The Preparation & Mounting of Microscopic Objects

by microscopist Thomas Davies

If the bone is not sufficiently hard in its nature to bear the above method of handling whilst grinding and polishing—as some are far more brittle than others—as thin a section as possible must first be cut with the saw, and one surface ground and polished. The piece must then be dried and united to the glass by heated balsam in the same manner as shells, &c. After which the superabundance of balsam must be removed from the glass; then rub down upon the stone and strop as before. When the polishing is completed the whole slide must be immersed in chloroform, ether, or some other spirit, to release and cleanse the section, when it may be mounted as the one above mentioned.

Some have recommended a strong solution of isinglass to affix the half-ground teeth or bones to the glass as causing them to adhere very firmly and requiring no heat, and also being readily detached when finished.

The reason why the sections of bone are usually mounted dry is that the “lacunæ,” bone cells, and canaliculi (resembling minute canals) show their forms, &c., very perfectly in this state, as they are hollow and contain air, whereas if they become filled with liquid or balsam—which does sometimes occur—they become almost indistinguishable. There are some dark specimens, however, where the cells are already filled with other matter, and it is well to mount these with balsam and so gain a greater degree of transparency.

To gain a true knowledge of the structure of bone, sections must be cut as in wood, both transversely and longitudinally; but with fossil bones, without the lapidary’s wheel, &c., it is a laborious task, and indeed can seldom be properly accomplished. In this place, also, it may be mentioned that by submitting bone to the action of muriatic acid diluted ten or fifteen times with water, the lime, &c., is dissolved away and the cartilage is left, which may be cut into sections: in caustic potash the animal matter is got rid of. Both of these preparations may be mounted in fluid.

The method of cutting thin sections of bone may be also employed with the stones of fruit, vegetable ivory and such like substances; many of which show a most interesting arrangement of cells, especially when the sections are transverse. Most of these objects present a different appearance when mounted dry to that which they bear when in balsam, owing to the cells becoming filled; and to arrive at a true knowledge of them we must have a specimen mounted in both ways.

To those who study polarized light, few objects are more beautiful than the sections of the different kinds of horn. We will briefly inquire into the best method of cutting these. There are three kinds of horn, the first of which is hard, as the stag’s, and must be cut in the same manner as bone. The second is somewhat softer, as the cow’s. The third is another and still softer formation, as the “horn” (as it is termed) of the rhinoceros. In cutting sections of the two last we should succeed best by using the machine invented for these purposes which I shall shortly describe when the method of cutting wood is considered. To aid us in this when the horn is hard it must be boiled for a short time in water, when the cutting will be more easily accomplished. The sections should be both transverse and longitudinal, those of the former often showing cells with beautiful crosses, the colours with the selenite plate being truly splendid. Of this class the rhinoceros horn is one of the best; but the buffalo also affords a very handsome object. The cow’s, and indeed almost every different kind of horn, well deserves the trouble of mounting. Whalebone, when cut transversely, strongly resembles those of the third and softer formation. All these are best seen when mounted in Canada balsam, but care must be taken that they have been thoroughly dried after cutting, and then steeped in turpentine.

An interesting object may also be procured from whalebone by cutting long sections of the hairs of which it is composed. Down the centre of each hair we shall find a line of cells divided from one another very distinctly. And (as recommended in the “Micrographic Dictionary”) if whalebone be macerated twenty-four hours in a solution of caustic potash it will be softened, and by afterwards digesting in water, the outer part will be resolved into numerous transparent cells, which will show more plainly the structure of this curious substance.

In a former chapter, hairs were mentioned, their many and interesting forms, and their beauty when used with polarized light. The sections of them, however, are no less a matter of study, as this mode of treatment opens to sight the outer “casing,” and the inner substance somewhat resembling the pith of plants.

It would be out of place to enter into the description of the different forms met with; but the ways in which sections are to be procured may be glanced at. If transverse sections are required, some place a quantity of hairs betwixt two flat pieces of cork, which by pressure hold them firmly enough together to allow the required portions to be cut with a razor. Others take a bundle of the hairs and dip it into gum or glue, which gives it when dry a solidity equal to wood. Sections of this are then cut with the machine mentioned a little further on, and these may be mounted in balsam. The human hair is easily procured in the desired sections by shaving as closely as possible a second time and cleansing from the lather, &c., by carefully washing. Most hairs, however, should be examined both transversely and longitudinally. It is not difficult to procure the latter, as we may generally split them with the aid of a sharp razor. In a great number of hairs there is a quantity of greasy matter which must be got rid of by soaking in ether or some other solvent before mounting.

We may next consider the best method of procuring sections of wood, which must be cut of such a degree of thinness as to form transparent objects, and so display all the secrets of their structure. There is no monotony in this study, as the forms are so various, and the arrangement of the cells and woody fibre so different, that the microscopist may find endless amusement or study in it. From a single section the class of trees to which it has belonged may be known, often even when the wood is fossil. The apparatus best adapted for cutting these sections is made as follows:—A flat piece of hard wood, about six inches long, four wide, and one thick, is chosen, to which another of the same size is firmly fixed, so as to form, in a side view, the letter T. On one end of the upper surface is fastened a brass plate, perfectly flat, in the centre of which a circular opening is cut about half an inch in diameter. Coinciding with this opening is a brass tube, fixed in the under side of the table (if it may be termed so). This tube is so cut at the bottom as to take a fine screw. Another screw is also placed at the same end of the “table,” which works at right angles to this, so that any substance in the tube may be wedged firmly by working this last screw. To use this instrument, the piece of wood or other object of which a section is required must be placed in the tube, when, by turning the screw underneath, the wood is raised above the brass plate more or less as wished, and by using the screw at the end, it is held firmly in the same position. With a flat chisel the portion of the object which projects above the surface of the brass plate may now be cut off, and by means of the bottom screw another portion may be raised and treated in the same manner. As to the thickness of which objects should be cut, no proper directions can be given, as this differs so greatly that nothing but experience can be any guide. The same thickness can be obtained by working the screw underneath in uniform degrees, the head being marked for this purpose; and when the substance to be cut is very much smaller than the hole in the brass plate, it may be wedged with cork.