THE mounting or “putting up” of microscopic objects signifies their preparation in such way that they may be preserved for future reference and observation.

As a general rule, objects should be mounted in that manner by which their structure is best and most clearly shown; but in certain instances the objects are mounted so as to make their structure difficult of detection, that they may form test-objects of the power and quality of the microscope.

Some objects require to be mounted in the dry state, while others are best mounted in liquid; some again as opake, others as transparent objects: these must be considered separately.

Dry opake objects were formerly mounted by gumming them upon small coin-shaped pieces or disks of cork, blackened upon the surface with a mixture of fine lamp-black and thin warm size, laid on with a hair-pencil. They were kept in a drawer, to the bottom of which a sheet of cork was glued, the disk being transfixed by the pin, so that the free or projecting pointed end of the pin could be thrust into the sheet-cork. This plan may still be adopted in the case of common objects, as seeds, &c.; but it is objectionable, on account of the facility with which the bare objects are knocked off or injured by dust.

Hence dry opake objects are usually mounted in such manner as to be enclosed in a cell, the sides being formed by a ring of glass-tube or cork, or a square piece of leather, cardboard, or paper, with a hole cut or punched out of the middle. The glass rings are best; but as they are expensive, some of the other substances are generally used. The size and thickness of the material from which the rings are made must obviously vary according to the size and depth of the object. The rings are cemented to the middle of ordinary slides; and it is best to keep a number of them ready prepared. The cementing material must vary according to the nature of the ring used. If this consists of glass, Canada-balsam or marine glue is best. In using the former, the ring is gently heated over the flame of the spirit-lamp, and a thin layer of the balsam applied to its upper or under surface, by means of an iron wire with a little balsam on its end; it is next warmed over the spirit-lamp, so that the surface is entirely and evenly coated. A clean slide is then slightly heated, the ring laid upon it, and gentle pressure is used to squeeze out the excess of balsam; and the slide is kept at a gentle heat, until on cooling the balsam becomes so hard as not to be indented with the finger-nail. Marine glue is applied in the same way as the balsam, except that prolonged heat is not required to harden it, for it becomes hard on cooling. The balsam may also be replaced by black japan or asphalte.

The pieces of cork, leather, or paper are best fastened to the slides with solution of shellac or sealing-wax in methylated alcohol, or with white hard varnish.

When the ring or piece has been firmly fixed to the slide by either of the above cementing materials, so as to form the sides of the cell, the bottom is to be covered with a piece of black paper, cut to fit it exactly, and fastened to the surface of the slide with a little gum, or of either of the above varnishes. As soon as this is thoroughly dry, the upper surface of the cell-wall, whether of glass or cork, &c., is thinly covered with varnish, and a clean thin-glass cover laid upon it, and very lightly pressed; the object is then permanently preserved.

The main points to be observed are, that the object and varnish are completely dry, and that the cell is thoroughly closed. If the latter be not the case, more varnish must be applied to any little openings which may have been left; and it is better to apply the varnish in very small quantities at a time, the application being renewed as soon as the previous layer is quite dry.

Dry transparent objects are usually small and delicate; for, unless they are so, their structure cannot be well seen. In mounting these, a square piece of note-paper or tracing-paper, with the centre cut out, may be fastened to a clean slide with a little paste, gum, or shellac varnish. When this is thoroughly dry, the object is placed in the vacant space, a clean dry cover laid on, and the varnish applied by means of a hair-pencil to the edges in very small quantities. This will run in between the under surface of the edges of the cover and the upper surface of the paper, and when dry will cement the two together.

Supposing that the object is so delicate that it cannot be removed from the surface of a slide, if it will not be injured by heat, a good plan is to draw a square or circle around the object with a little black japan, then to heat the slide gradually until the japan is not indented with the finger-nail when cold. A clean slide is then laid upon the ring of japan, the whole again gently warmed, until the varnish is softened, and the cover lightly pressed so as to be in contact all round with the varnish. The slide must then be rapidly cooled, by being laid upon a piece of metal, which prevents the varnish from running in so as to spoil the object.

Many dry transparent objects can be preserved by mounting in Canada balsam. This is the best process for mounting objects in general; but only those can be so preserved which are not injured by drying, and which are not rendered too transparent by the balsam. If the object to be mounted in balsam be small, it is thoroughly dried, and then a drop of oil of turpentine added to it upon a slide; the slide is then gently warmed, which causes the turpentine to evaporate. When this has nearly all evaporated, a drop of balsam is allowed to fall upon the object from the end of a wire held at a distance above the flame of a spirit-lamp. A warmed cover is next laid upon the balsam, and gentle pressure applied until the cover is sufficiently depressed. The slide is then kept at a gentle heat until the balsam is quite hard when cold. The superfluous portions may be removed with the point of a knife, and any residues cleaned off with turpentine or a little benzole on a cloth.

Another way consists in laying a cover upon the dry object on a slide, adding a drop of turpentine, and warming the whole over a spirit-lamp until all air-bubbles are displaced, then continuing the application of the heat until most of the turpentine has evaporated. A drop or more of the balsam may next be applied to the edge of the cover, when it will run in and mix with the turpentine. The whole is then gently heated until the balsam is hard when cold, more balsam being added if necessary, to replace the turpentine which has evaporated.

When the objects are large, they should be pressed as flat as possible without injury between two slides, being retained until dry by enclosure between the prongs of an American clothes-peg; or the slides may be fastened at the ends by sealing-wax. When perfectly dry, the object should be immersed in turpentine, kept in a common gallipot, until all the air-bubbles have been entirely displaced, and the object appears very transparent. It is then removed from the turpentine with forceps, drained, laid upon a slide, and melted balsam dropped upon it until it is quite covered. A clean dry slide is then laid upon its surface, and the two slides gently pressed together, the two slides fixed at the ends by sealing-wax, and the whole allowed to cool and dry. If requisite, more balsam is added to fill up any vacuities. When the balsam has become hard, the excess is cleaned away with a knife and turpentine, and the object is permanently mounted.

If the object should be spoiled by the presence of air-bubbles, the slides and object should be immersed in turpentine or methylated alcohol, until the whole of the balsam is dissolved; the remounting may then be proceeded with as at first. If the slides have been immersed in the alcohol (which is the quickest method), the object must be soaked in turpentine before the balsam is reapplied.

If, after an object has been mounted in balsam, on applying heat, bubbles resembling air-bubbles should be formed, the object must not be considered as spoiled; for these are merely bubbles of the vapour of turpentine, and will disappear spontaneously after a little time.

A quick way of mounting in balsam is to drop the melted balsam at once upon the dried object; but as air-bubbles are very apt to be produced in this way, the beginner had better previously apply the turpentine.

As balsam is very viscid, and adheres firmly to everything with which it comes in contact, some care is required in its use. Young microscopists very generally manage to soil the microscope, tables, chairs, papers, books, and even their clothes with it. It may be easily cleaned off, however, with turpentine or benzole.

Moist objects are best preserved, whenever practicable, in glycerine. There are, however, two important objections to its use: one is, that it makes objects very transparent; the other is, that it often wrinkles and distorts them, by withdrawing their watery contents. Hence only those objects can be preserved in glycerine, which are not too transparent, and which are sufficiently firm to resist the tendency to collapse.

When the objects are tolerably flat, and sufficiently firm to bear the pressure of the cover, they may be mounted by adding a small quantity of glycerine to them lying on a slide; the cover is then applied, and a little of the cement mentioned below applied warm with a hair-pencil around the edges of the cover to fasten it to the slide. Care is required that the glycerine applied be no more than sufficient; for wherever it has touched the cover or the slide, the cement will not adhere. Superfluous portions may be sucked up with a piece of clean moist sponge or a corner of blotting-paper.

When it is required to mount a large number of objects in a short time, the cement need only be applied to two opposite sides of the cover, leaving the other two sides open.

When the objects require to be protected from the pressure of the cover, the sides of a cell must be made with the cement or black japan upon the slide before the cover is applied, a further quantity being used to close the cell as usual.

A very strong solution of chloride of calcium may be used for the same purposes and in the same way as the glycerine. It has the advantage of not making the object so transparent; but it has the disadvantage of crystallizing slightly in a dry atmosphere. In most cases, I prefer it to glycerine.

A large number of interesting objects cannot, however, be preserved in either glycerine or chloride of calcium, without their value being impaired by the cause mentioned above. Many kinds of liquid have been recommended for preserving these, all agreeing mainly in being inefficient. The objection to them is, that they are evaporable; and after the object has been mounted for some time, the liquid creeps between the cement and the slide or cover, at some spot, and evaporates; and if the cement be not quite hard, the inner and more liquid portion of it runs into the cell, and spoils the object. A solution, containing a grain of salt and a grain of alum to the ounce of distilled water, is as good as any other; or simply distilled water in which a piece of camphor has been kept. In use, the cell is first formed by making a circle or outline square on the slide with black japan, and heating this carefully until it becomes solid when cold. The object is then laid in the cell, the liquid added, and the cover applied, any excess being removed with blotting-paper. The cell is to be closed with old black japan or gold-size, applied round the margins of the cover with a hair pencil. A second and a third layer of the varnish may be applied upon the first, when it has become hard outside. Although black japan and gold-size are generally used for the cement, I prefer that mentioned below.

When large preparations are mounted in liquid, the cell-walls are either formed of glass rings, or they are built up with four oblong pieces of glass, cemented to the slide and to each other with marine glue. A very good preservative liquid for large specimens is a solution of chloride of zinc, in the proportion of 20 grains to the ounce of distilled water. A mixture of spirit of wine and water, in the proportion of 1 part to 2, or 1 to 4, is often used for the same purpose.

When preparations are mounted, the cement and the adjacent parts of the slide and cover should be coated with a solution of sealing-wax in spirit, which hardens the exterior of the cement.

The cement above alluded to is made by melting together 5 parts of rosin, 2 parts of balsam, 1 part of bees’-wax, and 1 of red ochre. The cement is best kept in a little metallic cup, and melted over a spirit-lamp when used. It should be applied while hot, with a hair pencil; and cools very quickly.

The preservative liquids should be kept in corked bottles, a hair pencil being fixed into the under part of the cork; or, what is better, in stoppered bottles, the stopper being prolonged to a point nearly reaching the bottom of the bottle.

As soon as the preparations are mounted, they should be labelled, the labels being kept ready gummed. The balsam should be kept in a capped bottle, such as is used for holding solutions of gum, with an iron wire for removing portions as required. By keeping, the balsam becomes thicker; it may be thinned by the addition of oil of turpentine, and the application of a gentle heat.

The black japan, &c., may be procured at any oil-shop; the glass rings, cell-sides, covers, &c., from Mr. Norman, 178 City Road, or of the microscope-makers.

Mounted objects should be kept in shallow drawers, and be laid flat—not standing on edge.

Magnifying power.—Before entering upon the consideration of the objects themselves, a word or two must be said upon the magnifying powers. In the plates of this work, the serial number of each figure is expressed by large numerals placed above the objects, while the number of times the object is magnified is indicated by small numerals placed beneath. The latter must be understood to express the number of times the drawing is larger than the object in one dimension. Thus, considering fig. 13, Plate I. to be an inch in width (for it is really somewhat less), being magnified 150 times in the direction of the width, the object itself is about ¹/₁₅₀th of an inch in size; and it is represented magnified 150 times linear, or 150 diameters, as it is called.

A knowledge of the number of times the object is magnified is of the greatest importance in making use of the drawings; for, without it, the observer will be unable to apply such a magnifying power of the microscope as will enable him to see the structural appearances figured in the drawings.

The observer must also be acquainted with the magnifying powers of his microscope with the various object-glasses and eye-pieces. These are usually given when the instrument is purchased. Or they may be determined approximatively thus:—An ivory scale, with ¹/₁₀₀th of an inch engraved upon it, is placed on the stage, and viewed as an opake object, both eyes being kept open; and the size of the image of one of the gradations is measured with compasses, upon the stage as seen with that eye which is not applied to the eye-piece. The number of ¹/₁₀₀ths of an inch contained in the measure obtained with the compasses represents the magnifying power. Thus, supposing the image of the ¹/₁₀₀th of an inch on the scale appears magnified to the length of 1 inch on the stage; the magnifying power is 100 diameters, or 100 times linear. This proceeding is difficult to any one unaccustomed to the use of the microscope, yet by practice it becomes very easy. Other methods, which require the use of the camera lucida, are given in the Micrographic Dictionary.