and sometimes to the perviousness of some part of the cement, which has allowed a portion of the contained fluid to escape, and air to find admission. In either case, so soon as an air-bubble is seen in such a preparation, the attempt should be made to prevent its increase, by laying on an additional coat of varnish; but if this should not be successful, the cover should be taken off, and the specimen remounted, so soon as the fluid has escaped to such a degree as to leave any considerable portion of it uncovered.

140. Importance of Cleanliness.-The success of the result of any of the foregoing operations is greatly detracted from, if, in consequence of the adhesion of foreign substances to the glasses whereon the objects are mounted, or to the implements used in the manipulations, any extraneous particles are brought into view with the object itself. Some such will occasionally present themselves, even under careful management; especially fibres of silk, wool, cotton, or linen, from the handkerchiefs, &c., with which the glass slides may have been wiped, and grains of starch, which often remain obstinately adherent to the thin glass covers kept in it. But a careless and uncleanly manipulator will allow his objects to contract many other impurities than these; and especially to be contaminated by particles of dust floating through the air, the access of which may be readily prevented by proper precautions. It is desirable to have at hand a well-closed cupboard furnished with shelves, or a cabinet of well-fitted drawers, or a number of bell-glasses upon a flat table, for the purpose of securing our glasses, objects, &c., from this contamination, in the intervals of the work of preparation; and the more readily accessible these receptacles are, the more use will the Microscopist be likely to make of them. Great care ought, of course, to be taken, that the liquids employed for mounting should be freed, by effectual filtration, from all floating particles; and both these and the Canada balsam should be kept in well-closed bottles.

141. Labelling and Preserving of Objects.-Whenever the mounting of an object has been completed, its name ought to be at once marked on it, and the slide should be put away in its appropriate place. Some inscribe the name on the glass itself, with a writing diamond; whilst others prefer to gum a label' on the slide; and others, again, cover one or both surfaces of the slide with colored paper, and attach the label to it. In the case of objects mounted dry or in balsam, the latter method has the advantage of rendering the glass cover more secure from displacement by a slight blow or "jar," when the varnish or balsam may have become brittle by the lapse of years. Instead, however, of attaching the white label on which the name of the ob ject is written, outside the colored paper with which the slide is 1 Very neat gummed labels, of various sizes and patterns suitable to the wants of the Microscopist, are sold by the "Drapers' Stationers" in the City.

covered, it is better to attach the label to the glass, and to punch a hole out of the colored paper, sufficiently large to show the name, in the part corresponding to it; in this manner the label is prevented from falling off, which it frequently does when attached to the glass without protection, or to the outside of the paper cover. When objects are mounted in fluid, either with or without cells, paper coverings to the slides had better be dispensed with; and besides the name of the object, it is desirable to inscribe on the glass that of the fluid in which it is mounted. For the preservation of objects, the pasteboard boxes now made at a very reasonable cost, with wooden racks, to contain 6, 12, or 24 slides, will be found extremely useful. In these, however, the slides must always stand upon their edges; a position which, besides interfering with that ready view of them which is required for the immediate selection of any particular specimen, is unfavorable to the continued soundness of preparations mounted in fluid. Although such boxes are most useful, indeed almost indispensable, to the Microscopist, for holding slides which he desires (for whatever purpose) to keep for a while constantly at hand, yet his regularly classified series is much more conveniently stored in a Cabinet containing numerous very shallow drawers, in which they lie flat and exposed to view. Such cabinets are now prepared for sale under the direction of our principal Opticians, with all the improvements that experience has suggested. In order to prevent the warping of the thin wood of which the bottoms of the drawers are usually made, whereby their sliding action is obstructed, it has been found advantageous to substitute strained canvas or papier mache. Again, in order to antagonize the disposition of the slides to slip one over another in the opening or shutting of the drawers, it has been found preferable to arrange them in such a manner, that they lie with their ends (instead of their long sides) towards the front of the drawer, and to interpose a cross-strip of wood, lying parallel to the front of the drawer, between each row. It is very convenient, moreover, for the front of the drawer to be furnished with a little tablet of porcelain, on which the name of the group of objects it may contain can be written in pencil, so as to be readily rubbed out; or a small frame may be attached to it, into which a slip of card may be inserted for the same purpose.

SECTION 3. COLLECTION OF OBJECTS.

142. A large proportion of the objects with which the Microscopist is concerned, are derived from the minute parts of those larger organisms, whether Vegetable or Animal, the collection of which does not require any other methods than those pursued by the ordinary Naturalist. With regard to such, therefore, no special directions are required. But there are several most interesting and important groups, both of Plants and Animals,

which are themselves, on account of their minuteness, essentially microscopic; and the collection of these requires peculiar methods and implements, which are, however, very simple,-the chief element of success lying in the knowledge where to look, and what to look for. In the present place, general directions only will be given; the particular details relating to the several groups, being reserved for the account to be hereafter given of each.

143. All the Microscopic organisms in question, being aquatic, must be sought for in pools, ditches, streams, or other collections of water; through which some of them freely move, whilst others attach themselves to the stems and leaves of aquatic plants, or even to pieces of stick or decaying leaves, &c., that may be floating on the surface or submerged beneath it, while others, again, are to be sought for in the muddy sediments at the bottom. Of those which have the power of free motion, some keep near the surface, whilst others swim in the deeper waters; but the situation of many depends entirely upon the light, since they rise to the surface in sunshine, and subside again afterwards. The Collector will therefore require a means of obtaining samples of water at different depths, and of drawing to himself portions of the larger bodies to which the microscopic organisms may be attached. For these purposes, nothing is so convenient as a rod about five feet long, which may be divided into two pieces jointed together; and the farther extremity of this rod should be pierced with a hole, passing for some distance into its length. Into this hole, as a socket, may be fitted either of the three implements which the Collector may happen to require. If he desires to take up samples of the water, he will need a wide-mouthed bottle, containing about 2 oz. This may be attached to the extremity of the rod, by simply passing round its neck a strap of thin whalebone or sheet gutta percha, the two ends of which are to be brought together and inserted into the socket, in which they may be secured by a plug of soft wood or cork. The bottle being held sideways with its mouth partly below the water, the surface may be skimmed; or, if it be desired to bring up a sample of the liquid from below, or to draw into the bottle any bodies that may be loosely attached to the submerged plants, the bottle is to be plunged into the water with its mouth downwards, carried into the situation in which it is desired that it should be filled, and then suddenly turned with its mouth upwards. If, again, the organisms which it may be desired to collect, are of sufficient size to be strained out of the water by a piece of fine muslin, a ring-net should be fitted into the socket of the rod. This may be made by sewing the muslin bag to a ring of stout wire, furnished with a projecting stem which may be inserted by means of a cork into the socket of the rod. But it is more convenient

1 Cheap fishing rods are now sold at the toy shops, which answer this purpose extremely well, the last or slenderest joint being laid on one side; its socket in the last joint but one, being well adapted to receive the fittings above described.

that the muslin should be made removable; and this may be provided for (as suggested in the "Micrographic Dictionary," Introduction, p. xxiv) by the substitution of a wooden ring, grooved on its outside, for the wire ring; the muslin being strained upon it by a ring of vulcanized India rubber, which lies in the groove, and which may be readily slipped off and on, so as to allow a fresh piece of muslin to be put in the place of that which has been last used. For bringing up portions of larger Plants, either for the sake of examining their own structure, or for obtaining the growths which may be parasitic upon them, a cutting-hook, shaped somewhat like a sickle, may be fitted into the socket of the rod.

144. The Collector should also be furnished with a number of bottles, into which he may transfer the samples thus obtained. These it will be convenient to have of two kinds; one set widemouthed, and capable of being closely corked, for minute Plants; the other set with narrower mouths, having short pieces of tube passed through the corks, for the purpose of containing Animalcules without depriving them of air. The former kind, however, may be safely employed for Animalcules, if they be not above two-thirds filled (so as to leave an adequate air-space), and be not kept long closed. Such bottles should be fitted into cases, in which several may be carried at once without risk of breakage.1 Whilst engaged in the search for Microscopic objects, it is desirable for the collector to possess a means of at once recognizing the forms which he may gather, where this is possible, in order that he may decide whether the "gathering" is, or is not, worth preserving; for this purpose either a powerful "Coddington" or "Stanhope" lens (§ 19), or a Gairdner's Simple Microscope (§ 28), will be found most useful, according to the class of objects of which the collector is in search. The first will answer very well for Zoophytes and the larger Diatomaceæ; but the second or third will be needed for Desmidiaceæ, the smaller Diatomaceæ, and Animalcules.

The bottles in which smelling-salts are now commonly sold, having the corks fitted into disks of turned wood, are very convenient, both in size and shape, for the purposes of the Microscopist; cases containing 3, 4, 6, or 8 such bottles, are made by Mr. Ferguson, of Giltspur Street. The wide-mouthed bottles with screw caps, made by the York Glass Company, are also extremely convenient.

CHAPTER VI.

MICROSCOPIC FORMS OF VEGETABLE LIFE.-PROTOPHYTES.

145. IN commencing our survey of these wonders and beauties of Life and Organization, which are revealed to us by the assistance of the Microscope, it seems on every account the most appropriate to turn our attention in the first instance to the Vegetable Kingdom; and to begin with those humblest members of that kingdom, whose form and structure, and whose very existence, in many cases, are only known to us through its use. For those who desire to make themselves familiar with microscopic appearances, and to acquire dexterity in microscopic manipulation, cannot do better than educate themselves by the study of those comparatively simple forms of organization, which the Vegetable fabric presents; since a facility in minute dissection and in microscopic analysis may be thus acquired, which will save much expenditure of time and labor, that might be unprofitably applied, without such apprenticeship, to the attempt to unravel the complexities of Animal organization. But further, the scientific Histologist (p. 49) looks to the careful study of the structure of the simplest forms of Vegetation, as furnishing the key (so to speak) that opens the right entrance to the study of the elementary Organization, not merely of the higher Plants, but of the highest Animals. And in like manner, the scientific Physiologist looks to the complete knowledge of their life history, as furnishing the surest basis for those general notions of the nature of Vital Action, which the advance of science has shown to be really well founded, only when they prove equally applicable to both kingdoms. But further, a peculiar interest attaches itself at the present time, to everything which throws light upon the debated question of the boundary between the two kingdoms; a question which is not less keenly debated among Naturalists, than that of many a disputed frontier has been between adjacent Nations. For many parts of this border country have been taken and retaken several times; their inhabitants (so to speak) having first been considered, on account of their general appearance, to belong to the Vegetable Kingdom,then, in consequence of some movements being observed in them, being claimed by the Zoologists, then, on the ground of their