MANIPULATION 
 
 OF THE 
 
 MICROSCOPE 
 
 BY 
 
 EDWARD BAUSCH. 
 
 LUSTRATED. 
 
 TENTH THOUSAND. 
 
 PUBI.ISHKD BY 
 
 BAUSCH & LOMB OPTICAL COMPANY, 
 ROCHESTER, N. Y. 
 
Copyright. 
 
 BAUSCH & LOMB OPTICAL COMPANY. 
 1891. 
 
PREFACE TO FIRST EDITION. 
 
 It may seem to some persons an act of presump- 
 tion for a maker of microscopes and microscopic 
 accessories to enter the field of authorship and attempt 
 to supplement the valuable labors which in recent 
 years have made the use of the microscope an indis- 
 pensible aid in the advancement of science. 
 
 To such, if any, I submit, that being a producer of 
 microscopes and their accessories, I have had oppor- 
 tunity to become acquainted with the lack of general 
 knowledge of the fund imental principles of the 
 instrument and the best method of technique, even 
 among owners of microscopes Indeed, with so many 
 complications, with almost unlimited powers and uses 
 of the instrument, the beginner cannot fail to feel the 
 need of a guide and adviser. 
 
 In order to accomplish the greatest good, I have 
 started out in this little Manual with the supposition 
 that the purchaser, or owner, is a beginner, and abso- 
 lutely ignorant of the microscope and everything 
 which pertains to it, and therefore have attempted to 
 convey, step by step, in as simple language as I could 
 command, information which will, I trust, lead to ease 
 of manipulation and give both pleasure and profit to 
 those for whom it was specially written. 
 
 With these, its purposes and hopes, I beg for my 
 self-imposed labor a friendly reception. 
 
 EDWARD BAUSCH. 
 
PREFACE TO SECOND EDITION. 
 
 The demand for this book having considerably 
 exceeded the expectations of its author, and the 
 comments on its utility having been so favorable 
 leads to the view that it fills a gap in microscopi- 
 cal literature. 
 
 In preparing for a new edition an opportunity has 
 been given for enlarging on some of the subjects and 
 rewriting others, so as to make them conform to the 
 changes which the last five years have brought about 
 in the construction of apparatus. 
 
 While it may be true that many of the subjects 
 might be treated much more extensively, the writer 
 has purposely refrained from doing so, because he 
 has considered it beyond the province of his inten- 
 tion and because books giving more extensive infor- 
 mation are available. 
 
 An intending purchaser of a microscope finds it 
 more or less difficult to make a suitable selection, 
 and while it is always best to consult an exper- 
 ienced microscopist, the writer has endeavored to 
 convey information which, he hopes, will aid in 
 this direction. 
 
 May, 1891. THE AUTHOR. 
 
CONTENTS. 
 
 SIMPLE MICROSCOPES. 
 
 Purpose of the Microscope ; Kinds of Microscopes ; Magni- 
 fying Power ; Using Magnifiers ; Aberrations ; Achroma- 
 tism, ..---._ PAGES 9-13. 
 
 THE COMPOUND MICROSCOPE. 
 
 Description of Parts ; Base or Foot ; Pillar ; Arm ; Body ; 
 Nose-Piece ; Society-Screw ; Objectives ; Eye-Piece or 
 Ocular ; Draw-Tube ; Collar ; Coarse Adjustment ; Milled 
 Heads ; Fine Adjustment ; Stage ; Clips ; Centering- 
 Screws ; Mirror ; Mirror-Bar ; Sub-Stage ; Sub-Stage Bar ; 
 Diaphragm ; Optical Axis ; Object ; Slide ; Cover-Glass ; 
 Classification of Microscopes ; Mechanical Parts ; Stage ; 
 Mirror and Mirror-Bar ; Diaphragm, PAGES 14-25. 
 
 OBJECTIVES AND EYE-PIECES. 
 
 Objectives Classes ; Systems ; Angular Aperture ; Achroma- 
 tism ; Resolving Power ; Flatness of Field ; Penetration ; 
 Working-Distance; Magnifying Power ; Eye-Piece Huy- 
 ghenian ; Solid Eye-Piece ; Periscopic Eye-Piece ; Nomen- 
 clature ; Flatness of Field ; Size of Field, PAGES 26-46. 
 
 REQUISITES FOR WORK. 
 
 Working Table ; Room ; Light; Position of Light ; Which Eye 
 to Use ; The Ward Eye Shade ; Order ; Material, 
 
 - PAGES 47-^51. 
 
 HOW TO WORK. 
 
 To Set Up the Instrument ; Centering Stage ; Illumination ; 
 Attaching High Power Objectives ; How to Work ; Dark 
 Ground Illumination; Cover-Glass; To Draw; Camera 
 Lucida How to Use It; Determining the Magnifying 
 Power, - PAGES 52-73. 
 
ADVANCED MANIPULATION. 
 
 Dry-Adjustable Objectives ; Immersion-Adjustable Objectives ; 
 Test-Plate; Immersion Objectives on Test-Plate; Photo- 
 Micrography, ------ PAGES 74-89. 
 
 TO SELECT A MICROSCOPE. 
 Stands ; Objectives and Eye-Pieces ; Accessories, 
 
 PAGES 90-101. 
 
 SUB-STAGE ILLUMINATION. 
 
 Mirror ; Abbe Condenser ; Centering ; To Focus Condenser ; 
 Intensity of Light, PAGES 102-106. 
 
 CARE OF A MICROSCOPE. 
 
 To Take Care of a Stand ; To Take Cafe of Objectives and 
 Eye-Pieces, PAGES 107-110. 
 
 APPENDIX. 
 Considerations in Testing Objectives, - PAGES 111-118. 
 
SIMPLE MICROSCOPES. 
 
 Purpose of the Microscope. The Microscope 
 is an instrument which magnifies objects, so that we 
 are better able to examine their structure than is pos- 
 sible with unassisted vision. 
 
 Kinds of Microscopes. Microscopes may be 
 divided into two classes simple and compound the 
 difference between the two being that with the former 
 the object is viewed directly, while with the latter a 
 magnified image is observed ; while the first shows 
 the objects in their true position, the latter shows 
 them reversed, so that what is right in the object is 
 left in the image, and when an object appears to be 
 moving in a certain direction, the movement is in 
 reality the reverse, and must be moved accordingly 
 to keep it in view. 
 
 Fig. i. 
 
 Magnifiers. Simple microscopes are usually 
 termed magnifiers, and, when consisting of one or 
 more lenses, always remain simple. The most com- 
 
10 
 
 mon are those with one or several double-convex 
 lenses (Fig.l). The shorter the radii (the more curved 
 the surfaces) are in these, the greater will be the mag- 
 nifying power, and the higher this is, the less of the 
 object's surface can be seen at once. Each addi- 
 tional lens increases the magnifying power in pro- 
 portion to its curvature. The distance between the 
 lens and the object, when this is seen most distinctly, 
 is called the focus ; at the point where the object is 
 most distinct, the lens is said to be in focus ; when 
 indistinct or blurred, out of focus. 
 
 Magnifying Power. Unless a microscope is 
 known to come from the hands of a reliable firm, any 
 claim as to magnifying power should be accepted with 
 reserve. In former years, when the country was over- 
 run with cheap foreign productions, the most fanciful 
 claims were made in this direction. It is evident that 
 a lens magnifies an object equally in all directions ; 
 this is said to be in areas, and is the square of the 
 linear, so that if an object is magnified 4 times in 
 the linear, it is 16 times in area. The commonly 
 accepted term to express magnifying power of sim- 
 ple, as well as compound microscopes, is in diameters 
 (linear}. A single lens of 1 inch focus magnifies 
 about ten diameters ; one of 2 inch focus, about 5 
 diameters ; one of \ inch focus, 20 diameters, and so 
 on. In a lens of high magnifying power, the focus is 
 ordinarily about twice the diameter, so that if a lens 
 is \ inch diameter its focus is about 1 inch. This 
 may, however, be more accurately determined by pro- 
 
11 
 
 jecting, say a flame or window frame, upon a white 
 piece of paper ; the distance between the paper to the 
 center of the lens, when the image is most distinct, is 
 its focal distance. When a lens is two inches or more 
 in diameter, it is usually .termed a reading glass. 
 
 Using Magnifiers. In using magnifiers the lens 
 should be held close to the eye and such a position 
 taken that the object will receive the best illumina- 
 tion. In the lenses of equally convex surfaces, it is 
 immaterial which side is held toward the eye ; but 
 when plano-convex lenses are used, the plane side 
 should always be toward the eye, as it gives the flat- 
 test field. 
 
 Aberrations. Two factors arise which prevent 
 the advantageous use of more than about 25 diame- 
 ters in magnifiers ; they are called the chromatic and 
 spherical aberrations. The first is the term employed 
 when the object is apparently fringed with color, pre- 
 dominently blue and yellow ; the second, when all but 
 the central portion of the lens shows the object indis- 
 tinctly ; these 
 faults increase 
 with the magni- 
 fying power. In 
 the case of a 
 combination of 
 Fig- 2 - several lenses, Fi s- 3- 
 
 they may partially be overcome by interposing an 
 opaque plate with a small opening, called a diaphragm, 
 between them, which cuts off the outer or marginal 
 
12 
 
 rays, or the lenses may be made of a smaller diam- 
 eter. An incision may also be cut into the glass 
 equally between the two surfaces, when from the name 
 of the inventor, it is called a Coddington. Fig. 2 shows 
 a section of a Coddington, while Fig. 3 shows it in 
 its mounting. 
 
 Achromatism. The most approved method, 
 however, for eliminating these appearances, is by the 
 use of one or two con cave flint glass lenses in connec- 
 tion with the double convex crown glass lens. When 
 the color or chromatic aberration is thus removed, the 
 lenses are said to be achromatic, and when both the 
 chromatic and spherical aberrations are avoided, the 
 lens is called aplanatic, and is then said to be cor- 
 rected. An achromatic lens, composed of one flint 
 and one crown glass lens, is called a 
 doublet ( Fig. 4 ) ; one with two flint 
 glass lenses and one crown glass is 
 Fig. 4. called a triplet (Fig. 5). The latter is 
 the best form, as it gives the highest cor- 
 rection ; such a lens (it is thus called from 
 the the fact that the lenses are cemented 
 together and act like one) may be held 
 with either side toward the object with 
 equally good results, and may also be held Fig. 5. 
 at quite an obliquity, without loss of definition ; this 
 feature is important, as it is almost impossible to give 
 a lens a theoretically correct position to both the eye 
 and object with the unaided hand. 
 
Fig. 6. 
 
THE COMPOUND MICROSCOPE. 
 
 As was previously stated a magnified image is 
 observed in the Compound Microscope. Any two 
 lenses, one of short, the other of long focus, placed 
 sufficiently far apart, will attain this object, and this 
 was for years the method of its construction. 
 
 On any microscope, whether simple or compound, 
 the difficulty of holding it or the object steady during 
 observation increases with the increase in magnifying 
 power, and in the compound form with only a mod- 
 erately high power, it is utterly impossible to retain 
 sufficient steadiness to make any reliable observation. 
 Mechanical contrivances thesefore became a neces- 
 sity, and were applied in the earliest constructions of 
 the microscope. They were all made to fulfill the fol- 
 lowing three conditions : A platform for holding the 
 object ; a means of adjustment for properly focusing 
 on the object, and a mode of suitably illuminating the 
 same. 
 
 From what may now be called a crude attainment of 
 these three purposes, the construction gradually 
 became more complex. Many additions have been 
 made which have proven useful and have remained, 
 while others have been discarded, and these have led 
 to the present construction of the microscope. 
 
15 
 
 While certain parts are necessary to make up a 
 modern instrument, no one design of construction is 
 followed. The forms are innumerable, each maker 
 following his own inclination in variety, design, num- 
 ber of parts and material. For the latter brass pre- 
 dominates, although bronze and iron are used to a 
 considerable extent. The first two metals are usually 
 highly finished, and as they easily tarnish in this 
 state, are protected by lacquer, which is not only ser- 
 viceable in this direction when well done, but offers 
 a means of ornamentation. The latter metal is cov- 
 ered with a heavy coating of japan and being an 
 intense black is on this account often recommended 
 by instructors as being agreeable for the eyes. The 
 entire apparatus is called a microscope, whereas, with- 
 out the optical parts, it is termed a stand. 
 
 Description of Parts. As it is necessary for 
 the student to become conversant with the terms of 
 the various parts and to understand their use, we give 
 an illustration (Fig. 6) with letters, and append a list 
 giving the names. 
 
 A. Base on Foot. This is the foundation of 
 the instrument. It usually rests upon three points (or 
 should do so) and is of such a weight that it keeps 
 the instrument firm when it is in an upright or inclined 
 position. The revolving plate, when this is provided, 
 by means of which the upper portion of the instru- 
 ment is revolved, without changing the position of 
 the base, is considered a part of it. 
 
16 . 
 
 B. Pillar. It is that portion which is fastened to 
 the base and may be one or two, according to the 
 construction of the stand. It carries upon its upper 
 end the joint or axis. 
 
 C. Arm. This is connected with the pillar by the 
 joint and supports all theworhing parts of the instru- 
 ment. 
 
 D. Body. This is the tube-portion to which the 
 optical parts are attached. 
 
 E. Nose-Piece. This is an extra piece which is 
 attached to the lower part of the tube. 
 
 Society Screw. This is a standard screw which 
 is cut into the nose-piece, and is called so from the 
 fact that it was first established by the Royal Micro- 
 scopical Society of London. It is also called the /////- 
 versa! screw, and is in general use in this country and 
 England ; it has lately been adopted by some firms on 
 the Continent of Europe. 
 
 F. Objectives. This is screwed into the nose- 
 piece and is called so because it is nearest the object. 
 It is the most important of the two optical parts (of 
 the microscope proper) and upon its perfection the 
 distinctness of the image and therefore the value of 
 the instrument almost entirely depends. 
 
 G. Eye-Piece or Ocular. It is called so because 
 it is nearest the eye and is the remaining optical part. 
 It magnifies the image given by the objective. This 
 and objective will be treated more fully later on. 
 
17 
 
 H. Draw-tube. This is that portion of the body 
 which moves in the outer sheath and which receives 
 the eye-piece. It is provided for the purpose of attain- 
 ing different lengths, variations in magnifying power 
 and as a matter of convenience while working. 
 
 I. Collar. This is a ring which is attached to the 
 draw-tube and is usually provided with a milled edge. 
 
 J. Coarse Adjustment. This is a provision 
 
 for moving the body quickly back and forth for 
 adjusting the focus approximately. It is done by 
 a sliding rack and stationary pinion (not shown in 
 cut ) or a sliding body in an outer sheath. 
 
 K. Milled Heads. These are attached to the 
 shank of the pinion, which is revolved by means of 
 them and are usually large to give sensitiveness to 
 the movement. 
 
 L. Fine Adjustment. This is slow moving and 
 serves to get an exact focus. It is attained by a fine 
 thread, provided with a milled head, and acts upon 
 the body, either directly or by levers. This as well 
 as the coarse adjustment should be extremely sensi- 
 tive and should not have the least side or lateral 
 motion. The fact that either of them have it, is 
 evidence of poor workmanship. 
 
 M. Stage. This is the portion on which the 
 object is placed for examination and is attached to 
 the arm. 
 
 N. Clips. These are two springs which are 
 attached to the upper surface of the stage and 
 serve to hold down the object. 
 
18 
 
 Centering Screws. These are provided for 
 moving the stage in different directions to bring 
 the center of its revolving motion in the center of 
 the field. 
 
 O. Mirror. This is used for reflecting and 
 condensing light upon the object. As a rule two 
 mirrors are used, one plane and the other concave. 
 The first gives a comparatively weak light, while the 
 second concentrates it and gives it more intensity. 
 
 P. Mirror-bar. This carries the mirror and by 
 a sliding arrangement allows the variations in dis- 
 tance of the mirror to the stage_; it also swings in 
 a circle around the object in order to illuminate it 
 from any direction. 
 
 Q. Sub-Stage. This is a ring below the stage 
 to receive various accessories which may be required. 
 It is sometimes fixed to the stage but in the best 
 instruments it is separated from it and is provided 
 with an adjustment to vary its distance from the 
 object. 
 
 R. Sub-Stage bar. This receives the sub-stage 
 and permits its adjustment. In modern American 
 instruments this, as well as the mirror-bar, is on an 
 axis in the plane of the stage, so that whatever posi- 
 tion they may be in, relative to the object, the dis- 
 tance from this to the sub-stage or mirror does not 
 vary, except when made to do so. 
 
 S. Diaphragm. This is a perforated, revolving 
 disk, attached either to the stage or sub-stage. It 
 
19 
 
 has holes of different sizes so that 
 light from the mirror may be modified. 
 
 Optical Axis. This is an imaginary line which 
 passes from the center of the eye-piece through the 
 body, objective, stage and sub-stage to the mirror. 
 Whatever lies in it is said to be centered. 
 
 Object. That which is examined and placed 
 upon a slide. 
 
 Slide. This is a thin plate of glass, generally 
 3 inches long by 1 inch wide. 
 
 Cover Glass. This is an extremely thin piece 
 of glass, round or square, which is placed upon the 
 object, either for flattening or preserving it, or both. 
 
 Classification of Microscopes. Up to within 
 recent years microscopes have been divided into two 
 classes : the Jackson and Ross models. While the 
 latter was for many years very popular, particularly 
 with the English makers, it has been almost entirely 
 superseded by the Jackson form, and with good 
 reason. In the former the means of adjusting were 
 provided, as near as consistent with the construction, 
 to the body or tubes, whereas in the Ross they are 
 placed at the back or more distant point in the 
 instrument, thus increasing by means of the con- 
 necting arm the faults which might exist in the 
 adjustment. 
 
 A certain form of instrument which at the present 
 is very popular and called the Continental pattern, 
 from the fact that it was made originally by the 
 
20 
 
 manufacturers on the continent of Europe, is a com- 
 bination of both the Jackson and Ross models. 
 Whereas, the coarse adjustment when consisting of 
 a rack and pinion is placed closely to the tubes, the 
 fine adjustment is placed on the arm and although 
 being dissimilar from the original Ross in being 
 higher, it nevertheless has the disadvantage of mag- 
 nifying any lost motion in the adjustment by means 
 of the connecting arm. Considering the fact that 
 the Ross form alone is almost obsolete and many 
 instruments of the present day are a combination of 
 both forms, it appears to the writer that their desig- 
 nations have lost their value. 
 
 There is another direction, however, in which 
 microscopes are divided into two classes, which is 
 of far more import ance, and affects their utility in 
 a much higher degree. The writer does not know 
 that instruments have been so classified by others, 
 and knows that the subject has been given no 
 important significance. 
 
 In the Continental form just mentioned, a short 
 tube from 160.0 to 170.0 mm. (0.3 to 6.7 inch) is 
 used, whereas in the English form, and this is largely 
 followed in America, the length is from 8| to 10 in. 
 (216.0 to 250.0 mm.). The short tube of the Euro- 
 pean makers offers no optical advantages, but is 
 mainly used to contract the height of the instrument 
 to as great an extent as possible, as this is the vital 
 point throughout its construction. 
 
21 
 
 At the last meeting of the American Society of 
 Microscopists a committee was appointed to consider 
 the tube lengths as well as other subjects to be men- 
 tioned hereafter and reported in favor of the adoption 
 of two standards for tube lengths, 160.0 mm., or 6.3 
 inch for the short one, and 8| inch, or 216.0 mm., 
 for the long one. The American makers have 
 adopted these two lengths and we believe are gen- 
 erally following them. Practically, there are no 
 advantages in one or the other, except, perhaps, in 
 so far as the short tube might be considered advan- 
 tageous, but optically this recommendation of the 
 committee is far reaching, because an objective, par- 
 ticularly in giving considerable magnification, when 
 constructed to be used with a certain tube-length, 
 should be used with it only. When used with the 
 other standard it will fail to give satisfactory results. 
 This subject, with the optical results, will be touched 
 upon again later on. 
 
 Mechanical Parts As there will be but little 
 occasion to recur to the mechanical portions of the 
 microscope, we take the occasion to speak of them 
 in a more extended way in this connection. 
 
 Stage. This may be divided into three classes, 
 ordinary, semi-mechanical, and mechanical. 
 
 The first consists of a plain plate, either round or 
 square, on which the slide is held by means of the 
 clips. As the microscope is used principally in an 
 inclined position, the pressure exerted by the clips 
 
22 
 
 must be sufficient to retain the slide firmly in posi- 
 tion. This causes a certain amount of friction 
 between the lower surface of the slide and the upper 
 one of the stage, which at times is very annoying, in 
 that it is difficult to move the object to a point 
 desired, with certainty. The motion is "jerky" and 
 disagreeably harsh unless the stage is very clean. 
 
 A large number of plans have been devised to 
 overcome this difficulty, which have proven very 
 efficient. They may be termed semi-mechanical, in 
 that the fingers do not come in direct contact with 
 the slide. Usually glass is brought in contact with 
 the metal, but so arranged as to offer as few and 
 small points of contact as possible. 
 
 Fig. 7. 
 
 In Fig. 7 we show a so-called glass-stage and slide- 
 carrier, in which the stage is merely for the support 
 of the slide-carrier. This latter is arranged to 
 receive the slide and rests upon the glass by small 
 points. At its ends are two projecting tongues 
 which are bent downward and inward and act as 
 springs against the lower surface of the glass plate. 
 While the movements must be carried out by the 
 hands, they are smooth and steady and work with 
 ease, even under high powers. 
 
23 
 
 The mechanical stage is in every sense mechanical. 
 The hands do not come in contact with the slide 
 except to place it on the stage. Two movements 
 are usually provided at right angles to one another, 
 either both with rack and pinion adjustment, or one 
 with rack and pinion while the other has a screw 
 motion. Examinations can be carried out with ease 
 and reliability under the highest powers. The main 
 value of the mechanical stage is that systematic 
 examinations over the entire surface of an object 
 can be carried on. 
 
 Mirror and Mirror-bar. The proper illumi- 
 nation of an object is an important feature, and 
 although there are numerous accessories for properly 
 accomplishing this, which will be spoken of later on, 
 the mirrors alone are effective agents when properly 
 constructed and applied, particularly when no high 
 magnification is used. The plane mirror is usually 
 used on very low powers, and reflects light in about 
 the same intensity as its source. The concave mirror, 
 however, is intended to concentrate the light so that 
 all the rays which strike its surface are reflected 
 toward its optical axis and come together at some 
 point above it, and the rays from the surface being 
 contained within a comparatively small space, cause 
 an increased intensity, This point is called the focal 
 point, and is usually arranged to coincide with the 
 opening of the stage when parallel rays, such as 
 from the sky, are used. When the source of light 
 comes considerably nearer to the mirror, as for 
 
24 
 
 instance from a lamp and the rays are diverging, the 
 focal distance becomes considerably longer, and when 
 very close may be twice as long. Some of the intensity 
 is lost in consequence, as well as the degree of con- 
 vergence. For this reason mirror-bars are so arranged 
 that the distance of the mirror from the stage may be 
 varied to accommodate the variation in the source of 
 light. While this is of considerable aid, there is not 
 sufficient room for a complete accommodation, with 
 the result that, under certain conditions, the utmost 
 effectiveness of the microscope is not obtained. 
 
 Diaphragm. This is provided for regulating the 
 amount of light. While the mirror should work to 
 its utmost capacity, it very often occurs that for cer- 
 tain investigations a profuseness of light is more 
 harmful than otherwise. When too much light exists, 
 objects are said to be drowned in it, and often makes 
 it impossible to determine structures. An intelligent 
 use of the diaphragm is of great service. 
 
 Besides the revolving diaphragm there are a num- 
 ber of other forms which may be said to be better 
 for instance, the so-called cup diaphragms, which 
 require a separate piece for each aperture and which 
 are held by a special sub-stage receiver. Then the 
 dome diaphragm, which is a new application of the 
 ordinary revolving diaphragm. It consists of a sub- 
 stage fitting having a dome to which is fitted a 
 curved revolving diaphragm 
 
 The ideal regulator of light is the Iris diaphragm 
 consisting of a series of overlapping blades placed 
 
25 
 
 around a central opening, the size of which may be 
 varied by means of a lever or milled edge. In the 
 ordinary revolving form the aperture is of necessity 
 at some distance from the object, and does not fully 
 control the light on account of the stray rays, which 
 the other three forms accomplish. 
 
OBJECTIVES AND EYE-PIECES. 
 
 Although considerable magnifying power may be 
 attained by the use of two single lenses arranged in 
 a compound form, there is no advantage in it, from 
 the fact that the faults in the lenses are correspond- 
 ingly magnified, and these are so considerable that 
 they destroy what it is the purpose of the microscope 
 to give a distinct image. 
 
 Objectives, Classes. Objectives may be divided 
 into two classes, dry and immersion ; in the former no 
 intervening medium except air exists between the 
 cover and objective, while in the latter a fluid is used 
 to connect the upper surface of the cover to the front 
 surface of the objective. The use of immersion fluid 
 has several advantages, the first of which is that the 
 objective may be made to give better performance, as 
 will be explained later on ; the second is that more 
 light will be transmitted, as there is less loss of it by 
 refraction. 
 
 It should be understood however that no advantage 
 will be gained by using immersion fluid with a dry 
 objective. It does not increase its effectiveness one 
 particle, on the contrary it detracts from its quality. 
 When it is stated that an immersion objective has a 
 
2V 
 
 greater capacity, it is with the understanding that it 
 is so constructed as to give this result. 
 
 While many immersion objectives are constructed 
 to work both as dry and immersion, such a plan can- 
 not be said to be advantageous. Such objectives may 
 be made to work well in one direction and be of 
 indifferent quality in the other, or may be of medium 
 grade both ways. There is no question that the best 
 plan .is to have each objective selected with a view to 
 a specific purpose and use it for this purpose only. 
 There are two fluids in general use at the present 
 time, water and homogeneous fluid. The latter expres- 
 sion means of the same kind, and refers to the fact 
 that the fluid has about the same refractive and dispers- 
 ive power as glass, so that when this fluid fills up the 
 space between the two surfaces of glass, a ray of light 
 passes through the three mediums as if they were one 
 body. 
 
 The two large classes of dry and immersion objec- 
 tives may again be subdivided into two classes objec- 
 tives for long and short standard tube. As followed 
 by some firms at present and what it is hoped will 
 become a universal custom in time, each objective is 
 marked for the tube-length for which it is corrected 
 and with which it is assumed it will accomplish the 
 best results. 
 
 Objectives are sometimes called/<?7t'^;-.r, and in this 
 sense are divided into three classes : low, medium and 
 high. Dr. Carpenter classifies them as follows : low 
 powers, 3 inch, 2 inch \\ inch, 1 inch, f or J inch ; 
 
28 
 
 medium powers, T 4 ^ inch, \ inch, J- inch, ^ inch ; 
 high powers, \ inch, \ inch, -^ inch, r 1 ^ inch, ^ inch, 
 2 1 inch, ^g- inch. 
 
 As the objective is the most important of the two 
 optical parts, it follows that this must be as free from 
 faults as possible and all that human inge- 
 nuity and skill can devise is utilized to attain this 
 end. The advance in the perfection of the objective 
 has been step by step and each era was at the time 
 considered by many authorities the limit to further 
 improvement. Each advance was signalized by a 
 marked opposition and disbelief of its possibility. It 
 is therefore of inestimable credit- to the pioneer 
 objective-makers, and notably among these two Ameri- 
 cans, who by quiet but stubborn application disproved 
 previous claims and opened the way to further improve- 
 ments. A theoretical limit has been fixed on the 
 capacity of the microscope, which according to our 
 present knowledge can not even be reached. 
 
 While the introduction of water immersion made it 
 possible to obtain higher optical results than with the 
 dry objectives, as will be explained later on, the homo- 
 geneous immersion offers still greater possibilities in 
 this direction, and the advantages are so pronounced 
 that the former are gradually coming into disuse, 
 although for certain kinds of work they will be pre- 
 ferred and used by many persons. At present homo- 
 geneous fluid is made of either thickened glycerine 
 or cedar oil, and great care is required in keeping 
 
29 
 
 the front of objectives and cover glasses properly 
 cleaned, in which respect water has the decided 
 advantage. 
 
 It might be stated that such high power as ^th 
 and 2 V tn are vei 7 rare ly constructed at the present day, 
 and the^th may be considered the maximum, while 
 the T Vth is that most ordinarily used. This power will 
 give all the optical advantages, while higher powers 
 involve so many mechanical difficulties as to increase 
 the cost of production very considerably, and as a 
 rule detract from the optical qualities. 
 
 A modern objective of the highest capacity may be 
 considered a work of art, and there are a few pro- 
 ductions of the human hand which exact so much 
 untiring application, ingenuity and skill. 
 
 Systems. An objective is said to consist of sys- 
 tems which may vary in number from one to four 
 and five ; two and three are however mainly in use. 
 They are the individual portions consisting of one, 
 two or three lenses, which when more than one, are 
 cemented together and make up 
 the objective. An achromatic sin- 
 P gle system may consist of two or 
 three lenses, and a three or four 
 system objective may consist of 
 as many as seven or eight lenses. 
 The systems are called in their 
 A order : anterior or front, middle 
 and posterior. When one consists 
 of two lenses it is called a doublet, 
 when of three lenses a triplet. Thus in Fig. 8 ? A is 
 
the anterior, M the middle and P the posterior 
 systems ; thus also A is a single system, M a double 
 and P a triple one. 
 
 The various features which must be considered as 
 determining the quality of an objective are : angular 
 aperture, achromatism, resolving power, flatness of 
 field, penetration, working distance and magnifying 
 power. Although these attributes may be considered 
 separately, some of them go hand in hand. The 
 presence or extent of one necessarily involves or pre- 
 cludes another. 
 
 Angular Aperture. The angle which the most 
 extreme rays, which are transmitted through the 
 objective, make at the point of focus, is called its 
 angular aperture, or in short its angle, and of all the 
 qualities in an ideal objective, 
 this is the most important. Thus 
 p in Fig. 9, I) is considered the 
 point of focus, and C D E the 
 angular aperture. The above 
 definition has its limitations, 
 however. While in objectives 
 of proper construction it holds 
 true, there are many in which it 
 is not the case. For instance, 
 an objective may be so con- 
 structed that it may transmit a 
 considerable number of rays in 
 
81 
 
 excess of those which combine to form an image, and 
 it is evident that these should not be considered as 
 belonging to them. 
 
 As there are many objectives of the same power, 
 but of different angular aperture, there are again oth- 
 ers of varying power, but of the same angle. Other 
 things being equal, it is the angular aperture of an 
 objective which determines the quality. It is expressed 
 in degrees, and is also spoken of as being wide, med- 
 ium or narrow, although this is indefinite and 
 depends considerably upon the power of the objec- 
 tive ; while the angle may be excessively wide for a 
 low power, it may be narrow for a higher one. 
 
 For many years the extent to which angular aper- 
 ture could be carried was a matter of controversy, as 
 was also the use of objectives of wide and narrow 
 angles for different directions of work. It is, how- 
 ever, a matter of congratulation that the question is 
 at rest, although it has served a good purpose in pro- 
 mulgating a better knowledge of the subject. 
 
 All objects emit rays, and it is evident that those 
 coming from one point and contained in a large angle 
 are more numerous than those in a small angle ; also 
 that as the angle more nearly approaches 180 degrees 
 the rays will be larger in number. 
 
 It is assumed that two objects, B and B', are 
 equally bright, and therefore emit the same number 
 
of rays ; for the purpose in hand, it is sufficient to 
 consider only those which reach the plane surface of 
 the large lens ; if an equal space is imagined over B', 
 the same number of rays will be contained in this ; 
 
 C A 
 
 Fig. 10. 
 
 therefore, the cone contained in the angle A' B' C' 
 will contain as many rays as that contained in A B C; 
 but as the lens I)' E' is considerably smaller than A 
 C, only as many rays can enter it as are contained in 
 the angle I)' B' E'. As the rays contained in the 
 angle ABC and D' B' E' are carried through the 
 two lenses, which are supposed to be of the same 
 magnifying power, the image -formed by A C will be 
 considerably brighter than that formed by D' E', and 
 will therefore show more of its structure, as will be 
 shown hereafter. 
 
33 
 
 While this is not a theoretically correct explanation 
 of angular aperture, it will serve the purpose of show- 
 ing its effect. As increase in aperture means increase 
 of resolving power and it will be seen that every degree 
 thus added increases the effectiveness of the objec- 
 tive. It is in this direction that modern advance has 
 been signalized. Whatever views rrray be held on the 
 advisability of narrow or wide angle objectives, the 
 optical standard of excellence depends particularly 
 on this quality. 
 
 Objectives of the same angular aperture, but of dif- 
 ferent magnifying power (within ordinary limits), will 
 show the object equally well, provided they are other- 
 wise of the same quality, and it is also true that in 
 objectives of the same power but unequal angular 
 aperture, the one of wider angle will show an object 
 more brilliantly than the other, and, if the difference 
 be considerable, will show structure of which no trace 
 can be found with the narrow angle. These are facts 
 which are based upon natural laws, but there are 
 other conditions to be considered in connection with 
 them, which will be treated hereafter. 
 
 It very often happens that objectives from different 
 makers, but of the same angle, show a considerable 
 variation ; this does not prove that the above princi- 
 ple is wrong, but is evidence that greater care or skill 
 has been bestowed on one than on the other. 
 
 By arranging an objective with an immersion front, 
 its angular aperture may be considerably increased 
 over that of a dry front, and this explains why better 
 
34 
 
 results may be obtained with the former than with the 
 latter. 
 
 Prof. Abbe has introduced a new mode of deter- 
 mining 'and naming aperture. He calls it Numerical 
 Aperture, and this expression is now generally 
 adopted. Thus, 1.0 of numerical aperture is equal 
 to 180 degrees in air or about 82 degrees in homo- 
 geneous immersion, and has a direct relative value to 
 the resolving power of the objective. A complete 
 aperture table may be found in the Proceedings of 
 the Royal Microscopical Journal and larger works on 
 the microscope. 
 
 Achromatism. As has been stated before, when 
 single lenses are made to give a high magnifying 
 power, the chromatic and spherical aberrations pre- 
 vent corresponding advantages ; and as the objective 
 gives the image which is magnified by the eye-piece, 
 it is evident that if they exist in it, they are increased 
 by the ocular, and that especial care must be given to 
 exclude all faults as much as possible from it. Even 
 with the use of flint glass, it is impossible to free the 
 objective entirely from color ; there will remain a 
 residue of green and purple, and these colors will 
 fringe the object. These are called the secondary spec- 
 trum, and their presence in an objective is usually 
 evidence of the highest correction. 
 
 The amount of color in an objective depends some- 
 what upon the power of the eye-piece, and becomes 
 more visible as a higher power is used. -Color out- 
 side of the secondary spectrum is not always preju- 
 
35 
 
 dicial to an objective ; for, if in two, one shows the 
 structure of an object with a slight amount of it, the 
 other does not show the structure but gives a nearly 
 colorless image, it goes without argument to say that 
 the first gives the best results and is therefore pre- 
 ferable. 
 
 Within recent years new kinds of glass have been 
 found by means of which the amount of remaining 
 color has been reduced, and objectives constructed 
 with it are called apochromatic. 
 
 If on increasing the distance between the objec- 
 tive and object the latter shows a marked blue color, 
 and when the distance is decreased a yellow-red 
 color, the objective is chromatically under-corrected j 
 if, however, the conditions are reversed, if the object 
 shows a yellow color when the distance between it 
 and the objective is increased, and blue when 
 decreased, it is over-corrected. 
 
 Resolving Power. This is the quality in an 
 objective by which we are enabled to see the intricate 
 structure and finer details in an object. It depends 
 upon the amount of angular aperture, the correction 
 of the chromatic and spherical aberrations, and of 
 course upon the perfection of the mechanical work. 
 The power of resolving in an objective is indicative 
 of the perfection of the microscope, for it is almost 
 entirely dependent upon it for its quality. 
 
 When an objective is said to resolve a structure or 
 a certain number of lines, it means that it shows 
 them under certain conditions of light. It may 
 
36 
 
 resolve easily or only glimpse them the latter when 
 they are hardly to be distinguished. The angular 
 aperture of an objective indicates the resolving 
 power, and the theoretical capacity of every degree 
 has been mathematically determined. However, this 
 standard is only reached approximately and to a vary- 
 ing extent. It is not by any means said that every 
 objective of a certain angular aperture will have a cor- 
 responding resolving power ; it is at this point that 
 the acute accuracy of work and superior judgment of 
 the optician in making proper corrections will invari- 
 ably give the best results. 
 
 Minute structure such as bacter-ia can only be seen 
 by objectives having high numerical aperture, and 
 these are absolutely necessary in modern investiga- 
 tions. The many recent discoveries can only be 
 attributed to the increased resolving power. 
 
 It is an error to suppose that the resolving power 
 may be improved by merely increasing the magnify- 
 ing power. It is an invariable quality of an objective 
 and has a fixed limit. The extent to which it may be 
 approached depends upon the nicety of manipulation, 
 but no amount of increase in magnifying power by 
 the eye-piece or any other means will carry it beyond 
 it ; on the contrary, it will lose in this respect if car- 
 ried beyond a certain point. 
 
 Flatness of Field. Theyfc/;/in a microscope is 
 that portion which is observed in the eye-piece, and 
 its flatness may be observed when focused on a flat 
 object preferably a micrometer. It is said to be flat 
 
37 
 
 when all portions of the object are seen over the 
 entire field at once without further focusing. When 
 not flat, it will be found that as 
 the image approaches the edge 
 of the field it becomes more and 
 more indistinct, and that the 
 objective must be correspond- 
 ingly adjusted ; in many cases 
 it remains indistinct or blurred, 
 and this may be considered the 
 Fig. ii. most serious fault. In the case 
 
 of looking at straight parallel lines, such as in a 
 micrometer, they will appear to become more curved 
 as they near the edge, as shown in Fig. 11. 
 
 Flatness of field mainly depends upon the correc- 
 tion of the spherical aberrations, and as under the 
 best conditions the latter cannot be entirely eliminated, 
 it is impossible to attain absolute flatness, except with 
 eye-pieces especially made for this purpose. It may, 
 however, also be due to a faulty eye-piece ; in this 
 case it can fairly be determined, by observing 
 whether it shows equally in different objectives. With 
 beginners, especially, it is usually most complained 
 of, owing probably to the fact that it is the most easily 
 noticable. It is a desirable quality and indicates to a 
 considerable degree the quality of objectives. While 
 it is impossible to obtain absolute flatness, the opti- 
 cian's effort is to obtain the nearest approach to it. As 
 a quality in itself, without regard to resolving power, it 
 is most easily obtainable, but in connection with this 
 quality it becomes difficult to acquire. 
 
38 
 
 Penetration. This is the quality which enables 
 us to look into an object to observe different planes 
 at one time. In the mind of the writer, it is of no 
 special importance, or at any rate not as much as is 
 claimed for it, and if desired is easily attained. It 
 depends upon magnifying power and angular aper- 
 ture, and decreases with the increase of either of 
 these. Objectives are generally not constructed with 
 any reference to it ; it is a natural consequence of 
 certain conditions. 
 
 Penetration and resolving power are antagonistic, 
 or at any rate in an inverse ratio, and can only be 
 combined to a certain extent. In two objectives 
 of the same power and aperture, one cannot have 
 penetration as a special feature and the other resolv- 
 ing power ; they will be almost similar in these quali- 
 ties, provided that they are similarly corrected. How- 
 ever, if they are not similar in their angular aperture 
 the one of small aperture will have more penetration 
 than the other. In objectives of the same angle but 
 different power, the one of low power will have in 
 itself more penetration ; it will be similar in its action 
 to the eye, which, when an object is close to it, can 
 distinguish but one portion of it distinctly, while, as 
 its distance to the eye is increased, can distinguish 
 various parts of it lying at different distances, and 
 will finally see other objects outside of it. By look- 
 ing at an object at 5 feet distance, only this can be 
 seen plainly; but, at 10 feet, others quite a distance 
 in front or back of it can be seen as well. 
 
39 
 
 Working Distance. This term, strictly con- 
 sidered, is an invariable quality of the objective, and 
 is the distance between the front lens in the objective 
 and an uncovered object, when the objective is in 
 focus and is corrected for that object. All objectives 
 require a certain amount of projecting metal to pro- 
 tect the front lens, and this with a certain thickness 
 of the cover-glass lessens it. In objectives with fixed 
 mountings this may be, and with thick cover-glasses 
 is considerable. As it is comparatively unimportant, 
 however, for the working microscopist to know the 
 working distance per se of his objectives, but of con- 
 siderable moment to know what the actual space 
 between the objective and cover glass is, it would be 
 well, in the mind of the writer, to express it as avail- 
 able working distance. 
 
 In objectives of low and medium power, it is of 
 little consideration ; but where it must be expressed 
 in -j-^. or TTr V IT inch, it becomes a matter of importance. 
 
 Working distance is spoken of as being long or 
 short, and varies not so much with the power as with 
 the angular aperture ; generally the working distance 
 decreases with the increase in angular aperture, and 
 becomes greater as the aperture becomes smaller ; it 
 it was for a long time considered that these two prop- 
 erties varied according to a fixed rule, but this at the 
 present time is not considered to be the case. While 
 in objectives of the same aperture it may vary con- 
 siderably, it may in others of different aperture be so 
 that the higher one may have the greater working 
 
40 
 
 distance. The skill of the optician must in a consid- 
 erable manner determine the amount of it. 
 
 It will be seen from the above that working dis. 
 tance stands in no direct relation to the focal distance 
 of the objective, neither to its nomenclature or rat- 
 ing, and, it may be added, that it is never as great as 
 the focal distance of a single lens of the same magni- 
 fying power. 
 
 As may be imagined, there are a variety of opin- 
 ions as to what constitutes long or short working dis- 
 tance in a certain objective. No definite rule can be 
 laid down for this, as it is conditioned, by the skill 
 and requirements of the manipulator. Although it is 
 an important factor, the idea that it should in all 
 cases be as great as possible, is erroneous, for, while 
 it may be true in a dry objective, it may be the cause 
 of annoyance in one with immersion. On several 
 occasions it occurred in the experience of the writer 
 that after an objective had been completed, it was 
 found that its working distance was so large that the 
 immersion fluid would run out from between the 
 objective and the cover-glass when the instrument 
 was inclined, and it was necessary to change the 
 objective with a view to decreasing its working dis- 
 tance, in order to allow its convenient use. 
 
 Magnifying Power. This is a question, of vital 
 importance in a microscope, not so much as a quality 
 for itself, as in connection with the resolving power. 
 The inquiry should not be simply how many diame- 
 ters an instrument will magnify, but what the precision 
 
41 
 
 and extent of its definitions are under a certain mag- 
 nifying power. If a high magnifying power is all that 
 is desired, this may be obtained to an almost unlim- 
 ited extent by means of simple lenses which may be 
 procured at a small pecuniary outlay ; but these do 
 not give a distinct image nor do they make structure 
 visible, which, be it remembered, is the purpose of 
 the microscope to do. 
 
 The normal eye can distinguish from 200 to 250 
 lines to the inch, and in a microscope such magnify- 
 ing power should be used, which will apparently bring 
 the structure which is sought after at least up to this 
 figure. In illustration take a \ inch objective of 98 
 degrees and a \\ inch eye-piece. An objective of this 
 kind properly corrected, resolves pleurosigma angiila- 
 tum, in which the average lines are 00,000 to the 
 inch. With the above eye-piece it is utterly impossi- 
 ble to see them, while if it is replaced by a | inch or 
 | inch, they can easily be distinguished. This is not 
 owing to any peculiar quality of the eye-piece, but 
 merely to the fact that by increasing the magnifying 
 power, the dimensions of the object have been 
 increased to such an extent that the lines have appar- 
 ently been separated and become visible to the eye. 
 
 Beginners as a rule are apt to use too much magni- 
 fying power or amplification, and often attempt to 
 view a large surface with an objective which will show 
 but a small part of it. It must not be forgotten that 
 the apparent field of view is decreased as higher 
 powers are used, and that a low power will give a 
 
42 
 
 better impression of a large, coarse object and its 
 relative parts, not only because it makes a larger sur- 
 face visible, but because it has more penetration. 
 
 In objectives of the same power, but of different 
 angular aperture, the magnifying power and field will 
 always be the same. 
 
 The following table which has been compiled will 
 probably be of assistance to the beginner. After he 
 has become better acquainted with his instrument his 
 judgment will dictate to him what to do. 
 
 A power of 25 diameters will show a surface of 
 about J inch diameter. 
 
 A power of 50 diameters will show a surface of 
 about T L inch diameter. 
 
 A power of 100 diameters will show a surface of 
 about gV inch diameter. 
 
 A power of 500 diameters will show a surface of 
 about yi^ inch diameter. 
 
 A power of 1000 diameters will show a surface of 
 about TjJ-Q- inch diameter. 
 
 This table is approximately correct with a Huy- 
 ghenian eye-piece ; with a Periscopic almost double 
 the amount of such surface will be shown. 
 
 Magnifying power may be obtained by the eye- 
 piece or objective andthe desirability of using one or 
 the other for this purpose was for many years a mat- 
 ter of spirited discussion, but it is now generally con- 
 ceded that increased power should be obtained by 
 increasing the power of objectives and not go beyond 
 the equivalent power of 1 inch in the eye-pieces. 
 
 Objectives of the same angular aperture, but of 
 different power, will give identical results by bringing 
 them up to the same magnifying power, unless the 
 
43 
 
 difference is considerable. In both objectives and 
 eye-pieces the lenses decrease in size with the increase 
 in power and consequently give less light ; and while 
 this one objection exists in the objective an addi- 
 tional one occurs in the eye-piece, in that the eye 
 must be brought closer to the eye-lens and must be 
 kept more strictly in the optical axis, which at a long 
 sitting becomes fatiguing. 
 
 Between the 1 inch and 2 inch the choice should be 
 determined by requirements and individual prefer- 
 ence. All responsible manufacturers and dealers make 
 up such outfits of stands, objectives and eye-pieces, 
 which experience has taught them are most generally 
 useful. 
 
 It is a safe rule to follow in all work on recognized 
 forms ( objects of which the structure is known ) not 
 to use a higher power than is necessary to properly 
 study them. 
 
 Eye-Piece Huyghenian. This is now in gen- 
 eral use, and consists of two plano-convex lenses. It 
 ^~ -\ r ~^> receives its name from the 
 v/ \J inventor, who first applied it to 
 the telescope. The eye-lens is the 
 small lens nearest the eye, and 
 the fie 'Id- lens, or collective as it is 
 also called, is the large one near- 
 est the objective. A diaphragm 
 is placed between them, and 
 gives a sharply defined field. 
 This eye-piece is also called neg- 
 ative, as its focal point is between 
 
44 
 
 the two lenses (at the diaphragm) in contradistinction 
 to a positive, in which the focal point is outside of 
 and below the field-lens. 
 
 The Continental eye-piece is also a Huyghenian, 
 although it is mounted in a straight tube, in place of 
 the mounting, with the neck as shown in cut. The 
 American Society of Microscopists has recently 
 recommended that eye-pieces be made par-focal, that 
 is, that the equivalent foci coincide and that the par- 
 focal plane correspond with the upper end of the 
 tube. This is an excellent plan, as the focus on the 
 object is maintained whatever may be the change in 
 eye-pieces. 
 
 Solid Eye-Piece. This was the invention of the 
 late R. B. Tolles, and also belongs to the class of 
 negative eye-pieces. It is called solid 
 from the fact that instead of being 
 composed of two lenses, it consists 
 of one piece of glass, which is cut to 
 a cylindrical form, and on the ends 
 of which the proper curvatures are 
 ground ; the diaphragm is made by Fig. 13. 
 
 cutting a circular groove into the glass at the proper 
 distance between the two surfaces, which is then filled 
 up with an opaque pigment. 
 
 These eye-pieces are only made in high powers, as 
 optical glass is usually not of sufficient homogenuity 
 to make low powers, and their cost would be too con- 
 siderable, without a corresponding advantage. For 
 high powers they are superior to the Huyghenian, in 
 
45 
 
 that they give a better illuminated field, as there is 
 less loss of light by absorption through the glass than 
 by refraction at the two additional surfaces of the eye- 
 lens and field-lens in the Huyghenian. 
 
 Periscopic Eye-Piece. This consists of a triple 
 eye-lens and single field-lens. Its predominant fea- 
 ture is a very large and flat field, 
 with almost all objectives. In 
 this respect it has a considerable 
 advantage over the Huyghenian 
 and Solid. It is positive and 
 therefore well adaped for micro- 
 meter work, as it is focused like 
 a magnifier, and its magnifying 
 power remains constant, while 
 with the Huyghenian it is vari- 
 Fig. 14. able, from the fact that the eye- 
 
 lens alone is focused, thus varying its distance from 
 the field-lens, and consequently the magnifying power. 
 Nomenclature. The rating of eye-pieces was 
 formerly, and is to a considerable extent to-day, by 
 letters. This method, however, is arbitrary, as the 
 letters of different makers have a totally different sig- 
 nificance, so that nothing like a standard exists. This 
 fact induced the American Society of Microscopists 
 to endeavor to establish a universal method, and after 
 the matter had been given careful attention for sev- 
 eral successive years, it finally adoped the method 
 which rates them according to their magnifying 
 powers, the same as that which has been used in 
 objectives. .This gives an approximate idea of the 
 
46 
 
 magnifying powers ; thus, an eye-piece marked 1 inch 
 or by a letter signifying the same, shows that it mag- 
 nifies about 10 diameters ; one of | inch, 20 diame- 
 ters, and so on. 
 
 Flatness Of Field. Although this depends 
 mainly upon the objective, the absence of it may be 
 owing to a faulty construction of the eye-piece. If it 
 is so prominent as to be easily noticeable, and to the 
 same degree with a number of objectives, it may be 
 ascribed to the eye-piece. It must, however, be remem- 
 bered that an absolutely flat field has not yet been 
 obtained ; it may be closely approached by decreas- 
 ing the diameter of field to less than its normal size. 
 
 Size of Field. Quite a general but errorneous 
 idea prevails that the size of the tube has an influence 
 on the size of the field. Except in eye-pieces of very 
 low power, or with tubes with smaller than usual 
 dimensions, this is not so. It must be remembered 
 that a Huyghenian eye-piece admits of a definite size 
 of field, and this is regulated by the opening in the 
 diaphragm ; the same size of opening is used in all of 
 the same power, whether it is an eye-piece for a large 
 or small diameter. 
 
 A misconception also exists as to the definition of 
 field. Such inquiries are often made as : " As we 
 understand it, a wide-angle objective gives a larger 
 field ? " but it does nothing of the kind. The angular 
 aperture has no bearing whatever on the size of 
 the field, The field of view, or that which is shown 
 of the object's surface, is determined by the power of 
 the objective and eye-piece. 
 
REQUISITES FOR WORK. 
 
 It is the intention to make such recommendations 
 in this chapter which, if not absolutely necessary, will 
 be found convenient and will aid in facilitating work. 
 
 One of the first requisites for the proper use of the 
 microscope, is a thorough knowledge of its parts and 
 an acquaintance with the optical principles involved. 
 For this purpose the writer earnestly requests a 
 perusal of the preceding pages, and is convinced that 
 in cases where no previous knowledge of the instru- 
 ment has existed, work will be .done with far more 
 ease, in much less time, and with a greater degree of 
 satisfaction. Ignorance of the instrument's capacity 
 may lead to an idea that it is inferior and thus be the 
 means of its final abandonment ; and in place of the 
 anticipated pleasure there may arise a feeling of bit- 
 terness and disappointment for all future with every- 
 thing connected with it. There are innumerable cases 
 of this kind and they have induced a belief that it is 
 difficult to acquire a practical manipulation of the 
 microscope, whereas such is not the case when a lim- 
 ited time, properly applied, is devoted to it. 
 
 Working" Table. A firm table should be used, 
 preferably one with three legs, as this will always be 
 firm no matter how uneven the floor is, and if it can 
 
48 
 
 be arranged, should be devoted to this purpose only. 
 One with a round or square top of three feet provides 
 ample room. Although not necessary, a table with a 
 revolving top, provided with clamp, is very conven- 
 ient, as with this two or more persons may make 
 observations without changing their seats. 
 
 A very neat arrangement for a table-top is that sug- 
 gested and used by Dr. J. E. Reeves. He places upon 
 an ordinary table three or four thicknesses of white 
 paper and upon these a plate of polished glass as 
 large as the top ; this can be procured of almost any 
 glazier at a low price. It is pleasant to work upon 
 and will not soil. 
 
 As in almost all cities there is more or less contin- 
 ual vibration from wagons upon the paved streets, the 
 writer suggests an effectual remedy. Take a thin 
 board, say half an inch thick, of a sufficient size to 
 receive the microscope ; fasten on the upper side at 
 two opposite ends, cleats of 1 inch square and coun- 
 ter-sink into these through the board four spiral 
 springs of such tension that when they bear the 
 weight of the instrument, the bottom of the board 
 will be about \ inch from the table. 
 
 Have the working table provided with drawers and 
 arrange receptacles for the accessories, secure from 
 dust, but at a convenient point to reach. When the 
 instrument is not in use put it into its case or cover 
 it in a manner so that it shall be free from dust. For 
 this purpose a large bell glass is best. 
 
49 
 
 Room. If possible a room should be selected fac- 
 ing the north, as the light in this direction is most 
 constant. It will prove a great saving of time if all 
 or a portion of it can be permanently arranged to 
 receive the entire working outfit. It should also be 
 chosen with a view to its being free from disturbance. 
 
 Light As stated, the light from the northern sky 
 is most desirable, and that from a white cloud is pre- 
 ferable to that from a blue sky. On account of its 
 intensity, direct sunlight should seldom be used ; but 
 if modified by a white curtain or reflected from a 
 white wall it is excellent. 
 
 For lamp light an ordinary flat wick kerosene or 
 student lamp is well adapted. The Hitchcock lamp, 
 from its better combustion is still better, as its color 
 more nearly approaches white. The ideal artificial 
 light is that from an electric light. Gas light is not 
 desirable as it is seldom sufficiently steady. 
 
 Position of Light. The relation of the micro- 
 scope to the source of light is principally a matter of 
 personal convenience. With daylight it makes little 
 difference whether it is at the front or side of the 
 instrument, although the writer prefers it at the front, 
 as the manipulation of the object does not obstruct 
 it ; but the lamp should be placed at the right or left 
 side within, easy reach of the hand for the purpose of 
 controlling it. The writer suggests that the beginner 
 make it a habit at the outset to place it on the side of 
 the instrument opposite to the unoccupied eye, as the 
 tube then places the latter in the shadow. 
 
50 
 
 Which Eye to Use. In a binocular instrument 
 both eyes are used, but in a monocular only one is 
 used, and it depends upon a trial which is best suited. 
 A large proportion of persons are afflicted with astig- 
 matism, often without knowing it, and when this 
 exists it may be in one eye or when in both, may be 
 to a greater extent in one than in the other. Its 
 presence may prevent the eye from observing fine 
 detail ; but whichever eye is found to be best suited 
 should be used. When both eyes are alike it is some- 
 times advisable to change from one to the other. 
 
 It should be made a habit at the outset and strictly 
 adhered to, to keep both eyes open.- A little difficulty 
 may be found to do this, as the eye which is free will 
 probably observe the objects upon the table ; but as 
 soon as the mind becomes fixed upon what it sees in 
 the microscope, this impression disappears. After a 
 time it will be found to require no exertion and will 
 certainly add to the ease and comfort of the manip- 
 ulator while working. 
 
 The Ward Eye Shade, Fig. 15, will prove of as- 
 sistance in acquir- 
 ing the above men- 
 tioned habit, and 
 ebsides this, effect- 
 ually excludes the 
 
 Fig. 15. light from the eyes. 
 
 It is made of hard rubber and is attached to the tube 
 of the microscope. 
 
51 
 
 Order. Among the requisites for successfully 
 prosecuting work with the microscope are a strict 
 observance of the instructions, even if they appear 
 superfluous, a systematic way of doing work, and 
 cleanliness. Have a place for every article which 
 is required, so that the hand may immediately be 
 placed upon it ; after it has been used clean it before 
 putting it aside ; keep strange hands from your 
 apparatus unless you are assured that a knowledge of 
 its manipulation exists. 
 
 Material. Although the purpose of this manual 
 is to be a guide to the intelligent use of the micro- 
 scope and not the preparation or preservation of 
 objects, it may not be out of place here to enumerate 
 what every owner of an instrument should have at 
 the outset. The first should be a book on objects 
 giving proper instruction on their preservation. 
 There are many of these, and all of them good. Next 
 in order, slides, covers and labels are necessary. As 
 covers are easily broken in cleaning, a larger propor- 
 tion of them will be necessary. 
 
 A cabinet for slides, a large variety of which may 
 be selected from, will aid in starting work in a sys- 
 tematic manner. Forceps and a small pipette are 
 indispensible. For preserving objects, Canada bal- 
 sam or damar should be purchased, while the other 
 necessary material which may be gleaned from the 
 instruction book is easily obtainable. 
 
 When it is intended to do section-cutting a good 
 mechanical microtome, not necessarily expensive, 
 should be obtained at the outset. 
 
HOW TO WORK. 
 
 To Set Up the Instrument. Draw the instru- 
 ment from the case by grasping the base, and free it 
 from dust. If it has draw tube, bring it to its stand- 
 ard length, which is indicated by a ring, by as little 
 of a screw motion as possible ; if the draw tube is 
 highly polished, its surface will be best retained by 
 observing the above precaution. See that the mirror 
 and largest aperture of the diaphragm are in a central 
 position in the optical axis. After being convinced 
 that the eye pieces are clean, place one into the tube ; 
 then remove the objectives from their cases and after 
 first having increased the distance between the stage 
 and tube by means of the milled heads of the pinion, 
 attach the lowest power to the nose-piece, by using 
 both hands, being careful that it is as near as possible 
 in the optical axis while screwing it on. Then incline 
 the body by placing the left hand upon the base and 
 drawing with the right hand upon the arm ; be care- 
 ful not to pull on the tube, as it may prove too heavy 
 a strain upon this or the fittings. Incline the body 
 of the microscope until the eye piece about reaches 
 the level of the eye, so that when an observation is 
 made the position is as comfortable as possible ; the 
 
53 
 
 neck should not be strained, neither should the chest 
 be compressed. Next place the slide with a transpar- 
 ent object upon the stage, by sliding it under the spring 
 dips and get it as near as possible in the center of the 
 opening ; for an object anything near at hand, such 
 as a piece of printed paper or cotton fibres will do. 
 Watching the slide, adjust the mirror until it is seen 
 that the light strikes the object ; incline the head to 
 the level of the stage, and observing the objective, 
 rack it down to within i inch of the object. Again 
 placing the eye at the eye- piece, reverse the motion 
 of the milled heads and observing the field continue 
 the upward motion of the body until the image of 
 the object appears in view. 
 
 Centering Stage. If the microscope has a 
 revolving stage, turn this to see whether the object 
 or portion of it lying in the center of the field, 
 remains in the optical axis. It was true when the 
 instrument was shipped, but may have changed dur- 
 ing transportation. If not centered, loosen the screw- 
 holding it to the arm by means of the steel pin, just 
 sufficiently that by the exertion of a little pressure it 
 can be moved. After having observed first which por- 
 tion of the object remains stationary during its revo- 
 lution (this evidently is its center) move the stage so 
 that this point will be in the center of the field, and 
 then tighten the screw. If the point lies outside of 
 the limits of the field, its direction can be noted and 
 the stage moved accordingly. Where centering screws 
 are provided in the instrument, this is a simple mat- 
 ter. 
 
54 
 
 If the coarse adjustment does not prove sensitive 
 enough to focus easily, adjust by the fine adjustment 
 by taking the head of the micrometer screw between 
 the thumb and first finger and move toward the right 
 or left as may be necessary. 
 
 It may here be said in passing that the rack and 
 pinion should be so well fitted that they should per- 
 mit the adjustment of low power objectives with the 
 greatest ease, and should work without the slightest lost 
 motion with a \ or \ inch objective. This point is the 
 criterion of workmanship in an instrument, and if it 
 is found to have the least back-lash, or is not perfectly 
 smooth, it may safely be assumed that the instrument 
 is of inferior workmanship. 
 
 If the fine adjustment does not act, the screw has 
 either come to its stop, or has "run out," and must 
 be brought into action again ; the range of movement 
 in almost all fine adjustments is quite short and con- 
 stant care must be taken to keep it at about a medium 
 point. If the object is found not to give a full view 
 or is not in the center of the field, it must be moved 
 on the stage, but it must be remembered that a move- 
 ment in one direction causes an apparent opposite 
 movement in the field. At first this movement will be 
 in jerks, but after a little practice the necessary sen- 
 sitiveness of touch is acquired to give it more steadi- 
 ness. 
 
 Illumination. It should now be observed 
 whether the field is equally illuminated. Too 
 much stress cannot be laid on this point, 
 
55 
 
 as it is one which is easily overlooked and 
 is often the cause of considerable mischief. 
 If the light comes through a window a well defined 
 image of the sash is reflected by the mirror, and with 
 a lower power objective this can easily be seen ; 
 unless the mirror is correctly adjusted the field will 
 appear to be crossed by dark bands. In the case of 
 lamp light the flame is reflected and has a similar 
 effect. With high powers this fault is not so easily 
 noticed, and for this reason require the more care ; 
 proper resolution may in this manner be partially or 
 totally destroyed. The remedy is either by shifting 
 the mirror or by varying its distance from the object. 
 More information on this subject is given under the 
 head of Sub-stage Illumination. 
 
 Attaching High- Power Objectives. As the 
 
 difficulty of properly getting an object or a certain 
 portion of it in the field, increases with the magnifying 
 power, it is a good rule to use the lower power objec- 
 tive as a " finder;" after getting the point to be further 
 examined in the center of the field, remove the objec- 
 tive and attach the higher power, and after following 
 the procedure of focusing as with the low power, 
 except that the objective should be brought almost 
 in contact with the cover, the point will be seen in 
 the field or will be found to be close to it. 
 
 This plan of focusing as suggested above is always 
 a good one to follow and is observed by many of the 
 most expert manipulators. In many cases, however, 
 the focus is obtained without this precaution, by 
 
56 
 
 watching the field as the objective is brought down 
 toward the object, but is often followed by disastrous 
 results. Almost any person who has used the micro- 
 scope for any length of time is without question 
 aware that valuable preparations have been destroyed 
 in this way. 
 
 How to Work. It is now supposed that the 
 instrument is ready for work. To start, it is well for 
 the beginner to provide a few prepared specimens, as 
 these will help him considerably if it is his intention, 
 as it should be, to prepare them later himself. What- 
 ever branch of study he is going to follow, a slide of 
 pleurosigma angulatum, dry, will be valuable to prac- 
 tice upon and to determine the quality of his higher 
 power objective. In this latter respect, however, the 
 writer would advise the beginner to guard against 
 expressing an opinion too soon. He knows of many 
 cases where the optician's claims were flatly denied, 
 when often a few words of advice by lettter or a few 
 minutes of intelligent manipulation would resolve the 
 diatom, and would thereafter do it so easily that it 
 became a wonder how it could possibly be avoided. 
 
 For a low power objective, the proboscis of a blow- 
 fly is probably the most suitable and at the same time 
 most interesting object. Place this upon the stage, 
 and, after getting it as close as possible to the center 
 of the opening in it, focus by means of the coarse 
 adjustment. If only a portion of it can be seen and 
 if it is desired to see a larger surface, the length 
 of tube may be contracted by means of the draw-tube. 
 
57 
 
 In this case the object will be placed out of focus, 
 and another adjustment becomes necessary. If a 
 higher power is desired, the draw-tube may be 
 extended. Observe whether the field is well illumi- 
 nated, and if not, bear in mind what has been said about 
 properly adjusting the mirror. If the object appears 
 milky or the light is so intense as to be painful to the 
 eye, which is of usual occurrence to the beginner, the 
 diaphragm should be turned from one aperture to 
 another until a marked difference is seen ; or, the plane 
 mirror should be used. In this connection it is well to 
 state that the above precautions should always be 
 observed with low powers, unless the object is thick. 
 Now use the micrometer screw and note carefully the 
 beautiful structure which is opened to view. After 
 sufficient time has been spent, upon this, the objec- 
 tive may be replaced by a higher power and the 
 object by a slide of P. angulatum ; focus upon this, 
 being mindful of the suggestions previously given, and 
 do not fail to observe what has been said in regard 
 to a well illuminated field. If lamp light from a flat 
 wick is used, turn the edge of the flame toward the 
 mirror, and use the concave side of the latter. If the 
 diaphragm is in an adjustable sub-stage, bring it as 
 close to the stage as possible, or, whether here or 
 attached to the stage, it may as well be removed for 
 the present. Observe now whether outside of the cen- 
 tral rib any lines can be seen upon the surface of the 
 diatoms ; if not, vary the distance of the mirror from 
 the object; or, if lamp light is used, bring the lamp 
 
58 
 
 closer to or remove it from the instrument in one 
 line, so that the illumination will not disappear. If 
 this does not bring out the lines, swing the mirror-bar 
 from the central position into an oblique one, on the 
 side opposite to that of the light and readjust the 
 mirror ; in doing this grasp the ends of the mirror- 
 fork between the thumb and middle finger and move 
 the mirror by the first finger. If the field can not be 
 evenly illuminated, it is evidence that the mirror is 
 beyond the limit of angular aperture in the objective, 
 and it must therefore be brought back until it is. It 
 must here also be noticed that if the diaphragm is 
 still attached to the instrument and does not swing 
 with the mirror, it may also be the means of cutting 
 off light. By means of the micrometer screw carry 
 the fine adjustment back and forth beyond the plane 
 of the object and observe closely whether any lines 
 can be distinguished. It is very probable that they 
 will show ; but if not, the cause should be deter- 
 mined. It may be that the magnifying power is not 
 sufficiently great, and in this case a higher power eye- 
 piece should be used, or the cover glass may be more 
 or less than the normal thickness, which would cause 
 a spherical over or under-correction in the objective. 
 In this case th'e lines would appear when the diatom 
 is not in focus. If the objective is a non-adjustable 
 one, the proper correction may be approximately 
 reached by means of the draw-tube. If the lines 
 appear over the plane of the object, it shows over- 
 correction, and the length of tube should then be 
 decreased, or contrary when the lines show below or 
 
59 
 
 beyond the plane of the object. If the above direc- 
 tions have been followed, the lines cannot fail to be 
 seen with a moderately good j or 4 inch objective ; 
 but if they are not, the trial should be repeated. 
 Again, be careful to have no obstruction between the 
 course of rays from the mirror to the stage ; get good 
 illumination on the object ; observe well ; keep the 
 instrument in such a position that the object is not 
 illuminated from any other direction than from the 
 mirror. 
 
 When the diatoms are resolved in this manner, the 
 lines will appear to be diagonal in some ; longitudinal 
 or transverse in others, according to their position ; 
 anft, if the resolution is very good, these lines will 
 further resolve themselves in minute beads of a hexa- 
 gonal form. 
 
 It will now be well to bring the mirror more nearly 
 to a central position ; do this at intervals of about 10 
 degrees, and note the appearance at each decrease of 
 obliquity. It will be found that as the mirrror 
 approaches the optical axis the lines will appear to 
 become more faint, and may disappear before central 
 illumination is reached ; in this case it will be well to 
 begin again. An endeavor should be made to make 
 each attempt give better results than the preceding 
 one. Repeated trials will not only impress the var- 
 ious phenomena upon the mind, but will cause a nota- 
 ble improvement in manipulative skill, and thus a 
 better performance in the objective. 
 
 Until now we have assumed that transmitted light 
 has been used. We will now suppose that the object 
 
60 
 
 is not sufficiently transparent to use this method ; the 
 object is then said to be opaque, and requires a dif- 
 ferent procedure. . We will say that it is desired to 
 examine an insect ; it may be attached to a slide, or, 
 what is better still, may be fastened in a stage for- 
 ceps (Fig. 16) as irmay then be turned and viewed 
 from all sides. The low power objective should 
 again be attached ; after having been focused, it will 
 be found that the light is insufficient to illuminatate 
 n^w^-^n-^ it. The mirror- 
 bar should now 
 be swung upon 
 Fig. 16. its axis around 
 
 the stage to a point above it, so it will be at an angle 
 of about 45 degrees to its surface. If a lamp is used 
 and is in the same position as when used with trans- 
 mitted light, it is probable that the tube of the instru- 
 ment will obstruct the light, and it is then well to 
 move it toward the front. Using the concave mirror 
 adjust it so that the light will be concentrated upon 
 the object, by watching it directly, and then observe 
 through the tube. If it is not sufficiently illuminated 
 continue to adjust the mirror ; also vary its distance 
 from the object and swing the mirror-bar to a higher 
 or lower point. It often occurrs that, under the best 
 conditions, the need of better illumination is felt ; 
 in this case a bulls-eye condenser should be procured. 
 It will be found that this will become a useful and 
 perhaps necessary accessory in work outside of this. 
 Place it close to the instrument and set the bulls-eye 
 between the object and source of light, with \.\\e plane 
 
61 
 
 side toward the object ; if an ordinary hand lamp is 
 used, it will be necessary to elevate this to about the 
 height of the eye-piece, and if it is to be used often 
 in this position, a special support should be made for 
 this purpose. 
 
 Low power objectives are usually used on opaque 
 objects, but sometimes a higher power is desired. 
 Unless one is constructed with a view to opaque illumi- 
 nation its working distance is usually so short that it 
 will prevent the light from striking the object. A J 
 or i objective, of 75 degrees, has sufficient working- 
 distance, and its mounting are made conical in the 
 front, so that it will allow it. 
 
 Dark Ground Illumination. This method is 
 not in general use, probably because it requires a 
 special accessory, although it yields beautiful effects. 
 It is accomplished by means of a paraboloid (Fig. 17) 
 
 which is attached to the 
 sub-stage. As will be no- 
 ticed in the illustration, the 
 lower surface of the para- 
 poloid is plane, and the 
 light passes through this 
 without undergoing any 
 change. When it reaches 
 the polished parabolic sur- 
 face it is reflected to one 
 point, according to the sim- 
 ple optical law that the 
 Ffg. 17. angle of reflection is equal 
 
62 
 
 to the angle of incidence. An opaque stop, which 
 is cemented to the concave surface, prevents the 
 light from passing through the central portion of the 
 paraboloid. The object is thus illuminated on all 
 sides by such an obliquity qf light, that it does not 
 pass into the objective ; the object stands out in 
 relief, pleasantly illuminated on a dark back-ground. 
 In using the paraboloid, the plane mirror should be 
 used, and it is .necessary to vary its distance from the 
 object in order to attain the best results. 
 
 Cover-Glass. Thus far no attention has been 
 given to the use of the cover-glass, although it is an 
 important factor in reaching good -results. In prelim- 
 inary examinations of solid objects with low powers 
 it may be dispensed with ; but where fluids are used, 
 whether with low, medium, or high powers, it should 
 always be used. A drop or small quantity of fluid 
 placed upon a slide assumes a spherical form, and, on 
 viewing it with a low power, it will be found to give 
 a distorted field, and will cause disagreeable reflec- 
 tions and shadows. 
 
 As stated before, medium and high powers have a 
 comparatively short working distance, and the front 
 lenses will be so close to the water, urine, blood, etc., 
 that the capillary attraction will often cause an adher- 
 ence to the front surface of'the objective; besides 
 this, there is such a considerable depth to the fluid 
 that it obstructs the light, requires a great change in 
 adjustment for the various planes, and is usually in 
 such vibration that a sharp focus becomes impossible; 
 
63 
 
 by merely dropping a cover-glass upon it all these 
 objections are overcome. 
 
 The above are merely practical considerations, but 
 there are others of a theoretical nature and of as 
 much importance. After a high power objective has 
 been corrected to a certain thickness of cover, any 
 variation, not necessary considerable, has an injurious 
 effect upon the spherical corrections, and consequently 
 upon the resolving power. It is manifest that the 
 quality of the latter will decrease as the variation 
 increases, and when it reaches a point where no cover 
 is used, it may be so considerable as to destroy an 
 accurate perception of what is sought. 
 
 In this connection it is considered important to 
 state what thickness of cover-glass it is best to use. 
 As is probably well known, there are three grades, 
 which are designated as No. 1, No. 2 and No. 3. 
 Although they are classified, there is a variation with- 
 in the limits of different numbers The variation is 
 about as follows : No 1, T ^ to -^ inch thick ; No. 
 2, yffl- to T ^ inch thick ; No. 3, -^ to -^ inch thick ; 
 According to the prices of cover-glasses, when pur- 
 chased by weight, the No. 1 give the greatest number 
 and No. 3 the least. It may for this reason be 
 thought that the purchase of No. 2 is most advan- 
 tageous, but it must be considered that there is a 
 greater proportion of breakage by cleaning, as they 
 are very thin and sensitive. Considered only from a 
 optical standpoint, No. 2 should generally be used, as 
 the medium and high power objections are adjusted 
 
64 
 
 to this thickness and give the best results with the 
 thinnest of these. The same thickness is also used 
 on test objects, but they are generally not of as much 
 uniformity as might be desired. Objectives some- 
 times have such an extremely short working-distance, 
 that it is necessary to use the thinnest of No. 1, but 
 as these are usually provided with adjustment for 
 correction, their injurious influence is not so much 
 felt. The thickest covers are most comfortable to 
 handle and may be used with low power objectives 
 without much sacrifice of definition. 
 
 The writer takes the liberty of inserting in this con- 
 nection extracts from a paper which he recently read 
 before the American Society Microscopists and 
 which he hopes will give further information on this 
 subject. 
 
 " The cover-glass may truly be called a necesssary 
 evil ; for, while absolutely required in microscopic 
 investigations, there is no adjunct to the microscope 
 that has been and is productive of so much evil, and 
 has retarded the utilization of benefits made possible 
 by the advance in the construction of objectives so 
 much as it. 
 
 " It must be remembered that the majority of objec- 
 tives will always be dry, and especially so when such 
 improvements, which we hope are still to be made, 
 are accomplished. It is an unfortunate circumstance 
 that with this class of objectives the influence of var- 
 iation in thickness of cover-glasses is most apparent ; 
 but since it is so, we should, if possible, provide an 
 
65 
 
 agency which, eliminating the personal factor of 
 efficiency, will give, under all conditions, results 
 closely equal to those under which the objectives were 
 originally corrected. 
 
 "It is surprising to see how little attention is paid to 
 this subject in the large majority of standard works 
 on the microscope. Almost all books give carefully 
 prepared illustrations and descriptions showing the 
 effect on the course of light by the interposition 'of 
 the cover-glass, and after giving conclusive evidence 
 of its disturbing influence, still, in a general way, say 
 it is of little moment. 
 
 " With such statements to guide the microscopist, 
 it is not surprising that the subject should have 
 received so little attention, and that any efforts to lead 
 to improved methods of manipulating objectives 
 should have almost completely failed because of a 
 lack of the true understanding of their need and con- 
 sequent failure to create interest. The belief is quite 
 general that any time devoted to this subject is wasted 
 and might better be utilized in other directions. I 
 hope to be able to show that this is entirely wrong, 
 and may here say that, while I may be considered an 
 extremist in the other direction, my efforts emanate 
 from the desire to put it in the power of every micro- 
 scopist to obtain the highest possible results from his 
 optical battery and equal to those obtainable by the 
 optician. 
 
 "Outside of the differences of the lengths of tubes 
 used by different makers, which is also of great bear- 
 
66 
 
 ing on the spherical correction of objectives, one is 
 astounded by the difference in standard cover- glasses 
 used by different makers in correcting non-adjustable 
 objectives. With a thickness of 0.10 mm. for the 
 thinnest and 0.25 mm. for the thickest, it is only too 
 apparent that with the additional variation in lengths 
 of tubes it is beyond the power of the microscopist to 
 obtain even approximately the best results from his 
 objectives. More than this, a large quota of the 
 advance made in recent years in the capacity of objec- 
 tives has been lost. 
 
 "The greatest difficulty is met with non-adjustaleb 
 objectives. As is well known, compensation for thick- 
 ness may be obtained in the proper adjustment of 
 tube length ; but while not all microscopes are suit- 
 ably provided with draw-tubes, the requisite exper- 
 ience and skill is lacking with a large number of 
 microscopists to properly make the correction in this 
 manner, as well as in objectives specially provided 
 with collar correction. I am sure that microscopists 
 of long experience will bear me out in the statement 
 that results with adjustable objectives depend upon 
 individual skill, and that many such objectives now 
 in use fail to give results corresponding to their 
 capacity. It would seem, therefore, that any system 
 to permit the full utilization of the capacity of objec- 
 tives should depend on no personal factor in fact, 
 should be mechanical. 
 
 " In an objective corrected for normal thickness of 
 cover-glass there will be spherical over-correction with 
 
67 
 
 thick covers and under-correction with thin covers, 
 the amount of correction varying in a different ratio 
 to the amount of variations from the normal thick- 
 ness. The chromatic correction will also lose corres- 
 pondingly, but to not so high a degree. While a devi- 
 
 Fig. 18. 
 
 ation of a few hundredth millimeters in either direc- 
 tion will, perhaps, not signify, that which occurs in 
 covers classified in price-list under one number is 
 
68 
 
 sufficient to seriously affect and the high powers 
 totally obliterate the definition which under normal 
 conditions it may possess. The microscopist is there- 
 fore not obtaining such results as his objectives 
 ought to enable him to obtain, and the efforts of the 
 conscientious optician to provide classified objectives 
 of reliability and similar performance is almost 
 entirely nullified. 
 
 "The system which I have devised to aid in overcom- 
 ing these difficulties depends in the first instance up- 
 on a micrometer for measuring the thickness of cover 
 glass. See Fig. 18. 
 
 "In objectives provided with cover correction the 
 graduation is so arranged as to read to y i- mm. No 
 matter what the power of objective, the number gives 
 proper correction for a thickness corresponding to it. 
 Thus, with a cover glass of 0.20 mm. the collar of 
 such an objective need merely to be set at 20 to give 
 the proper correction and, consequently, the best 
 results. 
 
 "All the other scales give the correct tube length 
 in inches and millimeters for covers corresponding 
 to them, and in this manner offer a ready and definite 
 means of correction. The tube-lengths required for 
 the thinnest and thickest covers are so extreme that 
 probably no convenient means for obtaining them can 
 be practically arranged, but they can be so approxi- 
 mately if not entirely. At any rate, the micrometer 
 will detect the requirements before using the covers, 
 
69 
 
 and those deviating considerably from the normal can 
 be used on objects for use with low powers only, in 
 which case the effect will not be very appreciable. 
 
 "In this system I do not overlook the fact that var- 
 iation in tube length involves a variation in magnify- 
 ing power ; but, except in cases where micrometers 
 are used, I consider this of secondary importance, as 
 it always is in comparison to results obtained in resolv- 
 ing and denning power. 
 
 "This system involves four conditions : 
 
 First. That all cover glass be measured before 
 using them, and that the thickness be noted on the 
 preparation. 
 
 Second. That for convenience all draw-tubes be 
 marked in inches or millimeters or both. 
 
 Third. That adjustable objectives be corrected 
 according to this scale. 
 
 Fourth. That the same tube length and cover 
 glass thickness be used in all original corrections of 
 objectives." 
 
 To Draw Objects. It is very important that 
 the appearance of an object should be put upon paper, 
 especially of one which is not permanently mounted. 
 To do this does not require any great amount of skill 
 as the lines which are projected upon paper are 
 merely followed out ; but it is necessary that those 
 drawings be made truthful. Nothing should be put 
 down which is not actually seen ; neither should any- 
 
70 
 
 thing be omitted. Drawings thus made form a valu- 
 able record, not only for the individual, but for others 
 who are following the same line of study. 
 
 Camera Lucida How to Use it. The camera 
 lucida is the apparatus by means of which drawings 
 are made. There are various forms of these. The 
 methods of attaching to the tubes are also numerous, 
 but a very simple and effective device is that shown 
 
 Fig. 19- 
 
 in Fig. 19. The mounting is fixed to the cap of the 
 eye-piece by means of a flexible grooved ring. 
 
 In this form the pencil point is not plainly visible, 
 and a number of other forms are in use which are 
 
n 
 
 constructed with a special view to avoid this diffi- 
 culty. They are all, however, considerably more 
 expensive, but should be procured if means will 
 permit. 
 
 The procedure of working should be about as fol- 
 lows : Focus upon the object and then incline the 
 body, so that the center of the eye-lens will be 10 
 inches from the table. To obviate repeated measure- 
 ments, a standard stick of this length may be used. 
 If the instrument is so low that it will not allow the 
 inclination of the body to an angle of at least 45 
 degrees when at this distance, it should be placed 
 upon a box ; or, if not too high, upon the case of the 
 microscope. Now readjust the mirror and attach the 
 camera lucida from below and place the paper under 
 the instrument ; look into the camera lucida from 
 above, being careful that the eye is directly over the 
 center of its opening, and the image will be found to 
 be projected upon the paper. Possibly, and very 
 probably, it will appear faint. This is due to the fact 
 that the paper is almost as highly illuminated as the 
 field. To remedy this defect a cardboard should be 
 placed between the paper and light, so that the former 
 will be shaded ; the object will now come out in 
 strong contrast. Take a well pointed pencil and fol- 
 low the lines in the image. A little difficulty may at 
 first be experienced in seeing both the pencil and 
 image at one time, but after a little practice this is 
 overcome. 
 
It very often occurs that the pencil point can on 
 no condition be seen distinctly, but this is usually 
 due to abnormal sight, with which persons are often 
 afBicted. In these cases, the glasses which are 
 required for reading should also be used in drawing. 
 The difficulty is not experienced in the image, as this 
 can be adjusted to the eye. 
 
 Determining the Magnifying Power. Al- 
 though a magnifying table may be furnished, this 
 gives the powers merely approximately, as more or 
 less variation occurs in objectives and eye-pieces of 
 the same kind. As it is interesting, and sometimes 
 important, to know the exact magnifying power, a 
 simple method is mentioned. Procure a stage micro- 
 meter, divided into T i inch and TT5 Vo i ncn > an< 3 per- 
 haps ^oVo" inch, or, if prefered, any suitable division 
 in millimetre. Place the micrometer on the stage, 
 focus and incline the microscope, as if for drawing, 
 to within 10 inches from the table and attach the 
 camera lucida ; for low powers, T f J1F divisions may be 
 used ; for the higher ones TIJ Vo- or more ; the division 
 as now projected may be marked upon the paper and 
 then measured off with a rule divided into inches and 
 J^ inches ; if, for instance, the ^^ divisions are 
 used, and one division on the paper covered 1 inch 
 on the rule, it is evident that the magnifying power 
 is 1,000 times ; if it covered -f^, equal to , on the 
 rule, it would be 200 diameters, and so on. 
 
 Measuring the Size of an Object. A simple 
 and reliable way of learning the size of an object is 
 
73 
 
 by means of an eye-piece micrometer. As, however, 
 this does not measure the object directly, but only its 
 image, the first part of the process makes it more 
 complicated. However, this portion is usually 
 attended to by the manufacturers. The eye-piece is 
 provided with a slot, into which a micrometer is fitted. 
 A micrometer with the same divisions as the eye- 
 piece micrometer is placed upon the stage and the 
 objective focussed upon it. It is now observed how 
 many of the divisions of the eye-piece micrometer 
 are contained in the magnified division of the stage 
 micrometer, and the resulting figure is placed in the 
 sub-division under the objective. To determine the 
 actual size of an object, this is now placed on the 
 stage and, noting the number of divisions which cover 
 it, these are divided by the number on the card, and 
 the resulting figure gives the actual size. Suppose 
 the figure on the card is 8.0 and the image of the 
 object covers 40 of the spaces which are divided into 
 -jL millimeters, the size of the object would be ^ or 
 L millimeter ; or expressed in inches, (25.4 mm. equal 
 to 1 inch) about -fa. 
 
ADVANCED MANIPULATION. 
 
 Dry Adjustable Objectives. The information 
 which has thus far been given on the manipulation of 
 the microscope may be termed initiatory, as it is sup- 
 posed (at any rate hoped) to have disclosed some 
 new principles. These are comparatively simple, and 
 with a moderate amount of attention, are easily 
 acquired. It is the intention now to speak of some- 
 thing more complex and to give instruction in the 
 use of higher grade adjustable and immersion objec- 
 tives. The difficulty of doing this increases with 
 each step of advance, and whether it can be over- 
 come by means of written words, is, perhaps, an open 
 question. However, the writer is certain that if the 
 following instructions are faithfully adhered to, satis- 
 factory results will be gained. The highest^ attain- 
 ment must of necessity be the result of perseverance 
 and knowledge of the various properties of an objec- 
 tive, which are given in preceding pages. 
 
 It is assumed that a dry objective is used, say a |- 
 140 degrees or ^ 135 degrees, and provided with 
 screw-collar adjustment for cover-glass thickness ; it 
 is further assumed that in the first, the variation 
 
75 
 
 between the two first systems (anterior and middle) 
 is attained by means of a rectilinear motion to the 
 middle and posterior system and stationary anterior 
 system, while in the second the conditions are 
 reversed ; the two posterior systems remain station- 
 ary while the front is adjusted. Both are arranged 
 with graduations upon the screw-collar and have an 
 index. 
 
 In the objectives of the Bausch & Lomb Optical 
 Co., the graduations range from 10 to 35, and as pre- 
 viously stated these numbers indicate the proper 
 points of correction for the respective thickness of 
 cover glass. 
 
 In these objectives the open point, /. e., where the 
 objectives are most widely separated, is at 10, thus 
 giving the correction for practically the thinnest cov- 
 ers. As the objective is moved toward the higher 
 numbers the adjustment is closed and gives correction 
 for the thicker covers. In objectives of other manu- 
 facture where this system is not adopted, the figures 
 are arbitrary and have no special significance. While 
 the open and closed point can be learned in these by 
 trial, it should be given by the maker before work is 
 attempted with it. Before the objective is attached 
 the adjustment should be closed, as, if this is neg- 
 lected and the objective has a short working distance, 
 the front lens may come in contact with the cover 
 when it is endeavored to focus on the object. 
 
 Probably the best object for studying the effect of 
 the screw-collar adjustment and acquiring skill in 
 determining its best point is again P. angulatum. 
 
76 
 
 Place a slide of this upon the stage, and with a low 
 power eye-piece select a diatom which appears to be 
 flat ; such a one may usually be found when there 
 are a number on the slide. After the objective has 
 been attached to the nose-piece, focus carefully and 
 observe whether any lines can be seen ; if not, grasp 
 the milled edge of the adjustment collar between the 
 thumb and first finger of the left hand, keeping the 
 fingers of the right hand upon the micrometer screw, 
 or vice versa, if from the outset it was made a habit 
 to use the left hand on the fine adjustment ; turn the 
 collar slightly toward its open point, and as this will 
 place the object out of focus, move the fine adjust- 
 ment correspondingly ; continue to turn the collar, 
 little by little, and do not cease to observe closely ; 
 also, after each movement, focus above or below the 
 plane of the object, so that this will be indistinct, and 
 look for the lines. . Possibly after a little they will 
 begin to appear faintly ; but, if not, continue to bring 
 the collar toward the middle. The lines must now 
 soon make their appearance, and, when they do, it 
 will probably be above the plane of the diatom. This 
 is an indication that the objective is approaching its 
 correction for the cover. Now keep the lines in focus, 
 while the correction collar is being gradually turned, 
 until the lines and the outline of the diatom lie in one 
 plane; the objective is now said to be corrected for 
 cover. Observe which number corresponds to the 
 index, and note this upon paper ; again return the 
 collar to its closed point and go through the same 
 
77 
 
 proceeding as carefully as at first, 
 point is again reached, look for the number and see 
 whether it agrees with the first ; very likely it does 
 not, which is owing to a want in the faculty of per- 
 ception, due to a too slight acquaintance with the 
 phenomena. These trials should be repeated until 
 the proper sensitiveness of feeling in making the 
 adjustments is acquired, and until they can be made 
 to correspond with a certainty to at least within two 
 divisions. 
 
 Remove the eye-piece and attach one of higher 
 power. It must how, however, be remembered that 
 if there is a considerable difference in the powers 
 there will be a relative difference in their lengths, and 
 that this will cause a difference in the optical length 
 of the tube ; this not only will require another adjust- 
 ment for focus, but will partially destroy the correc- 
 tion as made with the low power. After some prac- 
 tice, the amount of variation may be fixed upon and 
 may be noted for the future ; but, to determine it, the 
 same plan as suggested with the low power eye- piece 
 should be followed. 
 
 When it is fo^nd after repeated trials that sufficient 
 skill has been acquired to bring the collar to within 
 one division, the number and power of the eye-piece 
 should be scratched with a diamond upon the slide or 
 with pen and ink upon the label ; thus, if it is found 
 that with a 1J inch eye-piece the index shows 5, and 
 with a I inch eye-piece shows 5, it should be marked 
 U-5 and |-54. In eye-pieces of par-focal construe- 
 
78 
 
 tion it will be unnecessary to mark with different 
 figures, as the correction will be the same for all 
 powers. For future examinations on the same slide, 
 this will facilitate work and give the assurance that 
 the best results are thus gained without further trial. 
 Mr. Wenham's general rule for obtaining the best 
 correction on objects in general is as follows : " Select 
 any dark speck or opaque portion of the object and 
 bring the outline in perfect focus ; then lay the finger 
 on the milled head of the fine adjustment and move 
 it briskly backwards and forwards in both directions 
 from the first position. Observe the expansion of the 
 dark outline of the object, both -when within and 
 when without the focus. If the greater expansion or 
 coma is when the object is without the focus or far- 
 thest from the objective, the lenses must be placed 
 farther assunder (or opened). .If the greater coma is 
 when the object is within the focus, or nearest the 
 objective, the lenses must be brought closer together 
 (or closed). When the objective is in proper adjust- 
 ment, the expansion of the outline is the same both 
 within and without the focus." * 
 
 Immersion-Adjustable Objectives. As was 
 
 stated before, immersion contact between the objec- 
 tive and cover-glass is made by either water or homo- 
 geneous fluid. The fluid should be kept in a small 
 bottle or phial, the cork of which is pierced to receive 
 a small pointed stick or match, and this should pro- 
 ject sufficiently so that it will enter the fluid about 
 
79 
 
 i inch. The fluid will then always be free from dust 
 and by withdrawing the cork the stick will always 
 carry a drop of fluid with it. 
 
 In fixing an immersion objective to the stand, the 
 latter should first be put in an upright position ; the 
 fluid should now be attached to the front lens, but 
 care should be taken not to put on too much ; it 
 should be merely enough to cover the surface. If 
 too much, a portion of it should be removed by allow- 
 ing it to adhere to the finger. The objective may 
 then be attached to the stand and brought down until 
 the fluid is in contact with the cover ; the stand is 
 now inclined and the objective focused ; if this 
 method is followed there is no danger of flooding the 
 entire cover with fluid, which sometimes may be the 
 means of destroying the object ; neither can the fluid 
 run out from between the two surfaces. 
 
 Extreme cleanliness should be observed in all work 
 connected with the microscope, and particularly in 
 the use of immersion objectives. The use of immer- 
 sion fluid in itself involves a certain amount of incon- 
 venience, but as in many cases it is absolutely neces- 
 sary, the observance of fixed rules will materially help 
 to overcome some of the disagreeable features. After 
 the work with an immersion objective has been com- 
 pleted, the objective should be removed from the 
 stand, and its front, as well as the slide, should invar- 
 iably be cleaned ; the fluid may be removed by a moist 
 piece of soft linen and then cleaned with a dry piece; 
 chamois skin is not suitable, as it does not absorb the 
 fluid. 
 
80 
 
 Test-Plate. Almost all microscopists who take 
 an active interest in the capacity of their instruments, 
 supply themselves with a set of test objects, of which 
 P. an^iilatiiin is in most general use, or with a so- 
 called test-plate. These plates consist either of a ser- 
 ies of bands of finely ruled lines ranging from 5,000 
 to the inch to 120,000 to the inch or with a series 
 of diatoms, upon which the markings represent 
 certain divisions of an inch. The one of these 
 which is principally used is made by J. D. Moeller, 
 and consists of a series of 20 diatoms. They are 
 furnished mounted both dry and in balsam, but 
 the latter is the most common. Below is a table giv- 
 ing the numbers, names of the various diatoms and 
 divisions on their surfaces to T y 1 o^ inch A specimen 
 of Eupodiscus Argus begins and ends the series : 
 
 Striae in 
 T oVo of an inch - 
 
 1. Triceratium Favus Ehrbg 3.1 to 4. 
 
 2. Pinnularia nobilis Ehrbg 11.7 to 14. 
 
 3. Navicula Lyra Ehrbg. var 14.5 to 18. 
 
 4. Navicula Lyra Ehrbg 23. to 30.5 
 
 5. Pinularia interrupta Sm. var 25.5 to 29.5 
 
 0. Stauroneis Phoenicenteron Ehrbg... 31. to 30. 5 
 
 7. Grammatophora marina Sm 36. to 39. 
 
 8. Pleurosigma balticum Sm 32. to 37. 
 
 9. Pleurosigma acuminatum (Kg.) Grun 41. to 46.5 
 
 10. Nitzschia Amphioxys Sm 43. to 49. 
 
 1 1. Pleurosigma angulatum, Sm 44. to 49. 
 
 12. Grammatophora oceanica Ehrbg = 
 
 G. subtilissima. . . 60. to 67. 
 
81 
 
 13. Surirella Gemma Ehrbg 43. to 54. 
 
 14. Nitzschia sigmoidea Sm 61. to 64. 
 
 15. Pleurosigma Fasciola Sm. var 55. to 58. 
 
 16. Surirella Gemma Ehrbg 64. to 69. 
 
 17. Cymatopleura elliptica Breb 55. to 81. 
 
 18. Navicula crassinervis Breb=Frustu- 
 
 lia saxonica Rabh 78. to 87. 
 
 19. Nitzschia curvula Sm 83. to 90. 
 
 20. Amphipleura pellucida Kg 92. to 95. 
 
 It may be said, and perhaps with truth, that a test- 
 plate does not belong to the necessities of an outfit, 
 but considering that it is a guage, on which the opti- 
 cian usually bases the quality of his objectives, it is 
 valuable to the owner of an objective to be able to 
 determine whether, under his manipulation, the objec- 
 tive will perform as well as is claimed for it ; due con- 
 sideration must, however, be given to the fact that 
 there is a certain amount of variation among differ- 
 ent plates, as is shown in the above table. Outside 
 of this, it is a continual incentive to determine the 
 extreme performance of an objective, and it thus 
 becomes the means of acquiring great manipulative 
 skill, which cannot be underrated. The writer is in 
 a position to know that there is great need of this ; 
 innumerable cases have come to his notice where sev- 
 eral objectives of the same kind and equal quality 
 gave unequal results in different hands, and would be 
 highly eulogized by the possessor of one and con- 
 demned by that of another. 
 
82 
 
 Immersion Objectives on Test Plate. To 
 
 determine the highest capacity on test objects, ordi- 
 nary daylight is hardly sufficient ; moderate sunlight 
 or good lamp-light is best suited, but the latter, 
 from the fact that it is always at hand, is preferable. 
 For the purpose of explanation, we will assume that a 
 flat wick lamp and a J, T V or Jg homogeneous immer- 
 sion objective is used. If the right hand is used on 
 the micrometer screw, place the lamp at the right side 
 of the instrument, about 10 inches from it, with the 
 edge of the flame turned toward the mirror. 
 
 The test plate may now be placed upon the stage, 
 and as the diatoms in balsam are very transparent, 
 and therefore very difficult to find, a lower power 
 objective may be used as a finder; bring No. 1, or 
 Tricerdtium Favus, in the center of the field, and after 
 the objective has been removed, attach the immersion 
 objective in the manner prescribed ; the adjustment 
 collar may be placed at zero, as this is about the cor- 
 rect point for standard length of tube. Get the best 
 possible illumination with the mirror at the central 
 point and move the test plate from diatom to diatom 
 until it reaches No. 11, P. angulatum, but observe 
 closely the structure of each one as it comes into the 
 field. Next see whether the objective is corrected ; 
 if the lines and outlines, or middle rib, do not appear 
 to be in one plane, adjust the collar until they are, 
 and then continue the advance toward the higher num- 
 bers until one is reached on which no lines can be 
 
83 
 
 seen. Swing the mirror-bar to an obliquity of 20 
 degrees, and, readjusting the mirror, observe the 
 effect. It is very probable that the lines will show, 
 and, if so, continue the advance ; if they do not, give 1 
 degrees or 20 degrees more obliquity, and after the 
 structure comes out, again go forward. A point may 
 thus be reached,, where with the greatest obliquity 
 which can be given and with the best possible illumi- 
 nation, the objective seems to have come to the limit 
 of its performance. From the claims which have been 
 made for it, it ought to do better. What is the cause 
 of failure ? Possibly the mirror is not correctly 
 focuse'd, or the adjustment collar may not be correct 
 for oblique light ; perhaps the eye-piece does not give 
 sufficient magnifying power to distinguish the striae. 
 It may be any one of these causes or all combined. 
 As to the eye piece, the manipulator must remember 
 the amount of separation of lines in the last object 
 which was resolved, and from the gradation in the 
 coarser specimens must judge whether the power is 
 sufficient ; it should be added that for any over No. 
 14 and under No. 18 a | inch eye-piece should be 
 used, and for those above No. 1 8 a power of \ inch 
 will probably be necessary, provided a -J- or ^ objec- 
 tive is used. After this condition has been complied 
 with, look to the correction collar of the objective ; 
 to obtain the highest results it very often occurs that 
 a different adjustment is required for oblique light 
 from that for central light. Note the number at 
 which it stands, and then work it back and forth, 
 
84 
 
 watching carefully for results. If this has no influ- 
 ence, return it to its number or to a point where the 
 outline of the object appears most sharp. Now look 
 to the illumination ; vary the distance of the mirror 
 to the object, and, if it conflicts with the stage or 
 does not give the desired results, vary the distance of 
 the lamp to the instrument and watch the effect of 
 the change through the tube. A great change in the 
 illuminating power can thus be produced ; the light 
 is best when it covers the least space, as it is then 
 most intense. The light may be quickly adjusted by 
 throwing it upon a point on the slide in the opening 
 of the stage and watching it there: If neither of these 
 changes give any improvement, recourse must be had 
 to another expedient. Place a bull's-eye between the 
 lamp and mirror with the plane side to the former, 
 and close to it so that the light is thrown on the lat- 
 ter. It. should be properly concentrated so that the 
 circle of light will not be larger than the mirror, 
 which can be determined by placing the hand or a 
 piece of paper back of it. Adjust when necessary by 
 moving the lamp or bull's eye. Keep it a little below 
 the line of the face of the stage, so that the light will 
 not strike it on its upper and as little as possible on 
 its lower surface ; if the light from the bull's-eye 
 directly reaches the object, it destroys the effect of 
 the oblique illumination. Great care should be given 
 to this point, as it is very important. 
 
 If all of these suggestions have been followed, a 
 great difference will undoubtedly be noticed in the 
 
85 
 
 performance of the objective ; but if it still does not 
 come up to the standard, patience must not be lost. 
 The slightest change in the mirror, bull's-eye, or 
 lamp, a touch to the correction collar or micrometer 
 screw is sometimes followed by astonishing results. 
 The beginner should sit down with the expectation 
 that he will fail at the first trial. At each succeeding 
 trial he can easily notice his improvement in manipu- 
 lation and the gain of corresponding results. He 
 should be able to bring the performance of the objec- 
 tive up to the claims made for it, if it has come from 
 the hands of a reliable optician, and should not rest 
 until this is accomplished. 
 
 The writer has often recommended sunlight with 
 generally successful results where ordinary means of 
 illumination have failed. The light is of course 
 intense, and great care will have to be used to modify 
 it by properly using the mirror, but success is often 
 attained and then creates confidence. It is, however, 
 only recommended for this purpose and not for gen- 
 eral use. 
 
 To the histologist it may seem strange that the 
 writer has thus far only spoken of working with 
 objectives on diatoms. This, however, was done 
 advisedly. They are thin, and therefore as suitable 
 as a thin section and far more preferable than a thick 
 one. Their form and structure are easily recogniz- 
 able, and there is very little variation among those of 
 the same kind ; therefore, rules laid down regarding 
 them are generally good. It is conceded by advanced 
 
workers that the time spent over diatoms for the pur- 
 pose of studying objectives is well applied, and the 
 most expert manipulators have acquired their exper- 
 ience in this manner. An objective which works well 
 on diatoms works equally well on other objects, and 
 therefore the manipulative skill which has been 
 attained on the former is as well applied on the lat- 
 ter. At the outset work may be done on other 
 objects than diatoms, and where ordinary working 
 objectives, such as a Student 1 inch and J inch, or f- 
 and | inch comprise the outfit, the road to good manip- 
 ulation may be as short as with diatoms The condi- 
 tions in both cases remain the same ; but it must be 
 cautioned that, if histological preparations be used, 
 only such be selected as are reliable. A poor speci- 
 men is perhaps as bad as none at all ; an abnormally 
 thick one obstructs light, makes it impossible for the 
 objective to penetrate through the various layers, and 
 leaves the impression that the latter is defective 
 
 Photo-Micrography. This subject does not 
 properly belong within the scope of this book, but 
 there are some points connected with it which may 
 be of value to mention. 
 
 Any person desiring to do this work should 
 endeavor to obtain some experience in ordinary pho- 
 tography and proper developing. First, as good 
 results in photography with the microscope are diffi- 
 cult to obtain, even with previous experience, a book 
 devoted to this purpose should be well studied before 
 attempting to obtain results. The beginner in this 
 
87 
 
 direction will find it somewhat difficult to make a 
 proper selection of apparatus. The most inexpensive 
 is to use the ordinary microscope in a horizontal posi- 
 tion and attach to an ordinary camera, in which care 
 will have to be observed to see that the ground glass 
 is at exactly right angles to the optical axis. As a 
 rule the ordinary view camera is not of sufficient 
 accuracy to use without specially adapting it to this 
 purpose, and the main difficulty lies in the lack of 
 coincidence of the ground glass with the film side of 
 the plate. Plates in themselves are irregular and the 
 shoulders on which they rest should at any rate coin- 
 cide. The best plan to determine this is by placing 
 a straight edge on the frame of the ground glass, 
 interpose a wooden wedge between it and the ground 
 surface of the glass and mark the point of contact by 
 a pencil mark. Follow the same procedure with the 
 plate holder containing the plate. The variation can 
 in this manner be seen with a nicety and proper cor- 
 rection easily made. The writer recommends that 
 this test be made with each plate, so that this is 
 always in exact coincidence. 
 
 Where means will permit, the best plan is no doubt 
 to obtain an apparatus which is complete in itself and 
 is used for this purpose only. The Atwood, as an inex- 
 pensive, and the Rafter as complete in every direc- 
 tion, may be recommended. 
 
 Whether an eye-piece should be used or not is a 
 matter of controversy. Both methods are followed 
 with good success. By the use of the eye-piece the 
 
88 
 
 microscope is brought to its normal condition and the 
 projection of the image into the camera carries with 
 it the faults of the eye-pieces, and these increase with 
 the length of the camera. Without the use of the eye- 
 piece the spherical and chromatic correction of the 
 objectives is disturbed in proportion as the plate is 
 distant from the standard tube length and will destroy 
 the definition of a high power objective unless com- 
 pensation can be made by means of collar correction. 
 
 The plan followed by a well-known worker is to 
 take a negative at the end of the tube without the 
 eye-piece, where the proper corrections are obtained, 
 and from this enlarge to any suitable size. 
 
 The amplifier should be used where the eye-piece 
 is not employed, as by its proper adjustment the 
 objective may be brought to its normal condition. 
 This is the principle involved in the Rafter camera. 
 Its use is therefore not that of an amplifier but that 
 of a corrector. 
 
 The question of suitable objectives is one of con- 
 considerable importance. The ordinary microscope 
 objective is not constructed with a view to photo- 
 graphic work, and unless nicely corrected there will 
 be a lack of coincidence between the visual and chem- 
 ical rays, /. e. between the image which is seen on the 
 ground glass and that which is photographically pro- 
 duced. For all this, practical experience has shown 
 that a number of Bausch & Lomb Optical Co. objec- 
 tives are well suited to this work. For low powers 
 the Student and Professional series may be success- 
 
89 
 
 fully used. For medium powers special objectives 
 are constructed, and for high powers any of the hom- 
 ogeneous immersion may be successfully employed. 
 
 Some favor the apochromatic objectives as these 
 are specially constructed with a view to using them 
 in this work, but their considerable higher prices ple- 
 clude their use in many cases. 
 
TO SELECT A MICROSCOPE. 
 
 When a person has concluded to obtain a micro- 
 scope, a suitable selection is a matter of considerable 
 importance to him. The varieties are innumerable, 
 prices without end, all sorts of claims made for them. 
 It is, therefore, easily explained -why this chapter 
 should be an important one in a manual of this kind, 
 and yet difficult to treat satisfactorily. 
 
 The variety of special lines of investigation involves 
 nearly as great a variety of requirements. The 
 amount of money to be expended ; what shall be the 
 stand ; what the objectives ; shall the entire outfit be 
 purchased at one time or little by little, are all ques- 
 tions of paramount importance which the writer does 
 not expect to solve, but hopes to give sufficient infor- 
 mation that a more intelligent selection may be made 
 than might probably be done otherwise. 
 
 Stands. Starting out with the assumption that 
 there are two classes of instruments to select from, 
 the long and short tube, the first decision to reach 
 will be this point. In a general way it may be said 
 that there are no optical advantages in either, but 
 whichever is adopted must be retained. 
 
91 
 
 The principal consideration is whether the instru- 
 ment is to be used in an upright or inclined position. 
 If the former, the short tube is most usually selected, 
 as it can be used comfortably on a table of ordinary 
 height. This one objection which might be and often 
 is raised against the long tube is easily overcome by 
 the friends of the latter providing a suitable table for 
 the same. As the instrument is used in the upright 
 position only in a few special lines of study, it is 
 really only of weight in this direction, as the instru- 
 ment may be inclined to the most comfortable point, 
 and when so is more comfortable than the upright 
 position. 
 
 The joint for inclination of arm is generally con- 
 ceded to be an advantage. While it may be the case 
 that many of the upright instruments are in use in 
 Europe, there are very few used in this country, and 
 the preponderence of instruments shown in catalogues 
 of foreign makers, would indicate the same tendency. 
 
 Almost all instruments for reliable work are pro- 
 vided with both fine and coarse adjustment. They 
 are both necessary, the only question being whether 
 the latter shall be by the sliding tube or rack and 
 pinion. The former, while perhaps having the advan- 
 tage of admitting a more speedy change of objectives, 
 has a decided disadvantage in the hands of the stu- 
 dents in endangering objectives and preparations. 
 Further than this, it is almost impossible for the 
 maker to center the nose-piece with the tube, so that 
 a change of objectives usually loses an object out of 
 
92 
 
 the field, and requires that it be looked for anew with 
 each change. In the rack and pinion the nose-piece 
 has an unvarying relation to the tube, and is not 
 liable to this difficulty, and offers a steady and agree- 
 able adjustment. The advantages of the rack and 
 pinion seem to be generally appreciated in this coun- 
 try, for there are few instruments sold and used with- 
 out it. 
 
 Whether an instrument shall be of japanned iron or 
 lacquered brass is probably largely determined by the 
 amount of money to be expended. As far as the 
 intrinsic suitability of the metals is concerned, there 
 is no difference. Brass, however, oilers the maker a 
 better opportunity for displaying his mechanical 
 skill, and while it is no doubt true that many highly 
 finished instruments are of poor workmanship in their 
 working parts, it is also a fact that a well made instru- 
 ment is always nicely finished. 
 
 The size of instrument is worthy of consideration. 
 If an instrument is to remain stationary in a practi- 
 tioner's office or laboratory, it may be large without 
 being cumbersome. If, however, it is intended to be 
 carried about, it should be of the smaller and more 
 contracted style. 
 
 Another important consideration is the space 
 between the stage and base, or table. While it is 
 advisible to have the stage low on account of the con- 
 venience in manipulating a slide, there should still be 
 sufficient space for the convenient attachment of sub- 
 stage accessories. As a rule the American pattern of 
 
93 
 
 instruments provide more room between the stage and 
 base on the lower side and stage ana ^trrwfon the 
 upper, than do the Continental. 
 
 As stated previously, a variety of stages are offered 
 on instruments of similar construction. The plain 
 flat stage while preferred to some, offers no advan- 
 tages over the ordinary round one, unless specially 
 made for examining specimens on larger slides than 
 the standard 3 by 1 inch. Some claim advantages for a 
 smaller stage than the length of the slide, so that this, 
 projecting, admits of the slide being grasped and 
 swung around the optical axis. These advantages, 
 however, are not generally appreciated, and even if 
 so, are offset by the drawback that in moving the 
 slide it is apt to be tilted. 
 
 Spring clips are usually of similar construction, 
 although varying in detail and curves. Properly con- 
 structed clips should have such thickness of metal 
 and be so bent as to allow specimens to be brought 
 under them without resistence and keep them prop- 
 erly in place without too much pressure and conse- 
 quent friction. 
 
 A glass-stage and slide-carrier may be considered 
 a good investment, as it admits of the convenient 
 manipulation of the slide without the grating feeling 
 which usually accompanies the direct movement of 
 the slide on the stage. 
 
 Where systematic examinations of a specimen are 
 to be made, a mechanical stage will be found a great 
 convenience and in petrographical work is almost a 
 necessity. 
 
94 
 
 A sub-stage may, in a general way, be said to be 
 preferable when it is adjustable, particularly in the 
 use of a condenser. It is absolutely necessary to 
 adjust the condenser for different objectives and this 
 must be done so nicely that a sliding movement is 
 hardly sufficient. In biological work the condenser 
 is at the present day a necessity, and this is now con- 
 structed as a separate attachment with all the neces- 
 sary adjustments, in which the rack and pinion is 
 deemed of first importance. 
 
 Objectives and Eye-Pieces. It is hoped 
 that the information given of the various qualities in 
 an objective will aid to make a suitable selection of 
 the optical parts. As the stands have been classified 
 in long and short standard tubes, the first quality to 
 look for is, after the stand has been selected, their 
 suitability to it. 
 
 As will have been seen under the proper head, a 
 variety of powers is obtained by a suitable combina- 
 tion of eye-pieces and objectives, and while power 
 alone can be obtained by increasing the power of the 
 eye-piece, it is not advantageous to do so. For ordi- 
 nary work no higher eye-piece than a | should be 
 used. In catalogues many outfits are made up of one 
 eye-piece and two objectives, but this is only for the 
 purpose of reducing price to a minimum. It is always 
 advisable, when means will permit, to select two eye- 
 pieces, preferably the 2 inch and 1 inch, and insist 
 they be par-focal, as this will be found extremely 
 convenient and will not disturb the optical standard 
 
95 
 
 length. If for any work J inch or higher powers are 
 desired, the solid eye-pieces may be recommended. 
 The periscopic are advantageous for micrometric 
 tests and other work where a large or flat field is 
 desirable. 
 
 For student's and practitioner's use, the outfits as 
 made up in catalogues are usually sufficient, except, 
 as above recommended, where but one eye-piece is 
 given, it is well to select two where means will permit. 
 
 Referring particularly to the catalogue of the 
 Bausch^c Lomb Optical Co., we will state the purpose 
 for which they were intended 
 
 Biological Laboratory, student and professional 
 use. 
 
 Large Biological Advanced laboratory, student and 
 professional use. 
 
 Harvard Laboratory, student and professional use. 
 
 Model Laboratory, student, professional and ama- 
 teur use. 
 
 Physician Professional. 
 
 Investigator Student, professional and amateur use. 
 
 Universal Student, professional and amateur use. 
 
 Concentric Professional and amateur use. 
 
 Professional Advanced laboratory, professional 
 and amateur use. 
 
 For ordinary professional use, including urinary 
 examinations, the J inch 27 degrees and \ inch 110 
 degrees objectives will be found sufficient. For 
 bacteriological examinations a higher power, such as 
 a ^ inch homogeneous immersion, objective will be 
 necessary. 
 
96 
 
 If means will permit, an investment in the f inch 
 40 degrees and -|- inch 130 degrees, or f- inch 40 de- 
 gress and i inch 140 degrees, will be well applied. 
 This latter is in very general use and may be highly 
 recommended. 
 
 In botanical work a lower power than those men- 
 tioned, such as a -2 inch 12 degrees or, preferably, 2 
 inch 15 degrees, will be necessary. 
 
 For amateur- use the ordinary outfit of } inch 27 
 degrees and -J- inch 110 degrees with the addition of 
 a 2 inch, preferably of the better grades, will do. If 
 the examination of diatoms will be followecf, the -J- 
 inch 140 degrees and T 1 2 inch homogeneous immer- 
 sion, will probably be required. 
 
 For the student, the f inch 28 degrees and inch 
 110 degrees, or f inch 27 degrees and -J- inch 110 
 degrees objectives, will ordinarily be ample. 
 
 Although from an optical standpoint it is true that 
 objectives give more detail as they increase in their 
 angular apertures, it will have been seen that the 
 highest class of objectives is not always recommended. 
 A great portion of everyday work does not require 
 this maximum of optical results, and can be accom- 
 plished completely and with comfort with objectives 
 of comparatively low aperture. Some years ago micro- 
 scopists were divided into two classes, the new school 
 of wide aperture, and the old school of narrow aper- 
 ture. The state of affairs existing then has happily 
 changed, concessions having been gradually made, so 
 that now the advantages of both classes of objectives 
 
97 
 
 are appreciated and there are few microscopists of 
 standing who would recommend only one or the other 
 kind. 
 
 In these days of competition, prices alone are too 
 often made the object of inducement, without any 
 reference to quality. Be distrustful of all such objec- 
 tives, arid if contemplating their purchase, always 
 reserve the right of having them examined by an 
 expert. Have a distrust especially of all "nameless" 
 objectives. It is safe to assume that if the maker can 
 not attach his name he is dpubtful of their superior- 
 ity. Any maker of responsibility will say without hes- 
 itation that he can produce objectives at less than 
 one -half their present cost, if he had the assurance 
 that they would be accepted as first put together, as 
 the cost of merely making and mounting lenses is 
 considerably less than the cost of making proper cor- 
 rections. In this case, however, they would be of 
 varying and inferior quality. 
 
 It is sometimes found that dealers offer the same 
 objectives of different quality at different prices. Too 
 great care cannot be observed in such cases, as the 
 very fact of the admission of a difference in quality 
 indicates that they are made by an unreliable maker. 
 This mode of offering objectives was in vogue many 
 years ago when the principles of optics and facilities 
 for making were limited, and when a higher price was 
 asked for those which might be termed a happy com- 
 bination. There is no excuse, however, at the pres- 
 ent day, for anything of this kind, because every con- 
 
98 
 
 scientious optician has his standard for every objec- 
 tive which is his guide. 
 
 In purchasing a microscope a beginner may be eas- 
 ily misled by the enticing appearance of an object, 
 which may be due not so much to the instrument as 
 to the object itself, and if the optical parts are infer- 
 ior, it will require but a short experience to become 
 convinced of it usually as soon as a comparison can 
 be made with reliable work. The investment in one 
 of these objectives is not only a source of disappoint- 
 ment, but usually proves to be a pecuniary loss, as it 
 is usually followed by a fresh outlay in responsible 
 work. 
 
 It is of ordinary occurrence that such objectives as 
 have just been spoken of are sent to the writer's firm 
 with the request to examine them and rectify the 
 faults ; but an examination almost invariably proves 
 that the cost of doing this is considerably greater 
 than purchasing a new objective of the same power, 
 and it would not even then be equal to the latter. 
 
 Accessories. As has been stated before, one of 
 the most useful accessories to the stand is the glass- 
 stage and slide-carrier. 
 
 Another accessory which is in equal demand, and 
 deservedly so, is the double nose-piece. By means of 
 it two objectives may be kept permanently attached 
 to the microscope, avoiding any loss of time from 
 changes. When properly fitted, an immediate change 
 can be made with the additional advantage of having 
 
99 
 
 the objectives centered and adjusted for focus, or 
 nearly so, which is a very decided consideration. 
 
 Fig. 20. 
 
 The same advantage holds good for the triple nose- 
 piece in the case of three objectives, or the quadruple 
 nose-piece for four. In these it is usually not con- 
 venient to arrange the objectives to correspond in 
 focus on account of the considerable difference in 
 focal distance. 
 
 The next in order of demand is the Abbe conden- 
 ser in its various forms of mounting. While some 
 able microscopists hold that the proper use of the 
 mirror will satisfy every demand, and that unless 
 properly used the condenser is disadvantageous 
 instead of beneficial, it is unquestionably true that it 
 will give results which the mirror alone can not pro- 
 perly give. Furthermore it enables results to be 
 obtained easily, which would require careful and long 
 experience by the mirror. The writer is in the best 
 position to know from the many inquiries received 
 from professional men, on how to increase the light, 
 that for this purpose and the better distinguishment 
 
100 
 
 of structure alone, it may be considered a boon. Bio- 
 logical work cannot be successfully prosecuted with- 
 out it. 
 
 The polariscope is absolutety requisite in petro- 
 graphical work, and in the examination of crystals in 
 connection with a selenite, it is an excellent appar- 
 
 Figs. 21. 
 
 atus in the hands of professional and amateur for 
 instruction and amusement, bringing out wonderful 
 coloration of many objects. Although the microscope 
 is really an instrument for scientific research, it does 
 not suffer by being used as a means for recreation 
 and pleasure, and in this direction no accessory will 
 aid more than a polariscope. Three selenites may be 
 used with it, giving the following combination of col- 
 ors : Blue and yellow, red and green, purple and 
 green. The polarizer is always arranged to resolve. 
 The bull's-eye condenser is mainly intended for the 
 illumination of opaque objects, and although modern 
 instruments have their mirrors so hung that they may 
 
101 
 
 be brought above the stage for this same purpose, 
 the results are not so satisfactory. The bull's-eye may 
 be used by interposing it between the mirror and 
 source of light, and with a very material increase in 
 its volume and easy regulation. 
 
 Fig. 22. 
 
 The Mechanical Stage, Micrometer, Camera Lucida 
 and other accessories have been treated separately, 
 and will therefore require no farther enumeration. 
 
SUB-STAGE ILLUMINATION. 
 
 While the objective and its proper application and 
 use will always be the most important part of the 
 microscope, the proper illumination of objects is 
 second in importance. 
 
 Mirror. As has been stated, the mirror alone 
 offers an excellent means of properly displaying an 
 object when intelligently used, for most of the low 
 and medium power objectives. On account of the 
 limited capacity of old objectives, efforts have been 
 made for the past fifty years to increase their utility 
 by the aid of lenses below the object. This was 
 mainly in the way of increase of intensity which at 
 the present day is not of so much consequence, as 
 our sources of illumination and light conveying 
 capacity of objectives is so much greater. 
 
 Abbe Condenser. The history of sub-stage con- 
 densers is very unique and interesting, and shows 
 how from having been the subject of no end of con- 
 demnation it is now the reverse. From single lenses, 
 compound, non-achromatic and achromatic, the use 
 of eye-pieces and objectives with any number of 
 
103 
 
 devices for regulating the light, the generally accepted 
 forms are those devised by Prof. Abbe. One of them 
 with a numerical aperture of 1.20 consists of a com- 
 bination of two lenses, and the other with an aper- 
 ture of. 1.42, of three lenses. These condensers are 
 applied to microscopes by each maker in a different 
 way, each seeking in his own manner to provide 
 means for controlling the volume or angle of light as 
 well as to obtain any degree of obliquity. To bring 
 the condenser to the minimum of cost, the Bausch & 
 Lo'mb Optical Co. quote both condensers in plain 
 adapters, giving the entire condensing capacity which 
 may be varied only by varying their distance from 
 the object. In this form they are incomplete, how- 
 ever, as their effectiveness depends upon the nicety 
 with which the light can be controlled. The most 
 
 Fig. 23. 
 
 simple device for this purpose consists of the mount- 
 ing with a slide in which various diaphragms wilh cen- 
 tral stops and openings may be dropped into it, and 
 
104 
 
 brought into the optical axis or passed diametrically 
 across the condenser, giving different degrees of 
 obliquity. 
 
 The vertical adjustment is obtained by sliding in 
 the sub-stage. 
 
 The best form is that which is provided with an 
 Iris diaphragm, which, when open, gives full volume 
 of light but may be reduced to nearly a pin-hole 
 opening. Then an addition which carries it laterally 
 under the condenser by rack and pinion, and another 
 rack and pinion for vertical adjustment either on the 
 sub-stage bar or as part of the condenser. As the suc- 
 cessful utilization of the condenser_depends upon the 
 nicety with which the various adjustments can be 
 accomplished, they can be none too complete. 
 
 Centering. In using the condenser the first con- 
 dition is the proper centering. For this purpose 
 the pin-hole cap should be placed over the top and 
 focused upon by means of a medium 
 power objective. The condenser 
 should be fixed so that the opening 
 will be in the center of the field. For 
 Fig. 24. objectives under 1.0 numerical aper- 
 ture it is not necessary to bring the condenser in 
 immersion contact with the slide ; for objectives of 
 wider aper-ture, however, it should be done by first 
 placing a drop of immersion fluid on the top lens and 
 then placing the slide in position on the stage, raise 
 the condenser until the fluid comes in contact with 
 the lower surface. 
 
105 
 
 It might be said here in parenthesis that tl 
 mirror should always be used with the conde 
 
 To Focus Condenser. An excellent plan for 
 focusing the condenser is to do so by means of a 
 low power objective, f to 3 inch, in the microscope 
 tube. After the slide is in contact with the condenser 
 or when not used with immersion and close to it, 
 focus the objective on the object. Now adjust the 
 condenser until the image which it projects of the 
 source of light is coincident with it. In the case of a 
 lamp, the flame will be projected. Daylight offers no 
 tangible image, and while the image of the window- 
 frame is not really quite correct, it is, in most cases, 
 sufficiently close for all practical purposes. 
 
 Intensity of Light. To say what amount of 
 light shall be used is very difficult on account of the 
 variety of specimens which involve different condi- 
 tions. While it is claimed that on stained bacteria 
 the best plan is to use the full volume of illumination 
 and thus differentiate the objects, it is certainly detri- 
 mental to do so on many histological specimens, diat- 
 oms and others, as they would be drowned in light 
 to such an extent as to be in some cases indistinguish- 
 able. The safest plan will be to use the minimum of 
 light and gradually increase until the point for pro- 
 per observation is reached. It will also be a good 
 plan not to use more light than is needed to accom- 
 plish the purpose desired. 
 
 To obtain oblique illumination, reduce the aperture 
 to i inch and gradually bring the opening out of the 
 
106 
 
 center up to the limit of aperture of the objective. 
 When beyond this, the object will appear illuminated 
 on a dark ground. The lateral movement should be 
 at right angles to the striae which it is desired to see, 
 and if this is not known, either the object should be 
 revolved or the mounting of the condenser. It will 
 sometimes be found advantageous to reduce or 
 enlarge the opening. 
 
CARE OF A MICROSCOPE. 
 
 Besides acquiring the ability to properly use an 
 instrument with its accessories, it is important to 
 know how to keep it in the best working condition. 
 It may be said without reserve that an instrument 
 properly made at the outset and judiciously used 
 should hardly show any signs of wear either in 
 appearance or in its working parts, even after the 
 most protracted use ; and further than this, every 
 good instrument should have a provision for taking 
 up lost motion, if there is a likelihood that this may 
 occur in any of the parts. 
 
 Especial care should be given to the optical parts, 
 in fact such care, that they will remain in as good 
 condition as when first received. Accidental injury 
 may of course occur to them, but if a systematic man- 
 ner of working is followed and a special receptacle 
 for each part is provided, this may usually be avoided. 
 The following rules refer mainly to the instruments 
 manufactured by the Bausch & Lomb Optical Co., 
 but are applicable to instruments in general. 
 
108 
 
 To Take Care of a Stand. One of the first 
 rules should be to keep the instrument free f ram dust. 
 This may be done in a manner formerly prescribed. 
 If dust settles on any part of the instrument, remove 
 it first with a camel's hair brush, and then wipe care- 
 fully with a chamois skin, with the grain of the finish 
 and not across it, as in the latter case it is likely to 
 cause scratches. Keep the working and sliding parts 
 absolutely free from dust, as this grinds and will 
 thus soon cause play. 
 
 Use no alcohol on any part of the instrument, as it 
 will remove the lacquer. As the latter is for the pur- 
 pose of preventing oxydation of the metals, it is 
 important to observe this rule. 
 
 In using the draw-tube impart a spiral motion. In 
 instruments which have no cloth lining, a straight up 
 and down movement should be employed, as the tube 
 will otherwise become scratched. 
 
 If it becomes necessary to lubricate any of the 
 parts, use a slight quantity of soft tallow or good 
 clock oil. 
 
 In an instrument which is in constant use, it some- 
 time occurs that the pinion works loose and occasion- 
 ally to such an extent that the body drops of its own 
 weight. Tightening screws are provided to take up 
 the play in the Professional, American Concentric, 
 Universal, Physician, Biological and Library Micro- 
 scopes these are in the back of the pinion. In the 
 Investigator, Model and Family Microscopes, they 
 are seen in the slide by removing the body. 
 
109 
 
 In using a screw-driver, grind its two large sur- 
 faces so that they are parallel and not wedge-shape, 
 and so it will exactly fit in the slot of the screw-head. 
 
 In inclining the stand always grasp it at the arm 
 and never at the tube, as in the latter case it may 
 loosen the slide or tear off some of the parts. 
 
 When repairs or alterations are necessary, always 
 have these made by the manufacturers ; they can, 
 from the system of duplicated parts, not only do it 
 cheapest, but best. 
 
 To Take Care of Objectives and Eye- 
 Pieces. It is as necessary to keep these free from 
 dust as the stand, in fact even greater cleanliness 
 should be observed. When indistinct, dark specks 
 show in the field, the cause may usually be looked for 
 in the field-lens, although sometimes in the eye-lens 
 also. The dust may be removed by a camel's-hair 
 brush, but when this is not sufficient use a well washed 
 piece of linen, such as an old handkerchief. From 
 its fine texture chamois skin is desirable, but as it is 
 fatty it should never be used until after it has been 
 well washed. 
 
 The same method applies to cleaning objectives. 
 Clean an immersion objective invariably after it has 
 been used, first by removing the fluid by a moist linen 
 and then by using a dry piece. 
 
 Keep the objectives especially in such a place 
 where they are not subject to extreme and sudden 
 changes of temperature, as the unequal expansion 
 
110 
 
 and contraction of glass and metal may cause the 
 cement between the lenses to crack. Also keep them 
 from direct sunlight. 
 
 Screw them into the nose-piece and unscrew, by 
 grasping the milled edge. 
 
 Avoid any violent contact of the front lens with 
 the cover-glass. Usually the latter suffers, but it is as 
 liable to occur to the former. 
 
 Above all, it should be made a rule that no one but 
 the owner handle the microscope and accessories. 
 One person may be expert in the manipulation of one 
 instrument and still find it difficult to work with 
 another. The fine adjustment particularly causes the 
 greatest difficulty as in some instruments it corres- 
 ponds with the movement of the micrometer screw, 
 while in others it is contrary and thus the objective 
 as well as object are endangered. 
 
APPENDIX. 
 
 (Reprinted from "The Microscope," Jan., 1885.) 
 
 CONSIDERATIONS IN TESTING OBJECTIVES. 
 
 EDWARD BAUSCH. 
 
 There is a laudable desire in almost all persons 
 possessing a microscope to become intimately ac- 
 quainted with it, and for this purpose it is not only 
 necessary to learn the use of its mechanical parts, 
 but to understand its optical capacity, which is .con- 
 siderably more difficult, and which involves more 
 considerations than would appear on first thought. 
 
 With all the care which may be bestowed upon 
 objectives, they are, to a certain extent, works of 
 chance, and depend upon the optician's judgment, 
 industry and skill, and upon the variations in glass, 
 for their excellence and uniformity. These condi- 
 tions are often so varying that in the case of sev- 
 
eral objectives of the same formula, made at the 
 same time there will be such great differences that 
 it can hardly be conceived on the first examination, 
 that they were to be similar. It is at this point 
 especially necessary to detect the errors, to deter- 
 mine their cause and apply the remedy, and to do this 
 properly often involves an inconceivable amount of 
 work, and in many cases the final results are reached 
 at a pecuniary loss. There are certain fixed tests for 
 each kind of objective, and to the best of my knowl- 
 edge all reputable opticians bring each objective up 
 to its standard before allowing it to pass their hands, 
 irrespective of the cost of doing_so. This must of 
 necessity be so, if only out of business consideration, 
 and not for a love of each production, for it is evi- 
 dent that a well-earned reputation would soon lose its 
 pre-eminence, and would acquire one for unreliable or 
 poor work, if on comparison, objectives of the same 
 kind would show a marked difference. There is some- 
 times a fortunate combination of circumstances which 
 makes a certain objective better than its fellows, but 
 this is a rare exception, and is positive evidence that 
 the exact requirements of the formula have been com- 
 plied with. As a rule, therefore, I believe that the 
 opticians' claim may be relied upon, and where the 
 results in the hands of the microscopist do not corres- 
 pond with them, .the cause may usually be looked for 
 in the lack of experience in manipulation or in con- 
 ditions, which differ from those under which the objec- 
 tive was completed. The belief, which I am aware is 
 
113 
 
 extant, that there are great differences in objectives 
 purporting to be similar, is, in my opinion, not justi- 
 fied, at any rate in the productions of those men who, 
 by general acknowledgement, are at the head of their 
 profession. I admit that, as in everything which 
 depends upon human skill, there is, strictly speaking, 
 no absolute uniformity, but also claim, that with few 
 exceptions, the differences are so slight, that anything 
 but the most expert manipulation cannot detect them. 
 
 It therefore appears to the writer that any informa- 
 tion which will tend to improve the knowledge of 
 testing objectives will not only prove beneficial to the 
 microscopist, but will prove advantageous to the 
 optician, in that his work will receive a fair trial, based 
 upon a knowledge of the principles involved, and 
 that he may be convinced that all his work which 
 deserves commendation will be the better appreciated. 
 The following points are by no means new, but are 
 often lost sight of in making tests. The writer will 
 speak of medium and high power objectives only, as 
 the deleterious influences are most noticable in these, 
 but they apply as well to the lower powers though in 
 a less degree. 
 
 The part of the instrument which has a strong 
 bearing in the performance of the objective is the 
 mirror. It should be adjustable on the mirror-bar, so 
 that it can be accommodated to the variations in dis- 
 tance of the source of light from the instrument. 
 When parallel rays are used, as with light from the 
 sun or clouds, its distance from the object should be 
 
114 
 
 decreased and increased when lamp-light is used. It 
 should be exacted that the focus of the concave mirror 
 be within the limits of its adjustment. The serious 
 disadvantage of its incorrectness in this respect can 
 easily be seen by taking, for instance, a -^ objective 
 which will resolve P. angulatum nicely with central 
 light, when the mirror is exactly focused. By mov- 
 ing the latter out of focus it will be seen that the 
 objective loses in performance, and if this is carried 
 sufficiently far it will arrive at a point where the 
 objective will cease to show any lines. The effect will 
 be the same on any other object, and is caused by the 
 lack of proper concentration of light on the slide. 
 When oblique light is used, unless the diaphragm 
 moves with the mirror, it should be removed, as the 
 advantage of obliquity is diminished or destroyed by 
 the loss of light. 
 
 The cover-glass exerts probably the greatest influ- 
 ence in testing as well as in general work. This 
 should be used of a thickness which corresponds to 
 that to which the objective (if non-adjustable) was 
 originally corrected. If thicker or thinner covers be 
 used, the objective will be spherically over or under 
 corrected, and will have to be moved correspondingly 
 above or below the plane ( outline ) of the object 
 to distinguish its structure, if the variation is con- 
 siderable the difference between the two planes will 
 be so great that it will cease to show any structure, 
 and it may then be said to be lacking in defining 
 power, although in reality it possesses it but is not 
 
115 , 
 
 properly used. Generally speaking the objective may 
 be said to be spherically corrected when it gives the 
 best defined image ; that is, when the outline and 
 internal structure of an object of extreme thinness 
 appear in one plane. When, after the objective has 
 been focused on the outline of the object, it is neces- 
 sary to increase the distance to focus on the structure, 
 it is evidence that the objective is spherically over- 
 corrected and that the cover is too thick ; in adjust- 
 able objectives the correction collar must be brought 
 to its closing point, which means that the lenses are 
 brought in closer contact. When the objective must 
 be focused to a point beyond the outline of the object 
 to see its structure that is, brought closer to the 
 cover-glass it proves that this is too thin, and is then 
 said to be spherically under corrected ; to give the 
 proper adjustment in an adjustable objective in this 
 case the adjustment is opened the lenses are sepa- 
 rated. It requires a certain amount of study to dis- 
 tinguish these phenomena, and although it can be 
 done in well prepared specimens, I know of none 
 better than coarsely marked diatoms, such as P. 
 angulatum. 
 
 Although I am aware that many eminent micro- 
 scopists do not favor adjustable objectives for every 
 day work, 1 must confess that I fail to see the force 
 of their arguments. From the foregoing it will be 
 seen that unless the cover-glasses are of a thickness 
 corresponding to that which was originally used, the 
 objective may be made to do imperfectly what is in its 
 
116 
 
 power to do well, and when pressed to its full capacity 
 may and is likely to fail. It must be remembered 
 that cover-glasses of the same number are not of the 
 same thickness. The selection of those of proper 
 thickness is expensive and tedious, whereas the 
 knowledge of correcting the objective is easily 
 acquired, and in the latter case it is in the manipu- 
 lator's power to command the highest performance of 
 which the objective is capable ; further than this, it 
 has the advantage that it may be used as a non-adjust- 
 able objective if desired. When homogeneous immer- 
 sion objectives were first introduced they were mounted 
 in fixed settings, as it was expected that the thickness 
 of the cover-glass would not affect the correction ; 
 although this assumption was correct, it was found 
 that even in these it was necessary. How much more 
 then, is it required in dry or water immersion objec- 
 tives ? 
 
 Another factor in the disturbing influences is the 
 variation in length of tube ; the deleterious results 
 are similar to those with varying cover-glasses. Objec- 
 tives are usually adjusted to 8| or 9 inches length of 
 tube, and although this in itself is a fixed standard, it 
 usually becomes variable by changing objectives and 
 eye-pieces. That this is so in objectives is patent, 
 and that it is so in eye-pieces can easily be determined 
 by making a change in powers, when it will be found 
 that a change in focus is required. By decreasing the 
 length of tube the objective will appear to be spheri- 
 cally under-corrected and vice versa when it is increased, 
 
117 
 
 so that it is apparent that by the use of the draw-tube 
 the effect of the cover-glass may be partially neutral- 
 ized ; for instance, when by the use of a thin cover 
 the objective is spherically under-corrected, it may, to 
 a certain extent, be corrected by causing a correspon- 
 ding over-correction in the tube by increasing its 
 length. The use of the draw-tube for the purpose of 
 changing the amplification or for the matter of con- 
 venience can hardly be commended, except in cases 
 where adjustable objectives are used. 
 
 Considerable also depends upon the perfection of 
 the eye-piece. I believe that, as a rule, too little care 
 is devoted to it ; at any rate, it is certain that while 
 any Huyghenian eye-piece for a telescope can be used 
 on a microscope, very few which have been made for 
 this can be used on a telescope ; and while it is true 
 that no such perfection may be required in the former, 
 it leaves such an indefinite range that it may become 
 difficult to place a limit for the perfect and imperfect. 
 In all work, and especially in testing, it should be seen 
 that the eye-lens, as well as the field-lens, are perfectly 
 clean. 
 
 Among the absolutely necessary conditions in judg- 
 ing of the quality of an objective are perfect speci- 
 mens, especially if they are sections. A thick object 
 obstructs the light and generally makes it necessary 
 to go through so many layers or planes that it is diffi- 
 cult to get any one distinct ; the impression may thus 
 easily be given that the objective is at fault. The 
 difference between two objects of the same nature 
 
118 
 
 may be so great that, while with one the objective may 
 be condemned as imperfect, it may with the other 
 appear to be of extraordinary excellence. 
 
 In conclusion, I will say that there may be other 
 conditions which may influence the performance of a 
 lens, and to acquire the power of eliminating them 
 requires considerable experience. When an objective 
 does not correspond with the claims of the optician, 
 judgment should not be passed upon it until after 
 repeated trials have been made, in all of which the 
 above points should not be lost sight of. 
 
Bausch & Lomb Optical Co., 
 
 MANUFACTURERS OF 
 
 MICROSCOPES, OBJECTIVES, ACCESSORIES, 
 
 PHOTOGRAPHIC LENSES, DIAPHRAGM 
 
 SHUTTERS, TELESCOPES, EYEGLASSES AND 
 
 LENSES, MAGNIFYING GLASSES, 
 
 AND A LARGE VARIETY OF 
 
 OTHER OPTICAL INSTRUMENTS. 
 
 FACTORY AND MAIN OFFICE, 
 
 531 TO 543 NORTH ST. PAUL STREET, 
 
 ROCHESTER, N. Y. 
 
 BRANCH OFFICE, 
 
 48 AND 50 MAIDEN LANE, 
 
 NEW YORK CITY. 
 
120 
 
 On the following pages will be found illustrations 
 of a number of instruments manufactured by 
 
 Bauseh & Lomb Optical Co., 
 
 ROCHESTER, N. Y., AND NEW YORK CITY, 
 
 Who issue the following Catalogues : 
 
 I Eyeglasses, Lenses, Magnifiers, Readers, etc. 
 
 2 Microscopes, Objectives and Accessories. 
 
 3 Supplement to Microscope Catalogue (i2th 
 Edition). 
 
 4 Photographic Lenses and Diaphragm Shutters. 
 
 Any or all of which will be sent to any address 
 free on application. 
 
121 
 
 AMERICAN STANDARD 
 
 PHOTOGRAPHIC LENSES, 
 
 UNIVERSAL 
 G. CLARK. 
 
 Both conceded to be, at least, equal to the best 
 of European manufacture. 
 
122 
 
 (Cut one-third of actual size.) 
 
 COMPACT DISSECTING AND MOUNT 
 ING MICROSCOPE. 
 
123 
 
 (Cut one-third of actual size.) 
 
 MODEL MICROSCOPE, 
 
124 
 
 Made in three forms. 
 
 ist with sliding tube, up- 
 right. 
 
 2d with sliding tube and 
 joint for inclination. 
 
 3d with rack and pinion for 
 coarse adjustment and joint 
 for inclination. 
 
 ' r : =, i 
 
 (Cut one-third of actual size.) 
 
 BIOLOGICAL MICROSCOPE. 
 
125 
 
 (Cut one-third of actual si/e.) 
 
 PHYSICIAN MICROSCOPE 
 
126 
 
 (Cut one-third of actual size.) 
 
 UNIVERSAL MICROSCOPE. 
 
127 
 
BRANCH OKFICB 
 
 OF THE 
 
 Bauseh & Lomb Qptieal Co., 
 
 IS LOCATED AT 
 
 48 AND 50 MAIDEN LANE, 
 NEW YORK CITY, 
 
 Where all visitors are cordially invited to call, and 
 will find in stock a complete assortment of goods 
 of our manufacture which will be shown with 
 much pleasure. 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 BERKELEY 
 
 IS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
 Books not returned, on time are subject to a fine of 
 50c per volume after the third day overdue, increasing 
 to $1.00 per volume after the sixth day. Books not in 
 demand may be renewed if application is made before 
 expiration of loan period. 
 
 OCT 28 1918 
 
 SE? 
 
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