetrating power, or focal depth, resolving power, and flatness of field) in the highest degree in which they are compatible one with another. But, as has just been shown, two of the most important viz., penetrating power, and resolv- ing power stand in such opposite relations to the angular aperture, that the highest degree of which each is in itself capable can only be obtained by some sacrifice of the other ; and therefore cf two objectives which are respectively characterized by the predominance of those opposite qualities, one or the other will be preferred by the microscopist, according to the particular class of researches he may be carrying on ; just as a man who is 7 Microscopy. 98 HOW TO SEE WITH THE MICROSCOPE. about to purchase a horse will be guided in his choice by the kind of work for which he destines the animal. Hence it shows, in the author's estimation, just as limited an appreciation of the practical applications of the instrument, to estimate the merits of an object- glass by its capability of showing certain lined or dotted tests, without any reference to its penetrating power or defining power, as it would be to estimate the merits of a horse merely by the number of seconds within which he could run a mile, or by the number of pounds he could draw; without any reference, in the first case, either to the weight he could carry, or the length of time during which he could maintain his speed, and, in the second case, either to the rate of his draught or his power of continuing the exertion. The greatest capac- ity for speed alone, the power of sustaining it not being required, and burden being reduced almost to nothing, is that which is sought in the racer; the greatest power of steady draught, the rate of movement being of comparatively little importance, is that which is most valued in the cart-horse ; but for the ordinary carriage- horse or roadster, the highest merit lies in such a combi- nation of speed and power with endurance as cannot coexist with the greatest perfection in either of the first two. The author feels it the more important that he should express himself clearly and strongly on this sub- ject, as there is a great tendency at present, both among amateur microscopists and among opticians, to look at the attainment of that resolving power which is given by angular aperture as the one thing needful. . . . OBJECT GLASSES. 99 ft is neither the only nor yet the chief work of the microscope (as some appear to suppose) to resolve the markings of the siliceous valves of the diatomacea ; in fact the interest which attaches to observations of this classier se, is of an extremely limited range. . . . And the more carefully we look into the history of those contributions to our knowledge which have done the most to establish the value of the microscope as an instrument of scientific research, the more clear does it become that for almost every purpose except the resolu- tion of the diatom tests, objectives of moderate angular aperture are to be decidedly preferred." * This quotation -essentially true when first published has been, and to a considerable extent now is, the orthodox faith ot the microscopist. It has been en- dorsed by everybody, both at home and abroad; the world of observers have rested peacefully at ease on its broad platform; the man of science was serene and content with his medium apertures; but it was the crowning glory of the quack that a " good hcnest," low-angled, " working" triplet cost only a dollar or two. Adid it was a fact, that so far as faith was con- cerned, the scientist and the veriest quack were in the same boat together, and both found pleasure in endors- ing the teachings of Dr. Carpenter. It seems to me that this singular state of things can be easily accounted for. The hard-working investiga- tors had little opportunity to study object-glasses, their time being completely occupied, and their eyes sorely *"The Microscope and Its Revelations" fifth edition, 18Y5, page 204. 100 HOW TO SEE WITH THE MICROSCOPE. taxed in the daily routine of labor and study. Their glasses were the best that could be procured when pur- chased, and, as far as their personal experience with them could attest, their experience was in harmony with the laws laid down by Dr. Carpenter ; and they had u no time to waste " in fussing over " diatom tests," etc. On the other hand, the empiric replies, " Your high- angled glasses are all well enough for the diatom man, but for my work give me reliable French triplets of moderate apertures;" and in this statement the author is in entire and perfect harmony ! The author repeats, that ten years ago the doctrines taught, as contained in the previous quotation of Dr. Carpenter, were essentially true; he has 110 fault to find with their original publication, but he does regret that Dr. Carpenter has allowed them to retain their place, unchanged or unre vised, in his late editions. Let us glance for a moment at the history of the ob- ject-glass as connected with its aperture. Years since it was known that the exhibition of surface-markings required to a greater or leeser extent the employment of lateral or oblique light; width of aperture hence became a desideratum, at least in such glasses as were to be used with this object in view. Whether right or wrong, demands were made on the optician for objectives possessing increased apertures. Notably, this demand was first met successfully by our own countryman , the veteran Charles A. Spencer, who produced a glass of wider angle than had been pre- OBJECT GLASSES. 101 viously accomplished and thereby astonishing the world, sorely punching the English opticians (who flattered themselves on secure ground at the head of the profes- sion) with an exceedingly sharp stick; with this glass Mr. Spencer succeeded in displaying both sets of lines on the diatom, now known as Navicula Spencerii. The London makers followed suit, giving especial at- tention to the extension of aperture, and, as might be reasonably expected, the problem was to get light of extreme obliqaity somehow through the lens, nor were they (the makers) very particular to stand on the order of its going; hence it came about that very many glasses were made and sold having increased angle, by virtue of which they were able to display certain ac- knowledged difficult tests ; but, on the other hand, the lateral pencils being poorly corrected (if corrected at all), these glasses could not compete by centrally dis- posed light with the well-corrected but narrow angles previously in use. It was, therefore, to the wide-angled glasses of that clay that Dr. Carpenter's remarks had force, the highest possible angle attainable being then limited by popular opinion to Ib5. What is now known and recognized as a high-angled glass of 175 was then not only unknoAvn, but would have been deemed a sheer im- possibility, and it therefore becomes obvious that any remarks at the time referred to, whether by Dr. Car- penter or by other authors, cannot be applied consist- ently to the wide-angled glasses of the present date. Those were the days, too, when the resolution of the 102 HOW TO SEE WITH THE MICROSCOPE. nineteenth band of the Nobert test-plate was said " to be a matter of faith rather than of sight." The curious reader is informed that the original of the quotation will not be found in the last edition of the ' ' Microscope and its Revelations ! " Again, the high-angled objective, as at present con- structed, is an entirely different affair from that of former times. In place of achromatism, then thought to be the ultima thule of perfection, the finest glasses now made can hardly be said to be achromatic. And then, again, the immersion system has been adopted with its manifold advantages. Improvement upon im- provement has almost marked the rolling months of the calendar, while each succeeding year, to the number of ten or more, has been richly laden with the fruits of the optician. In this wonderful march of progress, our own coun- try, I am proud to say, has ever been at the front, and with colors flying. As before stated, Spencer, with his resolution of the navicula, commenced this march of progress, and slackened not the pace until the reputa- tion of American objectives were duly acknowledged abroad. Then, indeed, he retired for a time, resting gracefully on his well-won laurels; and now appears Tolles at the van, who with almost superhuman genius and energy grapples with the very laws of optics, and bends them to his inflexible will ! Notwithstanding that the reputation of the country was quite safe in the hands of Mr. Tolles; Spencer, after a year or two of rest, again comes forward, assisted OBJECT GLASSES. 103 by his two sons, both of them, as a matter of course, " to the manner born," and in the very prime of life and activity. Thus far I have simply referred to Spencer and Tolles, because they have contributed largely to the production of high-angled objectives; but the list of American opticians is not thus complete; we have Wales, Zentmayer, Grundlach, and Grunow yet in reserve, whose productions in their several lines are not inferior to any imported. In the foregoing brief and general history of the American wide-angled objective, the conclusion is in- evitable, first, that during the past ten years something has been accomplished, and that in the interim our opticians have not been idle ; secondly, that the modern high angled glass is to be judged of strictly on its merits, and not by what was affirmed of a medium aperture ten long years ago. The converse of this is equally true, and the author, in presenting his views of to-day, declines to be held responsible for what the future may bring forth. In this place it may be well to consider what, in common parlance, constitutes a high-angled glass. The writer has already in print stated his individual views which have since undergone no change. As a rule, refer- ence being made to wide apertures, most persons are prepared for some great show of figures, such as "175" or "179"; others might call for nearly "ISO "; and then again, others there are who would insist on pass- ing the " impossible" 180 corner, and revel among the 104 HOW TO SEE WITH THE MICROSCOPE. balsam angles to the tune of 100, or even higher. In dealing with objectives of short focal distance, all this may be well enough; the writer, however, prefers to regard as high-angled, any, and all glasses, without reference to their focal lengths, which are endowed with the widest apertures obtainable. If this platform be accepted, then it will occur that a one-inch of 50 should be classed as a high-angled objective, and simi- larly, a two-inch of 25. And, again, it would also then occur that a one-sixth of 130, which fifteen years ago ranked as a wide, would now be classed as a glass of medium aperture. And furthermore it may possibly (yea probably} have place, that there are many observers to-day, loud in their denunciations of the " wide-angles," falling back on "The Microscope and its Revelations " for authority, who, in their habitual use of what are now known as medium apertures, are in truth the real culprits, to whom and for whom were Dr. Carpenter's original remarks intended. As has been suggested on a previous page, there may be seriously some question, not only as to angular aper- ture per se, but as to what constitutes the measure of the same. It is one thing to get light through an ob- jective, and quite another to bring said light into the traces, and render it of use to the observer. There are, too, scores of high-angled glasses (so-called) sold as having angle of 175, and possibly more, that are entirely worthless when worked with pencils beyond 130 ; indeed, many that I have seen would utterly fail when compared with a really good glass of 115. OBJKCT GLASSES. 105 The writer has before Jiim a glass of the latter angle, '(115,) made by C. A. Spencer & Sons. It is what they call one of their " professional series" a dry one- fourth ; the makers ought not to expect that this glass should be called on for work requiring oblique pencils greater than 100. Now this glass this professional one-fourth will give me real good shows, even when worked at all the obliquity obtainable on my Zentmayer stand; and thus have I seen with it the longitudinal markings on the balsamed surriella of the Moller test-plate, and this is a test that will defeat many dry eighths, engraved by their makers as having 160 or more angle. From this little experiment one that the writer has repeated scores of times in the interests of his friends :some curious conclusions might be arrived at, which, although possibly coherent and plausible in detail, become absurd when considered collectively; thus it misfht be held : r^ First. That both glasses having at least 160 of aper- ture, are in fact high-angled glasses. Second. That of the two named, the Spencer is the better glass. Third. That the Spencer objective has realty but an angle of 115, as marked by its maker; that its capa- bility of admitting working beams up to 140 gives it no real claim to those figures. Fourth. That, as any good eighth of 140 will -easily show the longitudinal markings of the surriella, it is proven that the one referred to has not that angle 106 HOW TO SEE WITH THE MICROSCOPE. at all, and is, in fact, but a medium power glass the general verdict being that it is an indifferent one at the best. Fifth. That the capacity of an objective to display markings on balsamed and difficult tests, at or near the limits of its aperture, is no index of angular aperture. In the case presented, if it so be that the eighth, when worked at 140, gave fair and distinct images of the sur- riella, then there is no reason to dispute the angle as claimed for the glass ; let the experiment be repeated an using angle of 120, and over dry as well as bal- samed mounts. The above may be taken as representing individual differences of opinion. Either of the conclusions pre- sented have been urged on our attention time and time again. To the second, third, and fourth, the author gives his assent, and has never allowed an opportunity to pass without exposing by actual demonstration the fallacy contained in the fifth. From the preceding, then, it becomes apparent that there is much difference of opinion as to what consti- tutes angular aperture or the measurement thereof. Until there can be some more precise plan arrived at, let the purchaser of any objective imperatively demand : First. That the objective, in general, work with its full vim fully up to the limits of the aperture claimed for it; that the images be strong, vigorous, brilliant, and without distortion, and that such images shall not be surpassed in any particular by any similar glass, without reference to its angle. If it be that you have OBJECT GLASSES. 107 a wide aperture in hand, see that its work by central or centrally disposed light is not excelled by any objective of narrow angle extant. In calling on the glass at the limits of its aperture, demand, and see to it particularly,, that there be no letting down of general performance,, that the images remain strong and vigorous, that the corrections are not impaired, and even with the widest apertured objective known, that there be no sensible distortion of the image. Again, if it so be that you desire thus to test an ob- jective claiming considerable aperture, say 170 or 175% and adjustable, note whether there be any special ad- justment required when worked at or near the limits of its aperture, other than necessary for its correct per- formance by central light; if this be found the case f the indications are that the two sets of pencils are not in harmony with each other as the Germans say, are not " married." It will be necessary to use a little discretion here, for some of the very finest glasses re- quire a slight change of adjustment under the condi- tions named. The less of this especial adjustment,, however, the better. Any objective that will stand acceptably the fore- going tests may be allowed to ' < pass muster " as to its aperture sans peur et sans reproche. In the act of writing the above, the author was inter- rupted by a friend with the remark, " Are you not screwing things down pretty fine? Don't you see that your method not only is a severe test as to working- 108 HOW TO SEE WITH THE MICROSCOPE. angle, but is also a severe test as to the general quali- ties of an objective?" In response to this interrogatory, I reply, that it's high time things were " screwed down." As to the latter portion of the remark of my friend, it may be observed that the directions given indicate a part, and part only, of the course to be pursued in testing the performance of a really first-class American objective of wide aperture. The plan proposed is liable to another, and, in the minds of some, a most serious objection; says one, " Don't you see that you have advanced no positive guage? Your idea simply is to compare one glass w r ith another in short, it means 'fighting objectives" Selah! SOMETHING FURTHER ABOUT OBJECTIVES. There is another matter of common acceptance which has in the past made some mischief; I refer to the fact that the focal or working distance of an objective has been and is considered the index of its capacity for cer- tain classes of work. Thus, the incii has been set apart for the study of such objects as required examination, with powers from 50 to 150 diameters, and where its comparatively long working distance was desirable, while the one-fiftieth, whose lowest power of 2,500 diameters and its exceedingly short working distance, could not perform the work of the inch the one- fiftieth being reserved for the investigation of the most minute organisms, and under the highest amplifications. SOMETHING FURTHER ABOUT OBJECTIVES. 10 ( J Now, in the instance cited the popular idea is correct. The inch cannot do the work of the fiftieth, nor can the fiftieth do the work of the inch, and each, as to the other, are to be appropriately brought into use. But, as is well-known, between and intermediate to the scope of the two glasses named, there are several objectives having not only intermediate but variable focal lengths. Among these latter, too, are to be found the objectives known as " medium powers," and it is with reference to these that it may be affirmed that the broad rule governing the inch and the fiftieth does not hold good. Some four or five years since, the author, in writing to a brother microscopist, hazarded the statement that the time would surely come when the optician would furnish one-sixths, capable of performing all the work then done with the one-fiftieth. The principal reason advanced at that date, in support of his opinion, was,. First, assuming the case of a perfect objective with a perfect eye-piece, he claimed that it made no difference to which end of the tube the power should be applied. Second, the nearer perfection arrived at in the con- struction of the objective and eye-piece, the higher may be the power of the latter ; and Third, as we have no right to expect absolute per- fection in the construction of objectives, it nevertheless seemed reasonable to infer that the optician could bet- ter handle and adjust a lens of sensible dimensions, such as are used in the manufacture of the medium 110 HOW TO SEE WITH THE MICROSCOPE. powers, than could be possible with the merest speck of glass forming the fronts of the one-fiftieth. Whether the ideas thus advanced were correct or not, the fact is patent that in less than two years from the date of the said letter, Mr. Xolles produced a one-sixth, that excelled for any and all work the performance of any one-fiftieth on record. This one-sixth is still in the possession of the author, who, ere the glass was thirty days old, pitted it against the finest fiftieth to be found in the country. The battle waged for an entire week, but the result was decisive. It was David vs. Goliath, and David had the best of it. Scarcely had another month elapsed before Mr. Tolles again sent the writer another glass this time a tenth which, in turn, eclipsed the previous inimitable work of the sixth; while at a still later day, Mr. Herbert Spencer produces a tenth, made on a somewhat differ- ent formula, the performance of which is not excelled by any glass yet made, " be it a fifth or a fiftieth." Without reference to the " impossible 180," it may be positively claimed that either of three glasses named have greater aperture than is possible (or has thus far been possible) to obtain with the fiftieths. As has already been stated in the introduction, the writer was the first to call public attention to the claims of American objectives of medium power. Statements so radically at war with the generally accepted popular belief were destined, as a matter of course, to meet with opposition. Microscopists from almost every section came in person to see for themselves, many of them SOMETHING FURTHEK ABOUT OBJECTIVES. 11] bringing their favorite high-power glasses for compari- son, and returning to their homes satisfied with the trip, leaving the one-sixth and the tenth to encounter the next comer. It being probable that there are others who yet re- main to be convinced as to the accurary or validity of the claims of the " medium powers," it may be stated that at this late day the writer is no longer in a minor- ity of one. Microscopists of note have studied the situation, arriving at similar results. About twelve months ago, Mr. John Mayall, Jr., a well-known and talented microscopist of London, wrote as follows: " I am not going to enter into a mass of details of the various trials I have made with Tolles' one-fourth and one-eighth. Suffice it to say that no lenses that have been in my hands have ever been so thoroughly tested against the best lenses by English, French, and German opticians (here Mr. Mayall presents a list of seventeen recent immersion objectives by the most re- nowned makers in Europe); and without reserve of any kind, I say these lenses are the finest I have ever seen. I affirm, then, that with cen- tral and oblique light on all the objects that are known here as tests, Tolles carried the palm. I find, on the most severe tests, there is in Tolles' lenses a better cor- rection for spherical aberration, the image is more crisp and clear. By difficult tests, I mean, for instance, sur- rirella gemma with central light, or amphipleura pellu- i is not a fixed and definite quantity nor one that can be fenced in and subjected to any fixed rules. Nothing definite in the way of rigid law can be applied to it. In the case just mentioned, another curious conclusion might be arrived at, and justly too. For instance, the decrease of aper- ture from that of the wider aperture to that of the lower would not only be accompanied (accepting the popular dogma), which in the case in question would hold true by an increase in the working distance, but the penetra- ting power of the glass would thereby be enhanced, and this, too (comparatively), without loss of angle. The facts presented are valuable, are significant, and worth careful thought and study. The author has never seen them in print, and they are, as suggested, the result of an active experience. And this brings us to the consideration of another matter; I refer to the popular dogma of * 'penetration. " This has been the biggest toad in the puddle, and has exercised an active agency in roiling and mystifying the mind of the microscopist. The doctrine of penetration SOMETHING FURTHER ABOUT OBJECTIVES. 119 as generally taken and accepted may be thus stated: objectives of low angular aperture are endowed with a peculiar inherent and intrinsic power, by virtue of which they enable the observer to see and study struc- tures situated in different planes of the object. For example, if the objective be focussed accurately to details occupying an intermediate plane of the object to be examined, then will the low-angled glass allow the observer, without change of focus, to study other details of the said object, situated in planes either nearer or more remote. We have been taught that this is a most valuable property, and one due to the employ- ment ot low angles only the idea thus conveyed being that the low angles possess a peculiar and accommodat- ing power of great value to the microscopist, to which the wide apertures stand inflexibly opposed, and defiant. In support of the doctrine of penetration , it has been customary to present the case of the optical principles governing the action of low apertures, contrasting the same relatively with similar conditions involved in the use of the high angles ; thus we have been taught that the narrow-angled glass admits as a matter of course, but a narrow cone of light, the pencils crossing at the focal point at a very acute angle. Hence it is "obvious" that it matters not whether the object to be viewed be placed exactly at the crossing point or a little within or without the said focus. The accompanying and sup- posed increase of working distance attributable to the narrow aperture of course is not lost sight of; and we are here admonished to keep in mind the fact that, with 120 HOW TO SEE WITH THE MICROSCOPE. an infinite working distance, there would be no need of special focal adjustment, and hence the longer the work- ing distance the better. On the other hand, we are told that objectives with very high apertures admit a much wider cone of light, the lateral rays of which cross in the focal point, at a more obtuse angle, and hence the necessity of placing the object to be viewed exactly in the focal plane. On all other planes, nearer or more remote, the object being out of the crossing of the rays, cannot be well defined; and here again, conversely the presumed decrease of the working distance due to the increase of aperture is held prominently in view. To all of the above, which has proved so acceptable to the world of microscopists, the author long ago pub- lished his dissent. He never did, and does not to-day, take the least stocK. in the aforesaid enunciation of the so-called doctrine of penetration. Admitting, as in the case of the two objectives pre- sented, that the cone ot light illuminating the field from the high-angled objective is wider, and that the lateral pencils cross in the focal point at a more obtuse angle than can occur in the case of the narrow-angled glass, it is nevertheless true (and singularly this little fact seems to have been entirely lost sight of), that the wider cone of light due to the employment of the wide aperture includes all of the central pencils present in the case of the narrow-angled glass. In other words, there are just as many central pencils at work (and remember that these are the fellows that cross the focal plane at such an acute angle, thus furnishing the beloved pene- SOMETHING FURTHER ABOUT OBJECTIVES. 121 tration) in the making up of the wider cone as can occur with the use of the narrow aperture ; furthermore, that it would be not only possible, but eminently prac- ticable, by the use of a diaphragm, to cut down the cone of the wider aperture objective to correspond with that of the low-angled glass ; hence it is obvious that in this latter case the two obje stives would be worked under similar conditions as respects the angle at the crossing of the rays, and, applying the argument based thereon, neither glass can be endowed with the greater penetration. Says one, "How about the working distance? " The relations of angular aperture to working dis- tance have already been discussed, and intentionally, with the View of preparing the mind of the reader for the above interrogatory. But there remain other con- siderations bearing on the matter of working distance, and the clinching argument on the part of the writer remains to be presented. In doing this, the author is compelled to deal in assertions dogmatically. , In the handling and compar- ing of object-glasses, he has had a very large experience, and he feels that he has the same liberty to speak ex cathedra as has been granted to others. Moreover, what he now has to say is " important if true," and he is as well assured ot their correctness as of any other fact within his knowledge and experience, nor is he alone in the matter about to be stated. Without exception, all who have experimented in the proper direction assent to all that will be here claimed, while 122 HOW TO SEE WITH THE MICROSCOPE. those who have not, may reasonably be expected to- know not. Let it be required to display an object under the microscope, and under a given amplification. It matters not what the object may be be it a diatom, or a bit of voluntary muscle, or what not; nor dees it matter as to the amplification be it 60, 600, or 6000' diameters, as the case may be. Now, to attack this object, we will provide two sets- of objectives, including all the focal lengths, say from the inch, upwards, to the one-fiftieth these glasses to- be the very finest of their kind made at the present day, and notably of low apertures ; the other set to be simi- lar -as to the range of focal length and quality, but notably to possess the highest apertures (respectively) known. Now choose your object, select your amplification,, and display the former, using the low angles with their very best foot foremost. This done, allow me to- remove the objective, replacing in its stead the suitable high-angled glass, and I affirm pointedly that the object shall be equally well displayed, under the same amplifi- cation, etc., and by an objective, too, having greater working distance than the low angle first selected. It should be contemplated, in any competitive com- parisons of this kind, that they be conducted without prejudice, and solely in the interests of science, and when so conducted, and by observers fitted for the emergency, the author apprehends that his statements will be found correct. BALSAM APERTURES. 123 We have thus again endeavored to make manifest that the idea that angular aperture is accompanied by a sacrifice of working distance has no real existence that is, in the form popularly accepted. Thus far we have discussed "angular aperture" in its- popular signification, and, in several of its aspects, from the definition given from the micrographic dic- tionary. Taken in conjunction with the remarks we have thus far had occasion to offer, the reader would probably infer as axiomatic that the range of apertures- would necessarily be confined within the axial pencil and the one striking the underside of the slide at near coincidence, thus traversing and limited by an arc measured by nearly 90 the latter being equal to 180 of aperture. This, too, is the precise aspect to which the author desired to restrict his observations. Now there is another kind of aperture of which very little is generally known, we refer to BALSAM APERTURES. From a theoretical or mathematical standpoint, the study of balsam angles fairly bristles with difficulties ; it has been to us a problem to which our school boy wrestlings with Euclid seem a pleasant and simple exer- cise. While we frankly admit our incompetency to properly present the subject, we have to remark, on the other hand that we were not willing to send forth this little book without at least some mention of the matter. Observers interested in the history of the American 124 HOAV TO SEE WITH THE MICROSCOPE. objective (and American observers ought to be, to a man) will find the subject ably discussed in the columns of the London Microscopical Journal, reference being made to the celebrated controversy on the subject of angular aperture between Mr. R. B. Tolles, of Boston, Mass., and Mr. Wenham of London. In this discus- sion, the American side of the question was ably assisted by Col. J. J. Woodward, of the U. S. Army, and Prof. Keith, of Georgetown, D. C. The entire controversy is well worth reprinting in a consolidated form, and should find an appropriate place in the library of every American observer. No attempt will, for the reasons given, be made to discuss the subject of balsam apertures in these pages. We shall, however, try and give the novice an idea or two connected with balsam angles without which some things which will hereafter be presented, would be wholly unintelligible. Suppose we put a ray of light down the tube of the microscope, thus reversing the usual order of things, and that said pencil have an angle of 41. This pencil traversing a suitable objective in position over a bal- samed mount, will find emergence into air at 90 ; equal to what is recognized as 180 of aperture. Such an objective would be said to possess a balsam angle of 82, in other words (rejecting fractions), the balsam angle of 82 is said to equal an air angle of 180. Now it is claimed by certain American opticians, that it is possible to construct immersion lenses that are capable, when worked over balsam mounts, of recogniz- BALSAM APERTURES. 125 ing interior pencils greater than 41. Mr. Tolles claims for some of his recent immersion objectives, balsam angles as high as 120.* We have devoted a great deal of time to the study of this class of objectives. These glasses were generally known here at home as " duplex," or four system immersions, as distinguished from the older form having a single front. Many of these objectives, ranging in their claims as to balsam angle from 82 to 100, have passed through our hands, and have been submitted to close and careful study ; one object on our part being to determine, if possible, whether a glass said to be of a low balsam angle was in any respect characteristically different from another claiming a higher balsam, angle, and in this way to arrive at some determination as to the existence or val- idity of the claims resting on the recognition of the angle itself. As a result obtained from close, tedious and pro- tracted observations, dating from the present date back to that of the first " duplex" made, we unhesitatingly affirm that it is quite possible to distinguish the per- formance of a duplex objective of 82 balsam angle, from a similar glass of 100, otherwise we would have had no occasion to have introduced the subject at all. Now, in the effect of balsam aperture, we recognize in the high balsam angles precisely what has been at- tributed to high air apertures, namely, a decrease of working distance, as the balsam angle is increased, and * Messrs. Tolles & Spencer are now (1880) making objectives of 120 balsam aperture. 126 HOW TO SEE WITH THE MICROSCOPE. a corresponding increase of definition, when worked by extremely oblique light over balsam mounts of " diffi- cult tests." These characteristic differences in perform- ance are so palpable, as to enable us to select in less than fifteen minutes' use of two glasses over the Moller & probe plate, the higher-angled glass (balsam) from the lower. It has been claimed by some who have used the duplex glasses, that the higher performance by central light is obtained with those of the smaller balsam angle. My own experience does not authorize me to endorse these conclusions. Certain it is, that when the higher angles are used by central illumination, their immense power of light will, if the matter receive not proper atten- tion, defeat the glass, and again, even with oblique illumination, the high-angled objectives require the most careful attention and expert handling. We are, therefore, prepared to endorse to some ex- tent, referring to high balsam angles, the remarks which have been quoted from " The Microscope and Its Reve- lations." FLATNESS OF FIELD. It is, of course, desirable that an object known to be flat should so appear when viewed under an objective. The optician, however, has thus far found it impossible to secure perfection in this respect, combined with the highest aperture obtainable, and this might be urged as an objection to the use of wide-angled objectives. The slight error shown by the glasses referred to, has much FLATMESS OF FIELD. 127 more weight on paper than occurs actually in practice, where the great increase in definition obtained causes O the slight deficiency in flatness of field to sink into utter insignificance. Hence, in testing the qualities of an object-glass, the flatness of its field would hardly be called into requisition. Nevertheless, flatness of field must have its due weight, and the performance of a first-class objective should not betray any serious error. It will be well, then, when examining an objective, to look after the quality referred to. The careful testing of an object glass in this particu- lar, is not such an easy and off-hand matter as might at a glance be presumed ; the manipulator may arrive at incorrect results, and thus condemn a glass without due cause therefor, and a word or two as to the proper mode of conducting this test may not be amiss. First, it is of the first importance that the object be in itself flat when presented to the objective. Errors may creep in first, because >the object is not in itself flat, or, second, because it is mounted on an improper slide, or on the cover thereof very few slides are flat, and covers are notoriously " in wind;" third, the stage of the microscope may not be at right-angles to the optical axis, and fourth, the eye-piece may not suit the objective. To guard as far as possible against these sources of error, proceed thus : Select a fine s.tage micrometer, having a band of lines just about as close as the objective can well display* when viewed with nearly central light, and if you have 128 HOW TO SEE WITH THE MIC11OSCOPE. no conveniences of your own, take this to the machin- ist, or the watchmaker, either of whom will allow you the use of a steel " straight edge," with which you will be enabled to ascertain with tolerable exactness whether one side of the micrometer is truly a plane. This done, by the aid of a suitable " guage," you will also deter- mine as to the other face. If, in a process of this kind the micrometer betrays defects, it should be discarded and another one chosen in its place. In our own prac- tice we always use a Nobert test-plate, which has been found to be very reliable. To avoid the third source of error, view the lines, selecting as close a band of lines as possible, and using, as before named, nearly central light, and the two-inch eye-piece, with the micrometer placed horizontally and vertically on the stage, examining the bands from end to end as they appear in the field. Repeat the experi- ment, but reversing the micrometer in each position end for end. Any error due to the pose of the stage is thus made manifest. Should you have reason to suspect trouble from the eye-piece used, repeat the entire test on the stand of some friend, always using the two-inch, or lowest eye- piece. This ocular furnishing the more severe test. Finally, let it be known that all the eminent makers, both American and English, furnish glasses that are not to be rejected for non-flatness of field. We would rather trust to the reputation of these gentlemen, than to the test conducted by the novice. A little practice on the other hand on the part of the latter will not be a waste of time. MOUNTING OF OBJECTIVES. MOUNTING OF OBJECTIVES. Having glanced briefly at the optical portion, a few words in reference to the mechanism of the instrument may not be out of place. Adjustable glasses are provided with movable or stationary fronts, that is to say, in the process of re- volving the correction collar the front lens also revolves, or remains stationary, as the case may be. The station- ary front being the mobt expensive mounting, it is gen- erally adopted in first-class American or London glasses of short focal distance. In the use of objectives of tolerably long focal dis- tance, the necessity for the stationary front is not so apparent, and some first-class makers adopt either form of mounting for such objectives. It behooves the buyer to keep in mind this difference in the cost of the two mountings, for of two glasses, both equal in optical performance, the one adjusting with stationary front ought to be the most costly. Generally, the mechanism of the collar adjustment should be first-class. There should be no "slip," "back- lash," or " dead-point;" the collar should rotate with a certain firmness of action, and yet run as "smooth as oil ;" there should be no undue rubbing or grating, nor "hitch or hindrance "of any kind. With a one-half inch, or a four-tenths, a slight " slip" or " back-lash" need not defeat an otherwise satisfactory objective; but, on the other hand, nothing of this sort can be allowed in the higher and first-class objectives. If the 9 Microscopy. 130 HOW TO SEE WITH THE MICROSCOPE. collar be adjusted by the maker to rotate under high pressure only, regard this with suspicion, and examine for back-lash closely, or what is perhaps the better way, get the opinion of a skillful mechanic. The models of some of our American objectives are, in the opinion of the writer, far too large and clumsy, and the fronts too large and too flat. We decidedly prefer the conical front the more conical the better. The new tenth of Mr. Herbert Spencer, which we have before found occasion to mention, was, beyond all cavil, the most beautifully mounted glass we had ever seen, and in which the last-named objections were almost wholly avoided. NOMENCLATURE OF OBJECTIVES. American and English microscopists usually class their objectives on the basis of their focal length, it being arbitrarily assumed that the inch ^lass worked with ten-inch tube and with two-inch eye-piece, should give an amplification of fifty diameters. Hence, the half-inch glass would give one hundred diameters, the one-fourth two 'hundred, the one-eighth four hundred, the one-tenth five hundred, and so on. It very seldom happens, however, that an object-glass will exactly re- spond to the designation given it by the maker some opticians over, while others under-rate their instru- ments; and then again with an adjustable glass the power- will change in different positions of the adjusting collar, the amplification being greater with the systems at closed than when at " open-point." Even when due NOMENCLATURE OF OBJECTIVES. 131 allowance has been made for this last condition, we have seen English eighths having really higher power than some American one-twelfths. Again, but a few weeks since, we handled a foreign one-sixth which was superior in amplification to a Spencer one-eighth. We, here at home, hyve scolded a good deal at this state of things, and at times have rated our English cousins for thus underrating their objectives. It is quite unneces- sary here to traverse the ground we have already dis- cussed, to render it evident that as the objective ap- proaches perfection, these nominal distinctions, based on focal length, fail to have particular force. If, for example, it were possible to produce a two-inch objec- tive, which, under extremely high eye-piecing, would more than do the work of our present tenths, then we could afford to work pretty much with one objective, and to the eye-piece look for the determination of the power. Until, however, some such " possibility" shall occur, our present nomenclature will be invested with some force, no matter how variable this force may be. It is, therefore, desirable sometimes to determine the actual rating of an objective. The method usually employed, is to place a stage micrometer in position, using a ten-inch tube, and pro- jecting the image by aid of the camera lucida on a screen ten inches distant. Knowing, then, the actual value of the divisions on the micrometer, we are enabled, by the measure of the magnified image thereof, to determine the amplification. This method is so well known and so often practiced, we content ourselves with its gen- eral mention. 132 HOW TO SEE WITH THE MICROSCOPE. The method has its drawbacks. For instance, all microscopes do not have tubes precisely ten inches in length, especially when the objective is placed in posi- tion; and then, again, if the problem be to determine which of two lenses is the stronger, it may occur, and pretty surely too, that one glass will have a shorter setting i. e., "mount," than the other. There are ways to dodge the difficulty, but the best method we have seen of measuring objectives, where strict mathematical accuracy is not a vital consideration, is by the use of a formula, modified by Col. J. J. Woodward, and by him given to the public through the columns of Silliman's Journal not long ago (Vol. III., June, 1872): Formula : ML where F focal length, M magnification without field-glass of eye-piece, and L length between stage micrometer, and micrometer in eye-piece. If the "B" (one-inch ocular with its field-lens re- moved, be selected, a micrometer ruled to one six-hun- dredth of an inch will be found to suit it very well. It will be seen that the formula is extremely simple, and quite within the comprehension of all ; we, how- ever, append an example or two, selecting cases which have occurred in actual practice. (1.) We lately had in hand a glass of Mr. Gundlach's, made for a half-inch, and we desired to be assured as to its true rating. Placing a suitable micrometer on the stage, and selecting a one-inch ocular, with its field- glass removed, but fitted with micrometer ruled to one NOMENCLATURE OF OBJECTIVES 133 six-htmdredths, and having brought the lines of the stage micrometer in focus, we then measured the dis- tance from micrometer to micrometer, finding the same to be just ten inches, and on comparing the divisions of the stage micrometer with those of the eye-piece, we found those of the stage micrometer to be amplified eighteen linear. Now, working out the formula, and substituting for the symbols their proper values, we have Focal length= !? = 1|? ='5 in. or i inch. Hence, we find the glass exactly to the rate given it by its maker. (2.) In a similar manner it was desirable to test a (so-called) one-fifth objective. Proceeding just as be- fore, simply changing objectives, we measured again the distance from micrometer to micrometer, getting a^ain just ten inches a matter of chance, however, and one not likely otten to occur. Comparing the two microm- eters again, we get an amplification of the stage lines of thirty-eight linear. Substituting values as before we now have Focal length = ??15 = .M. = .25 in. or i inch. For the purpose of developing the capacity and value of the formula, we will now repeat this example, but in place of using a distance of ten inches, we will arrange to secure in lieu thereof a distance of twelve and one-half inches. We now find the amplification, as shown by the two micrometers at this greater distance, 134 HOW TO SEE WITH THE MICROSCOPE. to be forty-eight linear, and substituting this value, we have Focal length = 48+12-50 = 600 2401 25 in. again. Either computation gives the same results (rejecting fractions too small to affect things sensibly), and we see, too, that the glass sold for the one-fifth is really but a true fourth; and, furthermore, it is evident that a change in the distance of the two micrometers in other words, working with different lengths of tubes does not practically affect the results. TABLE OF THE MAGNIFYING POWERS OF SINGLE CONVCX LENSES. The first column gives the focal length for parallel rays; the second, third, fourth and fifth, give the mag- nifying powers at ten, twelve and a half, twenty-five, and fifty inches respectively, the last three being from the American Journal of Science and Arts, as before mentioned, while the second was computed by the Hon. J. D. Cox, of Ohio, and by him presented to the author. Focal Length Magnifying Magnifying Magnifying Magnifying for Powers Powers Powers Powers Parallel Rays. at 10 inches. at lOVSi inches. at 25 inches. at 50 inches. 1 2 3 4 5 3 inches. T33 1-50 6-17 14-59 2 3-00 4-00 10-40 22-95 MAGNIFYING POWERS. 135 1 1-2 " 4-67 617 14-59 31-30 1 8-00 10-40 22-95 47-99 2-3 " 13-00 16-69 35-47 72-98 1-2 " 18-00 .... .... .... 4-10 " 23-00 29-21 60-48 122-99 1-4 " 38-00 47-99 97-98 197-99 \ 1-5 " 48-00 60-48 122-99 247-99 1-6 " 58-00 72-98 147-99 297-99 1-8 " 78-00 97-98 197-99 397.99 1-10 " 98-00 122.99 247-99 497.99 1-12 " 118.00 147-99 297.99 597.99 1-15 u 148-00 185.49 372-99 741-99 1-16 u 158-00 197.99 397-99 797.99 1-18 " 178-00 222-99 447-99 897-99 1-20 " 198-00 247-99 497-99 997-99 1-25 " 248-00 310.49 622-99 1247-99 1-50 " 498-00 622-99 1247-99 2497-99 By the use of the preceding table one is able to meas- ure the focal length of any objective, and without in- volving the trouble of computations. In ordinary cases it will be only necessary to see that the two microme- ters shall be exactly ten or twelve and one-half inches apart. The amplification in either case being noted, we enter the table under the proper head, and correspond- ing to the amplification will be found the focal distance. For instance, taking the first case we have presented, 136 HOW TO SEE WITH THE MICROSCOPE. that of Mr. Gundlach's objective, we worked it a dis- tance of ten inches, and observed an amplification of eighteen linear. Entering column two of the table, we find at a glance eighteen to be opposite to focal length of one-half inches. In the case of the so-called fifth, with the two micro- meters at ten inches distance, we observed a magnifica- tion of thirty-eight linear ; entering the second' column of the table again, opposite thirty-eight, we have the focal length, equal one-fourth inch. To the last com- putation we worked the same case, but with the micro- meters twelve and one-half inches distant ; in this case observing a magnification of 48. Entering the table in column third, opposite to 47.99, we have the same focal length as before, viz., one-fourth inch. It often occurs that many first-class stands cannot be worked at a distance of ten inches with objectives of ordinary lengths. In this case use the twelve and one- half inch distance, which can always be obtained by using the draw tube. CHAPTER III. OBJECTIVES CONTINUED. What constitutes a really superlative wide-angled objective? It has been our fortune to reply to this question upwards of ten thousand times, and it may be true that no two of these responses have been exactly alike. In the present essay let us get close to the reader, introducing a little gossip, and, at the same time, seek- ing a little relief by avoiding expressions which are getting to be stereotyped, such as the " writer," "this," " the author," "that," etc., helping ourselves freely to the first person singular, about as one would when writing to a friend. It has been already shown that there are two classes of high-angled glasses namely, those having balsam apertures, say up to 100, the other class responding to a ir angles, up, say, to the " impossible,''' 180. And, first, let us consider the objective ot high balsam angle. I have, in advance, stated that these glasses necessa- rily have a very short working distance. But this re- mark must not be swallowed whole; let it be taken rather with a "pinch of salt," keeping this little fact wdl in hand, that there are wide-angled balsam aper- tures, having greater working distance than will be found present in other objectives of the same nominal focal length, and having, too, no really wide air angles. 137 138 HOW TO SEE WITH THE MICROSCOPE. In comparison with such as these, the high balsam apertures have nothing to fear. As to working dis- tance, a vast amount of misguided effort has been ex- pended in the vain attempt of trying to compare one kind of an objective with another; for instance, a wide- angled air aperture with objectives of narrow air angles. Hence, we have had nothing but muddle and confusion. I repeat what I have before printed, that any attempt of the kind is as futile in its very nature as it would be to compare a " turnip with an orange." It has, therefore, been my aim, however imperfectly I may have succeeded, to lead my readers along one road at a time, and this " objective" point has been steadily held in view while writing this little book, and up to the present writing, whether success has attended the effort or not, I claim some credit for having essayed in this direction. " Revenons a nos moutons." I repeat, then, that of two superlative objectives having balsam angles ranging from 82 to 100, the rule presented holds good, i. e.> that the higher aperture will have the shorter working distance, or, in other parlance, the gain in aperture will be accompanied by a sacrifice of working distance. Now, we all know what is lost with the decrease of working distance. Let us, therefore, seek as to what is gained by the increase of aperture. First, we gain a wonderful increase in intensity of definition; an increase in definition too, entirely unapproachable from any other direction. In this particular these glasses stand alone and defiant. Secondly, their immense power of OBJECTIVES CONTINUED. light, superlatively corrected as they are, enable them to be used under wonderfully high eye-piecing, and with comparatively slight loss of either light or defini- tion, while by the aid of these high oculars, the greatest magnifications are obtainable, and in these particulars also, do these objectives again stand alone and defiant. ' Somehow, among the few who have paid attention to the claims of the balsam apertures, it has got to be the popular impression that it is the particular province of these objectives to bear high oculars, the impression having force in a restricted, or limited sense to wit, that there is nothing gained really by the use of highi balsam angles under low or medium oculars. Any idea of this sort is totally in error, for even under a low or medium eye-piece, the higher balsam angle will demonstrate its presence by an increase in the intensity ot definition, while its greater power of light is in itself & power capable of being turned to certain necessary and useful purposes. I have already stated that it is quite possible to distinguish the one glass from the other, and, I might have added, " under low or medium oculars." But, as I have said, with all this gain, there is an ac- companying loss of working distance. But this is not all; those who have been accustomed to work all their lives with objectives of low or medium apertures, might with good reason, claim that the high balsam angles are " exceedingly inconvenient to use." Palpable is the fact, that the slightest error in the collar adjust- ment, or in the management of the illumination, will 140 HOW TO SEE WITH THE MICROSCOPE. suffice to defeat the maximum performance of the bal- sam apertures, and the higher the balsam angle, the greater the " bother " attending their use. Nor is there anything so exceptional in all this. In any instrument of precision, when, by successive im- provements, we gain width of effective range, and, simultaneously, a nearer approach to accuracy of deter- mination, then, in ninety-five cases out of the hundred, do we, in like manner, increase the complications of the instrument; thus introducing " inconvenience " and " bother," and, be it known, an accompanying call for skilled manipulators. We often hear another remark to wit, that those owning and using the class of objectives we are now considering, do nothing else than " fight them." Before attaching any definite force to this remark, it will be well to inquire, " for what purpose is ihe battle!" Is it to determine which of two objectives, differing in construction, but supposed to be nearly alike in per- formance, is the better one, and with the purpose (avowed or otherwise) of rendering some contribution to a scientific end; or is it that "A" battles " B," the object being on the part of "A" to assure himself that his glasses are not behind the age in quality of per- formance; or, again, may it be that " C," being re- quested by his inexperienced Iriend "D" to select for him an objective in every respect fully up to the times? Or, to mention a very possible case, does "E,"who, having a few hours of daily leisure, desires to appro- priate the same to the study of object-glasses, with the SUPEELATIVE WIDE-ANGLED OBJECTIVE. 141 view of perfecting himself in the manipulations of the same, that he may in turn render his observations and experience of value to those who devote their spare time to investigations with the instrument? If these are the ends in view to be accomplished, then I say, " fight objectives," and furthermore that " E " need rot be ashamed of his occupation. On the other hand, if objectives are fought for other and baser purposes, that is no afiair of mine. I cannot help it, and the objective is not to blame. I claim, then, that the high balsam angles are indis- pensable in the studies of the advanced investigator ; they alone, " bother or no bother," are the ones for the work. Let the reader keep constantly in mind that, in the study of difficult and delicate structures, the slight- est superiority of definition is of vital importance. A first-class objective of high balsam angle will show, clearly and accurately, details in an object, which, from their extreme tenuity or transparency, would be totally invisible when viewed under an objective having but air angle. To sum up: We thus arrive at the conclusion that objectives of high balsam angles, have, as compared with others of less balsam angle, a shorter working dis- tance, and in general are " inconvenient and bother- some to use; nevertheless, that their use cannot be dis- pensed with in the study of delicate and difficult struc- tures, and it may be further stated that these are the glasses suitable for the study of difficult diatoms, the display of the Nobert nineteenth band, and the like. 142 HOW TO SEE WITH THE MICROSCOPE. Thus far, whatever force may attach to the elements of " inconvenience " and "bother," of which we have .heard so much, I have allowed full play and weight ; but let the reader not lose sight of the fact that those accustomed to the use of adjusting glasses never aban- don them to return to their first love, nor do they re- gard the process of adjusting a superior objective as being in any way or shape a " bother" or " inconven- ience." It may be well, right here, to slate a bit of personal experience. I am in the habit of putting ad- justing glasses in the hands of my pupils at the earliest possible moment ; not that they will accomplish more with them for a time than would be the case with those non-adjustable, but in order that the pupil be early brought in contact with them, and thus, by degrees, get accustomed to their use. By adopting this course, the tyro learns, in due and proper season, an important lesson, to wit, that there are some things to be clone besides putting an object at one end of the tube and the eye to* the other in order to see what's what. Now, it often happens that there may be more pupils in the laboratory than adjustable objectives, and thus, per- force, a student or two will have to fall back on the non-adjustable. When this state of things occur there is inevitably indications of discontent, and it is never on the part of those pupils occupied with the adjustable objectives. So much for the " inconvenience" and " bother." Now, let us get after that other elephant working distance. I have granted that with the higher balsam SUPERLATIVE WIDE-ANGLED OBJECTIVE. 143 apertures there is a loss of distance. Now I propose to discuss this "loss" from a practical point, and in so doing shall state facts, including names and dates. On the 22d of March, 1878, I received from Messrs. C. A. Spencer & Sons, a one- tenth objective of high balsam angle. This glass was made to my order, and for the Hon. J. D. Cox, of Ohio. Before sending it to its destination I " fought it." Its performance was as follows: Worked over the balsamed Moller probe plate, on the Zentmayer histological stand, with the mirror-bar swung to 31 from axis, I saw distinctly the striae of Nos. 18 and 19 of said plate, nor had I any doubt as to thus seeing the No. 20, the illumination being a common small coal-oil lamp. The light was taken direct from the mirror. Worked over the same Moller plate, but with the Wenham reflex illuminator (an instrument devised for shutting out all pencils having air transmission), j saw the transverse striae of the No. 20 of said plate handsomely using powers from 1,000 to 4,000 diam- eters. Tested over a fine slide of English podura, using ordinary illumination, and nearly central, I had the finest display I had ever witnessed. Worked over navicula angulata, with "dead central " illumination, it gave me instantly the markings very handsomely. Worked over a dry mount of ampliipleura pellucida. These shells surrendered at discretion ; ordinary oblique illumination. Substituting a balsam slide of this 144 HOW TO SEE WITH THE MICROSCOPE. diatom, placing- the same in position under the stage of the histological, I got handsome resolutions with the mirror, at say 80 from axis, and also when the swinging-bar was posed so that more than half the mirror was above the sta^e. C5 Tested for working distance. I found the same to bo .015, equal, say, to one-sixtieth of an inch. There were very many other " rounds" to this fight, which are purposely omitted as having no bearing on the subject in hand. The reader has before him the work of the objective by central, centrally disposed, tolerably oblique and decidedly oblique illumination. Now, thus having the work of the objective before us, it is palpably evident tliat it has all desirable work- ing distance. Surely no one but a bungler would at- tempt to cover a nice mount with glass over one one- hundredth of an inch in thickness, while, with such a cover, the one-tenth will have plenty of room and to spare; and if we compare the glass named with the usual run of immersion tenths, it will be obvious that there is no loss of working distance nor any sacrifice thereof in any way, shape, or manner. OBJECTIVES OF LOWER BALSAM ANGLE. These, when compared with the glasses we have just had under consideration, will be found to have greater working distance, as has been before stated; and with this increase of distance there will be greater focal depth the so-called " penetration " of the books. Now, in the preliminary investigation of many objects, OBJECTIV-LJS OF LCW'-Eli BALSAM AAGLE. 145 this is a desirable quality in a glass. It enables us to search through an extemporized mount in the least pos- sible time. And then, again, we are less liable to allow important details of structure to escape our attention. These are advantages, and must be recognized as such; but, per contra, let it be remembered that in subse- quent examinations it often becomes quite as important that this quality of penetration should be absent. For instance, when it is desired to study structures situated in one plane, and one plane only, the less of penetra- tion the better. I have not space to enlarge on this, but let the reader not forget the fact. The glasses of the lower balsam apertures are really the easiest to manage, and yet are effective and adequate for a large class of work. They have less power of light, and hence do not " stand up" so well under high eye- piecing, nor have they the same exquisite intensity of definition. In a certain sense, the diminution of defini- tion is very slight, indeed scarcely to be noticed (if at all) when examining tolerably vigorous tests. Hence, they are quite adequate for the resolution of nearly all the recognized test objects when mounted dry. It is, then, over exceedingly thin, faint, delicate, and trans- parent tests that the higher angles assert their suprem- acy. If the general definition as to what constitutes a high-angled objective, given on a preceding page, be accepted, then there remains room to discuss a class of objectives of such focal lengths, nominally, as defy any expert effort on the part of the optician to extend 10 Microscopy. 146 HOW TO SEE WITH THE MICROSCOPE. their aperture to any thing like an approach to 180. Among such may be included the two inch of 25, the one and one-half inch of 37, the one inch of 40, the two-thirds of 45, the one-half inch of 100, the four- tenths of 150, and so on. All the glasses above named may be considered as high-angled objectives. They should be nicely corrected, fully up to the limits of the aperture claimed for them, and, as compared with similar lenses of narrow apertures, they should possess a much greater power of light, and bear higher eye-piecing; and, let it be borne in mind, that of the above-named objectives, the one having the greater dis- tance will be "endowed" with the greater " penetra- tion." And right at this place let us see what the " power to bear high eye-piecing" means. To illustrate this, we will take a case such as might happen in practice. For example, I desire to see the transverse lines of pleurosigma balticum, and, as mounted on the Moller balsam test plate, their striations being from 31 to 34, in .001 Eng. inch. Now, I attack this with, say, a real good low-angled one-half inch, and find that I can just get a glimpse of the lines with the one-half inch eye-piece. I then apply the one-quarter inch eye-piece, and find that the view is not nearly as satisfactory as it was with the lower ocular; hence I conclude that the one-half inch eye-piece is as high as the objective will bear. Now, removing the one-half inch objective, we will substitute a two-thirds, but of higher aperture. With the latter glass, using the one-half inch ocular, I OBJECTIVES OF LOWER BALSAM ANGLE. 147 get better shows than was the case with the former ob- jective, and on applying the one-quarter eye-piece I have better definition yet. There is some loss of light, to be sure, but there is enough left to do the work, and in a more satisfactory manner than occurred when the lower eye-piece was in position. I take this case as an illustrative one, because I have used it repeatedly with my visitors, and the demonstration has, 1 believe, always been accepted as satisfactory. It will be noticed that thus using the two-thirds, the power applied at the eye-piece was greater than the rating of the objec- tive employed, and a tolerable test for an object-glass of such low nominal power. We have here, then, in the experiment cited, an illustration of what may be said to be *' power to bear high eye-piecing." There is a general and indefinite idea afloat, that there is something about high eye-piecing which ought to be condemned. Without making any special attack, it may be admitted that observers "came honestly by it." The facts warrant this much: power z. e., ampli- fication or magnification when not wanted, is to be condemned, and it matters not how obtained, whether at one end of the tube or the other. On the contrary, if any thing valuable is to be obtained by amplification, then we are authorized to use it, and to the best advan- tage apply it at either end of the tube, as the case may warrant. There seems no more propriety in the con- demnation of high eye-piecing per se than ought to ob- tain in the use of objectives of short focal length. 148 HOW TO SEE WITH THE MICROSCOPE. ADJUSTABLE OBJECTIVES. The generally received notion in regard to these glasses is, that they are provided with an adjusting collar, so that they may be " corrected for aberrations," due to the thickness of cover employed on the mount under observation. This is all true in a certain sense, and tc a certain extent only. If we examine the same object under the same objective (a first-class glass of high aperture), but through covers of varying thickness, we will find that the very best performance of the glass will be obtained when worked over a particular thick- ness of cover, and that any change from this particular thickness will interfere with the performance of the lens, no matter how perfect the corrections in each case may be made. My own experience teaches me that the maximum definition is only obtained when the objec- tive adjusts at or near that point in the collar adjust- ment which corresponds to the maximum aperture of the objective. Now, in most glasses of high angles the maximum angle will be found at only one position of the collar. I say " most objectives," for we have a one-tenth made by Mr. Tolles, that has maximum angle nearly at any point within the range of its collar ad- justment. We have also a one-sixth by the same maker, which has maximum angle only at the " closed point," the aperture decreasing rapidly as the collar approaches " open," and with the decrease of angle there is an accompanying decrease of definition and effective force of the objective. ADJUSTABLE OBJECTIVES. 149 Here we have an argument in favor of wide angles which has been in the past quite lost sight of, and was for the first time presented to the public by the writer in one of his monthly contributions years ago ; but all this- is foreign to our present purpose. Now, in the case of the one-sixth of Mr. Tolles, should it be attempted to use this glass over the thia cover of the Mollerprobbe plate, the result would only be to defeat the performance, for the glass would "cor- rect" within three divisions of "open-point." But, per contrary, if we desire to secure the best definition of the said objective, and over the said plate, it becomes necessary to supplement by thicker cover (making sure of optical contact by the use of water or glycerine), until the glass shall " correct" at or near " closed." Hence, it occurs that it is essential that the observer who proposes to employ objectives of wide apertures should pay some attention to the condition of things ; that he should know precisely over what covers he will be enabled to get the maximum performance. In short, to ascertain where he may find the maximum aperture of his objective. And to this end, we propose to aid and assist, as far as possible, in some future remarks we shall have occa- sion to make regarding the manipulations of the objec- tive. During the past twelve years the author has received hundreds of letters from as many individuals commencing the study of the microscope, desiring such information as to the best way of making an invest- ment as may pertain to the selection of stand, objec- 150 HOW TO SEE WITH THE MICROSCOPE. lives, etc. The said letters have met with a prompt response, and, as has before been stated, perhaps no two of these have been alike. In each and every case there has been some dissimilarity of circumstance, or, on the other hand, we have suffered some change in our own views. Be all this as it may, the experience of the last two years enables us to speak more precisely to the point than before, and we now endeavor to respond to the interrogatory " What shall I procure for an outfit?" There can be no general rule that will apply to all. Let us take the following as a typical letter for con- sideration : "DEAR SIR: I have read some of your contribu- tions to the American Journal of Microscopy. . * . I am a physician of ten years' practice ; am located in a town of some ten thousand inhabitants. I am satis- fied that I ought to know enough about microscopy to enable me to examine intelligently urinary deposits, cancerous growths, etc., and to this end do I propose to purchase the necessary equipment. Any informa- tion that you may be pleased to give me, will be with pleasure received," etc. Now, I would answer this letter, and did answer it, thus : When you buy a stand get one that you will have no occasion to sell at a ruinous sacrifice. I rec- ommend that you purchase one of the cheap and mod- erate-priced instruments, and, at the same time, one that will do any and all work. Such a stand ought not to cost, with one eye-piece, more than forty-five dollars ADJUSTABLE OBJECTIVES. 151 (and such are described in this little book). Now, as to objectives. You can do all the work named with a one-inch of tolerably low angle, costing you, say, some $7.00; and a real good three-tenths of 70 aperture, which will cost you, say, $11.00. The chances are, however, that as you become familiar with the use of the instrument, thereby learning its value to you in your daily professional practice, you will feel an inclin- ation to dip somewhat deeper into the problems which will most assuredly surround you. In your examina- tions of urines, you will get glimpses of bacteria ; you may meet with structures which you are almost assured are " pale hyaline " tube casts ; and you naturally desire a little more amplification and definition thereon to enable you to pronounce with certainty. In fact, you now want such a glass as a dry one-fourth of 110 or 120, and adjustable. Now, to use this glass to advan- tage, you have first to become familiar with its manipu- lations. It will be requisite that you arrive at some knowledge of this before such an objective can be of much avail to you. Now, if you purchase the one- fourth recommended, you will still have room for the employment of the three- tenths, but you will have to study the use of the one-fourth just the same as if you had never seen an object-glass. Hence, I had rather recommend that you purchase the one-fourth at the start, and thus get early accustomed to the use of ad- justable glasses ; and in this latter case it will almost assuredly occur that you will eventually desire to employ glasses of the widest apertures, and the expe- 152 HOW TO SEE WITH THE MICROSCOPE. rience you have gained with the use of the one-fourth will be of the utmost value to- you. Moreover, the one-fourth, too, will continue to be a useful interme- diate glass. And thus in replying to all my correspondents, I recognize the importance that one and all shall early become acquainted with the manipulations of adjusting glasses. To accomplish this they must use objectives of a reliable class, i. e., such as will respond promptly to change of collar-adjustment, keeping well in mind the importance of buying nothing to be discarded or thrown out of use in the future. In the case under consideration, it will happen, in nine cases out of ten, that in less than two years the buyer will feel that he needs a first-class inch, or perhaps a two-thirds, in which event the old inch will be of great service as a sub-stage condenser, providing that the stand has been selected with this end in view. It may further happen, in truth it will be likely to happen, that in the course of one or two years, my correspondent will either push his own observations or desire to keep pace with those of others, and over structures of the most delicate and " difficult" charac- ter, and now he will need a one-sixth or a one-tenth of the widest possible aperture. Allowing this to occur he will have expended but $110, which is less than the usual cost of a nominally first-class one-sixteenth, and all the glasses on hand will still be of service. Besides all this, he is well armed and equipped for any work requiring powers from 50 to 5,000 diameters, and there ADJUSTABLE OBJECTIVES. 153 will be no objectives on hand that will not be worth their cost. The above programme seems to me to fill the bill as well as any other that could be named, and is prac- tically the same as that followed in my own practice, having daily a large amount of work over urinary de- posits, a considerable portion of which does not require excessively fine definition, and time being very much of an object, I employ, in addition to the glasses named, a cheap, but good and reliable half-inch, say of 35 or 40. With this glass I am enabled to perform much prelimi- nary work over liquids, and am able to dispense with covers, and the objective, from its long working dis- tance, is out of the fumes of the re-agents constantly in use. The half-inch is thus of great service, as a time-saver in short, as a convenience. Finally, let it be understood that I have no war to make on objectives of medium apertures. We have stated our experience, and it must pass for what it is worth. The principal point which I desire to impress on the mind of the reader, is that, for the higher class of investigations, the objectives of wide apertures stand alone, unapproachable and unexcelled ; that they alone are the instruments for such work, and this, too, regard- less of the character of the illumination employed, be it central or oblique. Another point is that the use of the high angle enables the observer to cut down the numerical force of the glasses employed, thus saving unnecessary expense. We wish also to render the fact obvious, that a wide angle glass requires time, study, 154 HOW TO SEE WITH THE MICROSCOPE. and attention, and that the manipulations of the same are not acquired in a day. In my intercourse with microscopists, I have nearly always found one idea salient. The general question has been and now is, '* How shall we contrive to save expense ; where can we buy the cheapest? " It seems to me that these considerations must eventu- ally force the wide apertures into use. I was about to- say, " There's millions in it." For example, we know of a microscopist who has, by purchase, from time to- time, acquired a batter} 7 of some 35 or 40 objectives. Now, allowing the cost of these to average $10 each not a high estimate the owner has thus paid out some $350 or $400 for objectives alone 'I While one-half that money, expended in glasses of the highest apertures,, would have enabled our friend to have accomplished any and all work that can be eifected with his entire battery. And unless I am greatly mistaken, the wide apertures would carry the day, and with flying colors. This is no solitary case. We have in very many in- stances been called on to select wide-angled objectives for our friends and correspondents, and in each and every case there have been no exceptions we have received assurances from the parties interested, that there would have been money saved had the facts been known in time. Says one: " My one-fifteenth that I bought two years ago is a dead letter; what shall I do with it? I can't sell it for anything like what it cost, it would be perpetrating a swindle ! " Says another : " If I had met you thirty days ago I would be a hundred dollars better ADJUSTABLE OBJECTIVES. 155 off than I am to-day; you have made me sick of my ob- jectives." I say, therefore, that it seems to me that this- string" must pull, to the end that the microscopist, who desires to invest his money to the best advantage, will be driven to the use of the wide apertures, no matter how strongly he may have become prejudiced in another direction. We believe and earnestly hope, that in pre- senting the subject to the readers of this little work, we have done them a good honest turn, and one that will be appreciated. Before entering on another part of the subject of this- book, we desire to add that the views thus set forth are the result of practical expeiieuce. Here is no specula- tion ; but, on the other hand, the reader has before him the fruit of protracted and close study. The time was- when we were almost alone in our opinions, but that date is numbered with, the past. Among those who- had read his printed articles in their various periodical forms, and have subsequently visited him, " to see for themselves," he can name many of his dearest friends, all of whom have, in turn, placed the writer under countless obligations. And, right here, by the way, let me say, that, as an interesting and enjoyable pastime, that of " fighting objectives " is, when occasionally in- dulged in, second to no " 2.25 " race on record! But of any of the good things of this world " enough is a feast." 156 HOW TO SEE WITH THE MICROSCOPE. EYE-PIECES. It would be a real improvement to assign to individual eye-pieces their actual power expressed in inches, and to dispense with the arbitrary and indefinite nomenclature formerly, and, to a great extent, at present in vogue. Thus, let the two-inch have the same magnifying power as a simple lens of two inches focus, and similarly as to the other oculars used. Messrs. Spencer, Tolles, Sidle, and we believe Zentmayer, adhere strictly to this plan. We use principally the inch, half inch, and one-fourth inch. For those oculars above the inch in power, we greatly prefer the solid eye-pieces. We have also the two-inch and the solid one-eighth inch. The former is however, but very seldom used; in fact, in our own practice, it is seldom wanted or even thought of. The solid one-eighth, although not in general use, cannot be dispensed with when working with sunlight illumina- tion in conjunction with objectives of high balsam angles. We cannot too strongly recommend that every attention be bestowed as to the quality of the solid oculars, and, as a rule, that they be made expressly to order. It occasionally happens that stands fitted with poor objectives are placed in the market for sale, while to accommodate the quality of the objectives the powers of the oculars are let down considerably thus illus- trating the truth of the well-known proverb, c'est le premier pas qui coute. Let the purchaser look well to these points before consummating an investment. CHAPTER IV. MANIPULATIONS WENHAM'S REFLEX ILLUMINATOR. This instrument, and its various modifications, having come into very general use in this country, we give the following cut and description, taken from Dr. Carpen- ter's work on " The Microscope and Its Revelations:" "A very ingenious and valuable illuminator for high powers has been recently devised by Mr. Wenham, and constructed by Messrs. Ross. It is composed of a glass- cylinder, half an inch long and four-tenths in diameter, the lower convex surface of which is polished to a radium of four-tenths. The top is flat and polished. Starting from the bottom edge, the cylinder is worked off to a polished face at an angle of 64 ; close beneath the cylin- der is set a plano-convex lens of IJ-inch focus. When parallel rays are thrown up through this apparatus from the mirror, they impinge on the upper surface of a glas& slide at an angle of total reflection ; but if a suitable object adhere to that surface, the light reaches it on an angle that admits its passage. The object is then seen brilliantly lit up upon a dark ground, and many fine markings, that escape notice with other methods, become very distinct. It is advisable to rotate the apparatus until the best position is obtained. Some skill and practice are required to use this apparatus to advantage, but it will amply repay the trouble of mastering its 158 HOW TO SEE WITH THE MICROSCOPE. difficulties. It is best suited to thin and flat objects ; with those that are thick and irregular, distortion is unavoidable. Although specially designed as a dark ground illuminator, good effects can with care [and by the use of suitable objectives], be obtained for such ob- jects as difficult diatoms, in balsam or dammar. But the effect is that of very oblique transparent illuminQ.tion" We have preferred to give the above description from WENHAM'S KEFLEX ILLUMINATOR. 159 Dr. Carpenter. The interpolation within the brackets, and the italics are our own. As is above set forth, this instrument was designed by its maker for a dark ground illuminator, and for use with high powers; was made and sold for this purpose, and was generally so ussd. Shortly after their intro- duction into this country r Mr. Samuel Wells, of Boston, Mass., made the discovery that the " reflex" when worked with certain very high-angled objectives, could be made to give a brilliantly-lighted field, accompanied with beautiful resolutions of the severest tests, when the latter were mounted in balsam, the effect being, as Dr. Carpenter most justly observes, " that of very oblique illumination" As is evident from an inspection of the cut, the instru- ment can be variously modified in its working, simply by changing the angle of the facet. The fact, too, ap- pealed almost simultaneously to all, that a serious objec- tion to the instrument was that it could be only used over balsam mounts, and of course with objectives of the highest angles. The " reflex *' was therefore modi- fied by the London opticians to suit the angles of their leading objectives, and, at the same time, or at least shortly after, similar modifications of the instrument were made here. There has been an effort here at home to claim, on the part of our opticians, some originality of design in the construction of these various modifica- tions. The entire plan, however, belongs to Mr. Wen- liani, and the simple changing of the angle of the facet does not authorize any claim of invention The author 160 HOW TO SEE WITH THE MICROSCOPE. has used several of these instruments ; the first one was made by Mr. Tolles, the angle of facet being 26 An- other one made some little time afterwards had facet of 18. Subsequently Messrs. Spencer have made him sev- eral having angle of 15. The genuine instrument, as made by Mr. Wenham, is only suitable for use with objectives of high balsam angle, while those of the modern form can be employed with more or less suc- cess in conjunction with glasses possessed of tolerably wide air apertures. We have spent a great deal of time in the study of this instrument, including the several patterns named, and we find that when used with glasses of the highest balsam apertures, such as are made by Messrs. Spencer or Mr. Tolles, and over the severest tests, such as amphipleurapellucida, the resolutions are quite as strong and satisfactory when the illumination is- obtained from the modified " reflex" as when the instru- ment described by Dr. Carpenter is selected, while the latter has the serious disadvantage of being adapted for use over balsam mounts only. The instrument can be adapted to almost any good and reliable stand, even should the latter not be pro- vided with centring apparatus; yet if the sub-sta^e collar be itself well centered, a little ingenuity on the part of the observer will secure good results. It is, however, a great convenience, when using the " reflex," to employ a stand allowing the sub-stage to approach the slide or to be drawn away therefrom. To use the " reflex," place it in the sub-stage fitting, select a low power, say a half-inch or a two-thirds, and WENHAM'S REFLEX ILLUMINATOR. 161 one that you know centres well with the objective to be employed. With such a glass see if the central mark placed on the instrument by its maker occupies the centre of the field. If but slightly out of centre, a very slight tipping of the instrument in its sub-stage fitting will often be all that is necessary. If the stand has centring screws these will come well into play. We have found it desirable to secure perfect centring, and at the same time make sure that the plane face of the facet is parallel to the right-hand edge of the stage, and adjacent thereto. Having placed a small drop of gly- cerine (which is preferable to water, as it does not dry) on the top of the facet, and having placed the object and slide in position, the " reflex " is made to approach and the glycerine to contact the slide. It is better not to allow the top face of the instrument to actually touch the under surface of the slide, still keeping it, however, so close thereto that the drop of glycerine shall be considerably flattened. We are now ready to screw on the object-glass, and to make immersion contact with the cover. For illumina- tion we need one of the lowest and smallest kerosene hand-lamps that can be found. If the stand is small and low, it will become necessary to place it on a suit- able box, the tube inclined to a convenient angle. Re- moving the mirror, use the light direct from the lamp keeping the latter for the present in nearly or quite a central position. If, on looking through the eye-piese, there be found a lack of illumination, the trouble can be rectified by moving the lamp slightly. Having 11 Microscopy. 162 HOW TO SEE WITH THE MICROSCOPE. thus ^ot light enough, next proceed to find your object and to focus the same, and if the approximate correc- tion for the objective is known it will be well now to apply it. Next, we proceed to attempt the proper display of the object, Seize the lamp with the right hand, turn the edge of the flame edgewise to the illuminator, and move the lamp bodily, sliding it on the table in all directions, to the right, to the left, towards the stand away from it. If, while thus manipulating the lamp, the field should be lighted with a succession of colors, blue, red, green, etc., you may be pretty sure that things are working well. Making sure, now, that the edge of the lamp flame is turned to the " reflex," and still grasp- ing the lamp with the right hand, and the fine adjust- ment with the left, try the effect of very small move- ments of the lamp ; these will be followed with a change of color in the field. Supposing, now, that the distance of the lamp from the stand is just right. We will find that by pushing it horizontally and parallel to the front edge of the stage, towards the left, we shall shortly have a blue field, while with a little further shove in the same direction we lose our illumination. Now it is in such positions, and just at the point where the illumina- tion begins to decrease, that I get the strongest resolu- tions of severe tests. If we now retrace the path of the lamp, carrying the same towards the right, the field will become tinged with red, and by pushing the lamp still farther thereto this will deepen, while another move- ment of the lamp in the same direction will cause the THE WOODWARD ILLUMINATOR. 163 illumination to die away. Now , again , good resol utions can be obtained in the red similarly to those in the blue, i. e., just at the point where the illumination begins to decrease. It will now be quite time for those who have never before used the instrument to look carefully to the correction of the objective, and for the trial experi- ment the object selected should be one that the observer can master by the usual oblique illumination. It only remains now to assure ourselves that the lamp is at the proper distance from the stand, which is accomplished by simply moving it farther away, or, per contra, bring- ing it nearer, sliding from right to left, and trying the red and blue fields, as before instructed. The very best position of the lamp as to distance is generally attested by the general superior brilliancy of the illumination, together with the fact that when this distance is just right there will be room for greater lateral play of the radiant without losing the illumination. Care should be constantly observed to keep the lamp flame exactly edgewise. It may be further stated that when things are nearly in proper position, the smallest imaginable movement of the lamp will often produce marked effects. THE WOODWARD ILLUMINATOR. For this novel and useful accessory microscopists are indebted to Col. J. J. Woodward, of the U. S. Army. A paper, giving a detailed account of the instrument, by Col. Woodward, was read before the London Royal Microscopical Society, and subsequently published in the London Monthly Microscopical Journal. The paper 164 HOW TO SEE WITH THE MICROSCOPE. is written with singular tact and perspicuity, and is here reprinted unchanged : A Simple Devwefor the Illumination of Balsam-mounted Objects for Examination with certain, Immersion Ob- jectives whose "Balsam Angle" is 90 or upwards. By Surgeon J. J. WOODWARD, Brevet Lieut. Col. U. S. Army. [Taken as read before the ROYAL MICROSCOPICAL SOCIETY, June 6, 1877.] " Certain immersion objectives are so constructed that they are capable of admitting rays which enter the front lens at a greater angle with the optical axis than the limit for dry objectives. That this is not only theoretic- ally possible, but that such objectives have been success- fully constructed, was several years since demonstrated in the Monthly Microscopical Journal" both by Mr. Keith and myself,* notwithstanding which the contrary has often since been energetically asserted by writers in the same Journal. " Meanwhile, immersion lenses possessed of the exces- sive angle in dispute, continue to be put into the market by more than one maker ; and perhaps some of the pur- chasers will be interested in a simple device which I have used for some time with such objectives to illuminate test-objects mounted in balsam. This device consists merely of a right-angled prism of crown glass mounted beneath the stage in such a manner that its long side can be connected by oil of cloves, or some similar fluid, * June, 1873, p. 268; November, 1873, p. 210. March, 1874, p. 119; September, 1876, p. 124. r THE WOODWARD ILLUMINATOR. 167 with the slide on which the object is mounted. The details of the plan will be understood by the diagram on next page, in which the glass prism is seen in section just beneath the object slide F F. Just below it is an- other right-angled prism, of the same dimensions, made of brass ; the section of this prism is indicated by dark shading in the diagram . The right-angles of both prisms are truncated, and the facets are cemented together in such a manner that the long sides of the prisms are par- allel. The brass prism slips transversely in a groove in the top of a holder, C, which is fitted into the sub-stage of the microscope. D D is a blackened brass screen held in position by two brass arms, one of which is shown in the figure. This screen is parallel to the adja- cent face of the glass prism, and has in it a small cir- cular aperture, E, about the size of a large pin hole. The side of the glass prism next the scree'n is covered with black paper, in which is a corresponding pin hole. The two pin holes are so placed that a beam of parallel white sunlight (r) passing through both will be per- pendicular to the sides of the glass prism on which it impinges. " To use this apparatus it is adjusted in the sub-stage ot the microscope, a drop of oil of cloves is placed on the upper face of the prism, the glass slide F F, on which the object is mounted in Canada balsam under the usual thin cover, Gr, is placed on the stage, and the sub- stage is racked up until the drop of oil of cloves is spread out into a thin layer, I. " The object being thus arranged, it is evident that if 168 HOW TO SEE WITH THE MICROSCOPE. a beam of parallel solar rays (white sunlight), reflected from a plane mirror, be thrown through the two aper- tures upon the face of the prism, being perpendicular to that face, it will enter and pass through without refraction until it reaches the upper surface of the thin glass cover Gr. The parallel rays impinge upon this surface, as is evident from the construction, at an angle of 45 with the optical axis O O. If, now, the medium next above the thin cover, (r, be air, this obliquity will be greater than the critical angle, and total reflection of the rays will take place. If, however, the medium next above the thin cover be water, the obliquity will not be greater than the critical angle. Refraction hav- ing taken place, the rays will enter the water, H; and if an immersion lens of sufficient angle of aperture be focussed upon the objects mounted beneath the cover G, these rays not merely enter the front of the objective, but will form a well-defined image of the object on a brightly illuminated field, which will be visible through the eye- piece of the instrument in the usual way. Of course it is evident from the diagram that with no dry objective, or any immersion objective of less than 90 balsam angle, can anything whatever of balsam-mounted objects* thus be seen. " Immersion objectives may be divided according to their behavior, with this apparatus, into three classes: 1st. Those with which, since they do not have sufficient angle of aperture to admit the illuminating pencil, * The apparatus can be used, of course, to secure black-ground illumina- tion of suitable dry objects if they are mounted on the slide instead of the cover, as is usual. TI1E WOODWARD ILLUMINATOR. nothing can be seen, precisely as in the case of dry ob- jectives. 2d. Those which have sufficient angle of aper- ture to admit rays of this obliquity, but are incapable of bringing them to an image-forming focus ; with these the field appears well illuminated, but the objects are not well defined. 3d. Those which not only admit rays, of this obliquity, but form well-defined images with them. To this class belong not merely immersion ob- jectives with the so-called duplex fronts, but others; and I may add, not merely objectives of American make, but some constructed by a well-known English house. As might be expected, the quality of the image formed by the direct rays of the sun thrown through a pin hole at this excessive obliquity varies very greatly in differ- ent cases. I will state, however, that I have thus far found at least seven objectives, some of English, others- of American make, which define sufficiently well under these circumstances to resolve Amphipleura pellucid mounted in Canada balsam. With the objectives which performed best, the field was of exceeding whiteness and brilliancy, but by no means dazzling, the frustule undistorted, and the striae clean and black on the white ground, very little color aberration being perceived. With other objectives there was more ot less color aber- ration and distortion, both which faults were in one or two cases very conspicuous, although in the part of the frustule most sharply focussed upon the stria? \vere handsomely brought out. The objectives with which I thus succeeded ranged all the way from one-fourth to one-sixteenth immersion. I will add that the objectives- 170 HOW TO SEE WITH THE MICROSCOPE. which resolved Amphzpkura pellucida under these try- ing circumstances, when used in the ordinary way with this or other test-objects, displayed an exquisite perfec- tion of definition which it would be hopeless to expect to attain with objectives of less angular aperture. " As it is no part of my purpose in this communication to provoke ill-tempered discussion of the merits of indi- vidual makers, I will not append a list of the results obtained with the various immersion objectives I have tried in this way. The apparatus can be constructed for a few shillings, and those who take the trouble to use it will soon see to which of the three classes any particular objective they may test belongs." Subsequent to the date of the reading of the preceed- ing communication, Dr. Woodward proposed some slight changes in the form of his prism. Having had consid- erable experience with the prism as now used, we append the following description, and also the manner of work- ing the instrument. Essentially, it consists of a triangular prism of crown glass. In the form adopted by Dr. Woodward the ob- tuse angle is 98 and the two acute angles 41 each. The prism may be used unmounted, by simply attach- ing the same to the under surface of the slide containing the objects to be examined, a drop of glycerine or oil of cloves serving to secure optical contact, and at the same time acting as cement to retain the prism in place. Notwithstanding this is the arrangement employed by many observers, it is but a faulty plan, in fact, a regular THE WOODWARD ILLUMINATOR. 171 " make-shift " arrangement. With the prism thus mounted any movement of the object-slide will of course cause a corresponding movement and decentering of the prism; furthermore, such object-slides can only be well examined when posed in nearly the horizontal position on the stage; it often, too, occurs that the intermediate drop looses its hold, suffering the prism to slide or even to loose its attachment entirely ; and then, again, I have frequently got the very best work when the facet of the prism was slightly depressed from the under surface of the slide. It is far better, then, and for the reasons given, to have the prism mounted, and to those who propose to adopt my method of illumination, I will say that much depends on the proper mounting, and that any piece of sub-stage apparatus which shall impede the passage of rays from the lamp to the mirror, or from the mirror to the prism, will defeat the maximum working of the latter. Hence, as a rule, it cannot be well mounted in the usual sub-stage, the latter obstructing too much After much experiment, and with the hearty co-opera- tion of Mr. Sidle, I am now in possession of the Wood- ward prism, suitably mounted and adapted to the " his- tological" of Mr. Zentmayer, as will be seen from inspection of the cut on the following page. This accessory, as above delineated in plan and sec- tion, is easily placed in position on any of the histolo- gical stands. Provision is also made for centering in a lateral direction. The prism can also be revolved so as 172 HOW TO SEE WITH THE MICROSCOPE. to use either angle. It can at will be raised or lowered, or clamped in any desired position. a. Vertical vi^w. b. Sectional view. c. Prism three-fourths full size. In the adaptation of this useful accessory to the acme stand Mr. Sidle has been singularly fortunate. This neat and compact little device screws, as I have had occasion, on a previous page, to state, into the well-hole of the acme stage. It is thus " on and off'* THE WOODWARD ILLUMINATOR. 173 in a moment's time. All the necessary motions are pro- vided for, and it may be depended on for first-class performance. From what has already been stated, it is almost need- less to repeat that when either of the mountings de- scribed are to be used on the histological or acme stands the sub-stage must be removed. It is quite possible that the mountings presented may, with slight modifications, be fitted to other microscope stands. The form of prism (angles 98, 41 and 41), as sug- gested by Col. Woodward, will be found to do very acceptable work. It occurred to me, after having had considerable experience with this accessory, that there was nothing gained by having the acute angles equal, but on the other hand advantages would be insured by an inequality of these angles, either of which might be used as occasion required. I therefore begged Mr. Sidle to make me a prism with an obtuse angle of 93, one of the acute angles to be 47, the other to be 40, and this is the form I have adopted, believing it .to be the most seviceable .arrangement yet proposed, and especially adapted to the general run of modern wide-apertured objectives. To provide one's self with two or more forms of prisms will not involve serious expense. Either of the mountings 1 have described can be modified so that the prism can be removed and others substituted in its place. To use the Woodward prism we will suppose on the acme stand first, screw the accessory into the well- *hole of the stage securely ; next, place the slide to be 174 HOW TO SEE WITH THE MICROSCOPE. examined on the stage. With a one and one-half or two-inch objective (having raised the prism so as to contact nearly the under surface of the slide), focus entirely through the body of the prism, so that its lower edge may be seen in the field, and by turning the prism in its mounting make this line, by estimation, as nearly vertical as possible. If, perchance, this line appears to the eye considerably to the right or left of the centre of the field there is no harm done ; but you must make a note of the fact; this would indicate, however, that either the stage is out of centre, or the mounting of the prism has been injured. Next, rack back the objective and remove the slide, place a drop of glycerine on the top face of the prism ; replace the slide and raise the prism so as to make con- tact with its under surface. Having made this contact exactly, depress the prism, say, about one-fiftieth of an inch. Focus again with the low power, and bring the lower edge truly vertical, as before instructed. Remove the low power and substitute the wide-aper- tured objective, and by way of illumination provide a small kerosene hand lamp, the flame of which ought not to be higher than two-thirds the distance from the table to the stage of the microscope; remove, also, the sub- stage. Now, if the lower edge of the prism was seen to the right of the field place the lamp to the right of the stage ; on the contrary, if the edge was seen to the left, pla,ce the lamp to the left ; and in either case swing the THE WOODWARD ILLUMINATOR. 175 mirror away from the lamp, placing it so that the gradu- ating wheel shall read at about 34, 35 or 36 degrees. The lamp being passed to the right or left of the stage, as the case may be, and about four inches distant there- from, bring the edge of the flame to the mirror; now move the lamp to or from the front edge of the table, so that the edge of the lamp frame, prism and centre of mirror shall form approximately a straight line parallel with the front edge of the table, From the position described it will be seen that the ordinary sub-stage would be entirely in the way ; hence the necessity for its removal. Making immersion contact, focus your objective, and, without changing the position of the swing bar, manip- ulate the mirror so that the field may be nicely ill und- ated, select your object, which we take it for granted will be 3ome difficult lined test. Next, interpose the large bull's-eye condenser (flat side to the lamp), thus concentrating light on the mir- ror ; adjust the object glass. It will be well now to try the effect of various degrees of obliquity, remembering that any considerable move- ment of the swing- bar will necessitate a new adjustment ot the condenser. A slight change, too, in the position of the lamp will sometimes be attended with excellent results, keeping, in all cases, however, its edge to the mirror. The above includes the author's method of working the Woodward prism ; but as this accessory bids fair to come into general use he will now traverse the ground 176 HOW TO SEE WITH THE MICROSCOPE. over again, feeling sure that many there are who will not object to some further discussion of the subject. In the initial attempt to use the prism, the observer should select an object (balsam mounted) with which he is tolerably familiar. The collar adjustment, also, should have been previously ascertained. The next step is to decide on the proper position of the radial bar, i. e., its distance from axis, the extent of this distance will be demonstrated by the illumination becoming too feeble, the images, also, becoming generally unsatisfactory. The remedy is, in such cases, to cause the radial bar to approach nearer to an axial position, until the field can be suffi- ciently lighted and the object displayed with tolerable vigor. Thus the operator has the means of " gauging " his objective. The mirror being properly posed, it re- mains to obtain the best possible illumination, which is effected by slight changes of the mirror, condenser, and finally the lamp. When things are generally about right, a little movement of the lamp (grasping it firmly by the bowl), sometimes twisting it to the right or left, so as to get the flame exactly edgewise to the mirror (which is best determined in this way), will result in very nice effects, bringing out the strife on such tests as the Moller test-plaie in a very satisfactory style. Resolutions of difficult test objects are accomplished with this illuminator in a very handsome manner. It is an easy instrument to use, and will adapt itself kindly to the objective, of course; the higher the balsam angle of the object glass, the better the definition. It has, in its ease of adaptation a decided advantage over THE WOODWARD ILLUMINATOR. 177 the Wenham forms, and in the comparative examina- tion of objectives, the wedge illuminator is an exceed- ingly handy accessory. In the determination of the collar adjustment corresponding to the point of maximum aperture, the same holds true. We have spent two or three entire evenings in the at- tempt to determine which of the two illuminators is the most effective ; and our experience leads to the conclusion that the " reflex " is somewhat the superior instrument in the resolution of the most difficult lined tests; never- theless, we are glad to give the newcomer a place in the accessory box, and expect to make it very useful. It is easily made and mounted, and ought not to be expensive. Another matter closely allied to the new illuminator may as well be mentioned here. Learning that the Messrs. Spencers had just completed a new one-fourth inch objective, which was to be sent to the Paris Expos- ition, we wrote to these gentlemen asking the loan of the glass for examination, the Messrs. Spencers re- sponded promptly, and it occurred that we received the one-fourth and the new illuminator the same day. We were thus enabled to put the illuminator to practi- cal use at once, in this manner: First, we took our one- sixth, working it with the illuminator over the No. 20 of the Moller plate getting the radial bar as far from axis as the objective would allow and preserve a good display of the striae. This done, we substituted the one-fourth in place of the one-sixth, keeping the illum- ination, etc., carefully in the same position, (the cover of the plate was as well adapted to the one glass as the 178 HOW TO SEE WITH THE MICROSCOPE. other.) We found at once, that in order to obtain suf- ficient light, and retain the general vigor of the image, it was necessary to approach the radial bar to the axis und the required movement of the latter was quite per- ceptible. It was therefore accepted that the one-sixth had the higher balsam angle. The question then turned, as a matter of course, on the respective working dis- tances; that of the one-sixth was known. It remained, therefore, simply to measure that of the one-fourth, resulting as follows : The working distance of the one- sixth is twenty-four-thousandths of an inch; while that of the one-fourth was found to be thirty-two-thou- sandths of an inch, a difference of 33 per cent in favor of the one-fourth. Thus it will be seen that in this instance the question as to superiority may be further taken under advisement. We relate this bit ot " prac- tice" with the illuminator in illustration of the pre- ceding remarks. One thing was proven even by the above experiment, to wit: Having our tenth on hand, held in reserve for especial demands, we would greatly prefer the one- fourth as an intermediate ^lass. This fact is too obvious to need further comment, and, in general, we are glad to add that the new one-fourth of the Messrs. Spencer is indeed a lovely glass, and if properly exhibited in Paris, will be an honor to the talented makers. The modified illuminator above described (obtuse angle, 98, acute angles, 41 each) will work very well with objectives having wide air apertures only; hence, like the modified " reflex," it will work over dry mounts, THS WOODWARD ILLUMINATOR. 179 in such cases, in common with the modified " reflex," its action, to some extent, is crippled; nevertheless, nice resolutions are to be obtained with either instru- ment, with either of which we are able to instantly display the transverse striae of amphipleura pellucida, frustulia saxonica, etc. The simplicity of Dr. Wood- ward's device, its ease of working, and the facility it affords for the comparison of objectives are in them- selves strong points in its favor, and to these may be added the satisfactory character of the resolutions ob- tained. We gladly accord to Dr. Woodward our appre- ciation of the value of his illuminator. We are informed by General Cox that the pin hole apertures are only used when working with sunlight illumination ; they also serve a useful purpose as an assist- ance in measuring with greater precision the obliquity of the illuminating pencils employed, thus enhancing the value and capacity of the instrument. TOLLES' TRAVERSE LENS. It now remains to present to the reader the Tra- verse Lens, devised by Mr. K. B. Tolles. The follow- ing is the inventor's own description of this valuable accessory, and is taken from the American Journal of Microscopy : 1 ' With the advent of objectives of increased interior angle aperture, the indispensableness of equivalent ac- cessory means for the illumination of the object became immediately evident.* * See"M. M. J.," July, 1871, p. 38. 180 HOW TO SEE WITH THE MICROSCOPE. " In my first construction of such object-glasses I therefore required to provide means which proved so suitable that I have adhered to their use to the present time. "The first appliance was a deep plano-convex lens, cen- trally mounted below the object, and having its centre of curvature in the object place. Afterwards I adopted a plano-cylindrically convex lens, equal to a hemisphere less the thicknsss of the object-slide, which was placed in immersion contact with the base of the slide, so that the object itself formed the centre of curvature of this illuminating lens. Around the convex surface of this central Jens moved a shutter to regulate and limit the access of light, and it was provided also with a small plano-concave lens which, applied by its concave to the convex surface of the larger lens by immersion contact, Cancelled the refracting surfaces and allowed a perpen- dicular beam of light to reach the suitably immersed object without refraction.* " The device in a more complete form is represented in the annexed figure, where P is the basilar plate of the whole traverse system, having a circular groove and track in which the carriage, C, moves. On a projecting arm, A, of the carriage, C, are mounted whatever ap- pliances are to be used to modify or direct the light upon the traverse lens, T, in the direction of the object at the centre of the system. " In the figure, the concave lens, N, is shown in posi- tion on the arm. Thus situated, the interior convex * M. M. J., May, 1873, p. 213. TOLLES' TRAVERSE LENS. 181 and concave surfaces being of no effect, the two exterior plane surfaces of the traverse system constitute it a prism, and every slightest movement of this concave facet lens on the traverse lens, T, would give a different prism to infinite variety. In this arrangement the concave mir- ror can be used in the ordinary manner and condense light enough upon the object for all ordinary purposes. The full interior aperture of a dry objective would be reached at the very convenient obliquity of 41; i. e., at less than the critical angle, or angle of total internal reflection between crown-glass and air. L is a double convex condensing lens, that may be placed at about its principal focal distance from the object. " For a condenser, with the size of apparatus as drawn in the figure, a simple lens of 1J inch focus, and about ten (10) degrees of aperture, is convenient, and if the lens is movable along the arm, A, it can be focussed readily on the object, the position being fixed by inspec- tion. This would be well for parallel rays. If diverg- ing rays are used another lens of two or three inches focus, mounted on the arm, A, will conveniently take 182 HOW TO SEE WITH THE MICROSCOPE. up the rays from the radiant at the distance of the focus of this supplementary lens. " The plate, P, is graduated on its circular edge, as in the figure, to two degrees, and the arm, A, has a swing of seventy degrees of arc each way from the axis of the microscope. An index-line is marked on the bevelled edge of the carriage 10 from the axis of the condenser, which must be added to or subtracted from the real ob- liquity of the illuminating rays. " It is obvious that any observation made and duly re- corded as to its conditions, as of obliquity of incidence of illuminating pencil or ray, form of the pencil or beam, focal length and distance of the condenser, such obser- vation could be successfully repeated. The record of the obliquity of the most oblique rays reaching the ob- ject directly, and giving view of it at the eye-piece with luminous field, would express the * balsam ' aperture, or more correctly, the half interior aperture of the ob- jective when the front lens of the objective and the traverse system are of glass of similar refraction. " Having thus the * balsam ' angle, we readily calcu- late or learn the corresponding angle for glycerine, or water, or any medium of which we have the index of refraction. A corresponding notation, perhaps for air, might be engraved in juxtaposition on the basilar plate." CHAPTER V. ILLUMINATION. Ordinary daylight is the cheapest; and for a great many purposes the microscopist will find it amply suffi- cient. It will be found a great convenience to have the light enter the room considerably to the left of the microscope, in which case we naturally adjust the mirror with the right hand. Placing the instrument directly before the window is objectionable, and such a position should be avoided if possible. The quality, as well as the quantity of daylight illumination will, as a matter of course, vary with the particular aspect of the day. In bright sunny weather the light from a white cloud, as has often been recommended, is pure and pleasant to work with, and can be used with tolerably high ampli- fications with good success. In cloudy, rainy weather it is still quite possible to work with powers up to, say 200 diameters. The recent introduction of the s whiff- c5 ing sub-stage has worked somewhat of a revolution in our own practice. For years we have steadily eschewed the achromatic condenser as being a costly and incon- venient affair, making more " bother" than it was worth. The principal objection I had to urge against its use was that it was a fixture beneath the stage, thus preventing me from varying the obliquity of the illumination at will, as I desired, and, as a rule, practically I got better 183 184 HOW TO SEE WITH THE MICROSCOPE. effects without than with it. The former condensers were generally of short focal length, and of consider- able aperture. In the late stands having swing sub- stages, it being possible to swing sub-stage and con- denser together bodily, there seems to be no further use for condensers of wide angles, while on the other hand one is enabled to use in the place thereof cheaper and much less expensive instruments, and the lower the tingle the better, and one need not be very particular as to the matter of achromatism. On commencing the use of the little Histological, it occurred to the author (and probably to scores of others) that its swinging stage was a strong invitation to experi- ment again with sub-stage condensers, not for the pur- pose of resolving difficult tests by extremely oblique illumination, for in this work the achromatic condenser is of no manner of account, but, per contra, it seemed obvious that by the use of a narrow apertured lens placed below the stage, and so arranged that its inclina- tion might be jchanged at will, good effects might be secured in two directions : First, by the concentration of a narrow cone of light immediately upon the partic- ular portion of the object under examination, thus en- abling the observer to sharply illuminate a certain point of his object, and with less danger of drowning out details in a general flood of light. Secondly, such a contrivance would do good service by daylight in dark and rainy weather. It required but few experiments to demonstrate that there was force in the above reasoning, and the next thing in course was to ascertain what par- ILLUMINATION. 185 ticular form of condenser would be the best adapted for the purpose. In determining how far it would be practicable to cut down the angle of the condenser, thus reducing the illuminating cone of light, we have made countless ex- periments, while the low stage of the Histological ren- dered it imperative that the focal length of the lens should be such as would best accommodate the little stand. To sum up all these trials, we find that the cheapest inch objective made by Mr. Grundlach, or the inch of the Messrs. Beck's " National Series," are, either of them, well adapted for the purpose. Mr. Gundhich's inch has a rubber front which can be removed, while the setting of the Beck " National " is extremely short, and thus suited in this respect for the purpose. This, then, is the author's arrangement for work with low or moderate powers by daylight illumination, and the condenser described has become almost a fixture. In the darkest days there will be plenty of light, using the concave mirror, while in bright, sunny days the plane can be substituted. The general amount of illu- mination can be changed at will by merely raising or lowering the sub-stage, and the nicest effects in the way of definition obtained. The swing-bar can also be placed so as to afford central illumination, or it may (condenser and all) be swung laterally up, say to an angle of 40 or 50 degrees from the axis; and it further remains to say that either of these cheap objectives are real good, honest glasses for the money. 186 HOW TO SEE WITH THE MICROSCOPE. SUNLIGHT. In the study of very minute and delicate structures requiring- the utmost separating- or resolving- power of the objective, remarkable effects are to be secured by condensing sunlight on top of the object by means of the concave mirror, the object being mounted with a cover in the usual way. The objective used should of course have wide aperture. The mirror being posed slightly above the level of the stage, the sunlight is thrown on the surface of the cover, and making a very acute angle therewith. Although not absolutely neces- sary for this purpose, those stands furnished with swing- ing sub-stages, allowing the mirror to rise above the level of the stage, are extremely handy and convenient. By the employment of this illumination in conjunction with object-glasses of wide angles, the most difficult diatoms, such as amphipleurapellucida,frustulia saxonica* etc., are easily and forcibly displayed. The advantages attending the use of monochromatic sunlight, as ob- tained by the intervention of the cupro-ammonia cell, or a plate of blue glass, have long been known. This illumination is procured most easily as follows: Cut with a diamond, or the point of a file, a small piece of the blue glass roughly to fit the cap of the eye-piece, p.o that when the cap is restored to its place the blue glass shall be between the eye and the eye-lens of the eye-piece, and the light is thus modified before it reaches the eye. This is the handiest method of obtaining mo- ARTIFICIAL LIGHT. 187 nochromatic illumination we have ever tried, and the resolutions are quite as strong and effective as when the cupro-ammonia cell is used in the usual manner. In working with sunlight by either of the methods de- scribed, care should be taken to exclude the full strength of the solar beam ; that is, if the sun be clear and bright. Too much light, supposing the manipulations are tolera- bly well attended to, will be manifest by the appearance of a multitude of diffraction lines, and these as a rule may be recognized by their extending beyond the ob- ject observed. Under very high amplifications, involv- ing the use of powerful eye-pieces, we can of course help ourselves to a little more of the solar beam. When the sun is very clear, the beam being condensed on the top of the cover, as above described, there is danger sometimes, if the object be balsam mounted, of the heat starting the balsam. In this way we once ruined a Moller probbe plate. A very little attention will, how- ever, provide against accidents of this nature. ARTIFICIAL LIGHT. For the ordinary purposes of the microscopist the St. Germain or German Student's Lamp, O. A. Kleemann's patent, or a similar lamp made by the Cleveland Com- pany, will be found quite satisfactory. This style of lamp is too well known to require any extended descrip- tion. The flame is bright, clear and intense, and its height can be changed at will. It is easily kept in order, and has the advantage, too, of being well adapted for ordinary household purposes. The breakage of 188 HOW TO SEE WITH THE MICROSCOPE. chimneys has been a serious objection to its use ; a brand of chimney known in Cleveland as the " Crown" (each chimney having a crown ground in the glass) seem to be very free from breakage. Non-combustible wicks are to be obtained, fitting the Kleeman lamp. These are clean and handy, obviating the necessity of occa- sional cutting- and trimming ; but to our mind the light- is not so intense, and therefore we prefer to use the old style of wick. These lamps burn very steadily, and are not easily affected by occasional drafts, and this is a strong recommendation in their favor, as is also the ease with which they are kept in order. For investigations of -exceedingly difficult objects the circular wick is not so well adapted, and recourse must be had to lamps carry- ing flat wicks. The best lamp we know of, of the lat- ter style is the Mechanical Lamp, manufactured in New York City. The lamp stands about ten inches in height. The height cannot be changed, and this is an objection. It burns kerosene oil, without any chimney. The body contains a movement which, on being wound like a clock, drives a blast wheel, and thus supplies a current of air at the point of combustion. Although there is a peri- odicity noticeable in the burning of this lamp, never- theless the flame is very steady, is very intense, and superior to gas. Like the St. Germain, this lamp is very handy to have in the house, and it takes but little trouble to keep it in order. The movement should be cleaned once a year, and any one possessed of fair me- chanical skill will be competent to do this. While burning, the clock-work makes scarcely any noise. An- ARTIFICIAL LIGHT. other form of this lamp has the movement placed in the case flatwise, thus allowing- the flame to burn within three or four inches of the table. The lamp is thus ren- dered very handy for use when direct light is wanted. The author has found, as a result of thousands of ex- periments, that the very best artificial light for the pur- pose of the microscopist is only to be had from a small but very intense flame. The smaller the flame the better, owing to the fact that there is less light diffused. We therefore use and strongly recommend the smallest ker- osene hand-lamp procurable, and fitted with a well-be- haved burner of the smallest capacity. If possible let the lamp bowl be so low that the flame will be, say three or four inehes only above the table, thus adapting the lamp for use by direct light. On other occasions the lamp can be supported in a more elevated position. With a little lamp of this description, in proper order y all the most difficult tests known to the microscopist can be well displayed, provided^ obviously, that the ob- jective, etc., shall be competent for the work. It is of importance, when any lamp provided with a chimney is to be used, that the latter be kept scrupulously clean, especially from a whitish film that forms on the interior. A chimney may appear to be perfectly clean while cold, but when heated the aforesaid film can be detected, and should be removed, if delicate observations are in hand, in which case it will be well, too, if the wick be three or four weeks old, to remove the same and substitute a fresh one. Even in the case of the small pattern of lamp recommended there will be no occasion to force 190 HOW TO SEE WITH THE MICROSCOPE. the combustion to the fullest extent such a burner will afford. A flame with the lamp burning at one-half its capacity will be amply sufficient, and even this would be too much for the proper display of some of the most difficult tests. Attempts have been made to modify artificial illumina- tion by the introduction of blue tinted chimneys, white ground illuminators, etc. We have patiently tried the entire list, and reject them all, from the fact that there is no real advantage secured by their adoption which cannot be obtained in a simpler way without them. The neutral tint " Light Moderator," so called, is a pleasant thing enough for use with moderate amplifi- cations ; yet there is nothing seen with it that cannot be as well shown without it. The blue tinted chimney cuts down seriously the in- tensity of the lamp illumination to an extent which will defeat the resolution of any severe test, while, on the contrary, any and all work with the lower powers can be as well accomplished without its aid. The reader has thus before him all the various kinds of illumination we use. A great deal of the profes- sional routine of work not requiring, as a rule, the em- ployment of the highest amplifications (such as the examination of urinary deposits, malignant growths, etc.,) we try as far as possible to accomplish in the clay time, and by the use of diffused daylight. If the sun happen to shine, and it be desirable to cross-question some preliminary examination under the highest pow- ers, we generally use the sunlight condensed on the top ARTIFICIAL LIGHT. 191 of the cover, or perhaps with the aid of the bit of blue glass in eye-piece. For work at night we employ at times all the lamps we have described. . Should the routine examinations be prolonged into the evening", we use the German student's lamp for preliminary work, the same as we use diffused daylight in the day time. But should the higher amplifications become necessary, we bring the mechanical or the little hand-lamp into play. The German student's will still do service in the lighting up generally of the work-table at intervals. For the showing of such objects as the Nos. 18, 19, and 20 of the Moller plate by lamplight, of course the little hand-lamp, or the lower model mechanical is impera- tively employed, especially when the Wenham " reflex " illuminator is selected. There remains yet another method of sunlight illumina- tion which will be found useful at times. I refer to the use of the "reflex" illuminator with direct sun-light. In this case the solar beam can be received through a closed window (quite a boon in the winter season) and reflected from the plane mirror. This illumination is only suitable for work with wide apertures, and ever the most minute objects, and the mount must be free from surrounding objects of a coarse character, else, from the extremely oblique character of the illumina- tion these stronger and coarser objects will project their strong shadows across the field, causing nothing but contusion and chaos. With the genuine form of the Wenham " reflex " an epithelial scale would hardly be recognized were there several in the field. The princi- 192 HOW TO SEE WITH THE MICROSCOPE. pal advantage in the use of the " reflex " with sunlight is in arriving at a knowledge of surface markings, and for this purpose it is indeed very valuable. Thus work- ing the " reflex " by sunlight, the mirror must be manip- ulated so as to produce the same effects as have been described by moving the hand-lamp and conversely. The mirror may be substituted for the hand-lamp when working in the evening, but the most favorable results- are obtained with the light direct. This reflex and sun- light illumination is especially desirable when one wishes- to trace out structure situated in one particular plane,, to the exclusion of that lying in adjacent planes. In the general squabble to produce the so-called penetra- tion, this very important item has been lost sight of. We are now ready to consider a matter which has been alluded to on a preceding page. It has been already stated that the maximum performance of ad- justable objectives can only be secured when such object- glasses are worked at the point of their maximum aper- ture, and that this point is by no means a fixture but varies with different objectives. Every observer should then ascertain for himself as to the proper handling of his object-glasses in this particular. Methods will now be given which, although but approximate, are suffi- ciently precise for the use of the practical manipulator. For the purpose of testing the point of maximum aperture for object-glasses having apertures, say from 40 to 175, proceed thus: Place the objective in posi- tion on any good stand having a thin stage and mirror attached to radial arm. Commence by focussing any ARTIFICIAL LIGHT. 1 { J3 suitable object on the stage, with the mirror in a central position, the collar of the objective being set at the ex- tremity of its range. Now, by degrees, swing the radial bar carrying the mirror, meanwhile adjusting the mirror so as to secure all the illumination possible, just as would occur in arranging for greater obliquity of illumination, until the obliquity of the light becomes as great as the objective will bear; i. e., until the greatest degree of obliquity has been obtained that will secure a tolerably well-lighted field. Now move the radial bar a little, and but a little further from axis, meeting this change by the proper manipulation of the mirror, and so as still to secure all possible light. The object now ought not to have more than one-fourth the usual illumination, but should nevertheless be distinctly seen. Next, revolve the collar and notice the effect. If you get less and less li similar instrument invented by Prof. H. L. Smith, of Geneva, N. Y. Prof. Smith's device, however, is fur- nished with an interior metal reflector in place of the glass disk of the instruments of the Messrs. Beck. We had used the illuminator but a very short time when we discovered that the definition of the objective was very much improved by shutting off the area of the lateral aperture, thus allowing less light to enter. We also found that the actual amount of light needed de- pended on the objective employed. In most instances the higher the angle of the object-glass, the smaller should be the area of the lateral aperture. About the time we had arrived at the above fact, the Hon. P. H. Watson happened to call in to spend an hour or two " over the tube." The conversation turn- BECK'S VERTICAL ILLUMINATOR. 219* ing on the action of the Beck illuminator, we coupled the- instrument to my Tolles one-tenth and worked them over Mr. Watson's Nobert test-plate, and in a very short time succeeded in getting a most charming display of the 19th band, and while the latter aperture was nearly closed by interposing the circular edge of a large bull's eye condenser which happened to be at hand on the table. Mr. Watson and myself were both delighted with the exquisitely beautiful display of this so-called difficult object this 19th band. Other severe tests were also taken in hand and resolved. Among others we had a glorious show of podura under amplifica- tion of some 4,000 diameters. It was demonstrated, too, that the very best resolutions were only obtained when more or less of the lateral aperture was closed by interposing the circular rim ot the condenser, the clear aperture left being in the form of -a crescent. Subse- quently Mr. Watson devised the following described attachment, which answers the purpose fully : The lateral aperture of the instrument is somewhat enlarged and a hollow plug inserted therein, the opening in this plug being the same calibre as the original aper- ture. The plug is somewhat tapering, and fits the open- ing "spring tight." It also projects outward slightly beyond the circumference of the main tube. To the outer end of the plug is fitted a little shutter, while a narrow slit, about one-one-hunclred-and-fiftieth of an inch wider is pierced through the shutter ; the whole so arranged that the opening the plug can be partly or wholly closed^ or the little slit used by itself; and when low powers are 220 HOW TO SEE WITH THE MICROSCOPE. employed, or objectives of narrow apertures used, the entire attachment can be removed if desired. This illuminator, as originally designed, was intended for use with dry objectives and with moderate magnifica- tions; the idea of using it with immersion glasses of high angles and under high powers, originated with George W. Morehouse, Esq., of Way land, New York, who is well known as an expert and accomplished mi- croscopist. The special advantages obtained by the immersion system in the case in hand are too apparent to need further mention. It will be noticed, too, that the Beck is thus made to do duty like the Wenham reflex in a way quite foreign to the purpose of its original inventor. n The advantages derived by the use of the instrument are : First, we are thus enabled to view objects by the aid of reflected light; the so-called "opaque illumina- tion," under the highest amplifications and (if the proper objective be employed) with superb definition, as is attested by its ^unequaled work over the Nobert 19th band ; and by the employment of artificial light, a com- mon kerosene hand-lamp being all that is required. In this respect the instrument stands alone and inimitable. Second, the views given are surface markings only. There is no "penetration" here. The focus must be most accurately drawn, and on the surface of the object. The slightest deviation therefrom is instant and total O defeat. Thus we are enabled to locate structure, at times a most valuable assistance to the observer. Third, by a slight change in the position of the lamp, the mir- BECK'S VERTICAL ILLUMINATOR. 221 ror being at the same time brought into play, the illu- mination may be almost instantly changed to that of transmitted light, or from thence back to reflected light again. This cross-questioning under two methods of illumination is often of great advantage. Per contra : The vertical illuminator has but one drawback, and that is rather a serious one, to wit: Ob- jects to be displayed under it must be mounted dry, and also contact the cover. Hence it will be seen that a large portion of histological, pathological, as well as other permanent mountings, are excluded from use. Many of these can be temporarily prepared for study, and it is hoped that the attention of observers will be enlisted in this direction. Admitting the serious char- acter ot the objection named, let us bear in mind that there remain countless fields of research wherein the "Beck" will certainly prove an instrument of the greatest value. . The tyro will find the Beck illuminator a difficult in- strument to use, and his first attempts will probably result in failure. Nor is it an easy task to give any instruction in writing that will be of much aid. The instrument not being a costly one, and likely to be gen- erally used when it shall become better known, we will try and furnish a few hints that perchance may prove of value to the beginner. The novice will do well, at his first attempts, to select a dry mount, one that he is perfectly familiar with, and preferably scales from the lepidoptera; a dry mount of podura will answer very well, indeed. Select, also, the 222 HOW TO SEE WITH THE MICROSCOPE. highest angled objective, and with this examine the mount in the ordinary way and get a tolerable correc- tion for the glass. Next, removing the object-glass from the stand, couple it to the illuminator and screw the whole to the nose-piece of the stand, using trans- mitted light, as usual. Focus the objective. Now hunt through the slide; among the numerous scales will probably be found one or two which, in order tn bring in focus, the objective will require to be withdrawn from the cover slightly. In such a case the chances are that that particular scale is nearer the cover, and if in good -condition may be selected for further operations. Next, bring the lamp (a flat wicked one) towards the observer, revolving the tube of the illuminator so that the lateral aperture shall be in proper position to receive the light from the lamp, the latter being about seven or eight inches distant, and the flame about the same height as the aperture of the illuminator. Now grasp the little knob connected with the interior glass disk and turn it so that light shall be reflected to the rear of the object- ive; at the same time, and looking through the tube as you catch the first glimpse of light, revolve simultane- ously the main tube and also the little knob carrying the glass disk, the object being to secure as great an amount of light as possible. A little manipulation of this kind ought to result in illuminating the object with a horizontal (or nearly so) band of light. The next step will be by a slight movement of the lamp, keeping its edge exactly towards the aperture, to endeavor to make the band of light crossing the field as narrow as WORKING WITH LOW POWERS. 223 possible, and the outlines of the band clear and distinct. By this time the operator will have probably discovered that a slight rotation of the main tube will separate the horizontal band into two parts, or, as some of my pupils express it, " two tongues." The best position is when these are made to coalesce as completely as possible. It is also probable that in the attempts thus far made that the ima^e of the scale has been well seen. When this occurs it should be at once focussed. The next procedure is to correct the objective; the correction obtained by transmitted light will not suffice for the purpose in hand. It will be noticed that as the glass is made to approach the correct adjustment, the horizontal band of light will be correspondingly improved. So true is this, that one might almost be governed thereby in the adjustment of the objective, Having got thus tar along, and without any serious mishap, it will be easy, by closing the shutte'r, to admit the precise and most favorable amount of light, and also to try the effect of sundry very slight changes in the position of the main tube, glass disk and lamp. Very beautiful resolutions are sometimes obtained by bringing the lamp within five or six inches and interposing the bull's eye condenser, flat side to the lamp, in which case the shut- ter must be further closed. It will happen also, occa- sionally, that the best exhibition of striae on very diffi- cult objects, such as extremely close fnistulia saxonicas, is when the striae are placed at right angles to the hori- zontal band of light. Now, should the manipulator meet with tolerable sue- 224 HOW TO SEE WITH THE MICROSCOPE. cess, and get good shows of the lepidoptera, I recom- mend that he practice for some half-dozen sittings over the same mount. By this he will get a certain familiarity with the instrument which will be of great value to him. The chances are, too, that he will wonderfully improve in the manipulations, and in this alone will be well enough rewarded for his pains. Furthermore, he will learn what nice effects can be had by the slightest changes- in the position of the tube, glass disk, shutter or lamp,, and the necessity for the closest focussing will be taught practically. Having thus got tolerably acquainted with the instru- ment, let the operator try changing from reflected to transmitted light, and vice versa, which is accomplished merely by moving the lamp back to its first position, employing the mirror just as was recommended at the start. A little practice of this nature, working alter- nately by transmitted and reflected light, will soon accustom the observer to the situation, and enable him to make the change in illumination almost instantly. A fact worth knowing is this : The combined length of the illuminator and objective will, if the objective selected be one of the extremely long models, defeat its use oil some of the smaller stands. Thus we find that we can- not use the illuminator with a Tolles one-sixth or one- tenth on the little " Histological " of Mr. Zentmayer, there not being sufficient room between the "jacket'* in which the body tube slides and the stage to receive the illuminator and objective when coupled together. We BULL'S EYE CONDENSER. LO\V POWERS. 225 are therefore, when essaying the use of the Beck, com- pelled to fall back on a larger and heavier stand.* BULL'S EYE CONDENSER. This instrument, so well known, and accompanying almost every stand as sold, we make nearly constant use of. It should be simply a large plano-convex lens, say from two to three inches in diameter, and fitted with universal mountings. It has been too general a rule with the microscope makers to " adapt" the size of the con- denser to that of the stand. Thus, if a certain maker furnish five or six different sizes of stands, he will be pretty sure to have as many sizes of condensers to accom- pany the same. But let the reader insist that, however small may be the model of the stand selected, the con- denser be at least two inches in diameter, and that even three inches will be found none too large. Any of our opticians can furnish the instrument made to order. We have seen and worked with several made by Zent- mayer to accompany his large and intermediate stands, and also the large model furnished with the Messrs. Beck, all of which were effective instruments. WORKING WITH LOW POWERS. It has been our primary intention throughout this work to avoid repetition of instructions, hints or sug- gestions, such as may be found in the various text- books, while interspersed among the pages already writ- * The construction of the "Acme" admits the use of the Beck illumina- tor. 15 Microscopy. 226 HOW TO SEE WITH THE MICROSCOPE. ten the reader may perhaps recognize information as to every day work which, perchance, he may turn to some account. There remains therefore but little for us to add under the above caption. Without hesitating to repeat in a more condensed form the same ideas which have heretofore been scattered through our pages, we proceed to give other methods of working with the lower powers. First, we use and recommend a stand fitted with a swinging sub-stage, preference being given to the one that will allow the mirror to rise above the level of the stage. Stands there are which although allowing a lim- ited swing, do not afford the extreme range of motion, while scores there be fitted with stationary sub-stages and fittings. Of the last two named, that with the lim- ited range is infinitely to be preferred. One of the prin- cipal advantages arising from swinging the mirror above the stage is that we are enabled thus to condense either sun or artificial light on the top of the object, and it is possible to accomplish this otherwise than by the swing, by simply attaching a mirror to a separate adjustable stand of its own. In whatever arrangement which may be selected, let it be imperative that the mirror be attached directly to the swinging arm, and at the proper focal distance, and that the centre of rotation coincide with the plane of the object on the stage. Any adjustable and intermediate joints between the mirror and the swinging arm, allowing the former to be placed out of its focal position, is, in the opinion of the author, an intolerable nuisance, and one not to be submitted to un- WORKING WITH LOW POWERS. 227 der any conditions short of sheer necessity. Now of the two styles of stands, the one with the full swing and the other with the partial swing only, premising that both mirrors rotate in the plane of the object, there will not be a great deal of choice for general work. Nevertheless it costs no more to manufacture the one than the other, and hence the perfect stand becomes no more costly than one that is to a, certain extent imper- fect. Therefore we urge, why not procure the best, provided there are not especial and governing conditions patent to the purchaser. The substance of all this has been placed before the reader, and in again calling his attention, it is hoped that the repetition will gather force. Now all the information we have to offer as to working- o with the low powers will refer entirely to the use of the low-angled sub-stage condenser conjointly with such a stand as we have recommended. As has been before remarked, the introduction of the swing has worked a radical change in our ideas as to the value of the con- denser, and the instrument is now with us a constant fixture, and in daily use. With the use of the condenser we have at command a greater amount of light, but this is not the special advan- tage derived from its use ; and then again it is easy to make the bull's eye do duty in its place. Of the two accessories, the bull's eye is the more simple and convenient to use; nor will objects occupying the entire or greater portion of the field when illuminated with the condenser appear more brilliant and enticing than without it. The grand 228 HOW TO SEE WITH THE MICROSCOPE. advantage obtainable with the condenser is, that by a careful manipulation of the apex of the small cone of light we are often able to get just the right illumination on certain details of our object, while the field generally is kept in partial light only, " toned down" as it were. This effect may be produced in two ways: First, by raising or lowering the condenser; and, secondly, by swinging the sub-stage laterally to a point but just within the aperture of the objective employed. We sometimes use the one plan and sometimes the other; and then, again, we often use a mixture of both, and, as a rule, the better illumination will be attained when the condenser is somewhat within, or without the focal point. Now let the novice understand that in thus employ- ing the condenser there is no attempt to get " pretty dis- plays" of the object. On the other hand, the primary object is to obtain cool and reliable definition of struct- ture. Those who thus use it will, I think, be pleased with the results attained. It is somewhat more " bother " than taking the light directly from the mirror, or with the usual intervention of the diaphragm. Notwith- standing this, one soon becomes accustomed and addicted to its use. The swinging stage, too, is a valuable ad- junct to the old diaphragm, and with this latter instru- ment superior effects can be had working on the same general plan as with the condenser, and using a small aperture to the diaphragm. Should the light be too weak it can be assisted by interposing the bull's eye, or light can be shut off by depressing the diaphragm. WORKING WITH LOW POWERS. For ordinary work by daylight we use the plain mir- ror in conjunction with the condenser; and let me here again insist that the latter be such as has already been described, i. e., of low aperture, admitting but a narrow cone of light. In very dark days the light can be rein- forced by employ ing the concave mirror, a plan not gen- erally recommended, but nevertheless quite practicable. For reasons before given, we greatly prefer, for regu- lar right straight along daily work, to use the stand with the tube short, keeping, for the most part, the tube vertical, dispensing, too, with the stage clips, and simply laying the preliminary mount thereon, shoving it about in every direction required, by the fingers. Those who have been accustomed to confine the slide under the clips, as is generally done, will not, on the ' first trial, be likely to endorse my practice. Let me, then, to all such, especially recommend it. By thus allowing the slide to rest by its own gravity alone, one soon acquires a delicacy of finger manipulation that is of very general value; while, per contra, the clips are a real hindrance to fast work. Those who have the pa- tience to practice without them for one solitary week will not be likely to get back into the old rut. The height of the work table should vary with that of the observer ; such a table as one would naturally elect to write on will be about the correct height for microscope work. This, if the low angled condenser is to be generally employed, may be placed at a consid- erable distance from the window, but care should be taken that the light comes from the left. If there be 230 HOW TO SEE WITH THE MICROSCOPE. other glazed openings in the room liable to form cross- lights, they should be closed by shutter or curtain. The table selected should be solid and heavy. Any and all of the little light affairs in cherry and mahogany offered at the furniture stores are totally unfit for a mi- croscope table; but the crowning nuisance of the lot is the revolving table, made expressly for the microscopist, and sold at outrageous prices. Castors, also, are to be rejected. In short, anything that detracts from the sol- idity or rigidity of the work-table is to be eschewed. What the observer wants is a firm support for his stand, free from shake or tremble one that he can lean against freely when weary, and one that he may even run against accidentally without inaugurating any serious calamity. A couple of drawers placed in the front are a convienience for storage of accessories, etc., and these, being partly opened, form convenient rest at times for the forearm. The table should be sufficiently large and roomy. Three by four feet is none too large. THE SPENCER ONE-INCH OF 50 BROAD-GUAGE OBJECT- IVES, ETC. In the spring of 1878 we received from the Messrs. Spencers an inch objective of 50 aperture. This glass was made expressly to our own order. Our purpose in ordering the above glass was to deter- mine whether it was possible for the Messrs. Spencers to furnish an inch objective having angle and definition equal to their celebrated two-thirds. With the new inch our experience has thus far been BROAP-GUAGE OBJECTIVES, ETC. 231 necessarily limited; we have, however, instituted a few comparative tests which we now present to our readers. The working distance of the two-thirds worked with one-fourth inch solid ocular and standard ten-inch tubes is twenty-five-one-hundredths of an inch. The working distance of the new inch with same oc- ular, worked with ten-inch tube, is thirteen-one-hun- dredths of an inch ; when worked with the same eye-piece and the short tube of the " Histological," the working distance is increased to eighteen-one-hunclredths of an inch. In general, the definition of the inch is superior to that of the two-thirds. The very best definition of the latter is obtained by the use of the full length tube and one-fourth inch ocular, while the highest definition of the inch is reached by the use of the short tube and same ocular. The inch* worked over the balsamed Moller plate, gives readily nice shows of the transverse striae of P. Balti- cum, and with a little management both sets of lines are brought out. On the G. Marina, immediately pre- ceding the BaUicum, and really the more difficult shell, the inch gave me a very nice stand of the transverse striae from end to end of the shell, its performance over this diatom being manifestly superior to that of the two-thirds. On dry mounted shells of the P. Balticum the two sets of lines are very well displayed by the inch, as are also the transverse striae of dry angulatum, a favorable frustule being selected. 232 HOW TO SEE WITH THE MICROSCOPE, Over the Nobert test plate the inch gave me the 9th band, = 56 X lines in .001, Eng. inch. Working the two-thirds with the one-half inch ocular and full length tube, and the inch with one-fourth inch ocular and short tube, the amplifications were about equal ; the inch in every instance affording the better definition. According, then, to the inch manifest superiority in point of defining power, its shorter working distance, which in its most favorable aspect is 33 per cent, less than that of the two-thirds, must receive due consider- ation, this decrease, too, of working distance will be accompanied by some loss of "penetrating power." A mueh mort important point, however, to the author was to ascertain if the focal distance of the inch was sufficient to allow its being worked over wet and uncov- ered mounts without clouding from the evaporation. To test this, we used the glass an entire evening in reg- ular routine work over urinary deposits, and without experiencing the least trouble from the source named ; while on the other hand the general behavoir of the objective while thus engaged in practical work was most satisfactory. The reader is reminded that in comparing working distances of long focus glasses, the same percentage of weight does not obtain as would occur in comparisons of object-glasses of nominally short focal lengths, and giving high amplifications. A word or two concerning the standard " society BROAD-GUAGE OBJECTIVES, ETC. 233 After experimenting with the wide-angled inch, as above related, we wrote to the Messrs. Spencers, asking them if it was possible either to still further extend the single of the inch, or, maintaining the same aperture, to increase its working distance. In reply, the elder Spencer informed me that it would be difficult to fur- ther increase the aperture or the working distance unless the diameter of the objective should also be enlarged. From this it would seem desirable to increase the di- ameter of our low-power object-glasses, and this in turn necessitates a change in the construction of the micro- scope stand. Mr. Bullock's large stand, as also the Acme, are especially arranged to accommodate these " broad- gauge" low-power objectives. An one inch objective of large calibre is now in pro- cess of construction for the author. The optician hopes to endow it with an aperture of at least 62, maintain- ing a working distance of one-eighth of an inch. It is but a simple act of justice to say that the idea conveyed to me by Mr. Spencer had already occurred to Dr. W. W. Butterfield, of Indianapolis, Ind. While in attendance at the congress of microscopists held in Indianapolis in 1878, Dr. Butterfield showed me a broad-gauge four-inch made to his order by London opticians. He had also a stand then in process of con- struction, and designed for the use of this class of ob- ject-glasses. This, however, at the date mentioned, had not arrived; consequently there were no accommoda- tions for an examination of the objective. Should my own glass be completed be-fore these sheets go to press, 234 HOW TO SEE WITH THE MICROSCOPE. some account of its performance may herein be expected. Meanwhile, there seems to bo no good reason why, with the increase of calibre of our low-power objectives, there should not also obtain those advantages due to aperture 4.nd working distance. CHAPTER VII. For this class of work we have in the preceding pages- unhesitatingly expressed our preference in favor of ob- jectives of the widest aperture. Such are the instru- ments we ourselves use daily, and can confidently recom- mend to all who may be desirous of working with the best instrumentation obtainable. Therefore, whatever we may have to offer in the way of instructions sug- gested by the above heading will be solely applicable to the class of objectives generally known as " wide- angled," to which we have given in the past, and pro- pose yet to give in the future, a large amount of careful study and attention. And first of all it becomes neces- sary to disabuse the mind of the student of some of the popular fallacies which have found outlet and circulation, through the medium of the microscopical periodical literature of the present and past few years. These, as will be discovered by the attentive observer, are par- oxysmal in their nature ; in fact are veritable " chateaux en espagne," at once inconsistent in detail, and roundly absurd when contemplated as an entirety. Thus it oc- curs that at one moment the student is taught that wide- angled glasses are extremely inconvenient; that great attention has to be bestowed on the adjustment and illu- mination, etc. ; while on the other hand, another " au- 236 236 HOW TO SEE WITH THE MICROSCOPE. thority" subsequently insists that the so-called "hand- ling" (?) supposed to be necessary to the use of the wide apertures is simply a myth a downright farce; and that any one possessing a fair quota of intelligence -can easily acquire all that is to be acquired in the work- ing of an adjustL^le glass. Nor need one hunt long or dig very deep to find other " authorities" teaching that all this handling" " although essential to the optician" is r.o manner of use to the practical observer, unless he has so far degenerated as to aspire to the distinction of being simply a " handler " and a " fighter." Let the author, then, and in view of the situation as presented, inform those proposing to study the microscope with the intention of becoming in due time accomplished observers, that there is no " royal road " to success ; that to become even so sufficiently expert as to enable one to follow out (leaving original work out of considera- tion) the investigations already made and published by eminent microscopists, will require quite as much effort and study as would be called for in graduating from any college in the United States. The curriculum is a broad one in its very nature, involving a thorough knowledge of instrumentation, and when by means thereof we are enabled to see well, it then becomes a positive necessity to judge well of what is seen, and this in turn can only be well accomplished by those having eyes well trained to the work in hand. There is, moreover, work for the brain outside of the functions of the optic nerve. In the micrographic dictionary, by Messrs. Griffith & Henfrey, I find, (page 11,) the following: "Above all. WORK WITH THE HIGHER " POWERS." 231 however, it must never be forgotten that microscopic investigations require more time and patience than per- haps any others, even in regard to the determination of simple facts of structure and qualitative composition ; and although it is not very uncommon to hear those en- gaged in them sneered at as wasting their time over a very simple plaything, this may be regarded as arising from one of those prejudices which will exist so long as people will venture to express opinions upon matters with which they are unacquainted, and which are beyond their comprehension." The above quotation is well worth reprinting on it& intrinsic merits, and it may be that we shall find some especial use for it bye-and-bye. Meanwhile, this mat- ter of eye training calls for a word or two, for, among- the accomplishments tha,t go to make the first class ob- server, this education of the eye is generally supposed to be quite as much a myth as the capacity to " handle " an object glass. Let us take an item or two from our personal experience : Not lono- since the author had the honor to address a o select party of gentlemen at the parlors of a private residence. In the course of his remarks the matter of " eye-training " was brought prominently forward and its usefulness urgently insisted on. Subsequent to the close thereof a gentlemen present stepped up to him r saying, " I want a little talk with you about that eye business, which you seem to regard as a sine qua non. I do not see that thing as you do. You and I are about the same age. We have both of us necessarily been 238 HOW TO SEE WITH THE MICROSCOPE. using our eyes constantly all our days. If I look across the street and see a house, why, so can you; and thus we have been respectively seeing houses as well as other things all through life. In short, our eyes have been constantly at work, and have thus been as constantly trained. It may be that you have abused yours by over work with the microscope if that be the case, I reckon mine have the best of it. At all events, I can't see how you can establish any individual superiority as to vision." At this moment Prof. Huber had seated himself at the piano and was entertaining the company by his superb renderings from classical authors. In reply, I said: what you affirm as to the eye must be similarly true of the hands. Prof. Huber and yourself are apparently of the same age, and both of you have been using your hands " all your days." Whence comes that lightning rapidity of action; that wondrous delicacy of touch? Think you that the professor has abused his muscles by over work with his five finger exercises, or that you have any claim to digital superiority? But to confine the case strictly to the eye alone ; how comes it that the mariner can, not only detect a " sail " near the distant horizon, but can also state with accuracy whether it be a ship, brig, or schooner, and the direction the " sail" may be pursuing, to all of which the passengers present will be totally blind? Noi need we " go to sea" to find instances illustrative of the issue in hand. The eye of the artist recognizes, perforce of his experience with the esthetics of nature, WORK WITH THE HIGHER " POWERS." 239 beauties, to which that cf the shepherd boy is innocently enough a stranger, and of the two, permit me to inquire which would make the better rnicroscopist? As far as our auditor is concerned, we rest the issue on its merits. A few years ago we purchased for a gentleman well known in microscope circles, a wide apertured objective. The party was 110 novice, but on the contrary a real hard and close worker with the instrument; and fur- thermore, the gentleman had formerly filled the chair of microscopy in one of our most honored colleges. After working with this glass for about one year, he applied to me for instruction in the use of this instru- ment, proposing to spend his vacation with me, and for this purpose the author was delighted with the proposal and the arrangement was consummated by unanimous consent. Now the main point actuating my honored pupil was this: He had used his glass considerably enough to discover that there were conditions involved that he could not control as he desired. Sometimes he could see better than at others; sometimes the glass would work good naturedly, and then again, at others, it wouldn't work well. In the course of instruction which followed, a slide of navicula rhomboides was se- lected (this, by the way, was a diatom) and placed on the stand for examination with the objective named. In point of " difficulty" these shells would have been regarded as average specimens. We then look some little time to explain as well as we could the behavior of the object glass when in and out of adjustment, as 240 HOW TO SEE WITH THE MICROSCOPE. exhibited by the object selected. Our reason for mak- ing at this time the particular selection was this : When the objective was in perfect adjustment, the striae were admirably well seen when the inch ocular was employed,, but w r ere assumed to be invisible under the two inch. Next, the pupil was required to adjust the object glas& using the two-inch ocular only, acting, of course, under the general instructions he had received. This done he was to apply the higher eye-piece and learn of his suc- cess, practically. Our friend went at his task manfully, and fought that slide of diatoms three hours or more daily for more than a full week, constantly improving in its manipulations. Then it occurred that, getting somewhat weary of his protracted efforts over one and the same slide, he began to beg for a change to do some- thing else. Nevertheless, he was put off time alter time, until nearly at the close of a long evening's work he jumped up from his table, and running towards me, his eyes beaming with joy, exclaimed, " I have it! I see it! It's all plain sailing now!" "Well," said I> "what is it?" He replied, " I can see any shell on that slide, I care not how small nor how close the striae, and as well with the two-inch ocular as with any other; and more than that, I can put the correction collar right 011 the dot without humming or hawing, and do it every time." To this we responded: " You have now solved your problem, and are ready to tackle another mount at our next sitting. Now, reader, here is a practical case in point: My pupil had not only been improving himself in the "hand- WORK WITH THE HIGHER "POWERS." 241 ling" of the object glass, but all this time his eye was becoming educated. At the commencement it would have been an utter impossibility for him to have recog- nized the striae of the rhomboides with but the two- inch eye-piece, nevertheless the writer saw them splen- didly, and with that exquisite definition which per- tains only to the work of these high-angled glasses. Had 1 told my pupil this at the start, he would proba- bly have received the assertion in a becoming manner, meanwhile entertaining some " first class doubts " under his sleeve;" full fledged doubts, too, and simply wait- ing for a fine day to fly. Place the microscope in the hands of the shepherd boy ; its total defeat is established. Adjust, if you will, the objective with the utmost nicety, and arrange the illumination to perfection, and total defeat still reigns tiiumphant. He neither has the capacity of seeing well nor of judging well of what may be seen, neverthe- less, he can honestly and innocently look you squarely in the face and assert that he has as good a pair of eyes " as the next man." We have thus taken some little pains, and hope not without profit to the reader, to establish a fact well knwn to all who are expert in the use of the microscope. If our views are correct, it obtains that no inconsider- able amount of time and patient care and study are each individual elements in the outlay necessary in the effort to become an accomplished observer, and if this is to be considered in the light of an evil, then let it be remem- bered that " there are no evils unless attended with Microscopy 16 HOW TO SEE WITH THE MICROSCOPE. some corresponding good." And fortunate it is, in the case before us, there is a fascination accompanying the intelligent use of the microscope knowing neither limit nor bounds, and the task of becoming well acquainted with the use of the instrument is merely a labor of love. There is another aspect of the matter which deserves a word or two, to wit: The popular idea with many is, that if there be a certain amount of eye training essen- tial iii the use of the " high powers," this has no appli- cation to those who use the lower amplifications. Says one, " you are all right about your eye education when there is such nice work as showing the 19th band in hand ; but then you see most of my work is done with the inch, and that's quite another thing." Now there is just as much error here, but it is of a less serious character. The truth is, the expert can see more with the inch, and by the "expert" I mean (for the time being) those referred to who are able to display hand- somely such tests as the 19th band. Every day expe- rience with pupils in the laboratory demonstrate this fact pointedly. Thus: At the commencement of his practice, the novice is quite content with the meanest French triplet the premises afford, but in less than a month he will hold it in perfect contempt, and his sub- sequent progress will to a great extent be properly measured or indexed by the constantly increasing capa- city to handle even these non-adjusting glasses. In thus insisting on the necessity of the proper edu- cation of the eye, let us look for a moment to its im- portant bearing on a particular class of observations. WORK WITH THE HIGHER "POWERS." 243 We intended to refer to this bye-and-bye, but it finds an appropriate place right here. We may, however, refer to it at some future time. I allude to that class of work intimately connected with the use of object glasses of wide apertures, and over exceedingly difficult lined structures; for instance, the display of the striae of amphipleura pellucida. But let the reader remember that this class of work is not confined to the study of diatoms. To illustrate what we have in- hand, reference is made to the following sketch : Let C-D and G-H be a sec- tional view of some " difficult " diatom, such as amphi' pleura pellucida or the like, the short lines 1, 2, 3, 4 ? etc., representing in section the elevations of the striae; M and N being views " in plan" as seen conditionally in the microscope. Let us first consider the effect of illuminating C-D with direct central light, as indicated by the line A-B. The effect will be as shown in plan at N, to wit : There will appear but a series of exceed- ingly fine lines; so fine (mark the words, not necessa- rily close) that it will be impossible to see them with any glass extant. Recourse must then be had to " ob- lique illumination." Now let C-D, as duplicated at G-H, be illuminated by the oblique beam E-F. The effect of this is shown in section at 1', 2', 3', etc., and in plan at K, where we have the view as displayed in the microscope. Here we have tAVO things successfully accomplished; the striae which in the former case were so " fine " as to be invisible have now become broad and can easily be distinguished by the eye ; and, secondly, 123 244 WORK WITH THE HIGHER " POWERS." 245 this last display is as false, as to many it has been ac- ceptable. Now let down the obliquity of the illumina- tion, as indicated by the line I-J; the effect is noted on the sketch at L, and with a first class high balsam angled objective and an eye well trained, it may be that the strioe are not only discernable in the microscope, but the observer will further be able to note the intervening spaces also; and this, too, which the certain knowledge that of the varying condition set forth, the latter dis- play is not only the most satisfactory in general terms, but the most truthful. It is also apparent that the more we decrease the angle of the illuminating beam the louder the call on the defining power of the object glass and the greater the demand for education on the part of the eye. In other words, the expert, all other things being equal, with a pair of eyes trained by long practice in his profession, has the better chance of see- ing things as they actually are. From the illustration given we deduce several propositions, viz: First, it is always better to see structure somehow than not at all. Let those addicted solely to the use of narrow apertures ponder this well. Secondly, when engaged in investigations of " diffi- cult" structure similar to the case presented, and calling at the best for light of considerable obliquity, the less of the obliquity, as a rule, and within certain limits, the better ; and the more perfect the education of the eye the less will be the call above mentioned. Third, the higher the balsam angle of the glass the LESS will be the obliquity required. Let those who 246 HOW TO SEE WITH THE MICROSCOPE. favor work with centrally disposed light make a note ol this. Says one (alluding to the second proposition), I don't exactly see that thing as you do. Suppose, for instance, that the objective and the eye were both so perfect as to allow the illumination to become axial, as in your first illustration, wouldn't that be better still? We reply, ask the artist if he would prefer the land- scape thus lighted. The architect of the universe ex- pressly arranged things to prevent such a catastrophe. A certain amount of shade is as necessary as that of light. Such a thing as " dead central illumination," although often talked about, is a myth. Feeling deeply the importance of calling the atten- tion of the reader to an element which we regard as of vital importance, we have accordingly done so at the risk of being somewhat tedious. POSITION OF OBSERVER. Our experience is, that in sixty cases out of the hun- dred, having made some preliminary examination of an object under study, and thus demonstrating the neces- sity of the use of higher amplifications, that with the latter comes also the necessity of a long and protracted sitting ; in fact a downright seige is inaugurated. It is better in all such cases to postpone work until evening, or at least so to arrange that the observer shall have perfect immunity trom interruptions of any nature; and at the commencement of such work it is of para- mount importance that the operator adopt such a posi- POSITION OF OBSERVER. 247 tion at the work table as will allow him to observe for hours without serious fatigue. Such a table as has already been recommended, with two front drawers, will be just what is wanted. The novice is here re- minded that in working with the higher powers, after having placed the object in position under the objective, the latter being correctly adjusted, there will be no longer use for the coarse adjustment. Having, then, our table, place the chair adjacent thereto, but in place of putting its front edge parallel to the front of the table, as is generally done, turn the chair to the right until the front and left-hand sides form equal angles with the front of the table, the angular point formed by the meeting of the front and left-hand edges of the chair being adjacent to the table. Now let the observer seat himself, placing the shank of the left shoe on the left-hand round of the chair. Pull out both drawers, so that the edge of these, assisted by the edge of the table, shall form a double rest for the forearm. Now place the stand in such a position, that is, with refer- ence to the front of the table, that the left hand finds its way easily to the fine adjustment, while that of the right grasps the mirror bodily, the tube being mean- while adjusted to the standard length, and the whole instrument properly inclined. Let the reader practice these directions thoroughly until he shall be able to thus sit at his instrument firmly wedged to it. It will be noticed that, once in this position, either hand can grasp the object slide for the purpose of making any neces- sary change, and without seriously disturbing the double 248 HOW TO SEE WITH THE MICROSCOPE. adjustment for the forearm, while the right hand, being" thus so nicely supported, is enabled to manipulate the mirror with almost mathematical precision. At the first, as might easily be supposed, the sharp edges of the table and drawers will be a source of some little incon- venience. This can be remedied by placing a pair of napkins thereon; but the better way is to endure this slight annoyance for a little time, when the forearm will be found to have adapted itself to the situation. The position thus described we have represented as far as possible in the frontispiece, using such furniture as was at hand in the photographer's gallery This pose can be varied at times by bringing the left knee in use so that it may support the left elbow. Thus we get three rests tor the left arm, and sometimes we get the shank of the right foot on to the front round of the chair, spreading the knee open a bit and wedging it under the riofht hand draw. Other little chancres are O O practicable and need not be detailed here. Of all the manipulations connected with the use of the higher powers, the adjustment of the objective (sup- posing, of course, that it is a good one, one that Avill respond to the collar adjustment), is of paramount im portance. For reasons previously stated, two elements are involved, namely, the behavior of the object glass and the education of the eye ; and here the use of the diatommeca3 is imperative. We have insisted oil this for years. These little organisms are the most con- venient, and then, again, any little deviation from the perfect correction of the objective is sure to " stick out " POSITION OF OBSERVER. 249 be detected. This is not the case with other objects. Show the tyro a scale of podurae under a tolerably nar- row angled one-tenth, and likewise display the same with a similar object glass of the higher balsam angles. There will be as much difference in the quality of the two exhibits as there is (I was going to say) between light and total darkness; and yet, nevertheless, the nov- ice will be as well satisfied with the one as with the other ; but let the experiment be repeated, using in the place of the podura a balsam mount of surriella gemma, both glasses doing their best, as before, and the tyro, is no longer " at sea " as to his choice. Moreover, at this stage of his experience he will be fully prepared to blow his trumpet in the support of one of the most absurd and stupid errors that has ever been promulgated since the time of Adam, to wit, " High angled glasses are only fit for work over diatoms! !" When we say that diatoms are the most convenient objects over which to study the adjustment of the ob- jective, we mean it, and thereto attaches greater force than the casual reader may suppose. If it be imagined that these objects are " convenient " because their gen- eral proportions are about the thing because they can be purchased at slight expense, or, if preferred, pre- pared by the observer himself, or even be it granted that the markings on the more difficult of these shells will only surrender to a first-class objective in perfect adjustment, we admit the facts, but the story is not fully told. The grand, the culminating convenience attending 250 HOW TO SEE WITH THE MICROSCOPE. the use of diatoms in the study of the objectives used under high amplifications, is this : We are enabled to- display on one and the same mount, shells of the same family and species, differing only in size, and we thus are on the instant ready to study the work of the ob- jective over each. Now if it so be, and we make it a point that it shall so be that the smaller shell is in all respects the more difficult of the two. Then it occurs that the student, having mastered such smaller frustule can examine at his leisure the larger ones, and with the certainty that his objective is in at least approximate adjustment; hence he is further prepared to note the difference in the behavior of the object glass over the different diatoms, and thus he arrives at items of the utmost value; all this, too^without any change of the mount. Still other conveniences there are attending the use of the diatom, their extreme thinness preventing the shadow of one shell from interfering with the defini- tion of another, thus getting rid of a complication which would prove of serious detriment in the early studies of the student, while by and by he can essay an attack on the very problem named by merely selecting sueh positions of the amount as contain the little organisms huddled together. Thus learning their char- acter and being thus forewarned, is fore-armed against the time when he shall be brought in contact with other slides presenting the same difficulty, but in a more determined manner. But let the reader note this feet. It is one thing to. POSITION OF . OBSERVER. 251 look at diatoms, and quite another to study them with the especial object of becoming acquainted with the behavior of the objective, while it must be admitted that there is a fascination and charm per se, connected with diatom examinations under the microscope. It is equally true that the student can use them legitimately and for the purpose named, without establishing any claim to the functions of the diatomist. Referring to the especial purposes we have been con- sidering, the following list of objects will amply suffice, for the study of the one-sixth or one-tenth objectives, viz: 1. Navicular Rhomboides, Monmouth, Maine, Balsam Mount 2. Navicular Rhomboides, Cherryfield, Balsam Mount. 3. Frustulia Saxonica, Leipsig, Germany, Balsam Mount. 4. Frustulia Saxonica, Isle of Shoals, U. S., Balsam Mount. 5. Amphipleura Pellucida, Bridge of Allan, Scotland, Balsam Mount. 6. Amphipleura Pellucida, Aberdeen, Scotland, Balsam Mount* 7. Surriella Gemma, Balsam Mount. Of the above list, Nos. 1 and 2 can readily be ob- tained of the dealers. As to the others named, Prof. H. L. Smith, of Geneva, N. Y., has a large supply of the material, and has kindly supplied the author and his friends with excellent mounts and at figures much below the usual list prices. In addition to the diatoms, a genuine mount of English podura will be a capital thing to have on hand for occasional comparisons, but the student must on no- account select the slide himself. Poor scales are simply good for nothing, while perfect ones are held by the opticians in the highest esteem, and are constantly used 252 HOW TO SEE WITH THE MICROSCOPE. by them in the final corrections of the objective. Of all the scales of podura we have yet seen, a mount was shown us last summer by Mr. Herbert Spencer, which was perfection itself. As an index ot the value Mr. Spencer attached to this mount, I will add that he had once offered one of his first class one-tenth objectives in exchange for it ! Every microscopist needs a suitable stage micrometer and very few have reliable ones. We gladly state that Prof. W. A. Rogers, of Cambridge, Mass., has after years of careful study succeeded in making rulings on glass rivaling if not excelling those of Nobert himself. Not long since Prof. Rogers ruled for us a plate, con- taining lines 100, 1,000, 2,000, 5,000, 10,000, 20,000, 40,000 and 80,000 to the inch, which was a marvel in point of accuracy and delicacy. Subsequently Prof. Rogers ruled a plate for us up to 120,000 lines to the inch. This band the author has seen well and has shown to his friends. We therefore recommend first, that every student should be possessed of a micrometer; .and secondly, that the same be procured from Prof. Rogers. It will be well while one is about it, to order a plate ruled as high as 80,000 to the inch, inasmuch as the cost is not materially enhanced, the plate will thus do double service not only as a stage micrometer, but as a test plate for the comparison and study of objec- tives. In this connection the reader will remember that we have advised that eye-piece micrometers be ruled GOO lines to the inch. Hence, if our advice is followed, let the stage micrometer have a similar band. POSITION OF OBSERVER. The positive convenience resulting from this in the measurement of the focal length of objectives by the method previously described is too obvious to need further comment. The series of graduated diatoms by Moller of Wedel, Germany, and generally known as the Moller test Plate, is now to be found in the cabinet of nearly every niicroscopist, and can be advantageously supple- mented to the list above given. A table showing the mean of the measurement of ten of these plates will be QOW furnished and thus the plate can on a pinch be- made to do approximate duty as a stage micrometer. MEAN OF TEN MEASUREMENTS OF MOLLER TEST PLATES, BY PROF. E. W. MORELEY, M. D., OF HUDSON, OHIO. 1. Triceratium Favus '.. ..3.1 to 4. 2. Pinnularia Nobilis 11.7 to 14. 3. Navicula Lyra. . . . 14.5 to 18. 4. Navicula Lyra 23.0 to 30.5. 5. Pinnularia Interrupts, 25.5 to 29.5. 6. Stanronesis Phoenicenteroc 31. to 36.5. 7. Grammatophora Marina 36. to 39. 8. Pleurosigma Balticum 32. to 37. 9. Pleurosigma Acumiriatum 41. to 46.5. 10. Nitzschia Amphoyx 43. to 49. 11. Pleurosigma Angulatum 44. to 49. 12. Grammatophora Oceanica=G. Subtilissima 60. to 67. 13. Surriella Gemma 43. to 54. 14. Xitzschia Sigmoidea 61. to 64. 15. Pleurosigma Fasciola 55. to 58. 16. Surriella Gemma. Longitudinal 64. to 69. 17. Cymatopleura Elliptica 55. to 81. 18. Navicula Crassinervis, Frustulia Saxonica 78. to 87. 19. Nitzschia Curvula 83. to 90. 20. Amphipleura Pellucida 92. to 95. 254 HOW TO SEE WITH THE MICROSCOPE. THESE FIGURES DENOTING THE NUMBER OF LINES IN .001 OF AN ENGLISH INCH. As a matter of convenience to those having the cele- brated Nobert 19 band test plate, we present the fol- lowing tabulated values of the rulings. The first column contains the value in Paris lines. The second the number c f lines in .001 of an English inch, as ruled in the several bands. NUMBER. PARIS LINES. IN .100 ENO. IN. 1 1,000 11.26 2 1,500 16.89 3 2,000 22.52 4 2,500 28.13 5 3,000 33.78 6 3,500 39.41 7 4,000 45.04 8 4,500 50.67 9 5,000 56.30 10 o.COO. 61.93 11 * 6.0 may be slightly inclined upwards, but never downwards. A horizontal beam is not objectionable. Now experiment a little. Try and get the lamp as closely to the left of the stage as practicable, leaving room to work the con- denser. Next, place the slide of Cherryfield or Mon- mouth on the stage. Now reflect for a moment. Where is the point of maximum aperture of your objective? And how will the cover of your mount respond to it? If its point of maximum aperture be at nearly " closed," and your cover of such a thickness as will allow the glass to "correct" nearly at closed with glycerine, then you are to use the same. If so be, however, that the cover is thick enough thus to cause the glass to correct 286 HOW TO SEE WITH THE MICROSCOPE. near closed with water as the intermedium, then, of course, use water; or it may occur that the cover is an exceedingly thin one so thin that even with the glyce- rine the correction would obtain at some considerable distance from closed, in which case select another very thin cover to supplement that on the mount. All this has been discussed before, but in the preceding lessons one might have got along tolerably well without par- ticular observance of these conditions, but not so with the problems we are now about to tackle. Having then decided the above point make the im- mersion contact, and selecting a medium shell of the rhomboides, bringing it to the center of the field focus. Arrange the mirror so as to get the strongest illumina- tion ; let the lamp burn with a moderate flame ; if now there is obviously too much light, a perfect glare, bring the lamp closer to you (keeping the flame edge- wise to the mirror constantly) moving the same in a line parallel to the left hand edge of the stage, readjust the condenser and mirror thus lighting up the field again. It will be seen that the farther the lamp shall be thus moved the less will be the illumination. The point to stop at is when you have shut out the super- fluous sheen or glare. All this time \ve have supposed that the flat face of the condenser was parallel to the face of the mirror. Now change the position of the condenser causing it to assume a slightly diagonal pose, the nearer edge to be swung away from the stage; when the condenser is exactly right the mirror will have the appearance of being traversed by a line ol POSITION OF OBSERVER. 287 light, that is, it will not appear to be equally illumi- nated as was the case before. Now look to the adjust- ment of the objective, and by slight manipulations of the mirror display the striee, which will be right straight along work. Seize the base of the condenser, stand firmly, supporting the left forearm on the edges of the draw and table; move the condenser just a bit to and from the edge of the table, noticing the effect in the field, getting it thus in the best position possible. Look to your mirror again and see if that " line" is still there; this cannot be dispensed with. Now if the directions shal I be strictly carried out, the display will be much finer than possibly could have been ob- tained with the former illumination, the diatom will seem to swim in a lake of fire, or as my pupils some- times have said, " chain-lightning." Do not be satisfied with any apparent success, but repeat the method of illumination, time and time again, comparing notes. The student will not prob- ably meet with perfect success the first evening, although it may be that he will be quite satisfied with his efforts. If it so be that the condenser stands at quite an angle, more so than you think ought to be the case, never mind; if you have the right effects in the iield, you can study the rationale at your leisure. The next thing in order is to study all the appearances which have been pointed out in the preceding lessons. Now shifting the object-carrier, hunt up a valve next the very smallest on the mount, one that lies flat. If jou are using a one-tenth your inch ocular will do, but 288 HOW TO SEE WITH THE MICROSCOPE. if but a sixth, then you will need the " D" solid, the one-half inch. Try now and display the striae on the little shell ; remember that they are at best but very thin and faint. Nevertheless, if there has been suffi- cient practice with the " lesser illumination " previously mentioned, there ought to be now but little trouble in recognizing the lines. When once a good view has- been obtained, the observer will be astonished by a little experimenting, to find how far the lamp wick can be turned down just allowing the flame to peep out of the cone and still retain nice views of the striae. This entire position, including that of observer , stand, mirror, and condenser, we have tried to illus- trate in the cut on page 289, the diagonal pose of the condenser we have intentionally exaggerated. It is probable, too, that the student may find it necessary to make some slight changes to suit the tools he may have in hand. Let him keep this fact prominent, to wit,, although the condenser will give more light than the illumination first recommended, nevertheless there must be no more light used than is absolutely required to see the object without difficulty as respects illumina- tion. If it be desirable at any given time to employ a higher ocular, always turn up the wick of the lamp ; a slight turn will be found quite sufficient, and while on this subject let me say that nine out of ten microscop- ists use far too much light. They seem to be impressed with the idea that unless the object is bedazzled in a flood of light that it is not well shown. All this is in POSITION OF OBSERVER. 19 Microscopy POSITION OF OBSERVER. 291 error; when such illumination is employed the chances are that all fine detail of structure will be drowned out, while the effect on the eyes is most injurious. Keep then the field just as cool as shall be consistent with vivacity and life, show your objects brilliantly, and see that even the shadows are lively and transparent. Grasp the mirror firmly; get in the habit of this; when you touch it at all take hold of it boldly. There is considerable for the student to learn in the proper handling of the mirror, so as to combi ne boldness with delicacy and efficiency. Now as you change its posi- tion just a trifle, notice the action of the light. Does it slide by as it were, without taking hold? Or is there one particular position in which the light seems to catch and " nip," bringing out the makings with a vim. Now if the former is the case, there is something wrong (the objective being in adjustment) and perchance the whole system of illumination will require attention in detail. When the smallest shell of the Monmouth and the Cherryfield have been thus mastered (as to their transverse striae) those of the Leipsig may be taken in hand, and subsequently the smallest valves of the Isle of Shoals; these are quite as severe tests as an ordi- nary balsamed amphipleura pellucida. The slide of Leipsig ought to be thoroughly mastered previous to attacking the Isle of Shoals; all the peculiar appear- ances which we have from time to time set forth, are to be studied, and as the eyes become educated the amount of obliquity of the illumination is to be de- 292 HOW TO SEE WITH THE MICROSCOPE. creased. This latter item is to be a constant study ; bear in mind this one broad rule, that to use a wide- angled glass well is to work with the least obliquity possible, and when over easy tests, with more central illumination that can be effectively obtained with nar- row apertured objectives. In short the employment of the wide apertures points to the use of centrally dis- posed light ; this may be a novel doctrine to many, but not at all new to the author. Strive then to work with the most central illumina- tion possible ; get rid of all shadows not hidispensable ; as a rule where the markings are dependent on thick- ness of structure, the greater this difference of thick- ness, i. e., the more prominent the markings, the less will be the obliquity required; conversely the thinner and the fainter the markings the more obliquity is called for. Now the smallest shells of the Isle of Shoals are extremely thin, the striae are not so very fine, probably not measuring closer than 85 in .001 English inch, but they are so thin and so fine as to beat out any object-glasses, excepting those of the higher apertures. And even with these the mirror will have to stand (ar- tificial illumination being used as directed) at least (50 to 65 from axis; a fine one-tenth ought to show any and all of them, either with the one inch or one-fourth inch ocular. A similar one-sixth will require the half- inch. Now, in the study of the last four or five lessons, the student is to constantly endeavor to improve himself in the adjustment of his objective. The broad land-marks POSITION OF OBSERVER. 293 have been laid down for his guidance, and these should not be lost sight of. As his eyes become trained to this kind of work, he will begin to pick up items of value on his own account. Especially will he arrive nearer a due appreciation of what we mean by tone of objective and field. He will thus be able to recognize for him- self, confidently, too, when things are just right. Ditto, when they are not just right. Should the collar of his objective need to be changed, he must be able to make such change at once, and in the right direction. There must be no indecision no guess work. He will have learned, also, that when the adjustment is exactly cor- rect, not only has the display of the striae become more satisfactory, but that the general appearances of the entire shells have improved. This effort at improve- ment of the adjustment of the objective should become a constant and never ending study; in truth, after one becomes tolerably advanced therein, it is no longer a " study," but rather a pastime. We desire to insist with all the vigor we can com- mand, that there is force attached to the reciprocal re- lations above named, to wit: That when the student, perforce of intelligent practice, shall be in position to assert, dogmatically, that his manipulations are correct, he will, conversely, be just as competent to reaffirm the fact when things are not in proper adjustment. When this obtains to an acceptable extent, he will be compe- tent to use his glass for an}' field of investigation that he may elect, and, as a matter of course, he will regard those whom he is assured have not given proper atteu- 294 HOW TO SEE WITH THE MICROSCOPE. tion to this essential item, knowing 1 , too, that it is es- sential " cum grano satis" With our private pupils we have often tried amusing- experiments, simply to become assured of their proficiency in this matter, thus : We sometimes place a drop of mucilage on the interior surface of the field lens of the eye-piece, replacing- the same and watch for results. If the pupil is well trained there is no deceiving him. He will assert roundly that something is the matter. If I say to him, mildly, " why, that's pretty well, isn't it?" He replies at once, in tones that there is no dodging, " No, sir; there is something wrong, sure." And then, again, I have had the pupil pay me back in my own coin placing a similar drop of mucilage in the interior surface of my own oculars. We have already said that no two lenses work ex- actly alike. It remains, therefore, for the pupil to mske a specialty of his own object glasses, first assuring him- self, preferably by the advice ot some expert friend, that he shall not waste time over an inferior glass. While he thus becomes more and more familiar with his own objectives, he will also acquire a general knowledge of all, and will in due time be able to take a strange object glass and work it nearly or quite up to its maximum. Unless the observer be provided with a really super- fine objective, he can hardly make much headway in the examination of the smaller shells of the Saxonica from the Isle of Shoals. Should he have much trouble in getting the striae on the very largest frustules, these appearing quite obscure and of a generally uninviting aspect, the edges badly defined, and more or less distor- POSITION OF OBSERVER. 295 tion prevailing, he may be pretty sure that his glass is at fault. His time will therefore be much better spent in the examination of such objects as come within its powers. This point is well worth close attention, thus, perhaps, saving- a double outlay of time and patience; and there has been a vast amount of valuable time wasted in fruitless attempts to resolve severe tests, for which purpose the objective was totally unfit and incapable. The illumination we have just described is quite suffi- cient for the display of the transverse striae of either slide of Amphipleura. As we have before stated, the " Bridge of Allan" is the easier of the two and should be studied first, and the student should give preference to the cleanest shells on the mount. If the smaller scales of the Isle of Shoals are well seen, and the " Bridge of Allan " resist, the fault will certainly be in the adjustment of the objective over the latter mount; possibly it may occur that the observer has selected an exceptionally difficult shell of the Pellucida. The bet- ter way will be to look over the slide in detail, of course, as before advised, giving preference to the cleaner and most inviting frustule. If so be that the student has the Moller probbe plate, the illumination given is also quite sufficient to display the Nos. 18, 19, and 20. After he shall have become well advanced, there are some particular advantages connected with the study of this plate. He ought to see the Nos. 18 and 19 with but little trouble, and in a little time with the radial arm at 45 from axis. On this 296 HOW TO SEE WITH THE MICiiOSCOPE. plate passing from No. 19 to 20 is quite a jump. One may see the No. 19 in a very satisfactory manner, and yet fail of the No. 20. At the onset it will be better to secure all the obliquity of light possible. Should the Acme or the little Histological stand be used, the upper one-third of the mirror may rise above the stage, the lamp and condenser to be adjusted with the greatest nicety. If the No. 20 resist the attack, it is preferable not to spend much time over it, but go at once back to the No. 19 and endeavor, by some careful re-adjustment, to get a still better view of this ; and when satisfied that such has been accomplished then return to the attack on the No. 20. One will always succeed much faster by trying to improve what is seen than by blindly working over an object of which the desired details cannot be displayed at all. The ease with which the learner can run from one test to another renders the Moller plate very acceptable to the learner. Should it occur that the No. 20 make a protracted resistance, let the above programme be adhered to, spending nearly all of the time in perfecting the display of the No. 19. It may be worthwhile to add that the Moller Amphipleuras are about on a par, in point of difficulty, with those from Aberdeen, Scotland. If tiiere be any difference, my impression is that of the two the Moller No. 20 are perhaps the easier test. In studying the Aberdeen, or in fact any mount con- taining severe tests, it is always advisable to correct the glass approximately by selecting some fine object, of which the details can be displayed by such approximate POSITION OF OBSERVER. 297 adjustment. For instance, on the "Aberdeen" it is probable that oine specimens of Mtzschia may be found, by hunting for them, and this should be done at once, and the glass corrected as nearly as possible. An ap- proximate adjustment will suffice to reveal the trans- verse strias, and the initial display ultimately improved by force of a little well directed manipulation. It may occur, too, that some of the nitzschia are more difficult than others. This being the case, the stu- dent is to master these successively before attacking the amphipleuras. In the practice of all the preceding lessons, and over the test slides, it will be essential that the pupil, after having adjusted his glass as perfectly as may be, note the exact division of the collar graduations. This he should make a memorandum of, so that he can be able, if necessary, to readjust the objective to the mount without loss of time. But this is not all. He will find that as he acquires proficiency by practice, that he will, time after time, change these recorded numbers, and will be inclined, perhaps, to smile at the wildness attending his initial attempts ; and by thus comparing the present with the past, he may be encouraged as to the future. This habit of noting in black and white the best attained adjustment of the objective from day to day should be rigidly persisted in for at least a twelve-month. 298 HOW TO SEE WITH THE MICROSCOPE. WORK OVER DRY MOUNTS WITH HIGH APERTURES OBJECT- IVES. After, and not before, the student has become profi- cient in the preceding lessons, it will be advantageous for him to procure dry mounts of the acknowledged diatom tests, and study .well their peculiar action under the objective. At a glance it will be noticed that the dry frustules are much more vigorous than those mounted in balsam. They have more body, appear more solid,, the image is stronger every way. Hence it is that an objective may shew a dry mount tolerably well, and yet be utterly defeated by the same valves when balsam mounted. Should the student be well versed in our pre- vious instructions, he will find little difficulty in dealing with dry mounts. As a rule, the latter are to be illu- minated with pencils of less obliquity than objects mounted in balsam. In the dry mount, too, we have greater contrasts of light and shade. These differences will be .at once noticed by the intelligent pupil, who will find little difficulty in adapting himself to the situ- ation. He should keep in remembrance this one fact,. to wit: That, owing to the superior brilliancy pertain- ing to the dry mount, little differences in the collar ad- justment are not so perceptible as is the case with bal- samed objects; nevertheless, the difference is there, and well worth the study necessary to recognize these little differences. Again, should the learner desire to com- pare one objective with another as to their comparative WORK OVER DRY MOUNTS, ETC. 299 1 defining powers, always choose a balsam mount, and the thinner and weaker the shells (within reasonable limits) the better and the more palpable will be the comparisons. We advise every lover of a good objective to provide himself with a slide of the genuine English podura, as a matter of course dry mounted, and let him be careful to keep it dry! Amount that has once, only, been swamped in water will be probably badly damaged ; and should it occur that water leaks through the cement, it should be immediately dried by moderate artificial heat, and subsequently laid away in a warm place for several hours. Too much care cannot be taken with a really valuable slide of podura. Prof. Phin says, in his excel- lent work, " How to Use the Microscope," alluding to- the podura scale, " page after page has been written for the purpose of showing how the podura ought to look* and still the question seems to be undecided." A year or two ago a lively discussion sprang up in the London Monthly Journal of Microscopy ^ between Messrs. Piggott and Wenham, the former contending that the true resolution of the podura resulted in dots. Mr. Wenham, on the contrary, warmly taking sides with Ross and Beck, holding that the " exclamation points" were the proper thing to be shown. A month or two later a most curious article appeared in the *' Popular Science Monthly" contributed by Mr. John Michells, and entitled " The Microscope and its .Misinterpretations." The point of Mr. MickelPs article was to show the want of reliability attending observations made with the mi- croscope. To make this point salient, he referred to the 300 HOW TO SEE WITH THE MICROSCOPE. London controversy of the Messrs. Piggott and Wen- ham on the podura question, and also gave the state- ments of other English observers. Among the illustra- tions accompanying this article were five or six cute showing the same scale of Podura as seen in the micro- scope under different illuminations. Thus one identical scale was exhibited to the reader as widely changed in its aspects as could have occurred had dissimilar-scales been selected for the comparison. One or two of the results shown were evidently owing to distortion occur- ring from the use of a badly corrected objective. Now let it be remembered that Mr. Michells, in thus stating his authority for the misinterpretations of the microscope, worked with legitimate material, and the work over the podura presented was such as had been arrived at by English microscopists of acknowledged competency. There must be something wrong ; in what direction shall we look for the cause thereof? So far as the exhibition of " the markings " of the podura are concerned, they are an easy test; i. e.< almost any common-place glass will show them after a fashion, while what w r ould be known as a " real honest working o sixth or eighth " will show these markings with consid- erable force; and such a display w r ould likely be as acceptable to the novice (or to those who had never worked with better objectives), as the view given by a superb high balsam angled glass by Spencer or Tolles. But mark this: Hardly two of the " real honest work- ing glasses," owing to the hap-hazard character of their corrections, will give the same appearances, while the WORK OVER DRY MOUNTS, ETC. 301 objectives of Spencer and Tolles will have but one and the same constant story to tell. With these, properly manipulated, it is impossible to get aught over the po- clura but the exclamation points. With these superb glasses such a thing as the reproduction of the several appearances shown by one and the same scale, as ex- hibited in the cuts by Mr. Michells, becomes an impos- sibility. And here, in the opinion of the author, may be found the key of the problem, to wit: The better glasses showed the poclura tolerably well. The poorer ones, those " real honest working glasses,*' of which we have heard so much about, were bound to set forth their individual misrepresentations, and thus furnish material for contributions to periodical literature like that of Mr. Michells. As to the true structure of podura, there can be little doubt; there has never been but one opinion among American observers accustomed to the use of the finest American objectives, and the spines or ex- clamation marks are accepted as the true resolution of these seales. By the aid of electricity, these spines have been detached from the body of the scale; the detached spine together with the parent scale were photographed and thus presented to the readers of the " Lens." We have already had occasion to say that Mr. Geo. W. Morehouse, of Wayland, New York, was the first observer who demonstrated the capabilities of the Beck vertical illuminator; when used in conjunction with American object-glasses of high balsam angles , 302 HOW TO SEE WITH THE MICROSCOPE. Mr. Morehouse's results in displaying- objects by re- flected light under the higher powers of the microscope were truly astounding, and his paper read before the Dunkirk Microscopical Society of Dunkirk, arrested the attention of the author, who immediately repeated Mr. Morehouse's observations with the liveliest satis- faction. Mr. Morehouse particularly called attention to the unrivalled views thus given of the podura, .adding that they seemed to settle all question as to the nature of these markings. Mr. Morehouse observed also that near the ends of scales the spines were some- times to be seen projecting beyond the scale itself, and the author can affirm the same from his experience. As suggested by Mr. Morehouse the vertical illumina- tor, from the nature of its action dealing entirely with surfaces only not only deprives Mr. Mitchell's paper of argumentative force, but fairly turns his weapons against him, pointing to the podura, indeed, as a very proper object with which to demonstrate the reliability of microscope observations. The student will find the study of the podura a valu- able assistance in teaching him the behavior of fine object-glasses over dry mounts, and when he shall by dint of long practice become sufficiently expert to de- tect and appreciate the beauty of the display as given by balsam aperture objectives as contrasted with the work of medium angled glasses, he will suffer no re- grets as to the time and patience this accomplishment may have cost him. We have insisted on the use of the genuine English WORK OVER DRY MOUNTS, ETC. 303 podura, and as we have stated, this slide must necessa- rily be selected by some friend competent to the task. The American podura is quite inferior to the English, yet of the two a slide of the very best American should be accepted rather than a notably poor English specimen. The former will be much better than none at all, but let it be remembered that the English are the desired thing. Now as to the proper appearance of these scales under the objective, we must state that it is perfectly impossible to give any truthful representa- tion on paper. The following cut |(|| |j |i taken from Prof. Phin's book, |nr|J copied from the illustrated cata-Ly. logue of the late Richard Beck, J| may help matters some, and shows fey the podura with tolerable precision, 10 when the same is seen with a dry 1 '' one-eighth, say of 130 aperture magnified 1,300 diameters, and illuminated with light as central as possible. Now let the student take a half sheet of writing paper and roll this up so as to form a tube, and examine the cut attentively, looking through this tube, as is frequently done when examining draw- ings, etc. Letting a strong light fail from the direc- tion indicated by the shadowed sides of the markings, we think that most of our readers will have no diffi- culty in noticing that the " exclamation points " seem to " rise up " above (apparently) the general surface of the scale. Our principal design in calling attention to this feature is that he may thus recognize an effect to 304 HOW TO SEE WITH THE MICROSCOPE. which we have had occasion to allude to in previous pages, when treating of certain diatom tests. Now this same effect is easily seen in the podura when under the objective, and we have never been entirely able to get rid of it, especially when a glass of moderate angle is employed. Now the reader knows from personal experience just as much of this borrowed cut as does the author. We will, however, hazard the assertion that the il- lumination shown was about 20 from axis, the glass having aper- ture as above stated, of perhaps 120 or 130. Let us imagine then that we thus have such a display under the mi- croscope; and again r that we remove the objective and replace with another of high balsam angle. In the cut on this page we have endeav- ored to depict the changes. First it will be seen that with the radial bar in pre- cisely the same position we have the effect of greater obliquity; the shading is more decided and (over the tube) the lights are more intense and brilliant. Now examine this cut with the paper tube ; it will be ob- served that the exclamation points do not " rise " so WORK OVER DRY MOUNTS, ETC. 305 much as was the former case. They indeed appear in good relief, but they do not seem to float clear from the body of the scale; these appearances are to be watched under the microscope, and the obliquity of the illumination diminished to the least angle without allowing the spines to rise. When the adjusting collar and the illumination are just in the correct position, the small end of the " wedges" or " exclamation points are to be sharply defined. These scales may be exam- ined either in a vertical or horizontal position ; as a general thing the vertical will be the preferable posi- tion. Most mounts of podura will contain scores of scales, while for the most part there will be less than a half-dozen good ones. The student should look care- fully to this, and make the best selection possible. Ke- ferring to the use of the vertical illuminator, Mr. Morehouse called my attention to a curious fact con- nected with the study of these scales, namely, that the smallest and most uninviting of these, as shown by transmitted light, seemed under the vertical illumina- tor quite as strong, vigorous, and satisfactory as any. We found Mr. Morehouse's observation true as respect to many other insect scales we had occasion to examine. The following, mounted dry, will form useful objects and may be advantageously studied conjointly with the podura : Scales of Lepisma saccharina Degeeria Macrotoma major Petrobius maritimus Pontia brassica Morpho- menelaus Tinea vestimenti Gnat Hipparchia Janira Wing of Gnat any of the scales from the lepidop- 20 Microscopy. 306 HOW TO SEE WITH THE MICROSCOPE. ; tera will answer a good purpose, and in the summer time the student will find a plenty of these visitors, and from such material many excellent mounts can be prepared with little loss of time and at a slight ex- pense. We recommend that the pupil become well accustomed to handling insect scales before attempting the miscellaneous examination of anything and every- thing mounted dry that perchance may be directly at hand. After considerable proficiency has been secured, the student, may test his powers thus: Let him pro- vide himself with a dry mounted trachea, say of a bee; use the lowest ocular, select the strongest and most vigorous part of the object where the coils are the largest and the strongest, and adjust the glass as nicely -as possible thereon. This done, choose one of the longest continuous coils and see how far this can be followed towards the smaller end. Now having arrived at the limit, so that the coils can but just be perceived, manipulate the adjustment; now if you succeed in get- ting very much better definition,, the proof is before you that you were in error at the commencement. Make a note of the situation, and throwing the glass intentionally out of adjustment again, repeat the ex- periment until you shall be enabled to certainly make the initial correction with tolerable certainty and accu- racy. This is first-class practice, and will be of the utmost value in general work over dry mounts. The trachea, too, is a real good test object. The author has worked ten hours at a sitting endeavoring to trace these coils to the very last end, but has thus far met with defeat. WORK OVER DRY MOUNTS, ETC. 307 As to the general examination of the various dry mounts, or of histological preparations mounted in glycerine, nothing but the most general directions can be given. The previous study of the diatoms will become a valuable assistance to the student, and this study should be still kept up in the examination of dry mounts especially. Of such objects as have palpable size, and can be seen with the naked eye, as is the case with very many histological mountings, the pupil must expect more or less annoyance from the interference of the various shadows, from different parts of the struc- ture, situated in different planes. If, in such a mount it be desired to examine critically minute details, make it a point to select a bit of the object that has per- chance been accidentally removed from the main body, and when you prepare your own mounts keep this point in mind. A good general rule with such mounts is to keep the illumination as nearly central as possible. In the adjustment of the objective the same plan is to be adopted, selecting when available the smallest isolated fragment, with the illumination within 35 to 40 from axis (or less) the low angled sub-stage condenser will be valuable. It however requires some study, as we have found by practical experience, to work the condenser so as to secure its best effects. It is a matter, also, that calls for the outlay of time and no little patience. Nevertheless, the reward will repay the effort. Through the kindness of Mr. Zentmayer, who con- structed the apparatus we have been experimenting 308 HOW TO SEE WITH THE MICROSCOPE . with condensers of very small diameter, several of these of varying focal lengths were fitted to the same mounting. Either glass could thus be used at pleasure. The mounting was conical and the base being also of small diameter, say about one-fifth of an inch, our idea was by this arrangement to keep the angle of the con- densing lens as low as possible, and by the peculiar form of its mounting to illuminate as little as possible of the object and with an extremely acute cone of light and at the same time, allowing sufficient lateral range that the instrument might be placed in as oblique posi- tion as possible. The idea seemed theoretically sound, and we have spent much time in the experiments hoping to give our readers something of value, but we were doomed to disappointment. (Not the first experiment of ours which has failed). The instrument is very troublesome to use, requiring often some little time to coax the light through the little cone, and when once obtained, the slightest movement of the mirror will destroy what has been done. Nor when doing its best were the results any better than those obtained by the Gundlach, or Beck cheap inch which has already been recommended for general condensing purposes. We still believe that some improvement will be made in the direction of our experiment, and hope that other ob- servers will make similar efforts until further success shall be secured. OIL IMMERSION OBJECT-GLASSES. 309 OIL IMMERSION OBJECT-GLASSES. Since the introduction of the duplex objectives another form of four-system, wide apertured glass, constructed by Carl Zeiss, of Jena, Denmark, has made its appearance in this country. With the advent of these glasses came also the an- nouncement that their balsam apertures excelled those of the American duplex. A novel feature in the mounting of the oil immersions was that they were not provided with adjustment collar ; it being asserted that the only requirements necessary to secure the perfect action of the object-glass were to use the same with a tube of exactly ten inches length, and to employ cedar oil as the immersion medium. It was further stated that when these simple require- ments were alone attended to, that no further adjust- ment of the objective would be required, and that the performance ot the object-glass would be found un- equaled. As might be supposed, the above claims on behalf of the oil immersions were destined to receive attention at the hands of American microscopists. A fact well known to experts concerning wide aper- tured objectives is this, viz: An object-glass exactly corrected to the eye of A may not be in exact correc- tion to the eye of B without suitable change of the collar adjustment. During a sitting with Mr. Chas. A. Spencer at Buffalo last fall, the above fact became immediately 310 HOW TO SEE WITH THE MICROSCOPE. apparent. Mr. Spencer's adjustment of his duplex one-tenth objective (which we were then using) being invariably three divisions of the collar graduations nearer "closed " than my own. It is obvious therefore that an " oil immersion " suit- ably corrected to the vision of Mr. Zeiss when worked with ten-inch tube, might require a special correction when in the hands of Mr. Spencer, or myself. Feeling therefore assured of the error embodied in the popular representations concerning the oil immersions of Mr. Zeiss, the author solicited and obtained one of Mr. Zeiss' circulars, which, being official, will, it is believed, be read with interest. With the exception that a few typographical errors have been corrected, the following is otherwise a verbatim copy. ' NEW OIL IMMERSION ORTECTIVES. ONE-EIGHTH INCH. ONE-FIFTH INCH. " This object-glass is adapted to an immersion fluid > which in refraction and dispersion is equal as nearly as possible to the crcwn glass, a plan proposed by Mr. J. W. Stephenson, of London, as a devise for increasing the aperture and for dispensing with correction. " The lens constructed on this plan a four-fold sys- tem of pp. one-eighth focal length, calculated by Prof. Abbe shows an aperture of unusual amount, combin- ing a most perfect definition with a reasonable working distance. It works equally well through thick, thin,, and thick covering glass without needing special cor- rection, owing to the identic refraction of the immer- sion-fluid and the covering-glass. NEW OIL IMMERSION OBJECTIVES, ETC. 3 LI "The numerical aperture of the object-glass (accord- ing to Prof. Abbe's definition, the product of the sine of angular semi aperture with the refractive index of the medium exposed in front) is brought to the num- ber 1.25 exactly, which corresponds to a balsam angle of 113 and is in the ratio of 5:4 greater than would be the maximum aperture, 180 in air, of a dry. lens, considered in its numerical equivalent, the resolving power correspondingly affords a visible increase com- pared with immersion-lenses of the common system, which generally do not exceed 1.10 in the numerical equivalent of aperture. " As to the immersion-fluid a large number of ex- periences has shown the oil of cedarwood (ol. ligni cedri) to be the most fitted, though it is in a slight degree still less refractive than ordinary crown-glass ; other liquids of higher refraction exceeding too much in dispersive power. This oil (of which a sample will, be sent with the objective) may be got anywhere in sufficient purity. "For controlling its qualities or those of other liquids, which perhaps might appear convenient, a, special test-bottle is forwarded with parallel plane faces, and a prism of crown-glass, cemented to the stop of. this bottle. The vertical spar of a window seen, through the prism and through the oil beneath, should^ appear without a sensible defection and should show A bodies only slightly colored, if the liquid has the right, quality. "The pure oil, ligni cedri, will afford the best color- 312 HOW TO SEE WITH THE MICROSCOPE. correction for oblique light ; for observation with cen- tral light the image may be obtained still more colorless if a suitable quantity of a higher dispersive oil (oil of fennel seeds or anise oil) be mixed with that of cedar- wood. But in this case the refraction of the mixture should be reduced to the original refraction of the oil of cedar- wood by adding some pure olive oil ; the test- bottle being always applied for regulating the mixture. " A few other oils: The oil of copaiva-balsam and the oil of sandal-wood approach so close to the oil of cedar-wood in refraction and dispersion, that they may be used for it, instead of it, if they should be prefer- able in any respect, provided the test-bottle has stated the right quality of the sample. " The use of these oils will not be injurious to prepa- rations which are hermetically closed, as it is likewise necessary for water-immersion, although the black var- nish on some slides (not on all) will be slightly dis- solved by a prolonged exposure to the oil, its action will not be offensive within a moderate time; and in most cases any contact of the fluid with the edge of the covering-glass may be avoided, by the oil not being applied in a greater quantity than is necessary. A minimum drop on the covering-glass, and such a one on the front lens being quite sufficient for observation ; besides that, the varnish of the slides may be perfectly secured against the oil by a solution of shellac in alcohol. " A special advantage will result from this mode of immersion for the more difficult dominions of petro- NEW OIL IMMERSION OBJECTIVES, ETC. 313 graphic work; since rock-slices for inspection with the microscope will become perfectly transparent, using the oil without needing a polished surface or a cover- ing-glass cemented on, and may be observed to a greater depth than would be accessible to the higher powers of the ordinary system. 44 In every case the new object-glass, not considering the greater optical capacity in bringing out difficult structures, will prove exceedingly convenient for use, from dispensing with any trouble in finding the right correction. 44 Of course the full performance of the increased aperture can be effective only on preparations which are mounted in balsam (or any other medium exceed- ing 1.25 in the refractive index) or which, if mounted dry, perfectly adhere to the covering-glass. On objects separated by air from the covering-glass the lens will not work better than any good immersion-objective with an aperture equivalent to an air-angle of 180. 44 Besides this, for displaying the full performance in oblique light, the illuminating-apparatus must yield pencils of greater obliquity than are directly accessible to a slide from air. The most simple devise forgetting light on such an obliquity without needing a special apparatus (immersion condenser) is a plano-convex lens cemented to the under surface of the slide by a minimum quantity of oil. The ordinary mirror of the microscope brought to a moderate distance from the axis will now yield pencils of any wanted obliquity ; a lens fit for this use will be added to the object-glass. 314 HOW TO SEE WITH THE MICROSCOPE. " If ordered for English microscopes the lens will be perfectly corrected for a tube of ten inches exactly, and no sensible deviation from this length would be admis- sable without a loss of perfect definition. However, owing to the slight defect of refractive power in the oil of cedar-wood, some advantage may be found by, lengthening the tube for one-half to one inch while ob- serving through extremely thin covers (less than 0.004 inch), and by shortening it for one-half to one inch in the case of very thick covers (exceeding 0.008 inch).* " The object-glass is made with fixed brass work and with standard screw (like all my object-glasses) ; the price of it is 240 marks ; the price of the one-twelfth is 320 marks ; the aperture guaranteed to be not less than it is stated above. The lenses are screwed together with moderate pressure and may be unscrewed without great effort, but I caution expressly against unscrew- ing them ; owing to the great aperture the system is- extremely sensible to the slightest defect of centering the smallest particle of dust, or the least moisture get- ting ^into the screws, and the unavoidable difference of. pressure when screwing the lenses together, would cause a sensible loss in the performance of the glass." JENA, March, 1878. (Signed) CARL ZEISS. One can hardly read the foregoing somewhat contra- dictory document without arriving at the conviction that Mr. Zeiss has suffered severely at the hand of his translator. Be this as it may it will be noticed that he asserts that the oil-immersions practically NEED adjust- * Italics mine, J. E. S. NEW OIL IMMERSION OBJECTIVES, ETC. 315 ment, which adjustment he recommends be accom- plished by manipulating the draw-tube to a possible extent of two inches when working over covers varying from 0.004 to 0.008; and here let the reader be re- minded that these figures do not by any means express the maximum variations of cover-glasses. I have had an opportunity of critically examining but one of these objectives, a one-eighth, imported by a friend. This glass when worked with ten-inch tube over my usual covering-glasses required downright engineering of the draw-tube; furthermore, when worked over covers 0.008 inch thick, it was impossible to correct the glass at all, as the draw-tube of my stand when fully closed would not decrease the distance sufficiently to secure the proper correction of the ob- jective. The performance of this one-eighth (when exactly corrected) was very fine indeed, and closely resembling the work of the Tolles or Spencer duplex. Superb as its definition was, it did not excel that of my Spencer duplex one-fourth a glass of half its nominal power. The balsam aperture of the one-eighth was about the same as that of my duplex glasses. During my first evenings with the oil immersion the cedar oil behaved very well, but on the second and third sittings it made me trouble enough. With the tube inclined (as usual) the oil would most unaccount- ably take a notion to run away ; when this occurred, any attempt to reinforce by adding more of the oil, made things worse ; the only remedy was to clean the slide nicely and commence operations again, de novo. 316 HOW TO SEE WITH THE MICROSCOPE. The author learns from good authority that Mr. Zeiss has recently extended the balsam aperture of his oil-immersions improving thereby the performance. Oil-immersions are now made by Mr. Tolles and Mr. Spencer. These makers, however, supply the collar adjustment (as. should have been done by Mr. Zeiss). Mr. Spencer's objectives we are informed are arranged so as to work with oil or glycerine contact. While in the oil business as above stated, it occurred to me to try the effect of various oils with my duplex glasses. After a little experimenting, I found that ordinary kerosene, well washed in alcohol, worked most beautifully with both the Tolles one- tenth and the Spencer duplex one-fourth, with either glass when thus immersed I got the most exquisite displays of the most difficult known tests. It thus came to the surface that I had harbored oil immersions while ignorant of the fact. That the cedar oil diminishes the range required in process of adjusting for varying thickness of cover must be granted, hence the novice using the Zeiss im- mersions being necessarily restricted to this more lim- ited range might possibly get better initial perform- ance than would result in his first attempts to manipu- late a Tolles or Spencer four-system objective. In dispensing with the cover adjustment and its accompanying mechanism (such as furnished by Spencer or Tolles), falling back on the clumsy draw-tube as a substitute, the cost of construction is materially re- duced. Those affected with the res angusta dorni might NEW OIL IMMERSION OBJECTIVES, ETC. 317 doubtless contract with either Tolles or Spencer for objectives on the model of the Zeiss, and fully equal to- the latter in performance, and at figures considerably below those quoted in the Danish catalogue. Our experience with the Zeiss oil immersions has neither weakened our appreciation of the screw-collar adjustment, nor our high estimation of wide apertured objectives of American manufacture. CHAPTER VHI. A WORD OR TWO ON VOLUMETRIC ANALYSIS. The value of the microscope to the medical profes- sion is greatly enhanced by the conjoint use of a little chemistry; in the examination of urinary deposits it will often occur that neither the microscopical nor the chemical analysis per se would be complete or entirely satisfactory, while the combined results of these might lead directly to the information desired. Micro-chemical examinations of urines are of the utmost importance to the practitioner ; among the best English works on the subject may be named those of Gold- ing Bird , Beale , Roberts and Harley . The beginner will derive much satisfaction from the work of Dr. Bird, which although written twenty years ago is essentially good at this day. The reader should keep in mind in the perusal of either of the above authors, that micro- scopy has of late years suffered material advancement, hence it is with the superior objectives now at our com- mand, we are able to cross-question some of the plates contained in the above-named books. Almost anyone looking over the representations of ** tube-casts " as pictured by Dr. Beale, would natu- rally arrive at the conclusion that there could be no doubt as to the recognition of these objects in practice. It becomes my duty to say, therefore, that in neither of the works referred to, can be found a reliable repre- 318 ON VOLUMETRIC ANALYSIS. 319 sentation of a genuine l ( tube-cast " ; while on the other hand the learner depending- entirely on the information conveyed to the eye by the plates is surely liable to be misled. It is very far from our purpose to make any attack on the English authorities mentioned; from either of them we have derived a great deal of useful and valuable information, and they are to-day works, to which we make frequent reference. And then again we doubt seriously, if an accurate idea of a " pale hyaline cast " can be conveyed by means of drawings. The learner, however, may rely on the text of our favorite authors, and at the same time use due care that not every adventitious filament, hair, etc., be ac- cepted as a genuine " tube-cast." To detect with certainty a genuine " cast," one of the feebler order requires just as much care and study as would be required to show the lines on the Nos. 18 or 19 of the Moller test plate, and requires also just as good instrumentation. Having thus alluded to one serious source of error, we have to say that it is no part of our purpose here to write a treatise on urinary deposits, although we hope to accomplish something in this line at no distant day our present purpose is to give the medic a hint or two as to the little apparatus required in the volumetric analysis of urines, and to instruct him in the prepara- tion of the necessary chemical solutions employed in such analysis; $15.00 or even less will purchase a tol- erably effective outfit. 320 HOW TO SEE WITH THE MICROSCOPE. APPARATUS. Six test tubes ; let four of these be large size. Spirit lamp. Four or five two-ounce beakers. Bink's 250 grains burette. Two or three small glass funnels, say ot two-ounce capacity. Filters to match the above. Two or three glass rods. Two or 'three brushes for cleaning test tubes ; ditto, for burette. Litmus paper, blue and red. Two measuring pipettes, one-half, one and two fluid ounces. Urinometers. All of the above can be obtained of Messrs. Gr. Tie- man & Co., 67 Chatham street, New York, and will cost less than $7.00 Additional to this list we recommend the purchase of three or four " Marais " graduated tubes for approxi- mative analysis. These cost $1.50 each. At least one- of these should be provided. A delicate balance, one turning with one-fourth grain when loaded with an ounce in each pan, will be required. Most of the books treating on volumetric analysis de- mand that the balance should be quite an expensive one. Nevertheless, the simple little models which can be pro- cured almost anywhere, and costing less than $3.00, can be made to answer tolerably well. Those to whom the expense is not objectionable will find a really fine bal- ance enclosed in a glass case a luxury. Ours, made by Troemner, of Philadelphia, turning with, say one-fiftieth of a grain, costing $40.00, has given excellent satisfac- tion. Reliable instruments, by Becker, can be obtained in the cities, at prices from $10.00 upwards. Those who APPARATUS. 321 have used them speak highly of them ; but for the pur- poses of the practitioner, such instruments are not strictly a necessity. The small German scales found in the office of almost any physician can be made, with a little attention, to do very fair work. If such be se- lected, it will be of the very first importance to possess a reliable set of weights ; those furnished with the cheap instruments are not to be tolerated. Nor is it safe to appeal to the nearest druggist. The safest method is to make one's own, which is accomplished with no great outlay of time or money. First, purchase a set of Troemner's " aluminum " grain weights, costing fifty cents. These consist of a five, four, two three's, two, one, and one-half grain weights. With these in hand it will be easy to construct an accu- rate set. Specific directions are unnecessary. A thousand grain bottle will also be wanted. This can be purchased in the shape of a regular "specific gravity bottle;" its cost is $2.00. The practitioner can readily adapt an ordinary bottle to the purpose, simply selecting one of such capacity that when a volume of one thousand grains of rainwater at 60 F. is placed therein, the water shall rise part way in the neck, and its place marked with a file, we have on hand some half- dozen 1,000 grain bottles, which we have picked up from time to time, and which are in regular use, the specific gravity bottle being held in reserve as a standard, or for the determination at times of the density of liquids. Imprimis. Let me recommend to the practitioner that he eschew the entire system of weights as recognized 21 Microscopy. 322 . HOW TO SEE WITH THE MICROSCOPE. by the profession, and confine himself to that one simple unit the grain. This, in the case of chemicals, can be adopted with perfect ease, and much confusion thus avoided. In the following pages no other weight will be mentioned. Referring to liquid measures, the case would be somewhat modified, and we shall have occasion probably to speak of fluid ounces, drachms, etc. Now there are two fluid ounces in vogue ; the one is known as the English " imperial," and is equivalent to a volume of distilled water at 60 F., weighing 437.5 grains. This, also, is the ounce avoirdupois. The United States standard fluid ounce is quite another thing. This is equal to a volume of distilled water at 60 F., weighing 455.69 grains, sixteen such ounces making one pint. The United States ounce will be the one referred to in this book, when speaking of fluid ounces and drachms, eight of the latter being equal to one of the former. The practi- tioner should keep in mind these differences between the United States and the English fluid ounce, when reading English authors. In the selection of his chemicals for the preparation of the standard solutions, the utmost attention must be paid to their purity. They should not be bought at the " nearest drug store." It will be always advisable to purchase them from leading dealers. The formulas which will be given relate only to pure chemicals. Due attention also should be given to the manipulations, that the chemicals shall not become contaminated. Even after the standard solutions are successfully prepared, the careless operator may ruin a solution by thought- APPARATUS. 323 lessly pouring the remaining contents of the burette (after an analysis) into the wrong bottle. The same glass rod ought not to be used in different solutions without being cleansed. !Every bottle should have its own stopper, and these should not become interchanged. Every piece of apparatus used in an analysis should, at the termination thereof, be immediately cleansed. The practitioner will find it convenient to make his several standard solutions in quantities of 3,000 grains. With the exception of the one used for the determina- tion of sugar, they all have reliable keeping properties, and as 3,000 grains can be made with about as little trouble as 1,000, it is an economy of time to do so. Procure, if possible, suitable bottles for the standard solutions, furnished with a "pouring lip." These are so much handier in filling 1 the burette. Ours were ob- O tained from an ink manufactory, and answer the purpose perfectly. Analysis for Urea Standard Solution. Weigh thirty-eight and six-tenths grains of pure red oxide of mercury and place the same in a large test tube, add a little nitric acid, c. p., and apply the heat from the spirit lamp. The oxide will appear to crust and little inclined to dissolve. By keeping up an uniform heat, mean- while stirring with a glass rod, the oxide will become gradually dissolved. Should it, however, become neces- sary, add a little more acid carefully, little by little, maintaining the gentle heat and stirring with the rod until all of the oxide shall be dissolved, the object being to use the least amount of acid possible. The process 324 HOW TO SEE WITH THE MICROSCOPE. being completed, pour the whole into a thousand grain bottle, rinse two or three times with distilled water, adding this to the measuring bottle, and finally add dis- tilled water to make the whole to 1,000 grains. Let it stand for a few hours, after which time should there form a precipitate of the basic salt, add two or three drops more of the acid. The solution may now be transferred to the regular bottle for use. Two hun- dred grains of this standard solution are (should be) equivalent to one grain of urea. Baryta Solution. In one bottle make a cold solution saturated with nitrate of baryta; four ounces of dis- tilled water will be sufficient. In another bottle satu- rate eight ounces of distilled water (cold) with caustic baryta. When the solutions are fully saturated, which may be known by the baryta remaining in excess in the two bottles, allow a little time for the two solutions to become clear; then carefully decant as much as possible of the nitrate solution into your regular bottle, and to this add twwe the volume of the caustic solution, the two combined forming the baryta solution for use in analysis. Carbonate of Soda Paper. In a bottle prepare a saturated solution of carbonate of soda. When fully saturated pour into a large dish or platter, provide sheets of ordinary printing paper, saturate these and suspend the sheets until quite dry; cut into strips one inch wide by six or seven inches long, and preserve in a wide mouthed bottle fitted with a well-fitting stopper. Measuring Bottle. Procure one of the long and slim APPARATUS. 325 {bur-drachm bottles found at almost any druggist's, say with an interior calibre of one-fourth inch. Mark this with a file at heights corresponding to 50, 66, 100 and 200 grains. This is easily accomplished with the aid of the burette. The plain bottles without a neck are to be preferred. The Analysis for Urea. Take a sufficient quantity of the urine, and if albumen is present clear it from the same ; next, pour into the measuring bottle to the 100 grah A mark. Transferring this to a common wineglass, in a like manner measure 50 grains of the baryta solu- tion; add this to the other in the wine-glass, pour the whole on a dry filter, receiving the filtrate in another glass. Should the liquid come through clear, it is well; if not, it must be refiltered until it does. Three or four filtrations at the most will generally accomplish the de- sired end. While the filtering is in operation fill the burette to the " 0" mark with the standard solution. With the measuring pipette transfer one-half drachm of the clear filtrate to a clean wineglass, adding half its quantity of distilled water. Have in readiness a strip of the carbonate of soda paper and a glass rod. Deliver the standard solution from the burette to the filtrate as long as any precipitate is distinctly seen to form, stir with the rod, and place a drop from the glass in contact with the paper, waiting a moment to observe the reac- tion. If the paper continues white, add again from the burette, and stir again, placing a second drop on the paper, and thus continue carefully until the drop trans- ferred to the paper strikes a yellow color. Now look 326 HOW TO SEE WITH THE MICROSCOPE. among the previous drops on the paper and see if there are any indications of the yellow. If so, you have used too much haste, and the analysis must be repeated. If to the contrary the yellow is only to be observed at the last test made on the paper, the analysis is ended. Kead the burette, multiply this number by 12 -f- 100 and you Avill have the number of grains to the fluidounce of urine. It will be well, before making a practical use of the standard solution, to test the same. Proceed thus : Procure, say one-fourth ounce of pure urea. This can be had of the leading druggists at a cost of about twenty- five cents. Of this weigh carefully two grains, which dissolve in 200 grains of water. Take 50 grains of the solution and tritate as above. When the carb. of soda paper strikes a yellow color, desist and read the burette. Now the 50 grains of solution contained, of course, one- half grain of urea, and the burette should, if the stand- ard solution have been properly prepared, read at the 100 mark. If the reading should be less than the 100, the solution is too strong, and must be weakened by the fur- ther addition of distilled water. If, on the contrary, the reading should be more than 100, the solution is too weak, and more of the mercury must be prepared and added. For the purposes of the practitioner there is no necessity of being over precise. If the burette shows but a small error, plus or minus, it will be sufficient to note the fact on the label of the bottle, applying a proper correction to the future analyses. In the exam- ination of the reactions on the carb. of soda paper, es- pecially in the evening, we find a hand-magnifier of much service. APPARATUS. 327 Chloride of Sodium Standard Solution. Dissolve forty-four grains pure nitrate of silver in 3,000 grains of distilled water; employ a clear white glass bottle and set the same in the sunlight for eight or ten hours. A dark brown or black precipitate will probably form ; when this settles to the bottom, filter into a clean bot- tle, decanting carefully so as not to disturb the sedi- ment; 200 grains of this standard solution should equal one grain of chloride of sodium. Before being put to- practical use the solution must be tested. Solution of Chromate of Potash is made by saturat- ing three or four ounces of distilled water ; an excess of potash remaining undissolved is not objectionable. To Test the Standard Solution.. Procure a nice clean lump of " rock salt," crush, and select of the cleanest, ten or fifteen grains; powder roughly and dry with care, using but gentle heat. When thoroughly dry, dissolve two grains in two hundred grains of dis- tilled water, and of this measure fifty grains into a wine-glass, add a little water and one or two drops of the potash solution. Fill the burette and tritate. The addition of the first drops of silver to the salt solution may be followed by the appearance of a red precipi- tate, but on stirring with the glass rod this will re- dissolve; continue the addition of the silver little by little, stirring well after each addition, until the further delivery of the silver is followed by a permanent pre- cipitate imparting a red color to the contents of the wine-glass. This finishes the test, and if the standard solution is of the proper strength, the burette will 328 HOW TO SEE WITH THE MICROSCOPE. read at the one hundredth mark ; if too strong or too weak, add silver or water as the case may demand, and repeat the test. The indications of the burette furnish a ready guide as to the amout of correction necessary, hence the second test ought to be sufficient. Analysis of Urine for Chloride of Sodium. Filter say one-half ounce of the urine, and with the measur- ing pipette introduce , one fluid drachm of the filtered urine into a wine-glass, and add three volumes of dis- tilled water, and also two or three drops of the satu- rated potash solution. Now with a bit of litmus paper test the reaction of the mixture ; it should be rendered faintly alkaline by adding carbonate of soda, or dilute nitric acid as may be found necessary. Fill the burette and tritate according to the directions given above, until the permanent precipitate makes its appearance. Now read the burette, divide this number by twenty- five which will give you the number of grains of chloride of sodium per fluid ounce of urine. Should the specimen of urine be very high colored it may be somewhat difficult to detect the first appear- ance of the permanent precipitate, hence the accuracy of the analysis will be impaired. In such cases, there- fore, it is better first to decolorize the urine; this is tolerably well accomplished by adding a drop or two of a solution of permanganate of potash, and with the spirit-lamp bring the mixture nearly to the boiling point. A brown precipitate will be observed which is to be removed by filtration, and the filtrate used for the analysis according to the preceding directions. APPARATUS. 329 Analysis for Chlorides Approximately. Fill a Marais graduated tube to the ounce mark with, fil- tered urine, make strongly acid with nitric acid; pro- vide a solution ot nitrate of silver, forty grains to the ounce of water; add an excess of this to the urine in the tube, allowing the whole to remain quiet for twenty- four hours. At the expiration of this interval the volume of precipitate may be read from the graduated scale, each .2c.c. of precipitate will equal 0.19 grains of chloride of sodium ; in the above approximate anal- ysis the acid must not be omitted, otherwise a precipi- tate of phosphate of silver might vitiate the results. Analysis for Phosphoric acid. Dry carefully a suffi- cient quantity of chemically pure nitrate of uranium, using gentle heat; of this take 106 \ grains and add 3,000 grains of distilled water; 200 grains of this standard solution equals one grain of phosphoric acid. Solution of Acetate of Soda. Four hundred grains of acetate of soda are dissolved in six fluid ounces of water, and to the solution add 800 grains of acetic acid. The commercial article known as "No. 8" will answer. Solution of Ferro-cyanide of Potassium is made by dissolving one part of the salt in ten parts of water. Test Solution. Dissolve 50.4 grains of phosphate of soda in 1,000 grains of water; 100 grains of this solu- tion equals one grain of phosphoric acid. To Test the Standard Solution. Measure fifty grains of the above test solution into a beaker; add one-fourth volume of the acetate of soda solution, increase the 330 HOW TO SEE WITH THE MICROSCOPE. volume of this mixture three-fold by the addition of water; fill the burette with standard solution, and have in readiness a white plate or saucer, on which have been placed several separate drops of the ferro- cyanide. Heat the contents of the beaker over the spirit-lamp, and keeping the same tolerably warm,, tritate with caution, and when a drop from the beaker on being placed in contact with the ferro-cyanide strikes a brown color, the analysis is ended ; and if the standard solution is of the proper strength, the burette will read at the 100 mark. Any error of standard solution noticed is to be corrected and the analysis re- peated with the standard solution as amended. Analysis of Urine for Phosphoric acid. With the pipette measure two fluid drachms of the urine into a beaker, and to this add one-half drachm of the acetate of soda solution, also an extra drachm or so of water; have ready the white plate and the ferro-cyanide solution; warm the contents of the beaker over the spirit-lamp, and maintaining the heat, tritate as in the above test; continue the addition of the standard solu- tion until the drop fiom the beaker when in contact with a drop of the ferro-cyanide shall strike a brown color, which terminates the analysis. Read the burette, the number shown divided by fifty will indicate the number of grains of phosphoric acid to the fluid ounce of urine. Analysis with the Marais Approximate Tubes. The tube is to be filled to the ounce mark with filtered urine, add also two drachms of the acetate of sodasolu- APPARATUS. 331 tion, afterwards add an excess of a solution of nitrate of uranium (one to ten) ; placing the thumb over the end of the tube; mix thoroughly and allow the tube to- remain quiet for twenty-four hours, after which time the volume of precipitate may be read off, ever 2cc .016 grains of phosphoric acid. Analysis for Earthy Phosphates, using the Marais Tubes. Fill the tube to the ounce mark with filtered urine as above, add an excess of strong liquor ammonia ; mix well and set aside for twenty-four hours, at the end of this interval note the volume of precipitate ; each 2cc will correspond to .06 grains of the earthy phosphates. Analysis for Sulphuric acid Standard Solution. In 3,000 grains of distilled water, dissolve forty-five and three-fourth grains of pure chloride of barium; 200- grains of this standard solution are equivalent to one grain of sulphuric acid. If the solution be prepared with due care, it may be used without special testing* as it cannot be very well tritated. Solution of Sulphate of Soda. One part of soda to ten of water. Analysis of Urine for Sulphuric acid. With a pipette measure two fluid drachms of urine into a beaker, dilute this with twice as much distilled Avater,. adding also two or three drops of hydrochloric acid ; bring the beaker over the spirit-lamp until the contents become hot, fill the burette with the standard solution ; deliver from the burette into the beaker a few drops which will cause an instant precipitate ot the sulphate- 332 HOW TO SEE WITH THE MICROSCOPE. of baryta; this will gradually sink to the bottom of the beaker; continue thus the addition until no further precipitate can be detected by the eye. By waiting a few moments the liquid above the precipitate will be- come clear, when another drop or two of the standard solution may be added. Should this cause a further precipitate, continue in like manner to add the standard solution; when no further precipitate forms, place a few drops of the sulphate of soda solution in one of the smallest test tubes, and to this add a drop or two from the beaker; if a white precipitate appear in the test tube, the standard solution has been added to the beaker in excess ; and if the precipitate be very dense, the analysis will have to be repeated, using greater care towards the latter part. The number of grains finally shown by the burette, divided by fifty, indicates the number of grains of sulphuric acid to the fluid ounce of urine. The final determination in this analysis as to the pre- cipitation of all the sulphuric acid renders this tritation somewhat more troublesome to the beginner than the preceding. The practitioner should therefore, by re- peated trials, become acquainted with the details. * In examining the test tube as to the precipitate, it should be held to strong light, and a hand magnifier we have found to be of material assistance. Analysis for Sugar Standard Solution. Dissolve 51.98 grains of pure sulphate of copper in 500 grains of
  • X 0.50, -4- 228 = unity. Use of the standard solution in the ordinary testing for sugar. The practitioner will very often have occa- sion to simply test for the presence of sugar, when the exact amount is not necessary. In fact, after becoming thoroughly familiar with the behavior of the chemicals, he will be enabled to give a close guess as to quantity. The urine should in all cases be cleared from albumen. To merely detect the presence of sugar it is only neces- sary to pour a dozen drops or so of the copper solution into one of the largest test tubes (these are always bet- ter when there is boiling to be done), and to add twice the volume of the caustic solution. Bring to the boil- APPARATUS. 337 ing point, and add urine to the tube directly, continuing such additions and bringing to the boiling point sub- stantially as in performing the regular analysis. If, when the amount of urine thus introduced shall be equal to the original volume of the test solution, and there be no change of color, it may safely be assumed that sugar is not present. Even when the regular analysis is contemplated, the preliminary trial test should be instituted. For instance, we do not care to attempt the regular analysis, when by the rough test we are assured that there is no sugar in the specimen. Again, when, by the trial test, it shall be found that sugar is present, and in large amount, then it may sometimes be better to dilute the urine with an equal, or even six times its volume of water; of course, allowing for this reduction in the computa- tions after reading the burette; the accuracy of the analysis is enhanced by the dilution ot the urine. It may be further remarked that different specimens be- have somewhat differently. The directions given will, however, be found ample, and the practitioner having frequent use for the sugar test in a short time becomes perfectly at ease with the manipulations. Analysis for Albumen. The volumetric method, one of " trial and error," involving several filtrations, is somewhat tedious ; too much so for the practical pur- poses of the medical practitioner. The author was hence induced to experiment with the "Marais" ap- proximate tubes comparing results with those obtained by two of the regular methods. These were so far sat- 22 Microscopy. 338 HOW TO SEE WITH THE MICROSCOPE. isfactory as to lead to the employment of the approxi- mate tubes in his general practice ; and the degree of accuracy afforded by the use of these tubes is quite equal to the ordinary demands of the medical profession. The principal source of error arises, as we believe, from the fact that the coagulated albumen will at times pack closer in the tube than at others. By using three or four tubes in one and the same analysis, taking the mean of the results, the approximate process will, in most instances, be all that can be desired. Proceed as follows : In a clean evaporating dish, or a Florence flask, coag- ulate by heat three fluid ounces of the urine to be tested ; set aside until nearly cold ; shake well ; now take three inarais tubes, fill the first full, also the second with the treated urine and coagulum ; pour the remainder into the third tube, rinsing the dish with a little water, adding the wash to the several tubes. The three tubes can be thus made to contain the entire coagulum from the three ounces of urine; set aside for twenty-four hours, after which time read the several tubes; add the readings together and divide by three, and every two whole cubic centimetres will represent one grain of albumen to the ounce of urine. In the daily routine it will often suffice to use but one tube, in which case all that is necessary will be to coagulate a single fluid ounce of the urine; when cool, shake, and pour into the approximate tube, rinsing the dish as before, adding the wash to the tube ; at the end ot twenty-four hours read off the amount of coagulum ; APPARATUS. 339 every two whole c. c. will equal one grain of albumen to the ounce of albumen. Reaction. Urines being at times either acid, neutral, or alkaline, it is often interesting, not only to observe as to the fact, but also as to the degree of acidity or alkalinity; to accomplish this prepare the following solutions: Test Solution for Acidity. 1\\ 1,000 grains of dis- tilled water dissolve ten grains of pure hydrate of soda. Test Solution for Alkalinity . In 1,000 grains of distilled water dissolve 15.75 grains of pure oxalic acid. Equal volumes of these two solutions will exactly neu- tralize each other. To Test the Degree of Acidity in a Sample of Urine. Fill the burette with the soda solution ; measure one- half fluid ounce of the urine into a wine-glass ; deliver the soda from the burette into this a few drops at a time, stirring well after each addition and testing with litmus paper; when the mixture fails to affect the latter, read the burette twice, the figures read will indi- cate the number of grains of the solution employed required to neutralize a fluid ounce of the urine. To test for alkalinity proceed as above, using the acid solu- tion in place of the soda, and also red litmus paper in place of the blue. The daily variations in any particu- lar case can in this manner be determined and recorded. Proportion per tluid Ounce of certain of the Urinary Constituents. The estimates given in this part of the table are roughly ap- proximative, and represent the widest variations consistent with 340 HOW TO SEE AV1TH THE MIOROSCOPE. normal conditions. The variations, always considerable, are particularly marked as regards the uric acid . Urea 6*50 to 10'50 grains. Chlorine (1'30 to 3'60 sra. of chloride of sodium) 0'80 " 2'15 Sulphuric acid (1'30 to 3'20 grs. of sulphates) 0'66 " 1'62 Phosphoric acid (2' 10 to 4 '00 grs. of phosphates) 1'17 ' 2'25 " Do do combined with alkalies (phosphate of soda and phosphate of magnesia) 0'78 " 1'40 " Do do combined with earths (phosphate of lime and ammonio-magnesian phos- phate) 0-39 " 0-85 * Uric acid (0.40 to 0-70 grs. of urates) :. 0'23 " 0'40 FOB REDUCING THE INDICATIONS OF A GLASS UKINOMETEB TO STANDARD TEMPERATURE (60 Fahr.) WHEN THE SPECIFIC GRAVITY HAS BEEN TAKEN AT A HIGHER TEMPERATURE. (BIRD, Urinary Deposits. etc., Philadelphia, 1859 p. 7J.) No. to be No, to be No. to be Tempera- ture and added to the indi- Tempera- ture and added to the indi- Tempera- ture and added to the indi- degree. cation. degree. cation. degree. cation. 60 o-oo 69 0'80 78 1-70 61 0-08 70 0.90 79 1-80 62 0-16 71 1-00 80 1-90 63 0-24 72 1-10 81 2-00 64 0-32 73 1-20 82 2-10 95 0-40 74 1-30 83 2-20 66 0-50 75 1-40 84 2-30 67 0-60 76 1-50 85 2-40 0-70 77 1-60 APPENDIX. As a matter of convenience to the student we present the addresses of American dealers in Microscope Stands, Objectives, etc. The list is compiled from the various catalogues and other sources ot information which hap- pened to be in our possession. It is therefore probably incomplete, but nevertheless will be found to comprise most, if not all of the leading dealers. Of the latter, those marked with an asterisk (*) issue illustrated cat- alogues, which can be obtained on application, free of expense : NAMES AND ADDRESS OF DEALERS IN MICROSCOPES, OBJECT- IVES, ETC., ALPHABETICALLY ARRANGED. (*)Bausch &> Lomb Optical Company, 37 Maiden Lane, New York; factory at Rochester, New York. (*)R. & J. Beck, (London); American Agency in charge of W. H. Walmsley, Chestnut street, Philadel- phia, Pa. (*)W. H. Bulloch, 126 Clark street, Chicago, 111. F. J. Emmerich (Importer), 38 Maiden Lane, New York. E. Gundlach, L. R. Sextcn, agent, Rochester, New York. (*}T. H. McAllister, 49 Nassau street, New York. Pike (Dealer), 518 Broadway, New York. 341 342 HOW TO SEE WITH THE MICROSCOPE. (*) James W. Queen & Co., 924 Chestnut street, Phil- adelphia, Pa. William A. Rogers (Micrometer Plates, etc.,) Cam- bridge, Mass. (*)Messrs. C. A. Spencer & Sons, (Objectives, Ac- cessories, etc.,) Geneva New York. L. Schrauer, 50 Chatham street, New York. (*)J. W. Sidle & Co., Lancaster, Pa. (*)Robert B. Tolles, Hanover street, Boston, Mass. Charles Stodder, Agent, Devonshire street, Boston, Mass. (*)George Wale, Patterson, N. J. George Wale & Co., Hoboken, N. J., Agents. (*) William Wales, Fort Lee, New Jersey. (*)Joseph Zentmayer, 147 South Fourth street, Phil- adelphia, Pa. The following price list of stands, etc., is only in- tended to include those previously described in this book. Several of the makers therein mentioned manu- facture various models not included in our list. The stands ot the Messrs. Spencer's have been intention- ally omitted, as we learn that they are now devoting their entire attention to the production of objectives. The list of objectives is similarly incomplete. With the glasses of Messrs. Tolles and Spencer we have had a large experience. Several months ago the Messrs. Bausch & Lomb sent us a line of their objectives for study, all of which, after working with the same more or less for nearly a year, proved reliable glasses for their cost. APPENDIX. 343 The entire list may be understood to include only such objectives as we have used to a greater or less extent in practical work, and of which we can therefore speak from experience, and without prejudice to other reliable makers. We earnestly advise the student contemplating the purchase of object glasses, especially when ordering the same to be made by the optician, to seek the advice of some expert friend. Prctection is thus afforded both to the maker and the buyer, and it has often happened that the latter, through sheer incompetency, returns a really fine glass to its maker, with the assertion that it has proved unsatisfactory. While on the other hand, especially in glasses of high balsam angles, there IS a choice in the work of the very best makers. The ex- pert, too, will render valuable assistance in specifying the exact kind of objective desired. MICROSCOPE STANDS, BY W. H. BULLOCH, NO. 126 CLARK STREET, CHICAGO, ILL. Large best stand, Al, binocular, iris diaphragn, with the latest improvements, draw-tube, 5 eye pieces $300 00 Same as above, but monocular, iris diaphragm, with three eye pieces 250 00 Polished mahogany case, with side case for accessories. 25 00 Small best stand, A. B., binocular, 4 eye pieces and iris diaphragm 225 00 Monocular, two eye pieces and iris diaphragm 175 00 Polished mahogony case 20 00 kt D " stand, all brass, 2 eye pieces and case 75 00 " D " stand, all brass except the base, 2 eye pieces and case.., 67 00 344 HOW TO SEE WITH THE MICROSCOPE. MICROSCOPES AND OBJECTIVES BY THE BAUSCH & LOMB OPTICAL COMPANY. The Professional Microscope, with the following acces- sories : Hemispherical immersion condenser and ob- lique light projector, plain and concave mirrors, sub- stage, receiving accessories of standard English size, two revolving diaphragms, sub-stage with internal 4i society screw," 2 slot diaphragms for mirror, 3 Gund- lach's periscopic eye pieces (B, C and D), four object- ives, viz : 2-inch, 1-oth inch, 3-4th inch and l-8th inch immersion, with adjustment for cover, magnifying powers from 30 to 800 diameters, eye piece micrometer, camera lucida, bull's eye condenser, the whole in upright walnut case, with handle, lock and key, and drawer for accessories $200 00 Large Student's Microscope, plain and concave mirrors, sub-stage of extra size to receive standards, English accessories, revolving diaphragm, etc., all attached to the swinging mirror bar; sub-stage and mirror are re- movable ; three eye pieces, A, B and C, three object- ives, 2-inch, 3-4th inch and l-5th inch, magifying from 22 to 375 diameters, eye piece, micrometer, camera lucida, in upright walnut case, with handle, lock and key, and drawer for accessories 90 00 Family Microscope, concave mirror, adjustable for ob- lique light, one (B)eye piece, one first-class achromatic objective (one-half inch dividing) powers, 50 and 100 diameters, in upright walnut case, with handle, lock and key 20 00 The Messrs. Bausch & Lomb have brought out a new model which they term the Physicians' Stand, which may be thus de- scribed : Large, heavy cast iron foot, rack and pinion for coarse adjustment, draw-tube, allowing 2 1-2-inches shorten- ing, fine adjustment by a new frictionless motion, large hard rubber stage, sub-stage of standard size, carrying APPENDIX. 345 three diaphragms, which, when pushed up, will closely reach the object slide, plain and concave mirrors, ar- ranged so that their distance from the object may be varied, 2 eye pieces, A. and C, 2 objectives,3-4ths inch and l-5th inch, magnifying powers, when the tubes are completely drawn out, from 50 to 375 diameters, eye piece, micrometer and camera lucida, in upright wal- nut case, with lock and key 60 00 The past twelve-month has witnessed unusual activity on the part of microscope makers. Mr. Zentmayer, in the production of his " Histological," (as also the advent of the Acme Stand) stimulated other makers to increased energy in the production of small, low-priced and reliable stands capable of performing the work of larger and costly instruments. At the late (August, 1880) meeting of the American Society of Microscopists, held at Detroit, Michigan, two medium-priced stands were exhibited which were regarded bv competent judges as being quite equal to the Acme Stand. One of these was made by Mr. Bullock, of Chicago, and its cost as near as I can learn will be about $50. The other stand referred to was exhibited by the Messrs. Bausch & Lomb, of Rochester, N. Y. It is called the " Inves- tigator." Messrs. B. & L, have furnished the author with the following description of THE INVESTIGATOR MICROSCOPE. In this stand we confidently claim to have reached a higher degree of perfection than is possessed by any one approximat- ing it in price. We have combined in it the qualities of a first-class and high-priced stand, and at no sacrifice of its working qualities. The different parts are ingeniously com- bined, are strong and firm, and in the parts subject to friction we have introduced, as much as possible, new compensating bearings, which insure the retention of smooth and reliable movements with any amount of work. Working microscopists will understand the value of this quality. When contracted it stands but 11 high, but can be extended to 18 inches. 346 HOW TO SEE WITH THE MICROSCOPE. The base is of the tripod form, neatly japanned ; pillar and arm of brass, connected by a solid joint, which allows inclina- tion of body to any angle; rack and pinion for coarse adjust- ment, tine adjustment by our patent frictionless motion ; main tube with two draw tubes. This is an entirely new feature in microscopes, which is an unquestionable improvement. It per- mits the use of standard length of tube for quick adjustment in outside tube, same as in instruments without rack and pinion adjustment ; the same for any low power objective and the use of amplifier in either combination. The outside tube has a broad gauged screw, and adapter with society screw. The stage lies in the same plane as center of movement for mirror, is of brass, and has concentric, revolving motion with removable clips. It is thin to allow great obliquty, and as it rests upon a strong projecting arm, is perfectly firm under any manipulation. The mirror bar swings with a perfectly easy but firm motion, upon one bearing to any obliquity below, and above the stage for the illumination of opaque objects, and has affixed to it a secondary bar, to which the mirror is attached and which allows the separate use of the latter in any position of the sub-stage. It is provided with a sliding arrangement, whereby the mirror may be moved to and from the object. The mirrors are plain and concave, and of large size. The substage is adjustable along the mirror bar, and entire removable. It contains a diaphragm which may be brought directly under the stage. The ring is of standard size and is easily centered by a set screw. The stand is furnished with an eye piece of any power. In black walnut case, with receptacles for eye pieces and objectives, drawer for accessories, handle and lock, price $40.00. Same, with C Eye Piece, arranged for micrometer, camera lucida, micrometer, 3-4 inch and 1-5 inch objectives, $65.00. BAUSCH & LOME OBJECTIVES. Deer. Price. 2inch 18 $1200 2inch 12 600 linch 36 2000 linen.. 20 600 APPENDIX. 347 Deg. Price.. 3-4inch 27 800 l-2inch 72 2200 l-2mch 40 900 4-10 100 20 00 3-10 75 1100 1-4 100 .= 14 00 1-5 108 1500 1-6 inch 180 Adjusting 30 00- 1-8 Immersion 180 Adjusting 40 OO MICROSCOPES OF THE MESSRS. R. & J. BECK, OF LONDON. AMERICAN AGENCY IN CHARGE OF W. H. WALMSLEY, ESQ , 1016 CHESTNUT STREET, PHILADELPHIA, PA. Popular Microscope stand, binocular, with one pair of eye pieces, concave mirror, diaphragm, forceps, glass plate, pliers, etc $65 00 Same, but monocular 40 OO Economic Microscope, as furnished by the makers, mo- nocular, with sliding coarse adjustment, 1 inch and 1-4 inch object glasses, 1 eye piece, concave mirror, condensing lens, glass plate with ledge, brass pliers and diaphragm, in mahogany case 35 OO The same, with rack and pinion, for coarse adjustment, concave and plane mirrois, stage forceps, etc., in ma- hogany case 5000 (Both of the above stands are fitted with fine adjust- ment screw.) Extra eye pieces, each 4 00 The Messrs. Beck have just published a large and com- plete illustrated catalogue of their microocopes and other scientific instruments, which they furnish to any one on application. Among their later models of mi- croscope stands we notice the u New National," cost- ing, with one eye piece, concave and plane mirrors, diaphragm, stage forceps, glass plate pliers, etc 40 00 Mahogany cabinet for the same 10 00 348 HOW TO SEE WITH THE MICROSCOPE. Since the chapter on stands was written we have received the following description of BECK'S INTERNATIONAL MICROSCOPE STAND. The improved large best or international microscope stand has a tripod (A) for its base, upon which is placed a revolving fitting (B), graduated to degrees, by which means the micro- scope can be turned round without its being lifted from the table, and the amount of such rotation registered ; upon this fitting two pillars are firmly fixed, and between them the limb eported in the May number of the London Microscopical Journal. Alluding to the photographs of Fruatulia Saxonica, Mr. John Mayali said: " Every one who is familiar with the Frustul a Saxonica, photographs of which Dr. Woodward sent in illustration of his paper in December, knows it to be one of the most difficult test-objects a diatom that ranks next to Amphipleura pellucida. That particular form of Frustulia is one that I have rare y seen resolved, except by lenses of the highest excellence. I consider Dr. Woodward's photographs of it as in every way most remark- able, evincing first-rate skill brought to bear on one of the finest known lenses." It is hard to tell which is the most "reinarkanle," the photographs, or this statement of Mr. Mayall's. As there seems to be much difference of opinion as to these representa- tions of Frustulia Saxonica, both at h >me and abroad, a word or two may be admissable. When the lithographs first made their appearance in the Lon- don Monthly Microscopical Journal, one of our most talented experts wrote 390 HOW TO SEE WITH THE MICROSCOPE. Now, after seeing and learning what can be accomplished with a superior objective, when out of adjustment, it will be instructive, and I shall beg your permission, by the employ- ment of a superb glass, accurately adjusted, to show you a sim- ilar, but more difficult tiaxonica, as illuminated by the most oblique beams my extremely thin stage will admit. Thus han- dled the resolution of Frustulia Saxonica becomes one of the most charming and fascinating objects that can well be imag- ined. We shall thus see the frustule without sensible distortion the striae displayed with such exquisite beauty of definition as must command your admiration, and minus, too, the suspicion of a diffraction line I The attention of Andrew Boss, while examining Podura scales with glasses of his own manufacture, was called to the thickened edge, the " Nasmyth membrane " so plainly seen with the non-adjustable glass he then employed, and to him are we indebted for the collar adjustment now so common to all first- class objectives. Now, my friends, good object-glasses, like astronomers, have their " personal equation." They alone are the ones to be most affected by collar adjustment ; an ordinary glass, furnished with compensating screw, is scarcely superior to an ordinary non-adjustable lens ; one may turn the collar through its entire range, without sensible or corresponding change in definition. me saying that he was surprised that Dr. Wooaward should have allowed the publication of prints giving such indifferent ideas of the work of American lenses. I replied, that as I then understood the mat er, it was Col. Woodward's intention to show the work of the objective purposely placed out of adjustment, and I so continued to think at the date of the above lecture, a fact obvious to the reader. I also suggested that the litho- graphs, might not fairly represent the photographs ; that if it was the case. Col. Woodward would surely make the fact known. His silence, however, authorized the inference that the original photos had not suffered at the hands of the lithographer. I th nk I can now safely affirm that the general opinion is that one of these lithographs was intended by Col. Woodward to give a correct idea as to the appearance of P. Saxonica when properiy resolved,!, e., that it might be contrasted with the others then presented. Be all this as it may, I am prepared to assent that MO one of said lithographs gives any idea of the proper resolution of Ftvistulia Saxonica. J. E. S., January, 1878. SUPPLEMENT. 391 On the contrary, objectives of the widest apertures, and capable of yielding intense definition, require the strictest attention to their adjustment. * Nor is this all. It is quite possible to accurately adjust a first- class objective, and, nevertheless, in this very act defeat the maximum performance of a first-class glass ! When working with oblique light, the maximum performance of a superior glass will be obtained at maximum aperture ; if this maximum aperture obtains, with the systems ' closed," as is often the case, it is then manifestly our business to use covers of such thickness as will " correct" the objective at " closed," and to work such a glass over thinner covers, requiring the lens to " correct " at, or near " open point," would surely defeat its best performance. It being possible that we may have time in the course of the evening to practically discuss this point, I have brought here an objective which, in any position of its collar, has plus 180 of aperture ; its balsam angles are at " closed," 97i; at half way between " open " and " closed " 95 ; at open point, 85. It will be an easy matter to demonstrate that this glass is defeated by the thin covers of the Moller plates, over which the glass will " correct " near to " open point," and conversely, of the decided increase of definition obtained (and angle also) when worked through a supplemental cover of sufficient thickness to cause adjustment with the systems at, or near " closed." I have brought here another, and a totally different objective; this glass has a constant bals.im angle of 100 through nearly the whole run of its adjusting screw. Manifestly, when using this glass, there need be less attention paid to the thickness of cover, a fact which is demonstrated in practice. * In an article contributed by Mr. F. H. Wenham~3ee London Microscop- ical Journal for March, 18761 read, as follows : "The adjustment, seems to be a stumbling block for those advocating- an extra immersion theory. Wo have now in use thousands of serviceable im nersion object-glasses, capa- ble of denning most tests, and whion have no adjustment, as they are set for an average thickness of cover. They answer well, because in the im mersion system the errors of cover aberrations ara nearly eliminated, and with a balsam intermedium they would be inappreciable." (!!!) J. E. S. 392 HOW TO SEE WITH THE MICROSCOPE. These two objectives were constructed expressly for me, and were especially intended for conjunct use they are in truth, companion glasses ; either of them will display the 19th band by oblique use of artificial light, and also the markings of An- gulata by central lamp illumination. Of the two, the first- named has the greatest working distance, and, for reasons already given, will work at maximum performance over covers l-50th of an inch in thickness, and in the hands of one thor- oughly conversant with its functions, it is, perhaps, the most generally useful glass. I have here still another objective of plus 180 of aperture. This glass was intended to be a compromise, and to serve to a great degree the purposes both of the l-6th and the l-10th before mentioned. The glass I now refer to has at " closed " a balsam angle of 97 ; at open point, 92. The compromise is thus appar- ent : this l-6th was made to my order for the gentleman who tills the executive chair of this society. It asks no favors of M. Nobert, nor of a centrally posed Angulata, and when one is con* fined to al-6th alone, a glass of this construction will prove of great value. These conditions, which have still greater force when we include the work of various opticians, need attention and study, in default of which you may expect diffraction lines and dif- fraction borders; and, perchance, our old friend, the " Nas- myth," may put in an appearance I I repeat, that it was the dominant purpose of these desultory remarks to call your attention to the importance of microscope instrumentation, and to the manipulations connected therewith. To the fact that it is a study, a profession in itself, and an accomplishment that must be fully mastered at the outset, before one can investigate with precision, or have claim to the confidence of others. In point of fact, the most expert manip- ulator will constantly have something to learn there is no landing place where one can stop and rest. The world pro- gresses, and so should the microscopist pari passu. I take it for granted that nothing I have said will lead you to suppose that I am opposed to the legitimate use of low-angled objectives; nothing could be further from my purpose. When SUPPLEMENT. 393 properly used, for preliminary examinations, they are convenient and useful, but for advanced work they should be abandoned, and in favor of more inconvenient glasses of wider apertures. A wide-angled lens, incapable of receiving aad utilizing cen- tral beams, is a faulty and undesirable objective ; it is, in fact, but half a glass. And on the other hand, a low-angled objec- tive, incapable of receiving lateral beams, is. in my opinion, equally undesirable, unless, indeed, when we are prepared to :SciCrifice force .of definition to convenience of handling. It must be admitted that there are functional and character- istic differences in the performance of the two classes of objec- tives; for instance, a wide-angled glass of relatively shorter focal length will not see so far around an object as will one of narrow aperture and longer focal distance. This, as well as other considerations which might be mentioned, offer no rebut- ting testimony to the statement already advanced, that the principal advantage presented by the low angles is their con- venience in manipulation. In conclusion, I have to thank you for your kind attention to the rambling remarks I have had the honor of presenting. Dur- ing the few short hours that I have been in your city, the earth has not had time to make one rotation ; but I have had a plenty of time to experience and enjoy a generous hospitality. I am now ready to show you the promised objects, and shall be happy to see what you have to offer. You will find me ready, willing and ardent to be taught, and in microscope matters be assured I an: as absorbent as a sponge. A CHAPTER ON ELEMENTARY PHYSICS. (Dedicated to Prof. Hitchcock.) Part First The Sun. "The luminous orb, the light of which constitutes day ; the central body around which the earth and planets revolve ; a celestial body which can be seen almost any fair day through a piece of smoked glass." I beg my friend to read the above little monograph on ele- mentary astronomy carefully. Assuming his compliance with this request, I go on to say that just after sundown this evening, my attention was called to his reallv kind notice of myself, as 394 HOW TO SEE WITH THE MICROSCOPE. well as my Dunkirk lecture. Allow me to assure you, my dear professor, that although we have been pulling at opposite ends of the rope for some months, the generosity I have experienced at your hands has been vastly in advance of what I could have returned, and has placed me in position to receive any criticism that you might offer with the best possible grace. I now propose to jerk my end of the rope in a manner that will make things lively with you ! The fact is, when I get after you with a '* sharp stick " you become unusually brilliant with the jerk responsive I I like it. I admit that you are one " who demands accuracy of expres- sion in every branch of science " " that the scientist has no right to say what he does not mean, and he has no right to pre- sume that his readers will understand him if he uses incorrect language." I therefore have placed the above little monograph on elementary astronomy at your service, and in timely season r fearing otherwise you might be led astray by my use of the word u sundown " you will see the point. It's true that Joshua of old commanded the sun and moon to stand still, and the story is found in a book generally admitted to be a classic, but no matter. Now, professor, I fear that you do not practice what you preach ; for I read (page 92), " if this question were put to a thorough physicist * * * knowing nothing about balsam or fluid mounts, etc." Isn't there an " impossibility ' r here V A thorough physicist knowing nothing, etc. There's a screw loose somewhere. llight on your next line is another jumble. Here is a want of accuracy or what not which is it ? You say " Must the scien- tist be led by the nose," etc. If this expression is accurate, I want you to tell me just how this kind of a thing is accom- plished ; give me the p-r-a-c-t-i-c-a-l details, just as it would be done in the flesh, including the " adopt " and the " teach ' r before an audience, too, without " apology "and the why of it. " It's the right and duty of every man of science to ask this little question whenever it occurs to him." " The scientist has no- nqht to say what he does not mean." Let us have the facts. Part Second. 180 C> , plus 180, 180 + 4o. It is true, my dear SUPPLEMENT. 395 sir, that in my Dunkirk lecture I did use the words " plus ISO 01 of aperture/' Now that select and intelligent audience (com- prising physicists who did know something about " balsam angles,") so far from demanding an " apology," generously and unanimously gave me a vote of thanks. Curious, wasn't it ? I remember well my delight in there meeting the veteran Dr.- J^ W. Armstrong, Principal of the Fredonia Normal School, and one of the leading educators of your state; another physicist, too, who can talk intelligently as to " balsam angles," and who has made the apertures of objectives an especial study, and who afterwards became one of my most valued correspondents. Nor did we have any quarrel about the " plus 180 Q ." Most of the audience had read about Joshua, and doubtless would not baulk at such expressions as " sunrise '' or " sunset." After all, professor, admitting that the "plus 180" might have courted son,e such gentlemanly criticism as it finally got from you, I reckon I was as near right as Joshua. But before I can lift you over this stile, I must scoop you up ! get you together in some shape, so you can be handled. First of all r you seem to put the 180, plus 180, 180 x 45, all in one boat together; you regard them as synonyms. And, secondly, I have to learn some things from you. Messrs. Tolles and Spencer you denounce because they mark their objectives 180, which you say is not only " impossible, ' r but " absurd," and that one of these gentlemen (you don't say which) attempts to lead the scientist " by the nose." And you further say that one ot them (I don't know which, again, > " miff lit as well add 45 to said 180. Well, here is material enough to commence on. Now, my dear professor, you maintain it to be the " right and duty'" even of every man of science to ask this little question whenever it occurs to him the " why" part you know). That I have been engaged in a controversy with you for months, demonstrates- my claim as a " man of science." You can't dodge that, and the "occur " part is present and up to the high- pressure notch. Now, I want to know " why " it is that 180 of aperture i impossible ; "why " that plus 180 of aperture is impossible and absurd. We have your assertions, but minus the ghost of a dem- onstration. .396 HOW TO SEE WITH THE MICROSCOPE. Why do you accuse Messrs. Tolles and Spencer of perpetrat- ing acts that are absurd and impossible ; and why do you ven- ture to hint that one of these gentlemen attempts to " lead scientists by the nose," (whatever that may mean). The names of Spencer and Tolles are revered by American microscopists, and their unrivaled efforts in the improvement of object-glasses have won for them a world-wide reputation. The chances are that they know more of microscope optics than you and I put together; and these are the men who mark their objectives " 180." Why should there be " a law to prevent it ?" Now, professor, if this 180 is impossible and absurd, will you kindly inform the readers of this journal what figures Tolles and Spencer ought to engrave on their wide-angled objectives in place of the awful " 180." Will you be kind enough to name the extreme angle immediately adjacent, but not contact- inq the impossible and the absurd ? Again I read, " plus 180 does not mean 97 balsam." Did I assert this ? Now, I ask what does 97 balsam mean ? Let's have it, and the "why" thereof. Unless you can tell me exactly what the 97 balsam angle is, I shall not take any stock in your .above assertion. Once more, you say that " true apertures can be measured and definitely stated." I wish that you would tell Mr. Wenham how the thing is done ; he tried for a whole year to measure one of Tolle's objectives, without getting any two results alike! But I am after you, not Wenham. I desire to learn trom you precisely what " true aperture is." When thus taught, then 1 Desire to know by what physical process the same can be meas- ured "definitely" you know. "Accuracy" is the thing we scientists want. Now, my dear sir, I call on you to answer all of these inter* rogatories, and when you shall have proven that 180, plus 180, or 180 X 45, are one and the same thing ; when you shall show that Messrs. Tolles and Spencer are asserting absurd and impossible things, and more, are trying to lead scientists " by the nose ;" when you shall prove that a thorough physicist can know nothing of balsam or fluid mounts ; when you shall have demonstrated that I should have apologized to my Dun- SUPPLEMENT. 39? kirk audience ; when you shall have proven what 97 Q of balsam' angle is ; ditto as to " true aperture," and how to "definitely measure" the same then and not until then will I take the " plus 180 y " under advisement. Meanwhile we will be good friends, and with Gen. Grant say, " Let us have peace." Don't omit reading the reprint of Wenham's article on angular aper- ture, pages 74 and 75. CHOICE OF OBJECTIVES. Ed. Am. Jour. Microscopy. It was with real pleasure that I read the article entitled " Dr. Carpenter on Angular Aperture," by W. G. Lapham, Esq., in your May issue. First of all, it is very satisfactory, from the fact that Mr. Lapham speakes from a practical standpoint. The gentleman is an entire stranger to me, but the fact that his conclusions are drawn from his personal, and, I doubt not, protracted experi- ence, is evident from the ability with which he handles his subject. Your readers are well aware that I have often complained that I have been compelled to fight theory with practice and that no effort on my part sufficed to induce my opponents to abandon their theoretical ideas, and to examine practically as to the truth of the positions advanced by myself. In Mr. Lap- ham, however, I reaognize a co-worker, and although the results he arrives at are not in perfect coincidence with my own, I find nothing to criticize, but am simply desirous of further comparing notes with the gentleman, in the hope that some- thing to the advantage of microscopy may be developed. I propose to present a few thoughts for Mr. Lapham's con- sideration, requesting the gentleman to give them whatever study and attention he may elect, and to advise your readers as to the conclusions he arrives at; and, as before intimated, nothing that I have to say ought to be taken in the light of a criticism of his really excellent paper. First, I desire to ask if Mr. Lapham does not attach too much importance to the so-called " penetration." Is there in esse, any such thing as "penetrations?" Perhaps it will be well, first of all, to settle this point. I 398 HOW TO SEE WITH THE MICROSCOPE. Now I hold as follows : We will take, for example, two glasses, both possessing the same amplifications and the same apertures ; now of these two glasses, the one having the greater working distance will exhibit structures situated slightly out of the precise focal plane, or what amounts to the same thing, will be less susceptible to slight changes in the focal distance, or again, to use the words of Dr. Carpenter, will have the greater pene- tration. In extreme cases, the item of aperture may be dis- regarded. For instance, as stated in my Dunkirk lecture, a spectacle lens of sixty inches focal length will be endowed with greater penetration than a ToUes duplex objective. But in the Dunkirk address, I did not avail myself fully of the demonstration afforded by the spectacle lens, but now invite Mr. Lapham's attention, thus : If it so be that with the spectacle lens I am enabled to see with perfect clearness of vision objects across the road, it will surely occur that when I attempt to read fine print, that the lens will defeat me. I will, to be sure, see the lines of the print, will recognize the contour of the book, and larger objects in immediate proximity, etc. Here is an example from everyday life, to-wit : The glasses .suitable for reading will not answer for observations at a dis- tance. True it is that with such glasses, with which we read with ease, we may also see the general forms of things across the road, yet they suffice not to render clearly the details thereof. Hence we have all of us observed that many persons wearing convex glasses often use two pairs of spectacles, one for near and one for distant vision. Now in this everyday case we are taught a lesson, that, from some cause or other, has been a slow one to acquire. First "penetration," in its naked aspect, is simply a function depend- ing upon working distance ; and secondly that '"penetration," unless accompanied with a certain amount of definition^ is practi- cally worthless. I ask Mr. Lapham's close attention to this point, one that has been terribly overlooked. Now, if Mr. Lapham's one-sixth of 180 happens to just " turn the corner," i. e. have balsam angle, say, about 85, I would feel sure that a little observation would lead him to the same opinions I have arrived at, and I only fear that lie has SUPPLEMENT 399 chosen one of the highest balsam angles, in which case he will have to fall back on his four-tenths. Nevertheless, let him, if he will, try the experiment, and report as to whatever is gained in point of penetration by the use of his one-forth of 50 over and above what can be obtained by the use of the one-sixth, or the four-tenths. The point I have to make is this the one- fourth of 50 will have the greater penetration (so-called, like the spectacle lens) but will lack definition, to the end that more is lost in the latter element than is gained in the former. I state as a matter of fact, but with no desire to bias the further observations of Mr. Lapham, that I have used just such a one-fourth as he describes, but have ultimately discarded it, and in favor of a one-sixth marked by the maker 180 Q , which I find will do all the work (penetration included) of the one-fourth of 50 and a great deal more besides. But behind this one-sixth, I hold a one-tenth of 100 balsam angle in reserve, for work where " penetration" is ruled out. As to the other point suggested. Is there not undue weight attached to this "penetration?" With me, I have often been obliged to take special measures to get rid of this function, and for this purpose use the Beck illuminator, which gives me only surface structures. I mention the fact, but am willing to admit that a certain amount of " penetration" is at times desirable, and should be provided for as perfectly as possible. Mr. Lapham recommends a four-tenths of 10(R I have often thought that such a glass, or a half-inch of the same angle, would be desirable, and, as a luxury, am still of the opinion. It must be remembered that he very properly rules out the item of cost ; wnile on the other hand I have made it a study to avoid expense, where my opinion has been solicited in the matter of selection of objectives. Mr. Lapham recommends a certain number of low-angled glasses, all others to be of the highest angles, and of the best quality and well corrected. Why not amend this by insisting that all glasses have the highest attainable angles? selecting of such the one suitable for the work in hand. Again, Mr. Lapham states that we have no need to consider objectives of lower power than the half-inch for " they are not 400 HOW TO SEE WITH THE MICROSCOPE. made with angles sufficiently high to injure penetration." Here is involved an error in fact, one quite pardonable too. Not Jong: since a German gentleman remarked to the writer that it seemed to him that when the London opticians demonstrated a certain opticial law, some Yankee optician would be just mean enough to make an objective that would upset the whole arrangement! And it is perforce of this/act that Mr. Lapham becomes involved in his very pardonable error. Now I have in my possession a two- thirds of the Messrs. Spencers of 48 aperture. They have- lately made to my order an inch of 47 Q . The clear diameter of both glasses is about the same; the working distance of the two-thirds is 25-100ths of an inch, while that of the inch is but 13-100ths of an inch, and the kk penetration" of the latter (so- called) is less than that of the 2-3ds. In fact the penetration of the inch is "injured," as compared with one of 30. The reader will notice that Mr. Wenham's pet theory gets also into grief. I assure Mr. Lapham that this new inch is a glorious glass, pene- tration or no penetration. Finally, I desire to extend to Mr. Lapham personally my thanks for his instructive and interesting article. It will be a pleasure, and, I doubt not, profit to compare notes with him, and, in the words of my very generous opponent, Prof Hitch- cock, I will add that all I desire is "the facts." Mr. Lapham, by the way, will be pretty sure to catch it, about that 180, from Prof. H., on the " impossible" and the ' k absurd." It may be well enough to add that my first impulse was to write Mr. Lapham privately for an interchange of personal experiences, but on reflection chose this, the more public plan, hoping to enlist the attention of others in the same direction. OLEOMARGARINE AGAIN. Ed. Am. Jour. Microscopy: The article giving " The Micro- scopical Examinations of Oleomargarine," by Prof. Michels, which appeared in a recent issue of this journal, was perused by me with much interest, and I at once resolved to repeat the experiments as detailed. The first thing requisite was to secure authentic specimens of SUPPLEMENT. 401 oleomargarine. In order to do this, I solicited the friendly aid of a prominent gentleman of this city, who is also well known in business circles in New York. I also wrote, independently, to a gentleman in your city, urging him to obtain for me the samples required. Desiring also to procure a specimen of genuine dairy butter, I applied to a well-known citizen of this city, who procures his supplies directly from a farmer. After an interval of ten days, I was, in response to my solici- tations, ia possession of three samples of oleomargarine, and one sample of pure dairy butter. The three samples of oleo- margarine are directly from the manufacturers at Xew York city, and will be referred to as Kos. J, 2, and 3. The four samples were subjected to examination under the microscope ; tha objectives used were a l-4th inch of 100 by Tolles, and Spencers late duplex l-4th of 180. Samples Nos. 1, 2 and 8, and also the specimen of pure dairy butter, showed many crystals of chloride of sodium ; the crys- tals furnished by the pure butter were, however, cleaner, and more acceptable generally than those exhibited by Nos. 1, 2 and 3. Samples Nos. 1, 2 and 3, in addition to crystals of chloride of sodium, displayed other crystals, those of nitrate of soda being prominent ; while forms closely resembling crystals of chokste- nne were found in considerable numbers. Regarding Kos. 1, 2 and 3, it may be remarked that Kos. 1 and 2 were, to outward appearance, tolerably fair imitations of genuine butter, and might by the ordinary purchaser be accept- ed and bought as such. N"o. 3 was a poorer counterfeit, and would probably be rejected by most buyers ; but if mixed in equal parts with the genuine article, the mixture would be likely to deceive the purchaser. In samples Nos. 1 and 2 the microscope displayed the ' feathery crystals" (margarine) described by Prof. Michels, although these were not constantly present in every field exam- ined. By moving the slide, other fields were brought to view in which these crystals were much more prominent than those given in the cut accompanying Prof. Michels' paper. The crys- 26 Microscopy. 402 HOW TO SEE WITH THE MICROSCOPE. tals of nitrate of soda and cholesterine (?) were to be seen in every slide prepared from the samples named. Besides the crystals named, samples Nos. 1 and 2 gave " sus- picious cells" in large numbers, accompanied by shreds and tissue tibres, many in a broken down condition, while others seemed to be in a tolerably normal state, sufficient almost to establish the presence of voluntary muscle. Bundles of these fibres were closely examined with the duplex l-4th, under powres as high as 1,600 diameters, with the endeavor to bring out the transverse markings ; this was not accomplished owing, doubtless, to the nature of the vehicle (my observations in this direction will be continued). Many of the bundles seen were completely broken down, and the elementary fibres detached. Sample No. 3, when examined under the microscope, displayed fewei f the "feathery crystals;" nevertheless, this was the most u suspicious " specimen of the three ; there were multi- tudes of " suspicious cells," shreds and patches of tissue, in a more or less broken down state. In one field I felt tolerably sure of finding encysted hydatids. This slide unfortunately, was accidentally destroyed by the water getting under the cover. To give an intelligent description of this material (No. 3) would require far more time to its study than I have at my com- mand at present ; but I hope to attack it again, and at an earlp day. Suffice it now to say that this specimen contains verymani " suspicious " elements, and that its behavior under the objec- tive (here I also include Nos. 1 and 2), is with the exception of the crystals of chloride of sodium, and the presence of a few fatty globules totally different from that of pure dairy butter. The sample of pure dairy butter gave fields just as represented by Prof. Michels, with the exception that crystals of the chlor- ide were almost constantly present. Having established, to my own satisfaction, at least, the in- tegrity of the observations of Prof. Michels, I therefore hold that the gentleman should be regarded in the light of a -public benefactor ; the matter he has presented will be found worth serious and careful investigation. In one of my recent lectures before the medical class of this SUPPLEMENT. 403 College, on the Entozoa* I quoted (substantially) from Dr. liob- erts, as follows : " A marvelous light has been thrown in recent years on the zoological position of the Entozoa, chiefly by the researches of Siebold and Van Beneden. It has been ascertained that the hy- datid worm found in man, constitutes the encysted phase in the developement of a very minute tape-worm which infests the dog. The tape-Vorm in question is the Tcenia echinococcus ; the entire adult animal is so small that it scarcely exceeds the size of a millet seed. It consists of but three segments, of which three, the last only is fruitful. When this segment arrives at maturity, it is cast off, and a new one developed in its place. Myriads of these worms are sometimes found in the intestine of the dog, and their eggs are discharged in countless numbers with the excrements, the eggs so discharged are scattered far and wide ; and some of them find their way with the food into the stomachs of men and other creatures suitable for their de- velopment. Arrived there, the embryo is liberated ; and after penetrating the mucus membrane, it burrows its way, or is car- ried by the blood current to some distant organ, where it is arrested. Having thus lodged itself, it presently reappears as a hydatid vesicle, in which are developed the echinococci, as be- fore explained. Dogs in their turn become infested with the corresponding tsenia by feeding on the offal of slaughtered sheep, pigs, etc., which had been infested with hydatids. The echinococci therein contained develop in their intestines into the tsenia echinococci, and thus the circle of transformation and development recommences." By similar cycles of transformation and development, do we arrive at a class of parasites known in medicine as the ectozoa. This term may be said to include, or be applied to, worms or larvaB of insects that have been introduced into the intestinal canal by accident. Animalcules, such as the hair worm, grub of the fly, may be mentioned ; also the larva of the bee, the spi- der, etc. Among animals, the disease known popularly as the botts, to which the horse is frequently a victim, is caused by such animals swallowing the ova of the oestrus or gad-fly. That oleomargarine manufactured from refuse animal fats, 404 HOW TO SEE WITH THE MICROSCOPE. in the manner described by Prof. Michels, and at a temperature not above 120 F., may be a highway through which " eggs so discharged " find their way into the stomachs of men, is too pal- pably evident to need further comment. INDEX. Acetate of soda, solution, 329. Acme microscope, 84. Adjustable objectives, 148. Adjustable glasses, 129. American stands, 17. American stands compared with German. 18. American histological stand, 39. Analysis, chemicals for, 322. Analysis, chloride of sodium, 328. Analysis for albumen, 337. Analysis for glasses, 103. Analysis, earthy phosphates, 331 Analysis, phosphoric acid, 330. Analysis, sugar, 332. Analysis, sulphuric acid, 331. Analysis, urea, 323, 325. Analysis for sugar, 335. Angular aperture, what is it? 93. Angular aperture defined, 93, 370. Angular aperture illustrated, 94. Angular aperture, how to meas- ure, 95. Angular aperture, versus work- ing distance, 115, 121. Angular aperture and central illumination, 361. Angular aperture and penetra- tion, 118. Angle high glasses, 103. Apparatus for micro-chemical use, 320. Apertures, angular, 93, 405 Aperture, angular balsam, 123. Artificial light, 187. Balsam apertures, 128. Balsam angles, advantages of, Balsam angle, management of 257, Balsam angle, and working dis- tance, 142. Balsam angle, low objective 144. Baryta solution, 324. Bausch & Lomb's microscopes, 50. Bausch & Lomb's microscope, professional, 53. Bausch & Lomb's microscope, students, 56. Bausch & Lomb's microscope, stands, 344. Bausch & Lomb's objectives, 346. Beck's microscopes, 72. Beck's microscope stands, 347. Beck's vertical illuminator, 217. Beck's vertical illuminator, how to use, 221 Binocular, objections to, 30. Binocular, versus monocular, 29. Biological microscope, 68. Bull's eye condenser, 225. Bull's eye condenser, how to use, 227. Bullock's microscope, first- class, "A. 1." 59. 406 INDEX. Bullock's microscope, 59. Bullock's microscope, D stand, 57. Bullock's microscope, small, best, 67. Bullock's microscope stands, 343. Carbonate of soda, 324. Carpenter on object glasses, 97. Centennial stand, Zentmayers, 34. Centennial stand and A. 1. com- pared, 63. Central light and high angles, 257 Choice of objectives, 397. Choice of objectives for regular work, 202, Chemicals for analysis, 322. Chloride of sodium, 327. Ghromate of Potash, 327. Collar adjustment, objections to, 208. Concentric rotating stage, 25. Condenser, Bull's eye, 225. Condenser, small diameter, 308 Contributions to the Medical News, 358. Covering glass guage, 216, Covering glass, selection of, 213. Daylight, how to use it, 13. Diatoms, examinations of, 255. Diatoms tor test, 251. Diatoms, order for test, 254. Diatoms resolution as objective test, 249. Diatoms resolution, with high j powers, 24-5. Diatoms resolution, with low ! powers, 245. I Diatoms, resolution, 243. Draw tube, advantages of, 136. Dry mounts, list of, 305. Dry mounts, \\ith high objec- tives, 298. Duplex glass, the first, 9. Essentials of a reliable stand, 21. Examination of morbid pro- ducts, 206. Examination of urinary depos- its, 203. Examination of oleomargarine, 400. Eye pieces, 256. Eye pieces high, 146. Eye pieces fittings, 33. Eye pieces solid, 156. Eye pieces should fit loosely 204. Eye training, 287. Eye training illustrated, 239. Family microscope, 57. Fasoldt's micrometer, 357. Fehling's solution, 333. Ferro cyanide of potassium so- lution, 329. Field, flatness of, 126. Fighting objectives, 117. 140. Flatness of field, 126. Four system glasses, 9. Glasses, high angle, 103. Glasses, high angle history of 100, Glasses, testing aperture, 192. German stands compared with American, 18. INDEX. 407 German student lamp, 187. Handling objectives. 259. High angles, 103. High angles balsam, 125. High angles, discussion of , 359. High angles, objectives, 100. High, eye piecing. 146. High objectives, with dry mounts, 298. Higher powers, work with. 235. History of high angle objec- tives, 100. Histological stand, 39. Illumination, artificial, 187. Illumination, daylight, 183. Illumination small best, 189. Illumination, sunlight, 186. Illumination, varieties of, 183. Illuminator, Beck's vertical, 217 Illuminator, modified, 219. Illuminator, Wenham's reflex, 157. Illuminator, Wenham's with sunlight, 191. Illuminator, Woodward's, 163. Immersion, 125. Kerosene oil immersion lens, 316. Lens, Tolles' traverse, 179. Lessons on seeing with the mi- croscope, 261. Lesson first, 261. Lesson second, 262. Lesson third, 265. Lesson fourth, 266. Lesson fifth, 268. Lesson sixth, 270. Lesson seventh, 271. Lesson eighth, 273. Lesson ninth, 274. Lesson tenth, 280. Lesson eleventh, 282. Lesson twelfth, 284. Low balsam angles, 144. Low powers, working with, 225, Management of high angles, 257. Manipulations, microscopic, 157. Marais' approximate tubes, 330. Measuring bottle, 324. Medium power of glass 144. Micrometer, necessity for, 252. Micrometer, Fasoldt, 357. Micrometer, Rogers', 356. Micro-chemical apparatus, 320. Micro-chemical examination of urine, 318. Microscope, Acme, 84. Microscope, Bausch & Lomb, 51. Microscope, Beck's, 72. Microscope, Bullock's, 59. Microscope, dissecting, Beck's 79. Microscope, "D," 67. Microscope, economic, 72. Microscope, family, 57. Microscope, first-class A 1, 59. Microscope, international, 348. Microscope, investigator, 345. Microscope, new biological, 68. Microscope, new histological, 79. Microscope, monocular vs. bin- ocular, 80. 408 INDEX. Microscope, national, 29. Microscope, popular, 72. Microscope, professional, 53. Microscope students, 56. Microscope, Sidle's, 84, 351. Microscope selection, 147. Microscope, Tolles , 47. Microscope, Zentmayer, 34. Microscope, use and abuse of 375. Microscopic manipulations, 157 Mirrors, concave, 32. Mirrors, hangings strong, 38. Mirrors mountings, 32. Mirrors, plain, 61. Mirrors used as condensers, 229. Mirrors where attached, 226. Monocular vs. binocular, 29, 80. Moller's test-plates, measure- ments of, 253. Mounting of objectives, 129. N Names of microscopic dealers, 341. Nobert's test-plate, 254. Objections to mechanical stages 24. Objections to binoculars, 30. Objectives, adjustable, 148. Objectives, broad guage, 230. Objectives, balsam, 141. Objectives, choiceiof, 397. Objectives, Dr. Carpenter on 97. Objectives, fighting, 117, 140. Objectives, high angle, 97. Objectives, high angle, testing of, 107. Objectives, low balsam angle, 144. Objectives, one-sixth preferred to one-fiftieth, 109. Objectives, immersion, 125. Objectives, experience with, 202. Objectives, for physicians, 207. Objectives for regular work, 202. Objectives, test with diatoms, 249. Objectives, testing aperture of 192. Objectives, testing, 198. Objectives, wide angle, what is it ? 137. Objectives, working distance of, 108. Objectives, mounting, 129. Objectives, oil immersion, 310, Objectives, Spencer's, 353. Objectives, Tolles', 354. Objectives, performance of, 368. Observer's position, 246, 289. Oil immersion objectives, 309. Oil immersion, objectives, expe- rience with, 315. Oil immersion objectives of Zeiss, 310. Oleomargarine examinations, 400. Penetration and angular aper- ture, 118. Physicians, objectives best for, 207. Physics, elementary hints on, 393. Potlura, scale resolution of, 299. Position of observer, 246,289. Professional microscope, 53. Rising of the object, 276. INDEX. 401) Reaction of urine with tests, 339. Rogers' micrometers, 356. Selection of covering glass, 213. Selecting a stand, Sliding tube, advantages of, 44. Short tube stands, 25. Small light best, 189. Small kerosene lamp, 189. Society screw, 232. Society screw not sufficient for high angles, 88. Specific gravity and tempera- ture, 340. Spencer's objectives, 353. Spencer's one inch of 50, 230. Stage, thin, improvised, 39. Stage swing and mirror best, 01. Stage, concentric, rotating, 25. Stage, mechanical objections to 24. Stage, room necessary, 26. Stage used by the author, 27. Stand, Acme, 84. Stand, American, 17. Stand, American Centennial, 34. Stand, Bausch & Lomb's, 50. Stand, Beck's, 72. Stand, Bullock's, 59. Stand, essential of a reliable, 21. Stand, Economic, 72. Stand, Histological, 79. Stand, "D,"67. Stand, Investigator, 345. Stand, International, 348. Stand, large and small, 26. stand, large " BB," 47, Stand, large "A," 48. Stand, new biological, 68. Stand, new National, 80. Stand, first class " A. 1." 59. Stand, small best, 67. Stand, popular, 72. Stand, Sidle's, 84. Stand, Tolles', 47. Stand, students', 49, 56. Stand, small and cheap, best, 92. Stands, with short tubes, 25. Stands, Zentmayer's, 34. Students microscopes, 49, 56. Sunlight illumination, 186. T. Table for work, 229. Temperature and Urine specific gravity, 340. Testing high angles, objective, 107. Thin stage necessary, 90. Tight fitting eye pieces, 204. Tolles' first duplex, 9. Tolle's microscopes and objec- tives, 354. Tolles' traverse lens, 179. Tolles' large kk BB "stand, 47, Tolles' large k - A " jtand, 48. Tolles' students' stands, 49. Traverse lens, 179. Tube casts^ no genuine seen, 319. Tube sliding, advantages of, 44. TT. Urinnry constituents, propor- tion of, 339. Urinary deposits, examinations of, 203. Use and abuse of the microscope 375. V. Volumetric analysis, 318. 410 INDEX. W. Weights recognised, 321. Wenham's reflex illuminator, 157. Wide apertures, lessons in the use of, 261 Wide apertures, management of 258. Wide angle objectives, 137. Wide angle objectives and col- lar adjustment. 139. Wide angle objectives and illu- mination. 140. Wide angle objectives and working distance, 137. Work table, 229. Working with higher powers, 235. Working with lower powers, 225. Working distance of high an- gles, 137. Working distance of high bal- sam angles, 143. Working distance of objectives, 108. Working distance vs. angle aperture, 115,121. Woodward's illuminator, 163. Woodward's illuminator modi- fied, experience with, 177. Z. Zentmayer's stands, 34, 355. Zentmayer's American Centen- nial, 34. Zentmayer's American Histo- logical, 39. SPECTACELS ; AND HOW TO CHOOSE THEM. BY C. H. VILAS, A. M., M. D. Professor of Diseases of the Eye and Ear in the Hahnemann Medical College, and Ophthalmic and Aural Surgeon to the Hahnemann Hospital, Chicago, etc., etc. This book is full of interest and instruction to readers of all classes. Designed more especially for the profession, it never- theless does not deal in technicalities or obscure terms and will doubtless tind a large demand among non-professional people. In a clear and comprehensive manner it treats of a most important subject, a subject concerning which, far too many er- roneous and dangerous notions prevail. It's aim is to dissemin- ate knowledge, and to prevent the common haphazard custom of choosing Spectacles, a custom so often disastrous to vision or fruitful to discomfort. The Author points out how, within a comparatively recent period, what was once veiled in mystery, has attained a sure place among the sciences ; how the know- ledge of the proper construction, adaptation, and uses of Spec- tacles has grown to great proportions, and has led to a revolu- tion in the treatment of eye affections. Many popular delusions will be dispelled by reading this book. It is valuable alike to the physician and the layman. The former may utilize it in practicing his profession ; the latter will prepare himself to avoid errors fatal to vision. All the new and useful cases of trial lenses are described and illustrated and their respective merits and demerits are pointed out. It is clearly shown how the low prices, at which a good set of trial lenses can be obtained, will enable the physicians in general practice to give attention to the fitting of Spectacles and to treating ordinary defects of the vision, thus preventing the frauds so often practiced by itinerant venders. The Author's well known accomplishments in this depart- ment of science, have eminently prepared him to skillfully treat so important a subject. The work is illustrated and full of practical hints. Price bound in cloth, $1.00. DUNCAN BROS., Publishers. 131 & 133 8. Clark St., CHICAGO. THE NURSE; OR, HINTS ON THE CARE OF THE SICK; INCLUDING MOTHERS AND INFANTS, AND A DIGEST OF DOMESTIC MEDICINE. By CHARLES T. HAKEIS, A.M., M.D, Elegantly bound in Cloth - Price, 60 cents. "Every Nurse should have a copy." "Every Mother should possess this useful work." " It will not hurt physicians even to glance over the pages devoted to mothers and nurses, and this little treatise ought to become one of the tracts domiciled in every household." North Am. Journal of Homoeopathy. This is a valuable volume to professional nurses and mothers, the natural nurses. The author strongly advocates the importance of intelligent, self- sacrificing 1 nurses. The instruction the book contains is cle irly and con- scientiously given. Boston Inventors & Manufacturers' Gazette. Such a work is very much needed. It is simply and clearly written, giv- ing hints on all the points which a nurse ought to know. We cordially recommend this little work to the attention of mothers and nurses who will find it a valuable vade mecum. Monthly Homoeopathic Review, London. A neatly printed, handsomely bound work of over a hundred pages, written with the endeavor, as stated in the preface, to make their scope ' broadenough to cover the wants of two classes; the conscientious and ambitious professional nurse, and the faithful and loving mother." The matter of the worK, from the introduction to the last page, seems well and carefully written and very sensib.e. Ypsilanti Sentinel. It is written in simple and concise style and free from all technicalities, and abounds in valuable facts and directions to those who have health as well as to those who are called to nurse in the sick room. It is a book every mother could read with profit. Skilled and trained nurses are not a ways in reach, and the lives of those dear to the family are of ten imperiled by simple i-inorance of what every intelligent person should know. Good nursing, every intelligent physician will say, is of more value than drugs. The hand- some lit tie volume should find its wav into many homes, and by the intelligent observance of its rules, blessings alone will result. The News. 0^~May be ordered through your physician, or newsdealer, or will be sent to you direct on receipt of price. DUNCAN BROS., Publishers, 113 Madison and 131 & 133 Clark St., Chicago. HOW TO BE PLUMP. OR TALKS ON PHYSIOLOGICAL FEEDING. BY T. C. DUNCAN, M.D., Author of a Professional Treatise on Diseases of Infants and Children and their Homoeopathic Treat- ment; Editor " United States Medical Investigator, etc. etc" Finely bound in Cloth - - - Price, 5O Cents. It is an admirable work on hygiene, as far as ft relates to eating Ohio. Farmer. These " Talks " have their own charm, for most of us like to be plump. Women's Journal, Boston. Certainly all thin people should ponder well its suggestions and put them into practice. Voice of Masonry. A brochure which many persons of lean inclinations will be glad to read and may be able to profit by. Banner of Light. The little volume contains n.uch sensible professional advice upon healthy and morbid digestion, and the occasions of it. Zion's Herald. The book is intended to indicate how anyone may become fat, fair and jolly, and a careful perusal affords proof that the author is fully conversant with his subject, and that he has done it full justice. The Chicago Grocer. It is a common sense volume, that believes in rational and practic I methods of preserving health, beautv and happiness. It is not a receipt book of impossibilities; it is a bright, genial book, that understands itself from first to last. The Chicago Cosmopolitan. We do most warmly applaud its purpose, and especially commend its philosophy to theunnaturahy lean, and still mor ; to mothers in the training of delica e children whose physical stamina hardly equals that of a full- grown rye-stalk. Theirs is a leanness to be built up into steady strength and picturesque plumpness of limb and face. The Standard. CHICAGO, Sept 18, 1878 . The case referred toon page 45 of " How to be Plump" is our little boy, who was certainly rescued from death's door. This summer he began to run down again. After Dr. Dun an had tried several medicines without benefit he ; gain advised " inunction," and again with the same happy effect. The little fellow is now plump and well. CFIAHLKS WAT.ES. |3&~May e ordered through your physician, or newsdealer, or will be sent direct on receipt of price. DUNCAN BEOS., Publishers, 113 Madison and 131 & 133 Clark St., Chicago. " DOCTOR, WHAT SHALL 1 LAI t" A HAND BOOK OF DIET IN DISEASE. BY CHAS. GATCHELL, M. D., Formerly Professor of Practice, University of Michigan. It is highly endorsed by the Profession and Medical Press, How often do we hear that same question, Doctor: what shall I eat? and' though there are textbooks enough on the Pathology and Treatment of Diseases, there is hardly ever a word found in them about the Hygiene in Disease. Gatchell as a teacher, found put this want, and thus piobably originated this little treatise, for which we most heartily thank the author. We consider the recipes in it a little Materia Medica for itself, but follow instructions closely ; not quantity, but quality of food deserves our consideration. North American Journal of Homo&pathy. MILWAUKEE, Wis. "I consider your work on "Diet in Disease" to be the most practical, and therefore ihe most useful, work on the subject with which I am acquainted. No physician should be without it; every mother should have it. It is in use in many of the households in which I practice." C. C. OI^MSTEAD, M. D. President of ihe Wisconsin Homoepathic Medical Society. "This work is plain, practical and valuable, It is really a clinical guide on diet, and one the profession will find reliable and correct." United States Medical Investigator. "By far the best work on the subject in our literature. I find it of use every day. I want one in every household in which f practice." Oak Park, IU. F. G. FOLSOM, M.D. "Evidently much investigation, thought and carefulness have entered into the production of this work, and we believe it to be worthy a place in every household." The Magnet. * * * "We have carefully examined the work and s v all cheerfully recommend it for family use. The directions as to what food and drinks, and modes of preparation are very judicious." * * * Kesp. Yours, Janesville, Wis. DR. G. W. CHITTENDEN & SON Professor Charles Gatchell's Manual of 'DIET IN DISEASE' is the best book on the subject for the people. It contains in 160 pages an astonishing amount of condensed information on a subject of great importance, and one but little understood. Its style is admit able, pithy and to the point. The book has no padding about it and deserves an immense sale." SAMUEL POTTER. M. D., President, Milwankee Academy of Medicine, Aathor of Index of Comparative Therapeutics, etc. etc. |3F- Sent free on receipt of price, $l .OO. DUNCAN BROS., 131 & 133 S. Clark St. CHICAGO. THE FEEDING AND MANAGEMENT OF INFANTS AND CHILDREN, AND THE HOME TREATMENT OF THEIR DISEASES. By T. C. DUNCAN, H. D., PRESIDENT or THE AMERICAN P.EDOLOGICAL SOCIETY. CONSULTING PHYS- ICIAN TO THE CHICAGO FOUNDLINGS' HOME. CLINICAL, LECTURER ON DISEASES OF CHILDREN. AUTHOR OF "A PROFESSIONAL TREATISE ON THE DISEASES OF CHILDREN" (2 large volumes). EDITOR OF THE UNITED STATES MEDICAL INVESTIGATOR, 'ETC., ETC. MRS. J. E. JOICE writes : "I find it a very useful book and full of practical information, especially about diet for children." L. PAULY M. D. writes : "I am glad to know that we are going- to have a German edition of The Feeding and Management of Infants and Children. I know of no such book in German of a similar character and am confident that it will be welcomed by all thinking German mothers." MRS. T. WILCE, President of the Mother's Society of Chicago, says : "It is the most instructive book for mothers and < specially young mothers, that has come to my notice and I earnestly advise all prospective mothers to read it. Would to God that I had had such a book years ago. MRS. H. E.REDEKER of Kenosha, Wis., writes: "I think it is the bestwork of the kind I have seen. 1 am glad that I shall have Dr. Duncan at hand ready to consult at any moment for I have the greatest confidence iu all that he says. It is a book that it seems to me every mother would like to have." DR. EDWARD CRANCH, Secy, of the American Paedological Society (com- posed of the leading physicians interested in Children's Diseases), says : " I have carefully examined the work and unhesitatingly pronounce it a most necessary book for all families. It ought to go into every intelligent household where the health of the little ones is valued." 'Prevention is better than cure," and many a babe will fare better if the mother follows the advice given by 1 hat great friend of our bebies, T. C. Duncan. It is a work, whose first part we can fully endorse, and since I studied th^ se alkaline and acid babies, my endeavors are to have my little patients alkaline. The artificial feeding of children is a most interesting study even to an old physician and only by following closely the rules laid down, can we prevent the necessity of studying the secoud part. North American Journal of Homeopathy. The Organon of London, Eng., says: "Paedology is Dr. Duncan's Specia- Ute and we may be sure that what he writes on tiiis subject contains some- thing worth readin