T5 /'ly TS 1109 .L8 Copy 1 DEPARTMENT OF COMMERCE Technologic Papers OF THE Bureau of Standards S. W. STRATTON, Director No. 217 [Part of Vol. I6J PHOTOMICROGRAPHY OF PAPER FIBERS R. E. LOFTON, Associate Physicist Bureau of Standards AUGUST 2, 1922 1 n ^ '^ 6^2--( "« PRICE, 5 CENTS Sold only by the Superintendent of Documents. Government Printing Office, Washington. D. C. WASHINGTON GOVERNIVIENT PRINTING OFFICE 1922 ^fWWCJPf'' LIBRARY OF CONGrttSS RECEIVED 0X4 1922 DOCUMENTS DIViSfV PHOTOMICROGRAPHY OF PAPER FIBERS. By R. E. Lofton. ABSTRACT. This paper describes some of the more important factors in the jihotoiiiicroi^rapliy of vegetable fibers, especially of those used in the paper industry. As re^^'ards illumi- nation it is shown that the carbon arc can be advantageously replaced by an incan- descent stereoptican lamj). DilTcn-nt types of objectives are discussed, and it is shown that the working qualities of most objectives may be greatly improved by the use of proper light fdters. Three types of light fdters, with general directions for preparing them, are discussed, and directions are given for determining the quality of light transmitted by filters. The purpose and proper use of the substagc condenser and diaphragm are discussed. It is shown that, where an object lying in different planes is to be photographed, an objective of comparatively long focal length will give better resiilts than one of shorter focal length, and that this arrangement requires a longer bellows extension. Other advantages of a long bellows extension are also described. Diflerent types of photographic plates are described and suggestions as to the best type of plate to use for photomicrographic work are given. Suggestions for staining and preparing the material to be photographed are included, as well as some suggestions as to the value of photographs for permanent records anrl in the study and control of materials and mill processes. A short bibliography on ])h(jto- micrography and related subjects is given. CONTENTS. Page, I. Purpose nf publication 629 II. vSource of illumination 631 III. Light filters 631 I\'. I'se (if condensers 636 V. Use of substage diaphragm 636 VI. Choice of camera and objective O39 VII. Photographic plates and their development (144 YIII. Value of representative field nf fdjers 644 IX. Value of staining 641; X. Photomicrograj)hs as permanent records 647 I. PURPOSE OF PUBLICATION. The purpose of this paper is to put before those who desire to take up the microscopic and photomicrograpliic study of the vegetable fibers used in paper-making some essentials which are often overlooked, but without which the full measure of success in this field can not be had. Investigations of the value of stains, of light filters, of a long bellows camera as against one of shorter bellows, etc., have demonstrated that these factors are really 620 -9 630 Technologic Papers of the Bureau of Standards. COTTOU (Cos typiu n, iarhaJeme) ^ ' ^~^-««" rn w MSP£V (Po^u/us tr».r,:u/o,Vei) FLAXCLirum us-itatissimum) ^i.^.s) JACh flNE(Pn,ui dirarica-^a) SFRUCB cnouND MOOD SKIcer Qvr-minute exposure with this lamp. Finally, a new 4()()-watt I\Iazda stereop- tican lam]), recently put on the market, was substituted for the 300-watt lamp. This new lamp is much to be preferred to the 300-watt, as the six filaments furnishing the light lie in a vertical plane, and may thus be turned either edgewise to the object, giving an effect similar to that of the Nernst glower, for high magnifications, or may be turned flatwise so as to illuminate a larger field for low power photomicrography. III. LIGHT FILTERS. A study of the properties of light and of optical instruments will convince the reader, if he has not already been convinced, that all optical instruments have their limitations. The best that science ' .S,i: shiTt bibliusraiihy at end uf ll.is publicitiuii. 632 TeclDiologic Papers of the Bureau of Standards. [vd. ,t> and art can do in the construction of achromatic objectives with the different varieties of optical glass available at present is to make a perfect correction for two colors of the spectrum, called the "preferred" or "chosen" colors. These chosen colors are brought to a point focus nearer the objective than are the other colors, which are focused at various points along the axis of the objective. (See Fig. 2.) The colors chosen for visual work are yellow and green, usually spoken of as " yellow-green . ' ' or " applegreen," because the normal human eye is most sensitive to this por- tion of the spectrum. Since the ordinary photo- graphic plate is not at all sensitive to yellow-green light, but is most sensitive to the blue-viclet region of tiie spectrum, objectives intended e.xclusively for photographic use are corrected for blue light. (See Fig. 3.) As photographic objectives of 16 mm equivalent focus are comparatively rare, however, almost all photomicro- graphic work is done with achromatic outfits constructed for visual work, unless, perchance, an apochromatic outfit is at hand. It is evident then, since the visual focus and the photographic or chemical focus are not the same (see Fig. 4), that best results can not be expected if white light is used in conjunction with an achromatic outfit, since white light is composed of all the colors of the spec- trum. Those who attempt to make photomicrographs by using an achromatic outht and white light are faced at once with the problem as to how to make the visual and the chemi- cal fdci the same. If a good dry plate, sensitive to yellow-green only, were on the market, it would afford the most desirable solution of the problem ; for a microscope objective is supposed to give the best results, other things being equal, when transmitting only those colors of the spectrum for which it is corrected. vSince such a photographic plate is not to be had at the present time, -Shows objective corrected for photographic use, loscu (.(ilur" (See text lor explanation) ■ Lofion) PliotomicicxjKipliy of Paper Filn 633 however, the only alternative left is the use of ray or light filters, whose function it is to absorb all those rays of the spectrum not desirable for illuminating the object to be photographed. Light hlters may be purchased, or may be made in the photo- graphic workroom which will transmit any portion of tlie spectrum, usually within rather broad limits, however. A filter should be chosen which will transmit only a very narrow band of the color of the spectrum to be used; that is, the light transmitted should be as nearly monochromatic as possible. Unfortunately, however, hi ters transmitting a very nar- row band do not usually trans- mit Li large percentage of the incident light. Three different types of hl- ters are in common use: (i) Liquid hlters, made liy dis- solving various dyes, flepend- ing on the color of the light wanted, in their proper solv- ents; (2) filters made fromcol- ored glass ; and (3) filters made Ijy staining transparent films in xarimis dye solutions. Liciuid filters have the dis- advantages that they arc usu- ally not constant for any length of time in the quality and quantity of light they transmit, that the liquid of which they are composed frequently gets spilled or leaks out over the apparatus and thus sometimes causes great injury, and that they recjuire considerable time and care to keep in ]iroper working conditif)n. Quite a mmiber of filters made from colored films are on the market and some of them are good. There are also available glasses of various colors which make excellent filters when ground and polished properly. Neither the use of colored films, mounted on or between glass, nor that of glass filters has the disadvantages attending the use of litiuid filters. A suitable filter, often superior to most availal)le, can be made by treating an ordinary jihotographic plate in a plain fixing bath irocran '"' ■ P p 1 |H Ll t±u .i...,ti- ^^HH The- Ultcrs .It tiK- bolli.ni of i.ich s H-cl rocr.in to llic culur (if the liitl.t t;illitm on the .l.itc; UV vioUt; V. vinli-t; B. blue: G, crien ; V, yell ..r.aiBi-. .111(1 K. rid The number lciii;lh ..I tlR liuht in inillmmh.. ..f .l ir.llimet.r 634 Tcchnoloqic Papers of the Bureau of Standards. iVoi. 16 in the dark room until all the silver salts are removed, washing, and then immersing in an aqueous or aqueous-alcoholic solution of a suitable dye for a few hours. Although photographic plates may be had sensitive to all colors, there are various advantages in using only the shorter wave lengths of light, particularly blue light. All plates are more sensitive to blue and blue-violet light than to other colors, and so when exposed to light of this color require a shorter time of ex- posure. (See Fig. 4.) The use of filters transmitting only the shorter wave lengths also has the advantage of increasing the resolving power of the objective. The theoretical resolving power, R, oi a lens — that is, the minimum distance apart of two or more dots or lines at which the lens can form a distinct image of each dot or line — is given by the formula where X is the wave length of the light used and X. A. is the numerical aperture of the lens. By the use of light filters greater contrast is had between the material to be photographed and the background, so that it is possible in this way, especially with the use of stains as suggested below, to have the printed image stand out in bold relief with no background whatever showing. If a blue filter is being used, the greatest contrast l)ctween the background and the printed image is obtained by staining tlie material to be photographed with red or yellow dyes. Black dyes are suitable of course with any light, provided the fibers do not take them up in such quantity as to prexent all detail of structure from showing. Dense staining must be avoided in all cases, sometimes a mere tinting being sufficient. If one is to get the best results with light filters, there must be some practical method for determining the cjuality of light being transmitted by the filter used, since this can not be done by the unaided eye. The most convenient means of determining the transmission of light filters is a replica transmission grating of about I by 2 inches in size, having 5000 or 6000 lines per inch. To examine the quality of light transmitted by a given filter by this means, all light must be excluded from the grating except that passing through the filter. The grating must be held between the light filter and the eye, and the eye and grating directed some- what to one side and not directly toward the light filter. The LMfio,,] Photomicrography oj P,ipcr I'ibcrs. 635 various colors of the light trausniitted can then be seen and the quality of the filter determined. Another solution of the problem of making the chemical and visual foci identical is the employment of an apochromatic objec- tive and compensating ocular. If one wishes to use the objective without an ocular, however, __ a monochromatic filter ' should be used, as the apo- - IZZZZZZZZZIZ chromat is intentionally so — ZZZHIZZZZZZir constructed that, when ZZZZIZIZZZZZ ZZZZZHIIIZIII white light is used, it will . — not give an image free from — ~~~~~~~~' ' color except in combination — ^ with acompensatingocular. ' — Notwithstanding the finer ~~~~~ZIIIIIIIIZ. ~ ' corrections of the apochro- IZIZZll mats, it is doubtful if their ZZZZZIZZZZZZ ^_U performance in photomi- HHHIZZIZIZZZ ■ crography is much superior — ZZIZIZZ ; to that of the best achro- ZZZZHZZHZZZ ' mats when used Luidcr the ~~~~~IIZIIZZ most favorable conditions. Besides being less expen- ZZZZZZZZZZIZZ sive, the achromats have ~I~ZZZZIIIIII -- the advantage of giving a ../L- . ZZZIZ flatter image, that is, one ZZZZZIZZZZZZZ ^ __... ...Z 1 .1" having less curvature. ' - -.. — . Figures 5 and 6, which -— are photomicrographs at a , — - j— . — -— j magnification of 250 di- ameters of a scale ruled on Fig. =;. I-ig. 6. a glass slide, show some- .f^'-^f*- Theb(th.alf„fpIintnmirrosrnphsat amag- c> ' mhcationof rso-timesof a scale nilcdtin glass, show the rela- thing of the relative merits tive flatness ol image given by a 16 mm adimmaticcbiective . . .... il'"ig. ^) and b\' a lb mm apochromatic olijective (Fig. M of an achromatic objective and an apochromatic objective in giving llatness of image, l-'igure 5, the scale as photographed with the achromatic objective, shows that this objective gives a slighter flatter image than the apochro- matic objective, with which the same scale was photographed, as shown in Figure 6. Both oljjectives have an ecjuivalent focus of 16 mm. lU2.s;i4^— 2l' 2 tiiiiiiigement uith tu-o converging lenses. 6^6 TccIiHoloqic Papers of ihc Bureau of Sfandards. lVot.,6 IV. USE OF CONDENSERS. For shorter focal lengths than 8 mm a regular substage con- denser is not desirable, as better results are to be had by a con- denser arrangement which will send a parallel or nearly parallel beam of light into the objective. This may be had by removing the regular substage condenser and placing at the proper point i:i . the beam of light from the large condenser cither a converging or a diverging lens of short I'ocal length and rela- tively small diameter. The converging lens must be placed slightly outside the point, de- pending on its focal length, at which the rays are focused by the large condenser (see Fig. 7) ; the diverging lens, on the contrary, must be placed slightly within this point. (See Fig. 8.) This adjustment is easily carried out in a darkened room whose atmosphere contains lloating parti- cles of dust or smoke, as the path of the light rays can then be distinctly seen. V. USE OF SUBSTAGE DIAPHRAGM. It is a fact well known to all ])hotographcrs that to close dov.-n the diajjliragm of the camera in making a jihotograph increases the depth of focus or penetrating jiower of the lens. The only dis- advantage the ordinary photographer encoim- ters in decreasing the diameter of the dia- phragm is an increase in the time of exposure. In closing the substage diaphragm, however, the photomicrographer encoimters an additional and far greater evil, namely, the ap- pearance of diffraction or false images which mar the defmition and perfection of the real image produced by the objective. Diffraction eflects are most readily produced in using objectives of short focal length, but may be produced with comparatively long indcnscr arian:ii:nc:il uiih converging and diverging lenses. Photomicyography of Paper Fibers. L^fic,] Fliotomicrograpny nj raper riueiw. ^37 focal lengths if the diaphragm opening is made quite small. The purpose of the substage diaphragm is to exclude all those rays of light, from whatever source, which do not assist in forming the most perfect image of which the lens system is capable. Experi- ence will soon teach one that the best results are not to be had when the diaphragm is wide open, and tliat the less the numerical Fio. I'lt lo. iMBS. iffects Thediaplirasi ced lo times to iimatic objectiv glass, show ;m openings Phot. miiTf graphs at a magnification vi loo times ( : to the use ol substage diaphragm of too small apcrt were 14 mm. 4 mm. and i mm, respectively. I-'ig. 11 v\as suhsetnicntly ( distinctly the effects o( diffraction. All photographs were m.ide with an focus and numerical aperture of 0.25. aperture of the objective the smaller the diaphragm opening must be for the best definition. It has been found that best results are obtained with most objectives if the diaphragm is closed down until only about three-fourths or three-fifths of the back lens of the objective is lighted. The portion of the back lens which is lighted can be readily determined, if the mirror is properly ad- justed, by removing the ocular and looking down into the tube 63S Tcchnoloqic Papers of the Bureau of Standards. [Voi.io of the microscope. If the diaphragm opening is too large some of the rays of light enter at too great an angle with the axis of the optical system ; when the diaphragm is much too large a large proportion of the rays enter so obliquely that there is produced what is called "Hooding" of the objective with light, with the result that dellnition is almost or wholly destroyed. This latter C(Midition exists, however, only with substage condensers of relatively large numerical aperture. Fig. iia. — Chemical pulp from aspcn'Kioo. Stain . BiMnarck brown Mmmiing medium Venetian turpentine Objcttive 16 mm achromatic N. A. lo.2$ Diaphragm i mm Exposure 30 minutes Notice the diffraction bands— black and while lines parallel with the outlines of the fiber^^. " Flare pots." caused by dust particles being brought partially into focus by the aid of the small diaphragm, are evident- Compare with Fiu n and I'ig. 17, r'igures 9, 10, 11, and iia illustrate these facts. In Figure 9 the substage diaphragm was full open, in 10 it was closed to 4 mm diameter, and in 11 and iia- it was closed as far as possible, * The filter used was made in the laboratory from a photographic plate and a blue dye, by the method outlined in this paper. This filter transmits a rather narrow band in the blue and violet, and a small and relatively weak band in the deep red. Instead of the regular substage condenser, an achromatic lens of 5onim focal length was substituted and used throughout the^e experiments. In all photographs Seed No. 23 plates were used. Metol-hydroquinone developer of about double the usual strength was used for de- veloping the plates. The same kind of developer of the usual strength was used in making the prints. All prints weremadeon regular glassy velox, and were rolled down onferrotypeplatestodry. The Venetian turpentine used as the mounting medium was an artificial product. An attempt was made to secure some true Venetian turpentine, but without success. The artificial product, however, gave excellent results, although sufl'icicnt time has not elapsed since it was first used here to indicate anything as to its value as a permanent mountinij medium. The turpentine as used had a refractive index of 1.520 at 20° C. Photomicroqruphy of Paper Fibers. 639 to I mm. The sharpness of dehnition of Figure 9 is much better than that of 10; the lines in the latter are appreciably broader because of diffraction. In Figures 11 and iia diffraction effects are more apparent. Figure 11 is a lo-times enlargement of the original photomicrograph made at a magnillcation of 100 diameters. VI. CHOICE OF CAMERA AND OBJECTIVE. Successful ]5hotomicrographs can not be made with a camera provided with a very short bellows, especially if a comparatively large photograph, an S-inch by lo-inch, for example, is wanted at considerable magnification. If a short bellows is used, an objective of shorter focal length must be tised than if the bellows were longer, and an objective of short focal length is never desirable if one of longer focal length will give the detail required in the photograph. The objective of long focal length has the following advantages over one of less focal length: (i) Greater depth of focus (2) gives a flatter image, and (3) shows a larger field. By depth of focus, sometimes called penetrating power, is meant the capacity of a lens to bring to a sharp focus at the same time an object, or objects, lying in slightly separated planes perpendicular to the axis of the lens. The depth of focus of a lens is di- rectly proportional to the scjuare of its equivalent focus, or focal length, and in- versely proportional to its numerical aper- ture. A study of Figures 12, 13, and 14 will show that the greater the angle 1; which the perijiheral rays make it crossing at the focusing point the less will be the dejnh of focus. I'^igures 12 and 13 rejiresent lenses of the same focal length, but the diameter of 13 is much greater than that of 12, and it is very evident that the depth of focus of 13 is much less than that of 12. Figures 12 and 14 represent lenses of the same diameter, but 14 has a much shorter focal distance than 12, so that the peripheral rays through 14 make a greater angle than through 12; hence the depth of focus of 14 is less than that of 12. By tlie use of the diaphragm in crical aperture, n the marginal rays :it the principal fo( h hnsi-s..|diff.'i and different n (1= the angle \vl 640 Technoloqic Papers of the Bureau of Standards. {\\.i. ,6 photography the peripheral or marginal rays may be "stopped" out, thus decreasing the effective diameter of the lens, so that objects both relatively near to and relatively distant from the lens are in sharp focus. Figures 15, 16, 17, and iS illustrate the increasing depth of focus of objectives of increasing focal lengths. It is shown in treatises on light that all points of real images formed by convergent lenses do not lie in the same plane, even though the optician may have exhausted his skill in making the Fig. 15. — Chemical j^iilf^fipm aspi-n yioo. stain _ Bismarck brown .Mounting medium Venetian turpentine Objective 4 mm acliromatic X. A. /0.85 Diaphragm 4 nim Exposure 4 minutes necessary corrections. Although this curvature of image is usually not evident in lenses of comparatively long focal length, it is quite evident in lenses of such short focal lengths as high power, or even moderately high power, microscope objectives. When rays paral- lel to the axis are incident at all points on the surface of the lens, those rays passing through the margin are brought to a focus at points nearer the lens than are those passing through nearer its center. Also, of the oblique rays passing through the center of the lens those making the greatest angle with the axis are brought to the focus nearest the lens. For these reasons the image / of Lofion] Photomicrography of Paper Fibers. 641 Fig. lb.— Chemical f^ulp fww a.spen Xwo. Stain Bismarck brown Mounting medium \'enetian turpentine Objective S mm achromatic N. A. /0.50 Diaphragm 4 mm Exposure 1 minutes Fig. 17. -Chciulcil pulp f)0)n a.^pcn y lOO. Stain Hismarck brown Mounting medium \'enciian lurpcntine Objective 16 mm achromatic X. A. /'o.ss Diaphragm - 4 mm Kxposure i minutes 642 Technologic Papers oj the Bmean oj SiamlarJs. [Voi. the object will take the form as shown in Figure 19. Persons using the microscope have observed this condition, in tliat to get objects lying near the margin of the field of view in focus after the center of the field has been sharply focused upon one has to rack the objective slightly toward the object. In objectives of longer Fig. 18.— Chcmkul pulf>fiom Uipcn yiOO. Stain Bismarck brown Mounting medium Venetian turpentine Objective 48 mm achromatic N. A. /o. Diaphragm - - - 4 mm Exposure 4 minutes t?=nbiect: /= image; xplan.ui.m, Fig. iq. — Curviiliirc of i»uu:i' of a convex lens. -optical center of lens. .1 .! = axis . .1 lens . /■ - principal I. focal length this curvature of image does not exist to such an extent, so that it is particularly advisable to use objectives of longer ecjuivalent focus in photomicrography if a comparatively large photograph is to be made. But an objective of longer equiv- alent focus will require a longer bellows unless an eyepiece of higher U>fUm\ PhotoiniciOiji'uphy of Paper I-'ihci; ^'43 power be used, which is usually not desirable, since high-power eyepieces of ten unduly aggravate faults already existent, or even introduce new ones. Although compensating eyepieces must be used with apochromatic objectives unless monochromatic light is used, the use of eyepieces is not necessary with achromatic objec- tives, and not even desirable in most cases, if means are at hand IMG. 23. Fiss. .'o. n, 22, and -m. photon the greater flatness of image give achromatic i»bjective, Fij:. ji wit jective and Fis -m with a 4^ mm 21. Fig, 22 1-iG. 2.^ t a mufinificatii.nof 100 times of a scale 1 'es of greater focal leucth Fie ?o was chromatic objective. Fig. 22 with a 16 for getting the desired magnification without their use. If an eyepiece must be used, it should be of very low power, not more than about two times (2X). All the photographs shown were made with achromatic objecli\'es, with but one exception, without the use of eyepieces. Objectives of long focal length not only give greater depth of focus and less curvature of image, but they also give a larger field 644 Teclniologic Papers of the Bureau of Standarels. [ix. /a of view and cover a larger plate than those of shorter focal length, so that by their use a larger photograph can be made. It is also desirable that the total area covered by the image at the ground glass screen be much larger than the plate used to record the image, since then only the central portions of the image will be photographed and the best focus, as Hat an image as possible under the circumstances and the best definition will be secured. Fig- ures 20, 21, 22, and 2Ti show that the curvature of the image is less with objectives of greater focal length. The glass scale was not long enough, however, to show the comparative values of the 16 mm and 48 mm objectives in this respect. VII. PHOTOGRAPHIC PLATES AND THEIR DEVELOPMENT. The selection of the dry plate to be used in making a photo- micrograph is of almost as much importance as any other factor. Various plates were tried during this investigation, and it was found that good results were to be had only by using compara- tively slow plates. Even with these plates a contrast developer was used and was found not to give too great contrast if the stain- ing of the fibers was properly done. The slow plate usually has greater resolving power, due to the finer grain of the silver salts in the gelatine emulsion. These advantages will much more than compensate for the additional time of exposure required over that of the faster plate. Process plates, however, are not to be recom- mended for general work, as they give too much contrast. Whether one shoidd use an ordinary, an orthochromatic, or a panchromatic plate will depend, of course, on the colors of the material to be photographed, the purpose in \iew in making the photograph, and perhaps on other factors. VIII. VALUE OF A REPRESENTATIVE FIELD OF FIBERS. It is of first importance that the microscopist should know the distinguishing properties and characteristics of the fibers which he is about to photograph and that he should take all precaution to get a representative field under the microscope. It would be manifestly wrong, for example, to make a photomicrograph of a field of aspen soda pulp which does not show any of the character- istic pitted vessels chiefly by means of which all hardwoods are distinguished from the softwoods or conifers. It also gives a wrong impression to see a {photomicrograph of wood pulp showing an incorrect proportion of fillers to vessels, tracheids, or paren- chyma cells. The material to be photographed, whether it be Lofi.;A Pliotoinicicgiiiphy nj Paper Fihei.s, 645 pulp or other material, should lirst be studied suffieiently under the microscope to determine its properties and characteristics, and the correct proportion of the various cells or other components, by counting or by some other reliable method. A field for photo- graphing should then be chosen which will show the correct pro- portion of the various individuals m other characteristics which it is the pirrpose of the photographer to show. IX. VALUE OF STAINING. The utility and value of the use of various stains and dyes in the differentiation and study of plant tissues and elements has long been recognized by botanists. A great number of dyes and stains of recognized worth for botanical work are on the market, and this number is IxMiig graflually added to. A number of these stains are more or less selective in their action on jilant tissue, and many of them are emphatically so. Their great \-alue lies in the fact that because of this .selective property they dye certain tissues or certain parts of fibers more deeply than others, and thus bring out or accentuate character- istic markings, reactions, or other properties of the fibers. By their use the differentiation and identification of various fibrous materials, otherwise difficult or impossible, is made cjuite easy and certain. Their value in the photomicrography of paper-making fibers is greater, if possible, than in the visual study of plant fibers. One of the most serious limitations to the photographing of fibrous materials is that their finer markings and characteristics can not be shown as readily and as distinctly loy ])hotographic means as by visual means. Hence any means of showing more distinctly the characteristic markings of libers is a manifest advantage in making a photograph of these fibers; this means is supplied by the proper use of stains. Another great value oi the use (if a proper stain is the contrast afforded between the outlines of the fibers and the backgroimd. By this means the fibers may be made to stand out in bold relief, with U(j suggestion whatever of a V:)ackgromid in the finished print if the work is well done. Figures 24 and 25 show the advantages of staining in bringing out the physical features of fibers, and especially in offering sufli- cient resistance to the passage of the light vays through the libers to allow the plate to be cxposerl long enough to prevent the background showing in the finished print. 646 Technologic Papers of the Pjtreau of Standards. iVvii6 '^. •% Fig. 24. — Flax fibers Xioo. Stain Xone Mounting medium . . \'enetian turpentine Objective - 16 mm achromatic N. A. ars Diapliragiu ' 4 mm Exposure 2 minutes Fig. 25. — Flaxjihcis Xioo. Stain Methylene blue and salTranin Mounting medium Venetian turpentine Objective. 16 mm achromatic N. A. 0.35 Diaphragm 4 mm Exposure . 3 minutes Lrfio,,] Photomicrography oj Paper Fibers. 647 X. PHOTOGRAPHS AS PERMANENT RECORDS. The use to which jjhotomicrograpliy may be jiut in the control of various industrial processes and in recording the presence of adulterations are innunieralile. " \Vhere\'cr the niicroscojje finds application and permanent records of observations are required, photomicrography is the readiest, most convenient, and accurate means available. I'rints can be stored with the written notes of analyses and examinations, and have not only the value of records of known facts but of j^oints luiknown or unrecognized at the time, Fig. 26.— Fi7'./. fiom all ui.: 1,01 illlund.' ImiIii objective I>iaphragin. whicli would ine\-ilahly be missed in any written notes, but may become of the highest importance in future researches and afford accumulated evidence which it would be impossible to obtain otherwise." ' In the paper industry photomicrography may l)e advantageously employed in the study and control of the cooking and beating processes, in the e.xamination of pulps and pulp mixtures, and of sizing and loading materials, in recording adulterations frequently found in dyes, in studying bacteria and molds, dirt in pulp and = Hind and Raudk-s, Ha Tosraphy, p. ;iS. Limilu 64S Technologic Papers of the Bureati of Standards. [Voiie Fig. 27. — Fihers from oil rag rncdiion -dcight bond Y 100. Stain Zinc chloride— Iodine Mounting medium Zinc chloride— Iodine Objective 16 mm achromatic N. A. 0.25 r>iaphragm 4 mm Exposure lia minutes Fig. 28. — Fibers from all la^} thin bond Y loO . Stain Mounting mcdii Objective Diaphragm Exposure Zinc chloride — Iodine Zinc chloride— Iodine , 16 mm achromatic N. A. 0,25 i..'/'""l Photomicioqraphy of Puffer Fihos. 649 paper, and in many other ways. Whatever the eye can see Ijy the aid of the microscoj^e may be recorded photogra]ihica]ly if proper methods are used. Photographs 26, 27, and 28 show the differences in beater treat- ment for (26) ledger paper, (27) medium-weight bond, and (28) thin bond, and indicate the vahie of photomicrographs as perma- nent records. BIBLIOGRAPHY. A short list of books and articles on photomicrography and closely related subjects. Light axu Optics. r.ausch & Lonib Optical Co., Lenses, tluir liistun-, tlu-dry, Mini manufacture ; Ruches- tcr, N. v.. iqoo. Heck, Conrail, and .Andrews, Herbert, Photoijraphic lenses, 7th editiun; R. & J. Heck (Ltd., I, 6S Cornhill, London. Daniell, .\.. .\ text-liook of the iirincii)les of ])hysics: .Macmillan & Co., New York and I.flndon. Rdser, Edwin, Light for students; Macmillan & Co., London, 1907. ( Hleinentary, but very complete, i Lummer, O., and Thoni])Son, Silvanus, Contributions to ])hotoj;raphic o])tics; Alac- millan & Co., Mew York, kjoo. Watson, William, .\ te.xt liook of physics; Lon;- chemical microscopy; lohn \\'ile\- & Sons, New- York, 1020. Gage, Simon Henry, The microscope, an introduction to microsci>]iic methods ami to histolosy: The Comstock I'ublishins,' Co.. Ithaca, N. Y., lo.-o. (Yen," g I.l Hanausek, T. h'., and Winton, Andrew L , The microscopy of technical products; John Wiley S: Sons, New York, i.,07. Hogg, Jabe/:, The microscope, its histor)-, construction, and a|i]ilication; Geon;e Routledge & Sons (Ltd.), London, igii. (Very complete.) Lee, A. B., The microscopist's vade-mecum; P. Blakiston's Son & Co., Philadelphia, Microscopy of pulpwoods. Paper 26, 1!); April ji, 1020. vSpitta, Edmund, Microscopy, the construction, theon,-, and use f>f the microscope; John Murn,-, London, iqc(). Stokes, Alfred, Aquatic microscopy for beginners, John Wiley & Sons, New York, 19x8. The microscope, its design, construction, and applications, J. Roy. Microscopical Soc, Part 4; Dec, 11120, .See copies of above in Engineer, 120, p. 85-87, Jan. 23, iq2o, and in Engineering, 109, p. 86-87, ^1"^ 106/7, Jan. 16 and 2:;, 1Q20. (See current and b.ack numbers of J. Roy. Microscopical Soc. and of Tran. of Am. Microscopical Soc. for articles dealing with all phases of microscopical work. 1 WTiipple, G. C, The microscopy of drinking water; John Wiley S: S