When more than one specimen of some particular shell is obtained, it is better to place them upon the slide in different positions, so as to show as much of the structure as possible. I will conclude this subject by quoting a passage from T. Rymer Jones:—“It is, therefore, by no means sufficient to treat these shells as ordinary objects by simply laying them on a glass slide, so as to see them only from one or two points of view; they must be carefully examined in every direction, for such is the diversity of form that nothing short of this will be at all satisfactory. For this purpose, they should be attached to the point of a fine needle, so that they may be turned in any direction, and examined by reflected light condensed upon them by means of a lens or side reflector. In many of the thick-shelled species it will be necessary to grind them down on a hone [see [Chapter V.]] before the number and arrangement of the internal chambers is discernible; and in order to investigate satisfactorily the minutiæ of their structure, a variety of sections, made in various ways, is indispensable.”
Plants afford an almost inexhaustible treasury for the microscope, and many of them show their beauties best when mounted dry. When any of these also are to be mounted, care must be taken that they are thoroughly dry, otherwise the damp will certainly arise in the cell, and injure the object; and it may be here mentioned that long after a leaf has every appearance of dryness, the interior is still damp, and no way can be recommended of getting rid of this by any quicker process than that effected by keeping them in a warm room, as many leaves, &c., are utterly spoiled by using a hot iron or other contrivance. The safest way is to press them gently betwixt blotting-paper, which may be removed and dried at short intervals; and though this may appear a tedious operation, it is a safe one.
On the surface of the leaves, hairs and scales of various and very beautiful forms are found, most of which display their beauties best when removed from the leaf, and used with the polarizer. These will be noticed in another place; but a portion of the leaf should always be prepared in its natural form, to show the arrangement of the hair or scales upon it; which must almost invariably be mounted dry when used for this purpose. Many of them require very delicate handling. The epidermis, or, as it is by some termed, the cuticle, is the outer skin which lies upon the surface of the leaves and other parts of most plants. This is composed of cells closely connected, often bearing the appearance of a rude network. In many plants, by scraping up the surface of the leaf, a thin coating is detached, which may be torn off by taking hold of it with the forceps. The piece may then be washed and floated upon a glass slide, where, on drying, it will be firmly fixed, and may usually be mounted dry. Amongst the most beautiful and easily prepared of these may be mentioned the petal of the geranium, the cells of which are well defined and amongst the most interesting.
Closely connected with the leaves are the ANTHERS and POLLEN, of which a great number are beautiful and interesting subjects for the microscopist.
The mallow tribe will furnish some exquisite objects, bearing the appearance of masses of costly jewels. These are usually dried with pressure, but the natural form may be more accurately preserved by allowing them to dry as they are taken from the flower, with no interference except thoroughly protecting them from all dust. Sometimes the anther is divided, so that the cell required to receive them may be of as little depth as possible. The common mallow is a beautiful object, but I think the lavatera is a better, as it shows the pollen chambers well, when dried unpressed. The pollen is often set alone, and is well worth the trouble, as it then admits of more close examination. Often it is convenient to have the anther and pollen as seen in nature on one slide, and the pollen alone upon another. The former should be taken from the flowers before their full development is attained, as if overgrown they lose much of their beauty. Some pollens are naturally so dark that it is necessary to mount them in Canada balsam or fluid, as described in other places; but they are better mounted dry when they are not too opaque.
Here, too, we may also mention the SEEDS of many plants as most interesting, and some of them very beautiful, objects, requiring for the greater part but a low power to show them. Most of these are to be mounted dry, as opaque objects, in cells suited to them, but some are best seen in balsam, and will be mentioned in [Chapter III.]
The Corallines, many of which are found on almost every coast, afford some very valuable objects for the microscope. They must be well washed when first procured, to get rid of all the salts in the sea-water, dried and mounted in cells deep enough to protect them from all danger of pressure, as some of them are exceedingly fragile. The white ivory appearance which some of them present is given to them by an even covering of carbonate of lime; and should it be desired to examine the structure of these more closely, it may be accomplished by keeping them for some time in vinegar or dilute muriatic acid, which will remove the lime and allow of the substance being sliced in the same way as other Algæ. (“Micrographic Dictionary,” p. 183.)
The Scales of Insects.—The fine dust upon the wings of moths and butterflies, which is so readily removed when handled carelessly, is what is generally called scales. To these the wing owes the magnificent colours which so often are seen upon it; every particle being what may be termed a distinct flat feather. How these are placed (somewhat like tiles upon a roof) may be easily seen in the wing of any butterfly, a few being removed to aid the investigation. The form of them is usually that of the “battledore” with which the common game is played, but the handle or base of the scale is often short, and the broad part varies in proportionate length and breadth in different specimens. The markings upon these also vary, some being mostly composed of lines running from the base to the apex, others reminding us of network—bead-like spots only are seen in some—indeed, almost endless changes are found amongst them. These scales are not confined to butterflies and moths, nor indeed to the wings of insects. The different gnats supply some most beautiful specimens, not only from the wings, but also from the proboscis, &c.; whilst from still more minute insects, as the podura, scales are taken which were at one time esteemed as a most delicate test. The gorgeous colours which the diamond beetles also show when under the microscope are produced by light reflected from minute scales with which the insects are covered.
In mounting these objects for the microscope it is well to have the part of the insect from which the scales are usually taken as a separate slide, so that the natural arrangement of them may be seen. This is easily accomplished with the wings of butterflies, gnats, &c.; as they require no extraordinary care. In mounting the scales they may be placed upon slides, by passing the wings over the surface, or by gently scraping the wing upon the slide, when they must be covered with the thin glass. Of course, the extreme tenuity of these objects does away with the necessity of any cell excepting that formed by the gold-size or other cement used to attach the cover. The scales of the podura should be placed upon the slide in a somewhat different manner. This insect is without wings, and is no longer than the common flea. It is often found amongst the sawdust in wine-cellars, continually leaping about by the aid of its tail, which is bent underneath its body. Dr. Carpenter says:—“Poduræ may be obtained by sprinkling a little oatmeal on a piece of black paper near their haunts; and after leaving it there for a few hours, removing it carefully to a large glazed basin, so that, when they leap from the paper (as they will when brought to the light), they may fall into the basin, and may thus separate themselves from the meal. The best way of obtaining their scales, is to confine several of them together beneath a wine glass inverted upon a piece of fine smooth paper; for the scales will become detached by their leaps against the glass, and will fall upon the paper.” These scales are removed to the slide, and mounted as those from the gnats, &c. When the podura has been caught without the aid of the meal, it may be placed upon the slide, under a test-tube, or by any other mode of confinement, and thus save the trouble of transfer from the paper before mentioned. Another method is to seize the insect by the leg with the forceps and drag it across the slide, when a sufficient quantity of scales will probably be left upon it.
These scales are usually mounted “dry;” but Hogg recommends the use of Canada balsam ([Chapter III.]) as rendering their structure more definite when illuminated with Wenham’s parabolic reflector. Some advise other methods, which will be mentioned in [Chapter IV.] As most insects when undissected are mounted in Canada balsam, the different modes of treatment which they require will be stated in another place.