may be employed, which thoroughly and speedily cleanses glass from all traces of grease or balsam.

We have before said that any object to be viewed in the microscope must have its support; but if this object is to be preserved, care must be taken that it is defended from dust and other impurities. For this purpose it is necessary to use some transparent cover, the most usual at one time being a plate of mica, on account of its thinness; this substance is now, however, never used, thin glass being substituted, which answers admirably. Sometimes it is required to "take up" as little space as possible, owing to the shortness of focus of the object-glasses. It can be procured of any thickness, from one-fiftieth to one-two-hundred-andfiftieth of an inch. On account of its want of strength, and probable defect of due annealing, it is difficult to cut, as it is very liable to "fly" from the point of the diamond. To overcome this tendency as much as possible, it must be laid upon a thicker piece, previously made wet with water, which causes the thin glass to adhere more firmly, and consequently to bear the pressure required in cutting the covers. The process of cutting being so difficult, especially with the thinner kinds, little or nothing is gained by cutting those which can be got from the dealers, as the loss and breakage is necessarily greater in the hands of an amateur. It is convenient, however, to have on hand a few larger pieces, from which unusual sizes may be cut when required.

If the pieces required are rectangular, no other apparatus will be required save a diamond and a flat rule; but if circles are wanted, a machine for that purpose should be used (of which no description is necessary here). There are, however, other contrivances which answer tolerably well. One method is, to cut out from a thick piece of cardboard a circle rather larger than the size wanted. Dr. Carpenter recommends metal rings with a piece of wire soldered on either side; and this, perhaps, is the best, as cardboard is apt to become rough at the edge when much used. A friend of mine uses thin brass plates with circles of various sizes

"turned" through them, and a small raised handle placed at one end. The diamond must be passed round the inner edge, and so managed as to meet again in the same line, in order that the circle may be true, after which it may be readily disengaged. The sizes usually kept in stock by the dealers are one-half, five-eighths, and three-quarters inch diameter; but other sizes may be had to order.

For the information of the beginner it may be mentioned here that the price of the circles is a little more than that of the squares; but this is modified in some degree by the circles being rather lighter. If appearance, however, is cared for at all, the circles look much neater upon the slides when not covered with the ornamental papers; but if these last are used (as will shortly be described) the squares are equally serviceable.

The

As before mentioned, the thin glass is made of various thicknesses, and the beginner will wish to know which to use. For objects requiring no higher power than the oneinch object-glass, the thicker kinds serve well enough; for the half-inch the medium thickness will be required; while, for higher powers, the thinnest covers must be used. "test-objects" for the highest powers require to be brought so near to the object-glass that they admit of the very thinnest covering only, and are usually mounted betwixt glasses which a beginner would not be able to use without frequent breakage; but if these objects were mounted with the common covers, they would be really worthless with the powers which they require to show them satisfactorily.

It may be desirable to know how such small differences as those betwixt the various thin glass covers can be measured. For this purpose there are two or three sorts of apparatus, all, however, depending upon the same principle. The description of one, therefore, will be sufficient. Upon a small stand is a short metal lever (as it may be termed) which returns by a spring to one certain position, where it is in contact with a fixed piece of metal. At the other end this lever is connected with a "finger," which moves round

a dial like that of a watch, whereupon are figures at fixed distances. When the lever is separated from the metal which is stationary, the other end being connected with the "finger," of the dial, that "finger" is moved in proportion to the distance of the separation. The thin glass is, therefore, thrust betwixt the end of the lever and fixed metal, and each piece is measured by the figures on the dial in stated and accurate degrees. This kind of apparatus, however, is expensive, and when not at our command, thin glass may be placed edgewise in the stage forceps, and measured very accurately with the micrometer, or by the calliper eyepiece described by Dr. Matthews in No. 8, for October, 1869, of the Journal of the Quekett Microscopical Club.

Cleanliness with thin glass is, perhaps, more necessary than with the sides, especially when covering objects which are to be used with a high power; but it is far more difficult to attain, on account of the liability to breakage. The usual method of cleaning these covers is as follows:-Two discs of wood, about two inches in diameter, are procured, one side of each being perfectly flat and covered with clean wash-leather. To the other side of these a small knob is firmly fixed as a handle, or where practicable, the whole may be made out of a solid piece. In cleaning thin glass, it should be placed betwixt the covered sides of the discs, and may then be safely rubbed with a sufficient pressure, and so cleaned on both sides by the leather. If, however, the glass be greasy, as is sometimes the case, it must be first washed with a strong solution of potash, infusion of nutgalls, or any of the commonly used grease-removing liquids; and with some impurities water, with the addition of a few drops of strong acid, will be found very useful, but this last is not often required.

This method of cleaning thin glass should always be used by beginners; but after some experience the hand becomes so sensitive that the above apparatus is often dispensed with, and the glasses, however thin, may be safely cleaned betwixt the fingers and thumb with a cambric handkerchief,

having first slightly damped the ends of the fingers employed to obtain firm hold. When the dirt is very obstinate, breathing upon the glass greatly facilitates its removal, and the sense of touch becomes so delicate that the breakage is inconsiderable; but this method cannot be recommended to novices, as nothing but time spent in delicate manipulation can give the sensitiveness required.

It has been before mentioned that ordinary glass sides are sometimes worthless, especially for fine objects, from having a rough surface, which presents a dusty appearance under the microscope. This imperfection exists in some thin glass also, and is irremediable; so that it is useless to attempt to cleanse it; nevertheless, care should be taken not to mistake dirt for this roughness, lest good glass be laid aside for a fault which does not really belong to it.

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When any object which it is desired to mount is of considerable thickness, or will not bear pressure, it is evident that a wall must be raised around it to support the thin glass-this is usually termed a "cell." There are various descriptions of these, according to the class of objects they are required to protect; and here may be given a description of those which are most generally used in mounting dry" objects, leaving those required for the preservation of liquids until we come to the consideration of that mode of mounting. Many have used the following slides. Two pieces of hard wood of the usual size (3 in. by 1 in.), not exceeding onesixteenth of an inch in thickness, are taken, and a hole is then drilled in the middle of one of these of the size required. The two pieces are then united by glue or other cement, and left under pressure until thoroughly dry, when the cell is fit for use. Others substitute cardboard for the lower piece of wood, which is less tedious, and is strong enough for every purpose. This class of "cell" is, of course, fitted for opaque objects only where no light is required from below; and as almost all such are better seen when on a dark background, it is usual to fix a small piece of black paper at the bottom of the cell upon which to place them. For

very small objects the grain which all such paper has when magnified detracts a little from the merit of this background; and lately I have used a small piece of thin glass covered on the back with black varnish, and placed the object upon the smooth untouched side; but a solution of the best Egyptian asphalt in benzole of moderate thicknéss may be painted on with this further advantage, that in mounting such opaque objects as foraminifera, &c., it will be sufficient to arrange them in the positions they are to occupy, when by slightly warming the slide they will adhere to the asphalt.

Another method of making these cells is as follows:Two punches, similar to those used for cutting gun-wads, are procured, of such sizes that with the smaller may be cut out the centre of the larger, leaving a ring whose side is not less than an one-eighth of an inch wide. These rings may be readily made, the only difficulty being to keep the sides parallel; but a little care will make this easy enough. For this purpose close-grained cardboard may be conveniently used. It must have a well-glazed surface, else the varnish or cement used in affixing the thin glass cover sinks into the substance, and the adherence is very imperfect. When this takes place it is easily remedied by brushing over the surface of the cardboard a strong solution of gum or isinglass; and this application, perhaps, closes also the pores of the card, and so serves a double purpose. But, of course, the gum must be perfectly dried before the ring is

used.

For cardboard, gutta-percha has been substituted, but cannot be recommended, as it always become brittle after a certain time, never adheres to the glass with the required firmness, and its shape is altered when worked with even a little heat. Leather is often used, and is very convenient; it should be chosen, however, of a close texture, and free from oil, grease, and all those substances which are laid upon it by the dressers.

Rings of cardboard, &c., have been rejected by persons of