Chapter IV.

Next to the Diatomaceæ, no class of microscopic objects has been more looked into of late than the Foraminifera. These animals are almost all marine, having a jelly-like body enclosed in one or more chambers of shell generally composed of carbonate of lime. The shells are made with minute orifices, through which the pseudopodia (false feet) are extended by which the animal is enabled to lay hold of anything and draw itself along. From the possession of these orifices they derive their name, as _foramen_ means a door or opening. They have been found in every depth of sea hitherto sounded, each depth being abundant in certain species; the lowest beds containing the greatest number of specimens, though with less variation of kinds. In chalk they are found in a fossil state, and may readily be shown (see Chapter III.); in limestone and other hard stones they are abundant, and some mountains are composed principally of these shells.

The methods of obtaining Foraminifera are various. Many may be found upon seaweeds, which should always be examined as soon as possible after gathering. They are found in masses upon some coasts where the waves have carried and left them; but they are to be found the most abundantly in sand or mud dredged from the bottom of the sea. They must, however, be cleansed and separated from the mass of impurity with which they are usually mixed. This may be done in various ways, according to the nature of the accompanying matter. If sand alone, as is frequently the case, the whole mass must be _thoroughly_ dried, and then stirred up in clean water. The sand will soon subside by its own weight, but the chambers of the Foraminifera, being filled with air, will float upon the surface, and may be skimmed off. There is, however, one objection to this mode of proceeding--some of these objects are so minute, the chambers containing comparatively so small a quantity of air, that they sink and are cast away with the refuse sand. On this account it is preferable to take the trouble of searching certain soundings under the microscope, using the camel-hair pencil, or some other contrivance before mentioned, to extract those objects which are required. To clean the Foraminifera, Professor Williamson advises to transfer the specimens to an evaporating dish containing a weak solution of caustic potash. This must be boiled for “some moments,” when the organic matter will be entirely dissolved, and the calcareous shells left free from impurity. They must now be well washed in water, so that all alkaline matter may be entirely got rid of.

If the specimens are in _mud_, we must proceed in a different way:--Stir up the whole mass in water, and allow it to stand until the heavier portion has sunk to the bottom; the water may then be poured off and examined to see if there are any objects contained in it. This process must be repeated until the water comes off quite clear, when (if the search is for Foraminifera only) the solution of caustic potash may be used as before mentioned. However the soundings, &c., are cleaned, it is necessary to assort them under the microscope with the camel-hair pencil or other contrivance, as it is impossible to obtain them fit for mounting without undergoing this process.

The sea soundings taken by order of Government are drawn from the bottom in a kind of apparatus ingeniously made for the purpose, and the sand, mud, &c., are brought up in their original state. Common soundings, however, are taken by lowering a heavy piece of lead coated with tallow, which consequently brings up a small portion of the matter from the bottom. Mr. George Mosley, the late Secretary of the Manchester Microscopic Society, obtained numbers of the “scrapings” from the sounding leads. To make any use of these it is, of course, necessary to free them from all traces of the tallow. Mr. Dancer places the sounding in a basin and pours boiling water upon it, which causes the melted grease to rise to the surface. When cold, this may be removed, and the water carefully decanted. The operation may be repeated until no grease appears, when the water may be withdrawn and _liquor ammoniæ_ used, which will form a soapy solution with any remaining grease. This must be treated with hot water for the final washing. Care must be taken lest the finer forms be carried away in decanting the washing liquid. Should it be wished to make certain as to this point, each washing should be examined under the microscope. In some cases the process of Mr. Dancer will prove sufficient. Mr. Dale, however, gives a method of accomplishing the same result, which is much more readily completed; and as the results cannot be found fault with, I will here give it in full:--It is now well known that one of the products obtained from the naphtha of coal-tar is a volatile, oily substance, termed _benzole_ (or, by French chemists, _benzine_), whose boiling-point, when pure, is about 180° Fahrenheit, which is a perfect solvent for fatty substances. In a capsule, previously warmed on a sand-bath, Mr. Dale mixes with the tallow soundings benzole, whose boiling-point may be about 200°, until sufficiently diluted so as to run freely, pressing the lumps with a glass rod until thoroughly mingled; the solution and its contents are then poured into a paper filter, placed in a glass funnel; the capsule is again washed with benzole, until the whole of the gritty particles are removed into the filter. A washing-bottle is then supplied with benzole, and the contents of the filter washed to the bottom until that liquid passes off pure, which may be tested by placing a drop from the point of the funnel on a warm slip of glass or bright platinum, when, if pure, the benzole will evaporate without residue or tarnish; if grease be present, the washings must be continued until they are free from it. After rinsing through _weak_ acid, or alcohol, for final purification, the calcareous forms will be ready for mounting.

The filter and its contents may be left to dry spontaneously, when the latter can be examined by the microscope. Should time be an object, rapid drying may be effected by any of the usual methods; one of which, recommended by Mr. Dale, is to blow a stream of hot air through a glass tube held in the flame of a Bunsen’s burner. The lower the boiling-point of the benzole, the more readily can the specimens be freed from it. A commoner quality may be used, but it is more difficult to dry afterwards.

Pure benzole being costly, this may appear an expensive process; but, with the exception of a trifling loss by evaporation, the whole may be recovered by simple distillation. The mixture of tallow and benzole being placed in a retort in a hot-water, a steam, or a sand bath, the benzole will pass into the receiver, and the tallow or other impurities will remain in the retort. When the whole of the benzole has distilled over, which is ascertained by its ceasing to drop from the condenser, the heat is withdrawn and the retort allowed to cool before the addition of fresh material. Half a dozen to a dozen filters, each with its specimen, can be in process at the same time; and the distillation of the recovered benzole progresses as quickly as the filtration, which was practically proved on the occasion named. Great caution in the use of benzole is to be taken in the approach of lights to the inflammable vapour.

After the Foraminifera and calcareous forms have been removed, the residue may be treated with acids and levigation in the usual manner, to obtain siliceous forms and discs, if there are any present; but to facilitate their deposition, and to avoid the loss of any minute atoms suspended in the washings, I would suggest the use of filtration. The conical filter is unsuitable, as the particles would spread over too great a surface of paper; but glass tubes open at both ends (such as broken test-tubes) will be found to answer, the broad end covered with filtering paper, and over that a slip of muslin tied on with a thread to facilitate the passage of the water and prevent the risk of breaking the paper. Suspend the tube over a suitable vessel through a hole cut in thin wood or cardboard, pour in the washings which can be thus filtered and then dried. The cloth must be carefully removed, the paper cut round the edges of the tube, and the diatoms on the paper disc may be removed by a camel-hair pencil or otherwise, ready for mounting. Thus many objects may be preserved which would be either washed away or only be obtained by a more tedious process.

Such is Mr. Dale’s method of cleaning the soundings from the tallow, and as it thoroughly accomplishes its end, and is alike effective and not injurious to Foraminifera and diatoms, it may be safely recommended. The weak solution of caustic potash before advised for Foraminifera, must not be used where it is desired to preserve the diatoms, as they would certainly be injured, or destroyed altogether, if this agent were employed.

In fixing the Foraminifera upon the slide, no better plan can be followed than the “dry cells” and gum recommended in the early parts of this chapter. Owing to their thickness and composition, most of them are opaque objects only; but they are exquisitely beautiful, and require no particular care, except in allowing the cell, &c., to be perfectly dry, when the cover is placed upon it, or the damp will certainly become condensed upon the inner side, and the examination seriously interfered with.

Many of the Foraminifera require cutting into sections if it is wished to examine the internal structure, &c.,--“decalcifying” is also desirable in some cases;--both of these processes will be found described at length in the chapter on Sections and Dissection.

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.

In mounting blood of any kind to show the corpuscles, or, as they are often called, _globules_, which are round or oval discs, it is but necessary to cover the slide on the spot required with a coating as thin as possible and allow it to dry before covering with the thin glass. There is a slight contraction in the globules when dried, but not enough to injure them for the microscope. The shape of these varies in different classes of animals, but the size varies much more, some being many times as large as others.--Some of the _larvæ skins_ are beautiful objects; but, like many sections of animal and other fragile matter, are difficult to extend upon the slide. This difficulty is easily overcome by floating the thin object in clear water, immersing the slide and when the object is evenly spread gently lifting it. Allow it then to dry by slightly raising one end of the slide to aid the drainage, and cover with the thin glass as other objects. The tails and fins of many small fish may be mounted in a similar manner, and are well worth the trouble.

A few objects which are best shown by mounting _dry_ may be here mentioned as a slight guide to the beginner, though some of them have been before noticed;--many of the Foraminifera as elsewhere described. Some _crystals_ are soluble in almost any fluid or balsam, and should be mounted _dry_; a few, however, deliquesce or effloresce, which renders them worthless as microscopic objects.

The wings of butterflies and gnats, as before noticed, afford many specimens wherewith to supply the cabinet of the young student. A great variety also may be found amongst the ferns; indeed, these alone will afford the student occupation for a long time. On the under-side of the leaves are the reservoirs for the “spores,” which in many instances somewhat resemble green velvet, and are arranged in stripes, round masses, and other forms. The spores are usually covered with a thin skin termed the _indusium_, which is curiously marked in some specimens, often very like pollen-grains. The manner in which these spores with all their accompaniments are arranged, their changes and developments afford almost endless subjects for study; different ferns presenting us with many variations in this respect totally invisible without the aid of the microscope. The hymenophyllums (of which two only belong to England) are particularly interesting, and the structure of the leaves when dried makes them beautiful objects, often requiring no balsam to aid their transparency. Portions of the _fronds_ of ferns should be mounted as opaque objects, after having been dried between blotting paper, when they are not injured by pressure; but care must be taken to gather them at the right time, as they do not show their beauty before they are ripe, and if over-ripe the arrangement of the spores, &c., is altered. The spores may be mounted as separate objects in the same manner as pollen, before-mentioned, and are exquisitely beautiful when viewed with a tolerably high power. The number of foreign ferns now cultivated in this country has greatly widened the field for research in this direction; and it may also be mentioned that the under-sides of many are found to be covered with “scales” of very beautiful forms. A small piece of the frond of one of these may be mounted in its natural state, but the removal of the “scales” for examination by polarized light will be described in another place. The mosses also are quite a little world, requiring but a low power to show their beauties. The leaves are of various forms, some of which resemble beautiful net-work; the “urns” or reservoirs for the spores, however, are perhaps the most interesting parts of these objects, as also of the “liverworts” which are closely allied to the mosses. These “urns” are generally covered by lids, which fall off when the fruit is ripe. At this period they are well fitted for the microscope. The common screw-moss may be found in great abundance, and shows this denudation of the spores very perfectly. Many of these may be easily dried without much injury, but they should also be examined in their natural state.

The student should not omit from his cabinet a leaf of the nettle and the allied foreign species, the mystery of which the microscope will make plain. The hairs or stings may also be removed, and viewed with a higher power than when on the leaf, being so transparent as to require no balsam or other preservative.

There are few more interesting objects than the _raphides_ or _plant-crystals_. These are far from being rare, but in some plants they are very minute, and consequently require care in the mounting, as well as a high magnifying power to render them visible; in others they are so large that about twenty-five of them placed point to point would reach one inch. Some of these crystals are long and comparatively very thin, which suggested the name (_raphis_, a needle); others are star-like, with long and slender rays; while others again are of a somewhat similar form, each ray being solid and short. If the stem of rhubarb, or almost any of the hyacinth tribe, be bruised, so that the “juice” may flow upon the slide, in all probability some of these crystals will be found in the fluid. To obtain them clean, they must be freed from all vegetable matter by maceration. After this they must be thoroughly washed and mounted “dry.” They are also good _polarizing_ objects, giving brilliant colours; but when used for this purpose they must be mounted as described in Chapter III. A few plants which contain them may be mentioned here. The Cactaceæ are very prolific; the orchids, geraniums, tulips, and the outer coating of the onion, furnish the more unusual forms.

The Fungi are generally looked upon as a very difficult class of objects to deal with, but amongst them some of the most available may be found. The forms of many are very beautiful, but are so minute as to require a high magnifying power to show them. The mould which forms on many substances is a fungus, and in some cases may be dried and preserved in its natural state. A friend of mine brought me a rose-bush completely covered with a white blight. This was found to be a fungus, which required a high magnifying power to show it. Being a very interesting object, it was desirable to preserve it, and this was perfectly effected without injury to the form by simply drying the leaf in a room usually occupied. Amongst the fungi are many objects well worth looking for, one of which is the _Diachæa elegans_. This, the only species, says the “Micrographic Dictionary,” is found in England upon the living leaves of the lily-of-the-valley, &c. These little plants grow in masses, reminding one of mould, to a height of a quarter of an inch, and each “stem” is covered with a sheath, in shape somewhat like an elongated thimble. When ripe the sheath falls off and reveals the same shaped column, made up of beautifully fine net-work, with the spores lying here and there. This dries well, and is a good object for the middle powers. Amongst the fungi the blights of wheat and of other articles of food may be included. Many of them may be mounted “dry;” others, however, cannot be well preserved except in liquids, and will be referred to in Chapter IV. Amongst the zoophytes and sea-mats, commonly called “sea-weeds,” may be found many very interesting objects to be mounted “dry.” When this mode of preservation is used, it is necessary that all the sea-salt be thoroughly washed from them. As they are, however, most frequently mounted in balsam or liquid, they will be more fully noticed in other places.

The _scales of fishes_ are generally mounted “dry” when used as ordinary objects; but for polarized light, balsam or liquid must be used, as noticed in Chapter III. The variety and beauty of these are quite surprising to the novice. It is also very interesting to procure the skin of the fish when possible, and mount it on a separate slide to show how the scales are arranged. The sole is one of the most unusual forms, the projecting end of each scale being covered with spines, which radiate from a common centre, while those at the extremity are carried out somewhat resembling the rays of a star. One of the skates has a spine projecting from the centre of each scale, which is a very curious opaque object, especially when the skin is mounted in the manner described. The perch, roach, minnow, and others of the common fishes give the student good objects for his cabinet, and may be procured without difficulty.

Insects which are very transparent, or have the “metallic lustre” with which any medium would interfere, are mounted “dry.” The diamond-beetle, before mentioned, is a splendid example of this; the back is generally used, but the legs, showing the curious feet, are very interesting objects. Indeed, amongst the legs and feet of insects there is a wide field of interest. When they are of a “horny” nature, it is best to dry them in any form preferred, but to use no pressure; when, however, they are wanted flat, so as to show the feet, &c., extended, they must be dried with a gentle pressure betwixt blotting-paper if possible. But this will be treated more fully in