The Preparation & Mounting of Microscopic Objects

by microscopist Thomas Davies

Amongst fishes also may be found subjects for the same purpose, but they seldom furnish as good examples as those before mentioned, because the blood-vessels are not nearly so abundant, as in the foot of the frog, &c. The stickleback is, however, procurable almost in any place during the summer months, and may be laid in a shallow trough, loosely bound down as the tadpole. The tail may be covered with a piece of thin glass to prevent him curling it to the object-glass. The power needed with this will be about the same as with the other subjects—viz., half to quarter inch object-glass.

It is not absolutely necessary to go to reptiles or fishes for this curious sight, as some other animals serve very well. In the wings of the common bat may be found a good subject. These must be stretched out on something resembling the frog-plate before described, when those parts near to the bones will show the largest vessels very clearly. The ear of a young mouse is an illustration of the same phenomenon, but it is very difficult to fix it in a good position, as these animals are so very timid and restless.

Amongst insects also the same law may be observed, by placing them in the “cage,” or “live-box,” so as to keep them still, but not to injure them by too much pressure. In certain larvæ this is particularly well shown, as that of the day-fly and plumed gnat; but in some of these the blood is almost colourless. In the wings also of many insects this circulation is well seen, as in those of the common housefly; but as these parts become dry in a few days, the subject should not be more than twenty-four hours old.

Somewhat approximating to the forementioned phenomenon, is the “rotation” of fluid in the cells, or, as it is usually termed, the circulation of the sap, of plants. This is shown in certain vegetable growths as a constant stream of thick fluid, wherein small globules are seen; which stream flows round the individual cells, or up the leaf, turning at the extremity, and down again by a different but parallel channel. There is little or no difficulty in showing this in many plants; but some are, of course, better than others, and require a different treatment; we will, therefore, notice a few of these. Perhaps the best of all is the Vallisneria spiralis, which is an aquatic plant, frequently grown in, but not really belonging to, this country. As it somewhat resembles grass, the leaf is not used in its natural state, but a thin section cut lengthwise with a razor or other sharp instrument—this section, however, is much better when the outer surface has been first removed. It should then be laid upon a slide with a drop or two of water, and covered with a piece of thin glass. Often the cutting of the section seems to be such a shock to the leaf that no motion is visible for awhile, but in a short time the warmth of an ordinary sitting-room will revive it, and with a quarter-inch object-glass the currents will be rendered beautifully distinct. Where the “stream” is unusually obstinate the warmth may be slightly increased, but too great heat destroys the movement altogether. In the summer, any of the leaves show this “circulation” very well; but in the winter, the slightly yellow ones are said to be the best.

The Vallisneria requires to be cut in sections to show this “circulation;” but there are many plants of which it is but necessary to take a fragment and lay it upon a slide. The Anacharis alsinastrum is one of these: it grows in water, having three leaves round the stem, then a bare portion, again another three leaves, and so on. One of these leaves must be plucked close to the stem, and laid upon a slide with a drop of water. Thin glass should be placed upon it, and along the mid-rib of the leaves the “circulation” may be seen most beautifully when a good specimen has been chosen; but it requires rather more power than the Vallisneria. This plant is very common in many parts of the country, a great number of our ponds and streams being literally choked up by it. In the Chara vulgaris and two or three of the Nitellæ, &c., this phenomenon may also be seen with no preparation except plucking a part from the stem and laying it upon a slide as with the Anacharis. In using the Frog-bit, the outer part of the young leaf-buds must be taken to obtain the best specimens for this purpose; but a section of the stem will also show the “circulation,” though not so well. The plants before mentioned are all aquatic, but the same movement of the globules has been observed in several kinds of land plants, as in the hairs upon the leaf-stalks of the common groundsel; but these do not show it so well, nor are they so easily managed as the above.

Many microscopists who are not fortunate enough to be in the neighbourhood of these plants (indeed the Vallisneria is a foreign one) grow them in jars, so a few remarks as to the treatment they require will not be out of place. The Vallisneria requires a temperature not lower than 55° or 60°, and even a higher degree than this renders its growth quicker; and no great change must take place: the more equable the temperature the more healthy will the plant be. A glass jar should be taken, having an inch or two of mould at the bottom, which must be pressed down closely, and the plant must be set in this. Water must then be gently poured in, so as not to disturb the mould. As this plant flourishes best when the water is frequently changed, Mr. Quekett recommends that the jar should be occasionally placed under a tap of water, and a very gentle stream allowed to fall into it for several hours, by which means much of the confervoid growth will be got rid of and the plant invigorated. The Anacharis may be rooted in the earth like the Vallisneria, but a small detached piece may be thrown into the jar of water and there left until wanted. For months the “circulation” will be well shown by it, and it will probably grow and increase. It is also very healthy in an in-door aquarium. It is recommended that the jars in which any of the Chara are grown should be moved about as little as possible, as the long roots are very tender, and will not bear agitation.

An object which is interesting to the microscopist, as well as the unscientific observer, is the growth of seeds, as it is often erroneously termed. A shaving of the outside of the seed is taken and laid upon the glass slide; a thin glass cover is then placed upon it, and a drop of water applied to the edge of this. The water will then gradually flow under the glass and reach the section of the seed, when the transparent fibres will appear to spring out and “grow” for some minutes. This, however, is produced by the unfolding of a spiral formation in the cells, and, therefore, has really no similarity to the true growth. The seeds of the Salvias, Collomias, Senecio, Ruellia, &c., are well suited for the display of this curious sight.

To watch the development of the spores of ferns, and the fertilization and products, Dr. Carpenter recommends the following mode of proceeding:—“Let a frond of a fern, whose fructification is mature, be laid upon a piece of fine paper, with its spore-bearing surface downwards; in the course of a day or two this paper will be found to be covered with a very fine brownish dust, which consists of the discharged spores. This must be carefully collected, and should be spread upon the surface of a smoothed fragment of porous sandstone; the stone being placed in a saucer, the bottom of which is covered with water, and a glass ‘tumbler’ being inverted over it, the requisite supply of moisture is insured, and the spores will germinate luxuriantly. Some of the prothallia soon advance beyond the rest; and at the time when the advanced ones have long ceased to produce antheridia, and bear abundance of archegonia, those which have remained behind in their growth are beginning to be covered with antheridia. If the crop be now kept with little moisture for several weeks and then suddenly watered, a large number of antheridia and archegonia simultaneously open, and in a few hours afterwards the surface of the larger prothallia will be found almost covered with moving antherozoids. Such prothallia as exhibit freshly opened archegonia are now to be held by one lobe between the forefinger and thumb of the left hand, so that the upper surface of the prothallium lies upon the thumb; and the thinnest possible sections are then to be made with a narrow-bladed knife perpendicularly to the surface of the prothallium. Of these sections, which after much practice may be made no more than 1-15th of a line of thickness, some will probably lay open the canals of the archegonia, and within these, when examined with a power of 200 or 300 diameters, antherozoids may be occasionally distinguished.”

Another interesting object to the young microscopist is afforded by the spores of the equiseta (or horsetails, as they are often called). These may be obtained by shaking the higher portion of the stems when the spores are ripe. They will then fall like small dust, and may be placed under the microscope. The spores are then seen to consist of a somewhat heart-shaped mass with bands rather intricately curled around it. As they dry these bands expand, and are seen to be four lines at right angles, with the ends clubbed, as it may be called. If, whilst watching them, the spores are breathed upon, these bands immediately return to their former state, and are closely curled around the spore; but as they gradually dry again expand. This experiment may be repeated many times, and is a very interesting one.

The above are the principal objects which could not possibly be included in any of the former chapters, but would have left a most interesting branch untouched had it been neglected. There is another subject also which should not be passed by—viz., the production of minute pictures which serve as objects for microscopic examination.