CHAPTER VI

HOW TO MOUNT SPECIMENS

The method of procedure in mounting specimens for study varies according to the nature of the specimen, its preliminary treatment, and the character of the mount to be made. As to duration, mounts are either temporary or permanent.

TEMPORARY MOUNTS

In preparing a =temporary mount=, place the specimen in the centre of a clean slide and add two or more drops of the temporary mounting medium, which may be water, or a mixture of equal parts of alcohol, glycerine, and water, or some micro-chemical reagent, as weak Lugol’s solution, solution of chloral hydrate, etc. Cover this with a cover glass and press down gently. Remove the excess of the mounting medium with a piece of blotting paper. Now place the slide on the stage and proceed to examine it. Such mounts can of course be used only for short periods of study; and when the period of observation is finished, the specimen should be removed and the slide washed, or the slide washing may be deferred until a number of such slides have accumulated. At any rate, when the mounting medium dries, the specimen is no longer suitable for observation.

PERMANENT MOUNTS

Permanent mounts are prepared in much the same way as temporary, but of course the mounting medium is different. The kind of permanent mounting medium used depends upon the previous treatment of the specimen. If the specimen has been preserved in alcohol or glycerine and water, it is usually mounted in glycerine jelly. If the specimen in question is a powder, it is placed in the centre of the slide and a drop or two of glycerine, alcohol, and water mixture added, unless the powder was already in suspension in such a mixture. Cut a small cube of glycerine jelly and place it in the centre of the powder mixture. Lift up the slide by means of pliers, or grasp the two edges between the thumb and finger and hold over a small flame of an alcohol lamp, or place on a steam-bath until the glycerine jelly has melted. Next sterilize a dissecting needle, cool, and mix the powder with the glycerine jelly, being careful not to lift the point of the needle from the slide during the operation. If the mixing has been carefully done, few or no air-bubbles will be present; but if they are present, heat the needle, and while it is white hot touch the bubbles with its point, and they will disappear. Now take a pair of forceps and, after securing a clean cover glass near the edge, pass them three times through the flame of the alcohol lamp. While holding it in a slanting position, touch one side of the powder mixture and slowly lower the cover glass until it comes in complete contact with the mixture. Now press gently with the end of the needle-handle, and set it aside to cool. When it is cool, place a neatly trimmed label on one end of the slide, on which write the name of the specimen, the number of the series of which it is to form a part, etc. Any excess of glycerine jelly, which may have been pressed out from the edges of the cover glass, should not be removed at once, but should be allowed to remain on the slide for at least one month in order to allow for shrinkage due to evaporation. At the end of a month remove the glycerine jelly by first passing the blade of a knife, held in a vertical position, the back of the knife being next to the slide, around the edge of the cover glass. After turning the knife-blade so that the flat side is in contact with slide, remove the jelly outside of the cover glass. Any remaining fragments should be removed with a piece of old linen or cotton cloth. Finally, ring the edge of the cover glass with microscopical cement, of which there are many types to be had. If the cleaning has been done thoroughly, there is no better ringing cement than Canada balsam.

In mounting cross-sections, the method of procedure is similar to the above, with the exception that the glycerine jelly is placed at the side of the specimen and not in the centre. While melting the jelly, incline the slide in order to allow the melted glycerine jelly to flow gradually over the specimen, thus replacing the air contained in the cells and intercellular spaces. Finish the mounting as directed above, but under no conditions should you stir the glycerine jelly with the section.

If specimens, after having been embedded in paraffin or collodion, are cut, cleared, stained, and dehydrated, they are usually mounted in Canada balsam. A small drop of this substance, which may be obtained in collapsible tubes, is placed at one side of the specimen. While inclining the slide, gently heat until the Canada balsam covers the specimen. Secure a cover glass by the aid of pliers, pass it through the flame three times, and lower it slowly while holding it in an inclined position. Press gently on the cover glass with the needle-handle, and keep in a horizontal position for twenty-four hours, then place directly in a slide box or cabinet, since no sealing is required.

Glycerine is sometimes used to make permanent mounts, but it is unsatisfactory, because the cover glass is easily removed and the specimen spoiled or lost, unless ringed--a procedure which is not easily accomplished. If the specimen is to be mounted in glycerine, it must first be placed in a mixture of alcohol, glycerine, and water, and then transferred to glycerine. Lactic acid is another permanent liquid-mounting medium, which is unsatisfactory in the same way as glycerine, but like glycerine, there are certain special cases where it is desirable to use it. When this is used, the slides should be kept in a horizontal position, unless ringed.

COVER GLASSES

Great care should be used In the selection of =cover glasses=, however, not only as regards their shape but as to their thickness. The standard tube length of the different manufacturers makes an allowance of a definite thickness for cover glasses. It is necessary, therefore, to use cover glasses made by the manufacturer of the microscope in use.

Cover glasses are either square or round. Of each there are four different thicknesses and two different sizes. The standard thicknesses are: The small size is designated three-fourths and the large size seven-eighths.

[Illustration: FIG. 41.--Round Cover Glass]

[Illustration: FIG. 42.--Square Cover Glass]

[Illustration: FIG. 43.--Rectangular Cover Glass]

=Cover glasses= are circular (Fig. 41), square (Fig. 42), or rectangular (Fig. 43) pieces of transparent glass used in covering the specimens mounted on glass slides. A few years ago much difficulty was experienced in obtaining uniformly thick and transparent cover glasses, but no such difficulty is experienced to-day. The type of cover glass used depends largely upon the character of the specimen to be mounted. The square and rectangular glasses are selected when a series of specimens are to be mounted, but in mounting powdered drugs and histological specimens the round cover glasses are preferable because they are more sightly and more readily cleaned and rinsed.

GLASS SLIDES

[Illustration: FIG. 44.--Glass Slide]

=Glass slides= (Fig. 44) are rectangular pieces of transparent glass used as a mounting surface for microscopic objects. The slides are usually three inches long by one inch wide, and they should be composed of white glass, and they should have ground and beveled edges. Slides should be of uniform thickness, and they should not become cloudy upon standing.

SLIDE AND COVER-GLASS FORCEPS

Slides and cover glasses should be grasped by their edges. To the beginner this is not easy. In order to facilitate holding slides and cover glasses during the mounting process, one may use a slide and a cover-glass =forceps=. The slide forceps consists of wire bent and twisted in such a way that it holds a slide firmly when attached to its two edges.

[Illustration: FIG. 45.--Histological Forceps]

[Illustration: FIG. 46.--Forceps]

[Illustration: FIG. 47.--Sliding-pin Forceps]

There are various forms of cover-glass holders, but only two types as far as the method of securing the cover glass is concerned. First, there are the bacteriological and the histological forceps (Fig. 45), which are self-closing. The two blades of such forceps must be forced apart by pressure in securing the cover glass. The second type of forceps is that in which the two blades are normally separated (Fig. 46), it being necessary to press the blades to either side of the cover glass in order to secure and hold it. There is a modification of this type of forceps which enables one to lock the blades by means of a sliding pin (Fig. 47), after the cover glass has been secured. It is well to accustom oneself to one type, for by so doing one may become dexterous in its use.

NEEDLES

[Illustration: FIG. 48.--Dissecting Needle]

Two =dissecting needles= (Fig. 48) should form a part of the histologist’s mounting set. The handles may be of any material, but the needle should be of tempered steel and about two inches long.

SCISSORS

[Illustration: FIG. 49.--Scissors]

Almost any sort of =scissors= (Fig. 49) will do for histology work, but a small scissors with fine pointed blades, are preferred. Scissors are useful in trimming labels and in cutting strips of leaves and sections of fibrous roots that are to be embedded and cut.

SCALPELS

[Illustration: FIG. 50.--Scalpels]

=Scalpels= (Fig. 50) have steel blades and ebony handles. These vary in regard to size and quality of material. The cheaper grades are quite as satisfactory, however, as the more expensive ones, and for general use a medium-sized blade and handle will be found most useful.

TURNTABLE

[Illustration: FIG. 51.--Turntable]

Much time and energy may be saved by ringing slides on a =turntable= (Fig. 51). There is a flat surface upon which to rest the hand holding the brush with cement, and a revolving table upon which the slide to be ringed is held by means of two clips. In ringing slides, it is only necessary to revolve the table, and at the same time to transfer the cement to the edge of the cover glass from the brush held in the hand.

LABELING

There are many ways of =labeling slides=, but the best method is to place on the label the name of the specimen, the powder number, and the box, the tray or cabinet number. For example:

Powdered Arnica Flowers No. 80--Box A--600.

PRESERVATION OF MOUNTED SPECIMENS

[Illustration: FIG. 52.--Slide Box]

[Illustration: FIG. 53.--Slide Tray]

Accurately mounted, labeled, and ringed slides should be filed away for future study and reference. Such =filing= may be done in slide boxes, in slide trays, or in cabinets. Slide boxes are to be had of a holding capacity varying from one to one hundred slides. For general use, slide boxes (Fig. 52) holding one hundred slides will be found most useful. Some workers prefer trays (Fig. 53), because of the saving of time in selecting specimens. Trays hold twenty slides arranged in two rows. The cover of the tray is divided into two sections so that, if desired, only one row of slides is uncovered at a time. Slide cabinets (Fig. 54) are particularly desirable for storing large individual collections, particularly when the slides are used frequently for reference. Large selections of slides should be numbered and card indexed in order to facilitate finding.

[Illustration: FIG. 54.--Slide Cabinet]

## Part II

TISSUES CELLS, AND CELL CONTENTS

##