other substance preferred (as mentioned in Chapter II.). Cardboard is easily procured of almost any thickness; but sometimes it is convenient to find a thinner substance even than this. When thin glass is laid upon a drop of any liquid upon a slide, every one must have observed how readily the liquid spreads betwixt the two: just so when any thin varnish is used to surround an object of little substance, excessive care is needed lest the varnish should extend betwixt the cover and slide, and so render it worthless. The slightest wall, however, prevents this from taking place, so that a ring of common paper may be used, and serve a double purpose where the objects require no deeper cell than this forms.

Many objects, however, are of such tenuity-as the leaves of many mosses, some of the Diatomaceæ, scales of insects, &c.—that no cell is requisite excepting that which is necessarily formed by the medium used to attach the thin glass cover to the slide; and where the slide is covered by the ornamental papers mentioned in Chapter II., and pressure does not injure the object, even this is omitted, the thin glass being kept in position by the cover; but slides mounted in this manner are frequently injured by dampness, which soon condenses upon the inner surfaces and interferes both with the object and the clearness of its appearance.

The thin glass, then, is to be united to the slide, so as to form a perfect protection from dust, dampness, or other injurious matter, and yet allow a thoroughly distinct view of the object. This is to be done by applying to the glass slide round the object some adhesive substance, and with the forceps placing the thin glass cover (quite dry and clean) upon it. A gentle pressure round the edge will then ensure a perfect adhesion, and with ordinary care there will be little or no danger of breakage. For this purpose goldsize is frequently used. The asphalt and india-rubber varnish also will be found both durable and serviceable. Whatever cement may be used, it is well to allow it to become in some measure fixed and dried; but where no

cell or wall is upon the slide, this is quite necessary, otherwise the varnish will be most certain to extend, as before mentioned, and ruin the object. It may be stated here that gold-size differs greatly in its drying powers, according to its age, mode of preparation, &c. (Chapter V.): here gum dammar solution laid on in a very thin coating will be found most useful, as it dries so rapidly that it cannot run in unless laid on with an unsparing hand.

Should any object be enclosed which requires to be kept flat during the drying of the cement, it will be necessary to use some of the contrivances mentioned in Chapter II.

When the slide is thus far advanced, there remains the finishing only. Should the student, however, have no time to complete his work at once, he may safely leave it at this stage until he have a number of slides which he may finish at the same time. There are different methods of doing this, some of which may be here described.

If ornamental papers are preferred, a small circle must be cut out from the centre a little less than the thin glass which covers the object. Another piece of coloured paper is made of the same size, and a similar circle taken from its centre also, or both may be cut at the same time. The slide is then covered round the edges with paper of any plain colour, so that it may extend about one-eighth of an inch over the glass on every side. The ornamental paper is then pasted on the "object" surface of the glass, so that the circle shows the object as nearly in the centre as possible, and covers the edges of the thin glass. The other coloured paper is then affixed underneath with the circle coinciding with that above. And here I may observe, that when this method is used there is no necessity for the edges of the slide to be ground, as all danger of scratching, &c., is obviated by the paper cover.

Many now use paper covers, about one and a half inch long, on the upper side of the slide only, with the centre cut out as before, with no other purpose than that of hiding the edge of the thin glass where it is united to the slide.

The method of finishing, however, which is mostly used at the present time, is to lay a coating of varnish upon the edge of the thin glass, and extend it some little way on the slide. When a black circle is required, nothing serves the purpose better than the gold-size and lampblack, or the asphalt and india-rubber varnish, neither of which is liable to chip; but when used for this, the latter should be rather thinner, as before advised. Some of these varnishes are preferred of different colours, which may be made by using the different kinds of sealing-wax, as described in Chapter II.; but they are always liable to the defects there mentioned. This circle cannot be made in any other way than by one of those contrivances called turntables. A very little practice will enable the young student to place his slide so that the circle may be uniform with the edge of the thin glass.

The slide is now complete, except the addition of the name and any other particulars which may be desirable. For this purpose one of the methods described in Chapter II. must be employed.

Amongst the various classes of microscopic objects now receiving general attention, the Diatomaces may be placed in a prominent position. They afford endless opportunities of research, and some very elaborate works have already been issued concerning them. Professor Smith's may be mentioned as one containing, perhaps, the best illustrations. The young student may wish to know what a diatom is. The "Micrographic Dictionary" gives the following definition :-"A family of confervoid Algæ, of very peculiar character, consisting of microscopic brittle organisms." They are now looked upon by almost all of our scientific men as belonging to the vegetable kingdom, though some few still assign them to the animal. They are almost invariably so exceedingly small, that the unaided eye can perceive nothing on a prepared slide of these organisms but minute dust. Each separate portion, which is usually seen when mounted, is termed a "frustule," or "testule:" this consists of two similar parts, composed of silica, between and

sometimes around which, is a mass of viscid matter called the" endochrome." They are found in almost every description of water, according to the variety: some prefer seawater, others fresh, and many are seen nowhere but in that which is a mixture of both, as the mouths of rivers, &c. Ditches, ponds, cisterns, and indeed almost every reservoir, yield abundance of these forms. They are not, however, confined to "present" life; but, owing to the almost indestructible nature of their siliceous covering, they are found in a fossil state in certain earths in great abundance, and are often termed "fossil Infusoria." Upon these frustules are generally to be seen lines, or markings, of different degrees of minuteness, the delicacy of which often serves the purpose of testing the defining power of objectglasses. Some of the frustules are triangular, others circular, and, indeed, of almost every conceivable shape, many of them presenting us with exquisitely beautiful designs.

The markings, however, are seldom seen well, if at all, until the frustules are properly prepared, the different methods of accomplishing which will be given a little further on.

The collection of fresh diatoms is so closely connected with their preservation, that a few notes may be given upon it before we pass on. For this purpose a number of small bottles must be provided, which may be placed in a tin box, with a separate compartment for each, so that all chance of breakage may be done away with, The diatoms are generally of a light brown colour; and where they are observed in the water, the bottle may be so placed, with the mouth closed by the finger, that when the finger is withdrawn the water will rush in, carrying the diatoms also. If they are seen upon plants, stones, or any other substance, they may generally be detached and placed in the bottle. When there is a green covering upon the surface of the water, a great quantity of diatoms is usually found amongst it; as also upon the surface of the mud in those ponds where they abound. In these cases, a broad flat spoon will be found

very useful, and one is now made with a covering upon the broader portion of it to protect the enclosed matter from being so readily carried off whilst bringing it to the surface again. Where there is any depth of water, and the spoon will not reach the surface of the mud, the bottle must be united to a long rod, and being then carried through the upper portion with the mouth downwards, no water will be received into it; but on reaching the spot required, the bottle-mouth may be turned up, and thus become filled with what is nearest.

From the stomachs of common fish—as the cod, sole, haddock, &c.—many specimens of Diatomaceæ may be obtained, but especially from the crab, oyster, mussel, and other shellfish. Professor Smith states that from these curious receptacles he has taken some with which he has not elsewhere met. To remove them from any of the small shell-fish, it is necessary to take the fish or stomach from the shell, and immerse it in strong hot acid (nitric is the best) until the animal matter is dissolved, when the residue must be washed and treated as the ordinary Diatomaceæ hereinafter described.

Many diatoms are seen best when mounted in a dry state, the minute markings becoming much more indistinct if immersed in liquid or balsam; and for this reason those which are used as test objects are usually mounted dry. Many kinds are also now prepared in this way as opaque objects, to be examined with the lieberkuhn, and are exquisitely beautiful. Others, however, are almost invariably mounted in balsam; but as these will be again referred to in Chapter IV., and require the same treatment to fit them for the slide, it will not be out of place to describe the cleaning and preparation of them here. As before stated, there is much matter surrounding them which must be got rid of before the siliceous covering can be shown perfectly. As, however, we may first wish to become acquainted in some degree with what we have to do, it is well to take a small piece of talc, and place a few of the