QC 
 373 
 M4 
 P3 
 
 IRLF 
 
 THE OPTICS OF METALLOGRAPHY 
 
 By W. L. PATTERSON 
 
 BAUSCH & LOME OPTICAL CO. 
 
 of California 
 
 593 Market Street 
 
 San Francisco, California 
 

 1 IDftA 
 
 NOTE : Practically all of the important developments 
 in equipment for metallography, since the first publica- 
 tion of this article, are embodied in the new Bausch & 
 Lomb Large Metallographic Equipment. For this rea- 
 son, a rather complete description of this apparatus has 
 been incorporated in this revision. 
 
 In order that no confusion may exist as to which 
 equipment is referred to, all descriptions and illustra- 
 tions of this new apparatus are marked with an asterisk, 
 thus(*). 
 
 This is a reprint of an article presented to the American Society 
 for Steel Treating (Vol. II, No. 2, 1921) with additions and revisions. 
 
THE OPTICS OF METALLOGRAPHY 
 
 By W. L. PATTERSON 
 
 OBJECTIVES 
 
 The objectives undoubtedly constitute the most important part of the 
 microscope. Their quality determines the final results. 
 
 The objective is so called because it is nearest to the object. Figures i, 
 2, and 3 show respectively the 16, 4, and 1.9 millimeter objectives of the 
 usual type, but they are made in a variety of focal lengths, numerical aper- 
 tures, and styles of .mount by various makers. Following are given the 
 usual focal lengths and numerical apertures (N. A.) of achromatic objec- 
 tives: 
 
 Focal Lengths Numerical Type of Lens 
 
 Millimeters Aperture (N.A.) 
 
 32 o.io Sirigle achromatifc lens. 
 
 1 6 - 2 5 Two achromatic. doublets. 
 
 8 0.50 
 
 4 0.85 
 
 3 0.85 
 
 1.9 1.25 
 
 1.9 1.32 
 
 These three have non-achromatic front with 2 
 achromatic doublets. 6 lenses in all. 
 
 { Fluorite construction, semi-apochromatic. 
 
 Further reference will be made to the apochromatic objectives later. 
 The focal length of an objective does not indicate its working distance, but 
 means that the combination of lenses composing the objective is equal in 
 focus to a single lens of the stated focus. Thus the working distance for 
 the 
 
 1 6 millimeter focus = 7 millimeters 
 4 " = 0.30 
 
 1.9 " = 0.15 
 
 An objective may be said to possess seven qualities: magnifying power; 
 aperture, or numerical aperture; resolving power; depth of focus, or 
 penetrating power; illuminating power; flatness of field; and defining 
 power. To fulfill all of these requirements in a satisfactory manner 
 necessitates considerable skill upon the part of the optician. These quali- 
 ties will be discussed one by one. 
 
 Magnifying power usually is stated in the catalogues of various makers 
 in terms of the combination of a certain objective, certain eyepiece, and 
 tube length at a predetermined image distance. While tables given in the 
 catalogues are approximately correct, the magnification stated will be ob- 
 tained only if the conditions given are fulfilled. If the tube length, that is 
 the distance between objective and eyepiece, is increased or decreased, or 
 an increase or decrease in image distance is taken as in photography, the 
 magnification tables will not prove correct. Tables given in catalogues 
 are often for visual work and are based on an apparent or virtual image 
 being formed in space at a distance of 250 millimeters from the eye. This 
 
 Urn i ion A 
 
Fig. i 1 6 mm objective of usual type 
 
 Fig. 2 ^ mm objective of usual type. 
 
 Fig. 3 /.<? mm objective of usual type. 
 
 apparent size will vary with persons having near or far-sighted eyes, but 
 the normal distance is taken as 250 millimeters and the size of the image is 
 the same if projected on the ground glass of a camera at 250 millimeters. 
 
 Makers of objectives also give in their catalogues the initial magnifica- 
 tion of their several objectives, that is the magnification they give at a 
 certain distance unaided by the eyepiece. These tables of initial magnifi- 
 cation will be found to vary in different catalogues even though objectives 
 are of the same focal length, this being due to the different systems of 
 calculating magnifications. It will be found, however, that for the same 
 focus of objective, the same focus of eyepiece, and the same tube length, 
 the figures nearly agree. Some makers use a number system for both eye 
 pieces and objectives, but in making comparisons only focal lengths 
 should be considered. 
 
 In the large inverted forms of metallurgical microscopes the distance 
 between objective and eyepiece is usually about 40 millimeters longer 
 than in the ordinary table microscope, and therefore, the magnification is 
 increased some 25 percent even with the same focus of objective and eye- 
 piece; hence the difference in magnification tables for this microscope. 
 With the many variable factors in different outfits it is difficult to make 
 comparisons, and it is best to calculate the magnification for the particu- 
 lar outfit being used, especially as regards camera magnifications. To ob- 
 tain the exact magnification at the ground glass of the camera, and there- 
 fore of the photograph, it is best to project a stage micrometer upon the 
 ground glass and measure its magnified image. Micrometers graduated 
 upon metal may be obtained for this purpose. The rulings are in tenths 
 and hundredths of a millimeter. They are placed upon the stage and il- 
 luminated the same as a metal specimen. After focusing upon the ground 
 glass the magnification can be measured directly without further calcula- 
 tion. 
 
 The aperture, or numerical aperture (N.A.) is a property to which 
 many buyers of microscopes probably pay little attention, but it is never- 
 theless an important quality of an objective. The diagram in Figure 4 
 is shown in an attempt to simplify the theory of numerical aperture. If we 
 
assume that few, if any, objects are perfectly 
 smooth, then light is dispersed from every 
 point on the surface of an object in all direc- 
 tions up to 1 80 degrees. Only an extremely 
 narrow pencil of this can be received by the 
 unaided eye. The apparent problem of prac- 
 tical optics is to be able by means of lenses to 
 gather and bring to a focus as many of the un- 
 admitted rays as possible. In early investiga- 
 tion it was found that the objective which 
 gathered in the greatest angle of rays gave the AM - "" n 3in u ' 
 
 greatest resolving power. Fig. 4 Diagram showing 
 
 T r i j i i -11 theory of Numerical Aper- 
 
 If only dry objectives were to be considered, ture 
 
 we might make a comparison on the basis of 
 
 angular aperture, that is the angle of the cone of light which is admitted 
 to the front lens of the objective and reaches the eye. This is important, 
 as internal diaphragms reduce apertures, but these apertures may be 
 measured readily by special instruments. 
 
 A thorough study by Professor Abbe, however, proved that the only 
 exact method for comparison of objective apertures was by comparison of 
 the sines of the extreme admitted radiant pencils, and, when the media 
 differed between the object and the objective, of the refractive indices of 
 those media, which are i.oo for air, 1.33 for water, and 1.50 for cedar oil. 
 It is very evident that rays emanating from an object and passing to the 
 objective will travel in different angles according to the media through 
 which they pass, owing to the different refractive indices or bending 
 powers of such media. 
 
 In Figure 4 is shown a ray R x leaving the object at an angle of 30 de- 
 grees, passing through air and just entering the extreme edge of the front 
 lens of the objective. A second ray R 2 leaving the object at the same 
 angle and passing through oil will be bent so as to fall within the edge of 
 the objective lens, as shown by R 3 . It is obvious that a ray leaving the 
 object at a still greater angle than R 2 and passing through oil will be bent 
 so as to pass into the objective. Thus a wider angle of rays is collected in 
 case of the oil-immersion objective than in the dry objective. 
 
 It will be seen that angular aperture can not be a comparative measure 
 for the two kinds of media. Abbe found that a value could be expressed 
 for the different conditions by taking the sine of one-half the angle of 
 aperture and multiplying it by the refractive index of the media between 
 object and objective; hence the formula: 
 
 Numerical aperture (N. A.) = n sin u 
 
 where n = refractive index of media: air i.oo, water 1.33, oil 1.50, 
 and u = one-half the angular aperture. 
 Thus the numerical aperture on the air side would be 
 
 i.oo X sine 30 = i.oo X .5 = 0.50 
 and on the oil side 
 
 1.50 x sine 30 = 1.50 x .5 = 0.75 
 
 3 
 3 31 /O 
 
This gives a method of comparison between dry - and oil - immersion 
 objectives. 
 
 The resolving power, which is the property by which an objective 
 shows distinctly separated two small elements in the structure of an ob- 
 ject, is directly proportional to the numerical aperture; thus an objective 
 of 0.50 numerical aperture, as in the 8 millimeter, has twice the resolving 
 power of the 16 millimeter, or 0.25 numerical aperture. 
 
 If a very narrow central pencil is used for illumination, the finest detail 
 that can be shown by the microscope, with high enough magnification, is 
 equal to ^ where lambda is the wave length of the light used for il- 
 
 N.A. 
 
 lumination, say one-half micron. The wider the pencil used for illumina- 
 tion, the greater the resolving power until a maximum is reached, when 
 the width of the pencil is sufficient to fill the whole aperture of the objec- 
 tive. In this case the resolving power is twice as great, the finest detail 
 that the objective can show being now equal to ^ For a 0.50 
 
 2 X N.A. 
 
 objective the first would be one micron with reduced aperture, and the 
 second one-half micron with full aperture. 
 
 For practical purposes we may take the rule that the numerical aperture 
 multiplied by 100,000 will give the number of lines per inch which can be 
 resolved theoretically by a given objective. Thus, pearlite of say 25,000 
 laminations per inch should be easily resolved with an 8 millimeter ob- 
 jective of 0.50 numerical aperture, and a suitable eyepiece and bellows 
 draw can be used to magnify the image to the size desired, say 400 diame- 
 ters. On the other hand, an objective of say 0.25 numerical aperture 
 could scarcely be expected to resolve this structure no matter how high we 
 magnify the image by eyepieces or bellows extension. The magnification 
 should never exceed 1000 times the numerical aperture; preferably it 
 should be somewhat less, dependent upon the quality of the objective. 
 
 Depth of focus (known also as depth of sharpness or penetration) is 
 another important factor which is often not clearly understood. It de- 
 pends on the numerical aperture and the magnification, and is inversely 
 proportional to both. The formula used to express depth of focus is 
 
 i ; therefore, the higher the numerical aperture and the higher the 
 N.A. 
 
 magnification the less the depth of focus. It is beyond the power of the 
 optician to change these conditions. Every effort aiming at an increase of 
 the depth of focus, for instance, by inserting diaphragms above the back 
 lens of the objective, must necessarily decrease the effective diameter of 
 the back lens and thus decrease the numerical aperture, thereby lowering 
 the efficiency of the objective as regards resolving power. It may be ad- 
 visable at times to introduce such diaphragming for the work to be done, 
 and it will be shown later how such a change can be made by proper use of 
 the illuminating system. However, objectives that show greater depth of 
 focus than others of the same numerical aperture are not well corrected for 
 other qualities. 
 
 4 
 
The illuminating power of an objective is equal to the square of the 
 numerical aperture, and objectives can be so compared; that is, a lens of 
 0.25 numerical aperture has an illuminating power of 0.062, while one of 
 0.50 numerical aperture has 0.25 or 4 times as great, provided it is used at 
 the same magnification. "Flatness of field as such," said the late Dr. 
 Carpenter, "is an optical impossibility." But by using compensation or 
 hyperplane eyepieces to correct the margin of field, better effects aye ob- 
 tained, and by increasing the depth of focus by reducing the used aper- 
 ture, apparent flatness is secured but at the expense of resolving power, as 
 stated before. As an illustration of this we refer again to the statement 
 concerning the depth of focus, which is equal to i , hence, for an 
 
 N.A. 
 
 objective of 0.50 numerical aperture, the depth would be 2, while for an 
 objective of 0.85 numerical aperture, the depth would be 1.17. 
 
 As depth of focus also varies with the magnification, if we use the above 
 lenses at the same magnification we should naturally have greater depth 
 in the objective of 0.50 numerical aperture, and if this gives sufficient re- 
 solving power for the work in hand it may be used to obtain a flatter field. 
 
 Defining power depends upon the finest correction of spherical abbera- 
 tion and chromatic aberration, the perfect centering of the lenses of the 
 objective, and in fact on the general excellence of the mechanical work in- 
 volved in the making, and it is here that the optician must show his skill. 
 It should be borne in mind that the finer the definition of an objective, 
 the more sensitive it is to incorrect focusing and to slight changes of the 
 adjustment through vibration, etc. In constructing objectives their 
 formulae should be based upon rigorous computations, all elements of 
 construction, such as glass, radii, thickness of lenses, separation, etc., 
 being determined in advance wholly without recourse to experiment. 
 This method of construction is the only one insuring uniformity in optical 
 systems of such intricacy as a microscope objective. 
 
 It will be well to consider at this time the effect of tube length, that is, 
 the distance between the objective and the eyepiece with which it is used. 
 Most of the objectives used on biological and smaller metallurgical 
 microscopes are corrected for a mechanical tube length of 160 to 170 
 millimeters. By mechanical tube length is meant the distance from the 
 shoulder against which the objective screws to the shoulder against which 
 the eyepiece rests. In the case of the larger metallurgical stands of the 
 inverted type this distance is usually longer, owing to reflections neces- 
 sary to secure an inverted stage and a convenient position for the viewing 
 tube. Therefore, the objectives used on this type of microscope should be 
 corrected for this extra length. The use of objectives at incorrect tube 
 length, especially in powers over 100 diameters, will result in inferior 
 definition. This is especially true in metallographic work. 
 
 Objectives for metallurgical work must also be corrected for use without 
 coverglass, as specimens of metal are examined without covering. 
 
 All objectives used in biological microscopes are corrected for the re- 
 fraction which takes place when the ray from the object passes through a 
 
 5 
 
Fig. 5 Effect of coverglass upon focus. 
 
 thin piece of glass. These objectives are not suited for use on uncovered 
 objects, and if used in metallurgical work only inferior results will be ob- 
 tained except with the very lowest powers, such as 16 millimeters or lower. 
 To show the effect of cover-glass, Figure 5 has been taken from Carpen- 
 ter's book on the microscope. It will be seen that without the cover-glass 
 an ordinary microscope objective would focus at different planes for the 
 central and marginal zones, as shown at points Y and X, but with the re- 
 fraction of the cover-glass the rays all focus at the point O. 
 
 Aplanatic system. Under-corrected system. 
 
 Fig. 6 Underc orrected system obtained without use of coverglass. 
 
 Therefore if the ordinary objective is used without cover-glass, it will 
 work as an under-corrected system, as shown in Figure 6, whereas when 
 corrected for use without cover the objective should be perfectly aplana- 
 tic, as shown at the left, all rays focusing at one point. This correction 
 cannot always be accomplished by changing distances between lenses but 
 requires special computation. The oil-immersion objective, however, 
 needs no special correction as regards cover-glasses, because the immer- 
 sion oil and cover-glass are of the same refractive index, and if the cover is 
 missing, the additional strata is made up by a thicker layer of oil. The 
 tube length, however, must be correct even in this case. 
 
 As to the objective mounts, both long and short mounts are used in 
 metallurgical work. The short mounts should always be used with mirror 
 or prism illuminators so as to bring the illuminator as near to the back 
 focus of the objective as possible. The tendency of late, however, has 
 been to use short mounts for all kinds of illuminators. 
 
 The objectives previously described are of the achromatic type, but the 
 same characteristics are also found in the apochromatic objectives, and 
 they are even more sensitive to deviations from the standards for which 
 they are made. The usual focal lengths and numerical apertures are as 
 follows : 
 
Focal Length 
 
 1 6 millimeters 
 8 
 
 4 u 
 
 3 
 
 2 
 
 Numerical Aperture (N.A.} 
 apochromatic achromatic 
 
 0.30 
 
 0.65 
 
 -0-95 
 0.95 
 
 0.25 
 0.50 
 0.85 
 0.85 
 1.25 
 
 It will be seen that the numerical apertures are somewhat greater in the 
 apochromatic objective. This means increased resolving power and other 
 features attending higher aperture. These lenses have other improve- 
 ments than increased aperture. 
 
 The word apochromatic means free of color and is illustrated by Figure 
 7. If we were to form an image with an ordinary lens of non-achromatic 
 form, a rayof white light coming from the object would be broken up into 
 the several colors of the spectrum and images would be formed at differ- 
 ent distances from the lens, corresponding to each color. There would 
 thus be a series of images, one behind the other, ranging from violet to red, 
 the shorter waves being refracted more than the longer ones. 
 
 Fig. / Illustration of apochromatic lens. Fig. 8 Correction for spherical aberration. 
 
 In the standard achromatic objective, two of these colored images 
 would be combined, and in apochromatic objectives three colored images 
 would form in the same plane, and as the violet rays are brought to a focus 
 at the same plane as the applegreen visual rays, these objectives are ex- 
 cellent for photographic purposes, even when used without filters. As 
 shown by Figure 8, the correction for spherical aberration is more perfect. 
 Spherical aberration refers to the fact that in an ordinary uncorrected lens 
 the portion of a ray coming through the center and the portion coming 
 through the outer zones are focused at different planes. In the uncorrect- 
 ed lens this applies to all colors; in the achromatic objective the correction 
 is made for one selected color, say green, and in the apochromatic for two 
 colors. Therefore, for the very finest class of investigations the apochro- 
 matic objectives are essential owing to their finer color and spherical cor- 
 rections. These corrections are accomplished by the use of special glasses 
 
and a transparent mineral substance known as fluorite. Another series 
 of objectives now offered upon the market is the 4 millimeter dry and 1.9 
 millimeter oil immersions, known as semi-apochromatic objectives. They 
 have one fluorite element and are much better than the regular achromatic 
 objectives. They make a good compromise where cost will not permit the 
 purchase of apochromatics. 
 
 EYEPIECES 
 
 The second part of the optical equipment to be considered is the eye- 
 piece, so called because it is used near the eye. This may, however, lead to 
 some confusion, as the eyepiece is also necessary when forming the image 
 upon the ground glass of the camera. Some users of this class of apparatus 
 have been found who thought the eyepiece was needed only in visual 
 work. Small photographic lenses are used without eyepieces, as later ex- 
 plained. The ordinary eyepiece shown in Figure 9 is known as the 
 
 Fig. p Eyepiece of Huygenian form. 
 
 POSITIVE COMPENSATION OCULAR 
 (From Spitta, p no). 
 
 Fig. 10 Compensation type eyepiece. 
 
 Huygenian form after its designer. It consists of the two non-achromatic 
 plano-convex lenses with a diaphragm for limiting the field between them. 
 The upper lens is known as the eye lens and the lower as the field lens. 
 Huygenian eyepieces are made in a variety of powers, as 5 X, 6.4 X, 7.5 X, 
 and loX and 12. 5 X, these designations meaning that they magnify the 
 image formed by the objective by these amounts. Different makers used 
 different designations for their eyepieces and this is likely to cause confu- 
 sion if attempt is made at comparison. 
 
 In the case of Leitz and Zeiss the eyepieces are numbered from o to 5, 
 but the eyepieces of the Bausch & Lomb Optical Co. are marked with their 
 magnifying power as 5 X, loX, etc. 
 
 A new eyepiece known as the hyperplane produces a flatter image plane 
 than the Huygenian eyepiece and allows for a larger field of view, well 
 
adapted for photomicrography. These eyepieces have a compensation 
 about half-way between the Huygenian and compensation eyepieces de- 
 scribed below. 
 
 The hyperplane eyepieces are made in powers from 5 X to 20 X and may 
 be used with all classes of objectives. 
 
 Another form of eyepiece is known as the compensation- eyepiece, 
 shown in Figure 10, so called because it compensates for the variation in 
 size of the blue and red images given by the apochromatic objective. While 
 the apochromatic objective is so constructed as to bring these colors into 
 focus at the same plane as previously stated, the images formed are not of 
 the same size, and this variation is neutralized by the compensation eye- 
 piece. The latter should, therefore, always be used when one is using 
 apochromatic objectives, and may also be used to good advantage with 
 the high-power achromatic objectives. The compensation eyepieces of 
 various makers are not always interchangeable with different apochro- 
 matic objectives. 
 
 These eyepieces vary in construction and in the number of lenses ac- 
 cording to their power. They are made in powers from 5 X to 25 X, much 
 higher than the Huygenian type, and it is possible to use these high powers 
 with the apochromatic objectives owing to their finer color correction. 
 
 There is another type of eyepiece known as the projection eyepiece, in 
 which the eye lens or lens nearer to the photographic plate is adjustable 
 according to the bellows extension of the camera. With this adjustment 
 one is able to use the objective in the same relative position to the object 
 as it is when used for visual work. Some of these eyepieces have a very 
 small opening in the diaphragm, which is likely to indicate that the field 
 is unusually flat. 
 
 It is possible to get effects similar to those produced by the projection 
 eyepieces, with ordinary eyepieces, by partially withdrawing them from 
 the eyepiece tube when making photographs. This distance may be 
 found by the formula: 
 
 square of eyepiece focus 
 length of camera draw 
 
 Thus for a 5 X eyepiece of 50 millimeter focus and a bellows draw of 250 
 millimeters, the eyepiece would have to be extended 10 millimeters, or it 
 may be determined by experiment. It will be seen that the distance be- 
 comes greater with low-power eyepieces and short bellows draw. This 
 adjustment keeps the objective in the same relation to the object as when 
 used in visual work, as previously stated, and preserves its best spherical 
 correction, tending to flatten the fields. 
 
 Too much stress cannot be laid upon the necessity of keeping all of the 
 optical parts clean and free from dust, grease, or finger marks. Poor re- 
 sults are often due to this neglect. 
 
VERTICAL ILLUMINATOR 
 
 In Carpenter's book on the microscope it is stated that the first vertical 
 illuminator was made and used by Prof. H. L. Smith of Geneva, New 
 York. Smith's illuminator consisted of a piece of speculum metal so 
 placed as to reflect light down through one side of the objective, the image 
 returning through the opposite side. The plane-glass illuminator is said to 
 have been designed by Beck of England. In this form the light is re- 
 flected by the surface of the glass, through the objective and the image re- 
 turns from the specimen through the glass to the eyepiece. Formerly 
 much flare was experienced in using this type of illuminator, but it can be 
 eliminated by proper illuminating appliances and diaphragms. Both 
 forms of illuminator are in use today, the first in the form of mirrors and 
 the latter much in its original form. 
 
 The mirror and prism types are preferred by some workers, and while 
 they are satisfactory 'for low powers and relief work, they reduce the 
 available aperture of the objective by nearly one-half with consequent 
 reduction in resolving power. They do give increased illumination, due 
 to total reflection, and may be used for projecting metal specimens with 
 low powers when several observers wish to study the image. For high 
 powers and utmost resolution, the plane-glass reflector is best. 
 
 Vertical illuminators have appeared from time to time in many forms. 
 The one shown in Figure 1 1 is a base model to which can be fitted mirrors, 
 reflector, plane-glass reflector, lenses and diaphragms, as desired. For 
 visual work, side tubes with lenses and lamps may be attached and used 
 successfully, but when high-power illumination is used and the beam of 
 light must be carefully centered to the objective, the best results can be ob- 
 tained by having condensers and lamps on separate standards. A recent 
 addition to this type of illuminator consists in clamping screws, whereby 
 all adjustments can be locked so that once adjusted the illuminator cannot 
 be deranged easily. Care must be exercised to keep the glasses of the il- 
 
 Fig. if Vertical illuminator to which can be 
 
 attached mirrors^ reflector^ plane glass reflec- Fig. 12 Condenser and iris diaphragm 
 
 tor, lenses and diaphragms as desired. mounted on a separate standard. 
 
 10 
 
luminator clean and to prevent distortion by bending the mount when 
 cleaning as it will tend to destroy the definition. 
 
 Figure 12 shows a condenser and iris diaphragm mounted on a separate 
 standard in place of mounting it integral with the vertical illuminator. 
 This is termed by the makers a supplementary condenser, to distinguish it 
 from the condenser at the arc lamp. Its use and adjustment will be ex- 
 plained later. 
 
 Fig. /j Method for obtaining critical illumination. 
 
 Critical Illumination: It is generally agreed by expert microscopists 
 that the best results are obtained when the specimen is examined or photo- 
 graphed with critical illumination. Critical illumination is obtained when 
 the image of the illuminant and the surface of the specimen are about in 
 one plane, also, when removing the eyepiece and examining the back lens 
 of the objective through a pinhole cap, it is possible entirely to fill the lens 
 with light. In Figure 13 is shown how this kind of illumination may be ob- 
 tained. The arc or filament is nearly imaged on the supplementary con- 
 denser No. 2 by the aspheric condenser No. i with a distance of about 2< 
 to 30 inches between the two lenses. This should give an image of the il- 
 luminant of about 25 to 30 millimeters diameter. The image should be 
 slightly ahead of the supplementary lens No. 2 if an arc is used so as to 
 avoid imaging gas bubbles upon the specimen. 
 
 An image of the fully and evenly illuminated aspherical lens No. i, and 
 its iris diaphragm, referred to later, are projected into the back focus of 
 the objective by means of the supplementary condenser No. 2 and the 
 vertical illuminator. The supplementary lens is of such focus and set at 
 such distance from the objective, (that is, the same optical distance as the 
 eyepiece diaphragm from objective) that the image of the arc, the evenly 
 illuminated lens No. 2 and its iris, are imaged by the objective upon the 
 specimen when the objective is at proper focus. By closing the iris in 
 front of the aspheric condenser, the aperture of the light cone entering the 
 objective is reduced, similarly to that accomplished on some outfits by 
 placing an iris diaphragm on the side of the vertical illuminator. This re- 
 duction of the light cone below the aperture of the objective gives de- 
 creased illumination, and decreases the numerical aperture with attending 
 results; for example, less resolving power, greater depth of focus, and 
 greater flatness. Full apertures are obtained with iris opening of 16 
 millimeters for 4 millimeter objectives, and 18 millimeters for 16 and 32 
 millimeter objectives. 
 
 11 
 
Fig. 14 Tassln microscope with illum- 
 inating device attached to vertical illum- 
 inator. 
 
 Closing the iris of the supple- 
 mentary lens No. 2, when imaged 
 on the specimen, reduces the size 
 of the illuminated spot upon the 
 specimen, and hence the size of the 
 visible field. By just opening this 
 diaphragm enough to clear the 
 field of view, unnecessary rays are 
 cut off, so that they will not cause 
 reflections on the tubes of the ob- 
 jective, illuminator, etc. 
 
 The light returning from the 
 specimen after passing through the 
 objective and vertical illuminator, 
 is reflected by a stellite mirror to 
 the eyepiece. The use of a metal 
 mirror reduces all chance of flare 
 which might come from the plane 
 surface of a glass prism. 
 
 With the appliances just mentioned, the illumination scheme for 
 metallurgical work as described in the Zeiss circular, Micro No. 89, may 
 be obtained, although the adjustments are somewhat different. It should 
 be noted that in both visual and photographic work, decrease in illumina- 
 tion should be accomplished by filters and not by diaphragms which 
 reduce aperture, unless reduction of aperture is desired to secure certain 
 results. It will be shown later how these optical parts are adjusted to 
 secure the best results as regards centering. 
 
 MICROSCOPES 
 
 Any ordinary microscope with proper optical parts may be used in the 
 examination of metals, but it is, in this case, either necessary to mount 
 each and every specimen at a fixed height to avoid readjustment of the 
 illumination or to have an illuminating device attached to the vertical 
 illuminator, as shown in Figure 14. This stand is somewhat like the work- 
 shop microscopes of Swift and Watson, after the design of Mr. Stead. 
 The illuminating device consists of a 6-volt incandescent lamp, operating 
 on the regular no-volt circuit, and two condensers. As can be seen from 
 the illustration, it is attached to the illuminator and moves with it and the 
 optical parts. It may be applied to any ordinary microscope, and the 
 results obtained are good for magnifications up to about 200 diameters in 
 visual work. It may also be used in photography for about 100 magnifi- 
 cations, but of course cannot compare with the better instruments and 
 cameras. 
 
 The general form of stand for metallurgical work, however, is one in 
 which the stage and specimen are movable by rack and pinion, as by this 
 method varying thicknesses of specimen can be accommodated without 
 
 12 
 
Fig. i 5 Typical metallurgical microscope 
 for student use. 
 
 Fig. 16 Same as instrument in Fig. i$ ^ ut 
 
 with mechanical stage and horizontal camera 
 
 tube attached. 
 
 deranging the illuminating system. Instruments of this type require the 
 mounting of the specimen in wax or other form of holder. In the case of 
 steel specimens, a Sauveur magnetic holder may be used, the specimen 
 
 being held from the under side by 
 magnetism. 
 
 Figure 15 shows a typical form of 
 metallurgical microscope to which a 
 mechanical stage may be attached 
 if desired. The stand illustrated in 
 Figure 16 differs from the preceding 
 model in that it has a side tube for 
 connecting to the camera, so that 
 visual observations can be made 
 without disconnecting or moving 
 the camera, the only operation nec- 
 essary being the withdrawal of the 
 side tube, with its prism, from the 
 optical axis when making visual ob- 
 servation or in adjusting the light 
 and the specimen. 
 
 A later development of this type 
 of stand (Figure 17) provides more 
 space between the stage'and objec- 
 
 Fig. // Late model metallurgical microscope, 
 with unusual range for height of specimen. 
 
 13 
 
 tive, an improved type of vertical 
 illuminator and a series of adjust- 
 
ments and attachments which 
 makes it an almost universal in- 
 strument for general laboratory 
 purposes. It may be quickly con- 
 verted from vertical to oblique 
 or substage illumination, may be 
 fitted with polarizer and analyzer 
 for either opaque or transparent 
 specimens, and is adaptable 
 through the use of suitable at- 
 tachments for either binocular or 
 monocular wide field work at 
 comparatively low powers. It is 
 particularly adapted for metal- 
 lographic work in connection with 
 any of the standard photomicro- 
 graphic cameras as the stage ad- 
 justment permits focusing with- 
 out disarranging the alignment 
 of the illuminating equipment. 
 
 Many workers prefer a stand 
 of the inverted type, that is, one 
 in which the stage is above in- 
 stead of below the objective, and 
 in which it is necessary to prepare 
 one surface of the specimen only, 
 the polished face being placed, 
 face down upon the stage over 
 the objective. The inverted form 
 of microscope is credited to Le 
 Chatelier in the year 1897. 
 
 It has been made in a variety 
 of forms, first by Pellin of Paris 
 (Fig. 1 8), later by Dujardin & 
 Company (Fig. 19). Other models 
 have been made by Leitz (Fig. 
 20) of Germany and Reichert of 
 Austria. 
 
 *Figure 21 shows the latest 
 large inverted microscope of the 
 Bausch & Lomb Optical Com- 
 pany, which embodies all the de- 
 sirable features of a metallur- 
 gical microscope. This instru- 
 ment has been so designed that 
 relative vibration between the 
 specimen and the objective can 
 
a 
 
 3D 
 
 1 
 
 I 
 
Fig. 20 Inverted metallurgical microscope. E. Leitz. 
 
 not exist. Furthermore, the fine adjustment has been mounted so that it 
 is required to move a weight of only four ounces. When an illuminant 
 of high intensity is used, considerable heat is bound to be received by the 
 vertical illuminator. For this reason the objective is mounted independent 
 of the vertical illuminator and whatever heat reaches the latter and causes 
 it to expand will not change the distance between the specimen and ob- 
 jective and consequently will not cause the image to go out of focus. A 
 heat guard is provided to prevent any light other than the centered beam 
 from reaching any part of the microscope. 
 
 The body of the microscope consists of a casting in which is mounted a 
 stellite mirror which reflects the image received from the objective to- 
 wards the photographic plate. Over this stellite mirror is mounted the 
 vertical illuminator. The body, including the eyepiece, stellite mirror 
 and vertical illuminator are all in one rigid mounting. The pillar on which 
 this body is mounted is short and sturdy. It extends vertically downward 
 to the base and carries on it a rack slide. 
 
 The entire stage enclosing the microscope body is a single casting. At- 
 tached to the bottom of the casting is a rack and slide which 'moves on the 
 pillar. Vertical motion is accomplished by means of the rack and pinion. 
 The inner edges of the two stage posts nearest the camera are accurately 
 milled, forming a double slide. A pair of pins, mounted in an upright 
 which is securely fastened to the top of the body> maintain a constant 
 pressure against these milled edges. This even tension on both sides of the 
 stage posts, very near the surface of the specimen, assures perfect stability. 
 (Figure 22.) 
 
 On the microscope .pillar, opposite the stellite mirror position, is a 
 strong, broad fine adjustment slide on which the objective support moves. 
 (Figure 23). At the bottom of this slide is the fine adjusting mechanism 
 which is required to raise and lower the objective and its mount only. 
 The support for the objective is only slightly below the surface of the 
 stage, and the fulcrum about which vibrations would take place is in the 
 plane of the objective shoulder. This shoulder is level with the pair of pins 
 which steady the stage. 
 
 16 
 
18 
 
 *Fig. 21 Eausch &? Lomb Metallurgical Microscope. 
 
 1. Specimen Holder 
 
 2. Stage Aperture Plate 
 
 3. Mechanical Stage Scale 
 
 4. Mechanical Stage Adjustment 
 
 Heads. 
 
 5. Objective 
 
 6. Objective Handle 
 
 7. Iris Diaphragm Adjusting Ring 
 
 8. Filter Mount 
 
 9. Vertical Illuminator Mirror 
 
 Mount 
 
 10. Stellite Mirror Housing 
 
 11. Heat Shield Socket 
 
 12. Microscope Body 
 
 13. Observation Eyepiece 
 
 14. Camera Connector 
 
 15. Stage Casting 
 
 1 6. Coarse Adjustment Head 
 
 17. Fine Adjustment Head 
 
 1 8. Reducing Gear Lever 
 
 19. Coarse Adjustment Scale 
 
 20. Coarse Adjustment Lock 
 
 21. Stabilizer 
 
 17 
 
The screw that actuates the Bj 
 fine adjustment produces a ver- 
 tical movement of 0.125 mm per 
 revolution. A large diameter 
 button may be brought into con- 
 tact with a reducing speed shaft 
 by moving a cam. This provides 
 a more sensitive fine adjustment 
 for use in focusing the image from 
 the ground glass position. One 
 revolution of the rod in focusing 
 on the ground glass produces a 
 vertical movement of the object- 
 ive of only 0.03 mm. The fine 
 adjusting head is graduated to 
 read to 2.5 microns of vertical 
 
 *Fig. 22 Lateral stage support stabilizer. 
 
 movement but may be estimated much closer. 
 
 The coarse adjustment consists of a broad, heavy slide, on which the 
 stage casting moves, actuated by a heavy rack and pinion controlled by 
 knurled head. The position of the slide may be fixed definitely and 
 
 securely after an approximate focus 
 is obtained by means of a lock actu- 
 i ated by a convenient screw. This 
 support and lock at the bottom, and 
 the stabilizer at the top of stage sup- 
 port, as described above, effectually 
 prevent any possibility of vibration. 
 The coarse adjustment operates 
 by moving the stage and specimen 
 with respect to the objective. A con- 
 venient scale and index permits the 
 specimen to be brought into ap- 
 proximate focus for the objective 
 being used. 
 
 The fine adjustment (Figure 23) 
 consists of a broad dovetail slide, 
 actuated by a knurled head and 
 lever action, the same as on the 
 finest biological microscopes. The 
 slide is mounted on the pillar of the 
 microscope so that it is movable 
 with respect to the vertical illumin- 
 ator. The slide is carried on a sturdy 
 vertical rod, at the lower extremity 
 of which is the fine adjustment mech- 
 anism and at the top a horizontal 
 table on which the objective holder 
 
 *Fig. 23 Fine adjusting mechanism (Phantom 
 
 view). 
 
 18 
 
is mounted. The latter is so designed that the objectives are easily inter- 
 changeable and are held against a shoulder and stop in such a way that they 
 are positively centered with respect to the optical axis of the instrument. 
 
 This construction has a number of advantages over any previous de- 
 signs of any manufacture. The most important, of course, is the fact 
 that the fine adjustment carries the smallest possible weight so that re- 
 gardless of the weight of the specimen on the stage there is no additional 
 strain on the mechanism. In other constructions this mechanism carries 
 so much weight that, where long exposures are necessary, it is often im- 
 possible to prevent the microscope slipping or settling out of focus during 
 the exposure. 
 
 Another cause of change of focus in previous constructions is the ef- 
 fect of heat from the illuminant on the metal parts of the apparatus. The 
 beam of light first enters the vertical illuminator through the filter and 
 condenser, which absorb much of the heat and transmit some of it to their 
 respective mountings which expand as a result. Where the objective is car- 
 ried on the same fixture as the other optical parts, this expansion is bound 
 to displace the objective with respect to the specimen so as to destroy the 
 sharp definition at the plate. In this new construction, the objective 
 being mounted independently of the vertical illuminator, no effect of the 
 expansion of the parts is transmitted to the focusing adjustment. 
 
 A telescoping tube connector between the objective and the vertical 
 illuminator excludes extraneous light and reflections from this part of the 
 optical system.* 
 
 Fig. 24 Marten's metallurgical microscope. (Zeiss) 
 19 
 
Fig. 25 Rosenhain metallurgical microscope. R. & J. Beck. 
 
 Other types of metallurgical microscopes which should be mentioned 
 are those of Martens, made by Zeiss, (Fig. 24), and the Rosenhain micro- 
 scope, made by Beck (Fig. 25). The Martens' stand is of horizontal type, 
 provided with movable stage, and is in general form similar to the stands 
 first described. The specimen must be mounted so as to retain the pol- 
 ished surface in a vertical position. The illuminating apparatus is 
 placed at right angles to the axis of the microscope. The Rosenhain is an 
 enlarged and heavy form of usual type and has its vertical illuminator 
 built into the tube. 
 
 20 
 
ILLUMINATING SYSTEMS 
 
 Fig. 26 Arc lamp. Hand feed type. *Fig. 27 Arc lamp. Automatic feed type. 
 
 The direct current arc of low amperage is no doubt the best illuminant 
 to use if intensity is desired. The lamp may be of the hand-feed variety, 
 but it is better to use a lamp having an automatic mechanism for feeding 
 the carbons. Figure 26 shows a lamp of hand-feed type, and Figure 27 
 shows the automatic feed type, both lamps interchanging on the same 
 standard. Suitable condensers must be used to collect a wide angle of the 
 beam emitted from the lamp and condense it into the vertical illuminator. 
 Centering screws are usually provided so that the illuminant can be well 
 centered to the condensing lenses. 
 
 *Fig. 28 Illuminator and microscope mounted as one unit with 
 permanently aligned optical system. 
 
 21 
 
The carbons used on direct current for 4 to 5 amperes are usually 8 
 millimeters diameter for the positive or horizontal carbon and 6 milli- 
 meters for the negative. It is necessary to see that the positive wire of the 
 electric current is on the horizontal carbon. This can be determined by 
 noting the brightness of the carbons through the smoked-glass window, 
 the brighter one being the positive. 
 
 For alternating current the carbons should be 7 millimeters diameter 
 for both horizontal and vertical, and it makes no difference how the wires 
 are connected. 
 
 For visual observation and for photography where speed is not essen- 
 tial, a Mazda lamp with ribbon form of filament makes a very acceptable 
 illuminant. It has the advantage of giving a constant and uniform il- 
 lumination and requiring no care when once adjusted. This lamp, how- 
 ever, uses only 6 volts and is best used with an alternating current supply 
 in connection with a transformer, as the amperage rating is 1 8. 
 
 In earlier models it was customary to have all of the different parts ad- 
 justable relative to each other on the optical bed and in both vertical and 
 lateral directions. This was done so that the microscope, illuminating 
 system and camera could be all aligned by the operator. For the operator 
 who was trained in the manipulation of complicated optical apparatus 
 this construction presented few difficulties of technique but did consume 
 a great deal of time in the preliminary setting up of the apparatus every 
 time work was to be done. 
 
 *Experience shows that there is no particular advantage in this con- 
 struction. The experienced workman at the factory, by means of col- 
 limators and gauges can easily line up the illuminating and optical systems 
 so that they will work together at their highest possible efficiency and then 
 lock them securely in position. After this, no further adjustment is neces- 
 sary except to bring the light source into center. The most critical 
 technician can ask nothing more than a perfectly centered beam from the 
 illuminant into the vertical illuminator regardless of the kind of illumina- 
 tion used on the specimen. This is the principle on which the Bausch & 
 Lomb Large Metallographic Equipment is constructed and which is called 
 "permanently aligned." (Figure 28.) 
 
 In the new instrument all accessories such as the liquid cell, filter holder, 
 iris diaphragm and decentering device for oblique illumination are con- 
 veniently mounted between the light source and microscope and no ad- 
 justment of any of these parts can throw the beam out of its true center. 
 
 We believe, and experience has shown, that this construction is in every 
 way the most satisfactory forward step taken in the photomicrography of 
 metallurgical specimens since Le Chatelier introduced the inverted micro- 
 scope, because it permits anyone to obtain satisfactory results regardless 
 of their optical training or skill. 
 
 The mounting of the vertical illuminator on the microscope in a fixed 
 position makes it a part of the permanently aligned system of which men- 
 tion has been made above. Being in fixed position it is not thrown out of 
 center with respect to the axis of the beam when the objective is focused 
 
 22 
 
on the specimen by means of the fine adjustment, as is the case in instru- 
 ments in which the vertical illuminator and objective are both carried on 
 fine adjustment mechanism. 
 
 The vertical illuminator consists of a square metal box, mounted on the 
 upper surface of the microscope base and having three openings. Facing 
 the illuminant is a tube carrying the filter, iris diaphragm and condenser 
 lens. At the top is the opening for the objective connector, while facing 
 the operator is an opening carrying a shaft with knurled head at the outer 
 end, which controls the clear glass reflector and prism for directing the il- 
 luminating beam up thru the objective. Either may be used, being inter- 
 changeable by a simple adjustment. 
 
 The filter supplied in the vertical illuminator mount, produces practic- 
 ally monochromatic light equivalent, to the E line of the mercury spec- 
 trum, which is the wave length for which the achromatic objectives are 
 corrected. This wave length has been found to be the most satisfactory for 
 all around metallographic work, particularly for iron and steel. In case 
 
 other Wratten filters are to be used, they 
 will be placed in the filter mount on the 
 front of the liquid cell attached to the illu- 
 minating unit. 
 
 The full numerical aperture of the ob- 
 jective may be used, or the aperture may 
 be reduced by means of the iris diaphragm 
 in the tube directly back of the filter. The 
 maximum resolution with any objective will 
 be obtained when the full aperture of the 
 objective is used. The flatness of field can 
 be increased, with a sacrifice of resolution, 
 by using a smaller opening. The ILS Mi- 
 croscope can be diaphragmed down to give 
 a flat field but in no photomicrographic 
 outfit, metallographic or otherwise, can 
 full resolving power and perfectly flat field 
 be had at the same time. 
 
 The square-, object stage together with 
 the support at each corner is cast in one 
 solid piece, to increase the rigidity. This 
 casting is mounted on a heavy L shaped 
 part, the longer side of which is the slide 
 which moves along the pillar of the microscope under control of a heavy 
 rack and pinion, forming the coarse adjustment. The weight of the 
 stage and specimen is located directly over the center of the coarse ad- 
 justment slide. (Figure 29.) 
 
 The slide of the coarse adjustment is dovetailed in cross section and is 
 accurately milled and fitted. A clamping device on the coarse adjustment 
 head locks the slide securely in place so that no sagging or vertical motion 
 
 23 
 
 *Fig. 29 Microscope stage. Weight 
 
 of stage and specimen centered over 
 
 coarse adjustment slide. 
 

 I. 
 
 *Fig. jo Mechanical stage. Adjustment heads on both 
 sides of stage and all acting in same plane. 
 
 can take place under the weight of the specimen during the longest ex- 
 posures. As a precaution against the vibration of the stage relative to the 
 other parts of the microscope, there is provided a stabilizer which prevents 
 any lateral movement of the upper part of the stage, so that the latter is 
 supported both top and bottom. The stabilizer consists of a Y shaped 
 brace attached to the top of the microscope. In horizontal position at the 
 top, it carries two ball ended shafts which bear against milled surfaces at 
 each side of the stage supports. This brace is practically in the plane of 
 the objective and effectually prevents any vibration of the specimen rela- 
 tive to the objective. 
 
 The mechanical stage, (Figure 30) which directly supports the speci- 
 men has two horizontal movements at right angles to each other, making 
 it possible to thoroughly and systematically explore the surface of the 
 specimen. These movements are controlled by a screw and rack and 
 pinion actuated by two shafts rotating in the horizontal plane. The 
 shafts bear knurled heads at each side of the microscope so that manipu- 
 lation may be accomplished with either hand and in the most convenient 
 position. 
 
 There is a \y^" diameter opening in the center of the stage plate which 
 is turned with a shoulder to hold a metal aperture plate. This plate has a 
 long tapering opening which permits the use of any opening between 4mm 
 and iimm. 
 
 Projecting in the horizontal plane from the stellite mirror mount is the 
 body tube of the microscope. This carries the eyepiece at its outer ex- 
 tremity, together with a light tight connector to exclude stray light when 
 using the camera. Extending at right angles from the body tube is an 
 auxiliary tube for visual observation. In visual observation a prism 
 diverts the emergent beam from the stellite mirror into this observation 
 tube, which is also equipped with an eyepiece. When focusing on the 
 ground glass this prism is withdrawn from the optical axis. This arrange- 
 
 24 
 
ment permits preliminary orientation and focusing of the specimen before 
 the final focusing on the ground glass screen of the camera is undertaken. 
 The entire arrangement is such that observation in the auxiliary tube, 
 manipulation of the mechanical stage and focusing with the fine adjust- 
 ment are all accomplished simultaneously in the most comfortable and 
 convenient position. 
 
 The illuminant and its accessories are supported on a rigid stand that is 
 a unit with the pillar which supports the inverted microscope. This 
 stand supports, in order, the illuminant in its adjustable centering mount, 
 the condensing lenses in a spiral focusing mount, the iris diaphragm and a 
 water cell and auxiliary filter mount combined. 
 
 The illuminant regularly supplied with this outfit is the automatic feed 
 arc lamp which must be used with a suitable rheostat for the ordinary 1 10 
 V current. The clock feed mechanism of the arc lamp operates the carbon 
 holders by means of a constant chain drive, maintaining a steady and 
 uniform arc of high intensity. The excursion of the carbon holders is 
 sufficient to accommodate the entire length of 6" carbons with one setting, 
 giving nearly two hours of steady light. 
 
 Interchangeable in the same mounting is a 108 watt ribbon filament 
 Mazda lamp in special housing. This provides a steady and uniform 
 light, that is always in alignment. It is preferred by many because of its 
 convenience and freedom from attention. As the intensity of the light 
 from the Mazda is much less than that from the arc the length of expo- 
 sure must necessarily be greater. However, if the equipment is to be used 
 for extended periods of observation only, this lamp is recommended. 
 
 The large iris diaphragm is mounted in the optical axis of the instru- 
 ment, just in front of the condensing lenses. It is held on a mount which 
 is adjustable for rotation about the principal axis, for decentration in any 
 direction to the extent of one half the radius of its total aperture and for 
 total swing-out clear of the aperture of the condenser. The diaphragm 
 will give any opening from less than two millimeters to over thirty milli- 
 meters and this construction permits the opening to be centered at any 
 point in the total cross-section of the beam from the illuminant. This 
 makes possible the illumination of the specimen by oblique illumination 
 from any direction and a central stop may be readily inserted to provide 
 for conical illumination. 
 
 The liquid cell is mounted on an arm between the diaphragm and the 
 microscope. It consists of a metal box with glass windows in the light 
 path. The glass plates are burnished in against rubber washers and are 
 liquid and trouble proof. The cover of the cell is so designed as to also 
 form a limiting aperture over one of the glass plates and a mount for a 
 Wratten filter on the opposite side of the cell from the illuminant. The 
 cell may be easily removed for cleaning or filling. It may be used either as 
 a water cell for protecting the Wratten filters or as a liquid filter cell. 
 
 In the photomicrography of transparent specimens, two methods of il- 
 lumination are in common usage. These are critical illumination, in which 
 the image of the light source is focused directly on the specimen, and 
 
 25 
 
Fig.jz A camera especially 
 designed for use with the 
 side tube microscope. 
 
 Fig. 32 Simple type of vertical camera. 
 
 Fig. 33 A form of low stand with 
 vertical camera. 
 
 26 
 
Fig. 34 Small metallographic outfit for student use. 
 
 Koehler illumination in which some evenly illuminated surface such as the 
 condenser surface is imaged on the specimen. Where opaque specimens are 
 concerned the critical system of illumination is preferable owing to the 
 higher intensity of light on the surface of the specimen. 
 
 In this equipment the illumination is critical. The light source is first 
 imaged by the two lens condenser. In the opening of the vertical illumina- 
 tor, facing the lamp, is a lens having a focal length such that this source 
 image appears at the microscope objective, to come from the plane of the 
 diaphragm of the eyepiece. In other words, the source image is again 
 imaged by the lens in the side of the vertical illuminator in a position con- 
 jugate to the specimen. Thus, when the objective receives the light, it 
 forms an image of the light source practically on the surface of the speci- 
 men, presenting critical illumination as described above. 
 
 The image of the source on the specimen varies in size with the ob- 
 jective used. The size of the image, or illuminated area is slightly larger 
 than the area of the field covered by the objective and 5 X eyepiece. No 
 light goes to waste and when the objective is focused on the specimen the 
 light is also focused on the specimen. Furthermore, the source image 
 formed on the specimen is free from spherical and chromatic aberration. 
 It is these reasons that make the illumination ideal in metallurgical micro- 
 scopes. The highly corrected objective, with which the detail in the speci- 
 men is brought out, performs a double duty; it is first a condenser and then 
 an objective.* 
 
 CAMERAS. The cameras and supports available for this work proba- 
 bly are as great in variety as the microscopes. A few types will be re- 
 ferred to, all of which embody supports to carry an illumination system of 
 the type described. Figure 32 shows a simple type of vertical camera 
 which has been in use for a number of years. It differs from the ordinary, 
 
 27 
 
Fig. 35 Complete Eausch & Lomb Large 
 Metallographic Equipment with shock 
 absorbers and stand. 
 
 in that the camera and all accessories are mounted on a single base board 
 to which they can be clamped after adjustment. It is annoying in this 
 work to have a number of loose parts which are readily disturbed. Figure 
 33 shows a low form of stand with vertical camera. When resting on the 
 floor the ground glass is at a convenient height for observation. The 
 
 camera may be swung to one 
 side when making adjustments of 
 the specimen and illumination. 
 
 Figure 31 shows a type espe- 
 cially designed, after suggestions 
 of Professor Sauveur, for use with 
 the side-tube microscope. As 
 stated, when referring to the mi- 
 croscope, the side tube is with- 
 drawn for visual observation and 
 again pushed into the path of 
 light when making photographs. 
 The light-tight connector be- 
 tween camera and microscope is 
 so made that neither camera nor 
 microscope need be moved in 
 making the adjustment from vis- 
 ual to photographic use. All the 
 
 *Fig. 3 6-Supplementary plate holder and view- camera ^ supports described are 
 ing device with viewing tube in position for made with either 4 by 5 or 5 by 
 observation. j inch cameras. 
 
 28 
 
*Fig. 38 Attachments for macro-photography, 
 showing arc lamp tilted for oblique illumination. 
 
 . 37 Shock absorbers^ showing frame, 
 spring and adjustable rubber damper. 
 
 f Fig. 39 Attachments for macro-photography, show- 
 ing arc lamp in position for vertical illumination. 
 
 The camera shown in figure 34 is a small outfit for student use and for 
 industrial plants where only a small amount of work at low and medium 
 powers is required. The microscope is of the inverted type and is mount- 
 ed over the camera. The specimen is examined by means of a side tube, 
 which, with its prism, may be withdrawn from the optical axis, permitting 
 the image to fall upon the plate in the camera. While the image may be 
 focused upon an opaque screen in the camera, it is possible so to adjust the 
 camera eyepiece that the image is sharp upon the plate when focused for 
 visual observation. The illuminant is the 6-volt, io8-watt Mazda lamp. 
 The mountings for lamp and the condensers are of much simpler form 
 than in the previous outfits, but will give satisfactory results in magnifica- 
 tions from 50 to 400. 
 
 *Figure 35 shows the complete Bausch & Lomb Large Metallographic 
 Equipment, the microscope and illuminating unit of which has already 
 been described. 
 
 The camera parts consist of a tapering bellows of 40" extension, with 
 front and rear supports and center frame to prevent sagging. The camera 
 back is reversible and is provided with a hinged door, single book form of 
 plate holder for 8 x 10 plates with reducing kits for smaller sizes down to 
 3^ x 4/4* on e ground glass screen with clear center for use with focusing 
 glass and a clear glass screen with graduated cross lines. The front sup- 
 port is fitted with an exposure shutter and light tight connector for con- 
 necting with microscope. An extension rod for moving the fine adjust- 
 ment while observing the image on the ground glass, and a focusing glass 
 are included in the camera equipment. 
 
 29 
 
The supporting base is made of two parts a heavy, steel double bar 
 for the camera parts and a section of accurately milled optical bed for the 
 microscope. They are carefully joined together and provided with low 
 supporting feet at either end. The supports for the camera, slide very 
 easily along the bar because of the long bearing surface and are prevented 
 from tipping by a tongue running in a groove on the side of the bar. The 
 bar is graduated in millimeters for the easy determining of bellows draw. 
 
 A new camera back, Figure 36, has been developed to make the focus- 
 ing of the image easier and more positive, and to eliminate vibration in the 
 manipulation of the dark slide. 
 
 It consists of a frame interposed between the rear bellows frame and the 
 camera back proper, in which there is mounted a magnifier with a right 
 angle prism which faces toward the eyepiece of the microscope. To focus, 
 one has but to look in the magnifier and adjust the fine adjustment of the 
 microscope until the image is in focus on the cross hairs. 
 
 As these are located the same distance from the prism as the plane of 
 the plate, the image must be in focus in that plane. The magnifier is 
 mounted on a pivot support so it may be moved across the entire field. 
 
 In this new back there is a supplementary dark slide which is raised by 
 counter balanced weights running in cylinders on either side. With this 
 device it is possible to withdraw the regular dark slide from the plate 
 holder, permit the equipment to come to rest, check up on the focus with 
 the magnifier, swing it aside, close the shutter, release the catch on the 
 supplementary dark slide, permitting it to automatically rise without any 
 vibration and then make the exposure. This arrangement will be readily 
 appreciated by all photomicrographers. It can be attached to any of the 
 type G cameras now in use. Trial exposures may be made by withdraw- 
 ing dark slide, giving the plate an exposure of 15 seconds; replacing the 
 slide one inch at a time, giving each succeeding inch 15 seconds, so that 
 the final exposure on a 5-inch plate would be 75 seconds, 60 seconds, 45 
 seconds, 30 seconds and 15 seconds for the various parts. After some ex- 
 perience it is easy to judge the correct exposure for a given class of ob- 
 jects. 
 
 It is a recognized fact that the vibrations caused by machinery, street 
 traffic, etc., must be absorbed before successful photomicrographs can be 
 taken. The Bausch & Lomb Shock Absorbers (Figure 37) consist of four 
 unit supports, each unit of which is made up of a sturdy frame, one coiled 
 spring, a sponge rubber damper and a threaded secondary support for ad- 
 justing the pressure on the rubber damper or absorber. 
 
 Figure 38 shows an arrangement for attaching low-power photo- 
 graphic lenses, of the 32-, 48-, and 72-millimeter foci, to camera, and 
 special device for holding and focusing the specimen. The focusing is 
 done by extension rod from rear of camera while observing the ground 
 glass. It is intended for the photography of fractures, etc. This also 
 shows the illuminant as used for oblique illumination. Plane glass holders 
 are supplied for vertical illumination with these low-power lenses, as 
 shown in Figure 39.* 
 
 30 
 
>L 
 
 12 13 D 9 
 
 Fig. 40 Metallographic outfit with parts numbered for reference. 
 
 ADJUSTMENTS 
 
 Many of the difficulties in the use of metallographic equipment are due 
 to improper adjustment of the illuminating and optical parts. The 
 later models of the Bausch & Lomb Large Metallographic Equipment are 
 permanently aligned at the factory by means of collimators and gauges so 
 that the greatest possible efficiency is obtained. As the various parts 
 are fixed in position there is no possibility of this arrangement being 
 disturbed and no time is lost in making the preliminary adjustments each 
 time the apparatus is to be used. 
 
 This model is, however, comparatively recent and there are many 
 of the older models as well as those of other makes in use, in which it 
 is necessary to align the optical parts. For this reason a brief description 
 of the process is here given, the principle of the operation being sufficiently 
 
 FLAM GLASS 
 
 MIRROR 
 
 CORRECT /NCOffRECT CORRECT INCORRECT 
 
 Fig. 41 Method of adjusting light spot with Fig. 42 Method of adjusting mirror 
 plane glass. illuminator. 
 
 similar in all cases to make these directions applicable. Number refer- 
 ences are to parts indicated in Figure 40. 
 
 One of the main points is the centering and focusing of the illuminating 
 pencil or cone of light. If this is incorrect, good results cannot be ob- 
 tained. First see that the plane glass of the vertical illuminator is set an 
 angle of 45. Place a highly polished, unetched specimen on stage face 
 down, focus objective, and while observing in the eyepiece bring the 
 small spot of light to the center of the field. The spot should appear as 
 the one in Figure 41 marked correct. If it is off center, the vertical il- 
 luminator should be adjusted either by knob of plane glass No. 6, or by 
 rotating the illuminator upon the verticle axis until the spot is centrally 
 
 located. 
 
 31 
 
Going back to Figure 40, focus the objective roughly and remove eye- 
 piece, No. 2. Place over the eyepiece tube a pinhole diaphragm, for cen- 
 tering the eye with tube. Place supplementary lens, E, in position about 
 6 inches to right of microscope, and while observing through pinhole 
 cap, adjust this condenser until the bright spot is in center, as just shown. 
 Then replace eyepiece and close iris, No. 16, of supplementary lens. If 
 the iris is not sharply defined, slide the supplementary lens tm its base, 
 No. 1 8, to or from the microscope until the blades of iris diaphragm can 
 be distinctly seen in the field, when objective is in focus. In making these 
 last observations, a smoked-glass cap should be used over the eyepiece. 
 The image of this iris should be concentric with diaphragm of the eye- 
 piece, and will be if set-up has been correctly made. 
 
 If the arc is now focused on the supplementary lens, the light condition 
 should be as shown in Figure 13. The arc is nearly focused on supple- 
 mentary lens No. 2, by lens No. i. Lens and iris No. i are focused at 
 back of objective by lens No. 2. Arc, lens, and iris No. 2 are focused on 
 specimen by the objective. This gives critical illumination. When these 
 results are obtained, good images are secured. After a little practice it is 
 not difficult to set up the apparatus in the manner described. Some users 
 introduce a ground glass in front of the arc. This diffuses the light, and 
 while giving even illumination, it does not give critical illumination as 
 previously described, but is preferred by some for lower powers and is 
 even recommended by some workers. 
 
 In adjusting the mirror illuminator, or if a prism illuminator is used, the 
 same conditions prevail. The spot of light is to be centered in the field as 
 before, and the supplementary lens then placed in position and the loca- 
 tion of the light pencil adjusted as shown in Figure 42. In this case it 
 will be noticed that in the correct position the circle of light is located in 
 the middle of the hemisphere and not centrally with the optical axis. The 
 mirror illuminator is excellent for projecting with low powers. A little 
 practice will enable one quickly to make these adjustments, and the re- 
 sults will amply repay for the time spent. 
 
 Fig. 43 Diagram showing illuminating system with short focus condenser. 
 
 *The most recent models (1927-8) are arranged, as shown in Figure 43, 
 with the source of illumination 9^ inches from the axis of the microscope. 
 Such an arrangement permits critical illumination and makes a more 
 
 32 
 
compact instrument with fewer adjustments. This reduction of the dis- 
 tance from the light source to the vertical axis is accomplished by a short- 
 focus condenser and a supplementary lens to magnify and form a virtual 
 image of the light source. 
 
 The light source S (Fig. 43), is imaged at S' under a magnification of 
 about 1.5 by the short-focus condenser A. B is an auxiliary condenser 
 placed very close to the axis of the microscope and of such focal fength as 
 to form a real image of the condenser A in the back focal plane of the 
 microscope objective. The auxiliary condenser B must also form a virtual 
 image of the light source S' in the conjugate focal plane of the objective. 
 That is, the lens B places the light-source image to the left, at a distance 
 from the objective equal to the distance to the right from the image 
 plane to the objective, or at a position such that the objective will image 
 the light-source S on the specimen O. Thus, critical illumination results, 
 and we have a short system of illumination performing the same as that 
 represented in Figures 13 and 40.* 
 
 The elimination of vibration is one of the difficult problems that most 
 metallurgists have to overcome. No matter how rigid the apparatus, 
 short sharp blows will produce vibrations which make it impossible to 
 make good photomicrographs. There are a number of different devices 
 in use ranging from simple to very elaborate appliances. Each user seems 
 satisfied with his particular device. Probably the simplest and easiest to 
 install consists of two planks, of a size suitable to hold the entire appara- 
 tus, and between them two inflated 3^2 inch inner tubes or two circles of 
 tennis balls. 
 
 Another device much in use consists of a platform suspended from the 
 ceiling, some consisting of simple chains or ropes, others of springs ar- 
 ranged with oil dampers below platform to check the spring movement. 
 Another device, designed by Mr. Pafenbach of the Simonds Stell Co., 
 Lockport, New York, consists of a submerged platform level with the 
 floor, but suspended upon springs held by angle pieces projecting down- 
 ward from cross beams, also level with the floor. It seems that each one 
 might best provide such means as are necessary in his particular location, 
 but vibration, if it exists, should be eliminated. Vibration can be detected 
 easily by observing the reflections from a small dish of mercury set upon 
 the apparatus. 
 
 A focusing glass is a desirable addition, and almost necessary if one ex- 
 pects to obtain a sharp image upon the photographic plate, especially in 
 high magnifications. As to the proper color of filter to use in metallogra- 
 phy, there seems to be some differences of opinion, no doubt due to dif- 
 ferences in plates, illuminants, objectives, and specimens used by dif- 
 ferent workers. Different colors have been experimented with, and it has 
 been found that on iron and steel specimens a combination of green and 
 yellow dominant wave length of 5500 Angstroms give the best results. 
 The Angstrom unit equals one ten-millionth of a millimeter. Often the 
 appearance of the image upon the ground glass is made the basis of the 
 proper filter to secure contrast desired, but suitable plates must be used to 
 
 33 
 
Green 
 
 Red 
 
 |GY| 
 
 NORMAL SPECTRUM SHOWING THE SENSITIVENESS OF ORDINARY 
 PHOTOGRAPHIC PLATES. 
 
 (After Mees, and magnified as in fig. 139). 
 
 Fig. 44 Sensitive region^of the ordinary photographic plate. 
 
 X0.6- X7- 
 
 NORMAL SPECTRUM SHOWING THE SENSITIVENESS OF ORTHO- 
 CHROMATIC OR' ISOCHROMATIC PLATES. 
 
 (After Mees; magnification as in fig. 139). 
 
 Fig. 45 Sensitive region of orthochromatic or isochromatic plates. 
 
 NORMAL SPECTRUM SHOWING THE SENSITIVENESS OF PANCHRO- 
 MATIC PLATES. 
 
 (After Mees; magnification as in fig. 139). 
 
 Fig. 46 Sensitive region of panchromatic plate J r or a wide range of the spectrum. 
 
 record such an image. Much difficulty is found in obtaining solid glass 
 filters that will give the color desired, so it becomes necessary to use the 
 glass-mounted, gelatine-stained variety. In the matter of plates one 
 must select a plate suitable to the work and filter used. 
 
 Figure 44 shows the sensitive region of the ordinary plate, and if filters 
 are used which transmit only green and yellow with very little blue, the 
 exposure will be very long and the results not satisfactory. Figure 45 
 shows the sensitive region of the orthochromatic or isochromatic plates, 
 and if filters are used, such as the B & G Wratten, transmitting wave 
 lengths around 5500, this type of plate gives good results. The slower 
 orthochromatic plates have a finer grain and are therefore best for 
 photomicrographic work. Such plates as the Stanley Commercial or the 
 Cramer Slow Iso will give good results. If heat-tinted specimens are 
 used and there is much variation in colors, all of which one may wish to 
 record, then the panchromatic plate should be used. Figure 46 shows 
 that these plates are sensitive to a wide range of the spectrum. 
 
 The many factors involved, such as objectives, aperture, magnification, 
 illuminant, filter, etc., make the keeping of records almost imperative. A 
 method of standardizing a photomicrographic outfit has been described 
 by Prof. Alexander Petrunkevitch of Yale University in the Anatomical 
 Record, Vol. 19, No. 5, October 1920. While his article refers particularly 
 to transparent photography, the same rules can be applied, namely, to 
 
 34 
 
determine the best location for each and every part of the apparatus, dia- 
 phragm apertures, for different objectives, ray filters, plates, etc., and 
 after establishing such record, one may make a picture of a given kind of 
 object in the minimum amount of time and be practically sure of the re- 
 sult without recourse to cut and try methods every time a picture is to be 
 made. 
 
 The surface of the polished specimen must be flat. Any rocking on the 
 grinding or polishing wheel will produce a cylindrical surface and the 
 field in the microscope will have a band in focus while both sides may be 
 out of focus. Also, the etching must be done properly to produce the con- 
 trast necessary to show the structure desired. Light etching has been 
 found to give the best results with high powers. It is well again to em- 
 phasize the necessity of keeping all parts of the apparatus clean, not only 
 all lenses, but all bearing surfaces. The apparatus should be protected 
 with a rubber cloth covering when not in use. The instrument should be 
 placed, if possible, where it is not exposed to the fumes of the chemical 
 laboratory. 
 
 35 
 
Table of Magnifications For ILS Inverted Microscope 
 
 OBJECTIVES 
 
 Eyepiece 
 Power 
 
 Distance from eyepiece to screen in CM 
 
 E.F. in MM Magnification 
 
 25 
 
 50 
 
 75 
 
 100 
 
 Achromat 
 32 
 
 ic System 
 5.2 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 25 
 33 
 39 
 52 
 65 
 78 
 
 52 
 66 
 78 
 104 
 130 
 156 
 
 78 
 99 
 117 
 156 
 195 
 234 
 
 104 
 132 
 156 
 208 
 260 
 312 
 
 16 
 
 12.5 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 62 
 80 
 94 
 125 
 
 156 
 
 188 
 
 124 
 160 
 188 
 250 
 312 
 376 
 
 186 
 240 
 282 
 375 
 468 
 564 
 
 248 
 320 
 376 
 500 
 624 
 752 
 
 8 
 
 26 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 130 
 166 
 195 
 260 
 325 
 390 
 
 260 
 332 
 390 
 520 
 650 
 780 
 
 390 
 498 
 585 
 780 
 975 
 1170 
 
 520 
 664 
 780 
 1040 
 1300 
 1560 
 
 4 
 
 52.3' 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 262 
 335 
 392 
 523 
 654 
 785 
 
 524 
 670 
 784 
 1046 
 1308 
 1570 
 
 786 
 1005 
 1176 
 1569 
 1962 
 2355 
 
 1048 
 1340 
 1568 
 2092 
 2616 
 3140 
 
 1.9 
 
 116.5 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 583 
 746 
 
 874 
 1165 
 1456 
 
 1748 
 
 1166 
 1492 
 1748 
 2330 
 2912 
 3496 
 
 1749 
 2238 
 2622 
 3495 
 4368 
 5244 
 
 2332 
 2984 
 3496 
 4660 
 5824 
 6992 
 
 Fluorite 
 4 
 
 System 
 51.6 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 258 
 330 
 387 
 516 
 645 
 774 
 
 516 
 660 
 770 
 1032 
 1290 
 1548 
 
 774 
 990 
 1161 
 1548 
 1935 
 2322 
 
 1032 
 1320 
 1548 
 2064 
 2580 
 3096 
 
 1.8 
 
 120 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 600 
 768 
 900 
 1200 
 1500 
 1800 
 
 1200 
 1536 
 1800 
 2400 
 3000 
 3600 
 
 1800 
 2304 
 2700 
 3600 
 4500 
 5400 
 
 2400 
 3072 
 3600 
 4800 
 6000 
 7200 
 
 Apochrom 
 8 
 
 atic System 
 
 28 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 140 
 179 
 210 
 280 
 350 
 420 
 
 280 
 358 
 420 
 560 
 700 
 840 
 
 420 
 537 
 630 
 840 
 1050 
 1260 
 
 560 
 716 
 840 
 1120 
 1400 
 1680 
 
 4 
 
 58.2 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 291 
 372 
 436 
 582 
 
 728 
 873 
 
 582 
 744 
 872 
 1164 
 1456 
 1746 
 
 873 
 1116 
 1308 
 1746 
 2184 
 2619 
 
 1164 
 1488 
 1744 
 2328 
 2912 
 3492 
 
 3 
 
 74 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15.0 
 
 370 
 477 
 555 
 740 
 925 
 1110 
 
 740 
 948 
 1110 
 1480 
 1850 
 2222 
 
 1110 
 1422 
 1665 
 2220 
 2775 
 3330 
 
 1480 
 1896 
 2220 
 2960 
 3700 
 4440 
 
 2 
 
 111 
 
 5.0 
 6.4 
 7.5 
 10.0 
 12.5 
 15 
 
 555 
 710 
 
 833 
 1110 
 1388 
 1 fifiS 
 
 1110 
 1420 
 1666 
 2220 
 2776 
 3330 
 
 1665 
 2130 
 2499 
 3330 
 4164 
 4PPS 
 
 2220 
 2840 
 3332 
 4440 
 5552 
 6660 
 
 NOTE: Above objectives' are short mounted and corrected for 215 mm tube length. 
 The first column of magnifications represent visual magnifications as well a 
 25 cm. 
 
 Form No. E-214 VIII 28 
 
 36 
 
 that on the ground glass at 
 Printed in U. S. A. 
 

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 QC 
 373 
 vh 
 P3 
 
 Patterson, W.L. 
 
 The optics of metallography,