The Preparation & Mounting of Microscopic Objects

by microscopist Thomas Davies

As this instrument is peculiarly adapted for cutting wood (though used for other substances, as before mentioned), I shall notice a few particulars concerning this branch of sections. It may here be remarked, that to obtain anything like a true knowledge of the nature of wood, it should be cut and examined in at least two directions, across and along. The piece of wood is often placed in spirits for a day or two, so that all resinous matter may be dissolved out of it; it must then be soaked in water for the same length of time, so as to soften and render it easy to cut. Sections are obtained in the manner described above, but often curl to such a degree as to make it necessary to immerse them in water, from which they may be taken and dried under slight pressure. They are often mounted dry, and require no care beyond other objects, as in [Chapter II.] Some, however, are best mounted in balsam, particularly the long sections when used for the polariscope; these must be soaked in turpentine, and the greatest care taken that all air bubbles are got rid of. Others are thought to be most useful when mounted in shallow cells with some of the preservative liquids mentioned in [Chapter IV.]—weak spirit and water, chloride of calcium solution of the strength of one part of the salt to three parts of distilled water, &c.

The above “section-cutter” may not be within the reach of every student, nor is it absolutely necessary; though where any great number of specimens is required it is very useful, and insures greater uniformity in the thickness. Many employ a razor for the purpose, which must always be kept sharp by frequent stropping. Sections of leaves also may be procured by the same means, though, as before mentioned, they are sometimes easily divided by stripping the coatings off with the fingers. The cells which come to sight by cutting some of the orchideous plants are most interesting. To cut these leaves they may be laid upon a flat piece of cork, thus exposing the razor to no danger of injury by coming in contact with the support. It may be mentioned here that the razor may also be used in cutting sections of the rush, than which a more beautiful object can scarcely be found when viewed transversely, as it shows the stellate arrangements of the ducts to convey the liquids to the different parts of the plant very clearly. This should be mounted dry. In the same way sections of the leaf-stalks of ferns may also be cut, some of which, as Dr. Carpenter states, show the curious ducts very beautifully, especially when cut rather obliquely.

When sections of the softer substances are required, no instrument can be compared with “Valentin’s knife,” which consists of two steel blades lying parallel with one another and attached at the lower end. The distance of separation may be regulated at will by a small screw near the handle. When, therefore, a section is wanted, the substance must be cut through, and betwixt the blades a thin strip will be found, which may be made of any thickness, according to the distance of their separation. By loosening the screw the blades may be extended, and the section may be floated out in water if the damp will not injure it. The knife cuts much better if dipped in water immediately before use and also when the substance to be operated upon is wet, or even under water altogether; but care must be taken, after use, to clean the blades thoroughly and oil them before laying by, if the place is at all damp. This instrument is most useful in such subjects as anatomical preparations where the sections are required to show the position of the different vessels, &c.; but, as before stated, is very valuable for all soft substances. As an instance of this, it may be mentioned, that it is frequently used in cutting sections of sponges; but as these are often very full of spicula, &c., it is much better to press the sponge flat until dry, and then cut off thin shavings with a very sharp knife; these shavings will expand when placed in water. After this they may be laid betwixt two flat surfaces and dried, when they may be mounted as other dry objects, or, when desirable, in balsam.

Valentin’s knife is very much used in taking sections of skin, which are afterwards treated with potash solution, acids, &c., to bring out in the best way the different portions. Dr. Lister’s mode, however, of getting these is thus given in the Microscopic Journal:—“But I afterwards found that much better sections could be obtained from dried specimens. A portion of shaved scalp being placed between two thin slips of deal, a piece of string is tied round them so as to exercise a slight degree of compression; the preparation is now laid aside for twenty-four hours, when it is found to be dried to an almost horny condition. It then adheres firmly by its lower surface to one of the slips, and thus it can be held securely, while extremely thin and equable sections are cut with great facility in any plane that may be desired. These sections, when moistened with a drop of water and treated with acetic acid, are as well suited for the investigation of the muscular tissue as if they had not been dried.”

There are many who almost confine their attention to polarized light and its beautiful effects. Such would not deem these efforts to aid the student in cutting sections complete without some notice of those which are taken from various crystals, in order to display that curious and beautiful phenomenon, the rings with a cross. The arrangement of these is somewhat changed by the crystal which affords the section; but nitrate of potash gives two sets of rings with a cross, the long line of which passes through both, the short line dividing it in the middle.

The process of cutting these sections is rather difficult, but a little care and perseverance will conquer all this. The following is extracted from the Encyclopædia Metropolitana:—“Nitre crystallizes in long six-sided prisms whose section, perpendicular to their sides, is the regular hexagon. They are generally very much interrupted in their structure; but by turning over a considerable quantity of the ordinary saltpetre[F] of the shops specimens are readily found which have perfectly transparent portions of some extent. Selecting one of these, cut it with a knife into a plate above a quarter of an inch thick, directly across the axis of the prism, and then grind it down on a broad wet file till it is reduced to about one quarter or a sixth of an inch thick, smooth the surface on a wet piece of emeried glass, and polish on a piece of silk strained very tight over a strip of plate-glass, and rubbed with a mixture of tallow and colcothar of vitriol. This operation requires practice. It cannot be effected unless the nitre be applied wet and rubbed till quite dry, increasing the rapidity of the friction as the moisture evaporates. It must be performed in gloves, as the vapour from the fingers, as well as the slightest breath, dims the polished surface effectually. With these precautions a perfect vitreous polish is easily obtained. We may here remark, that hardly any two salts can be polished by the same process. Thus, Rochelle-salt must be finished wet on the silk, and instantly transferred to soft bibulous linen and rapidly rubbed dry. Experience alone can teach these peculiarities, and it is necessary to resort to contrivances (sometimes very strange ones) for the purpose of obtaining good polished sections of soft crystals, especially of those easily soluble in water.

“The nitre is thus polished on both its surfaces, which should be brought as near as possible to parallelism.”

[F] Sometimes the saltpetre of the shops is nitrate of soda, and as this is slightly deliquescent, it is well to be certain that we have the nitrate of potash, which is free from this defect.

Some sections of the naturally formed crystals also show the “rings” very well,—as Iceland Spar, which gives a single ring and cross; but the difficulty of cutting and polishing them is almost too great for the amateur, and must be left to the lapidary. This curious phenomenon, however, may be seen by using a plate of ice uninterruptedly formed of about one inch in thickness.

Before concluding these remarks on sections, I must mention a few difficulties which may be met with, and their remedies. The foremost of these is the softness of some objects, which have not resistance enough in themselves to bear cutting even with the sharpest instruments. This may often be removed by soaking in a solution of gum, and then drying, which will render the substance firm enough to be cut, when the sections must be steeped in water, and the gum thus got rid of. Small seeds, &c., may be placed in wax when warmed, and will be held firmly enough when it is again cold to allow of them being cut into sections, &c. And, lastly, where a substitute for a microscopist’s hand-vice is required, a cork which fits any tube large enough may be taken and split, the object being then placed between the two parts, and the cork thrust into the tube, a sufficient degree of firmness will be obtained to resist any necessary cutting, &c.