Cleanliness with the thin glass is, perhaps, more necessary than with the slides, 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 affixed 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 nut-galls, 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.
The above 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 was before mentioned that the ordinary glass slides were sometimes worthless, especially for fine objects, from having a rough surface, which presented a dusty appearance under the microscope. This imperfection exists in some thin glass also, and it is irremediable; so that it is useless to attempt to cleanse it; nevertheless, care should be taken not to mistake dirty glass for this roughness, lest good glass be laid aside for a fault which does not really belong to it.
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 made use of the following slides. Two pieces of hard wood of the usual size (3 in. by 1 in.), not exceeding one-sixteenth 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.
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 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 becomes 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 great experience, because they are of such a nature that dampness can penetrate them. This fault can be almost, if not totally, removed by immersing them in some strong varnish, such as the asphalt varnish hereinafter mentioned; but they must be left long enough when affixed to the glass slide to become perfectly dry, and this will require a much longer time than at first would be supposed.
There has, however, been lately brought out what is termed the ivory cell. This is a ring of ivory-like substance, which may be easily and firmly fixed to the glass slide by any of the commonly-used cements, and so forms a beautiful cell for any dry objects. They are made of different sizes, and are not expensive.
Sometimes slides are used which are made by taking a thin slip of wood of the usual size (3 in. by 1 in.), in the centre of which is cut a circular hole large enough to receive the object. A piece of thin glass is fixed underneath the slide, forming a cell for the object, which may then be covered and finished like an ordinary slide. This has the advantage of serving for transparent objects for which the before-mentioned wooden slides are unsuitable. A slight modification of this plan is often used where the thickness of the objects is inconsiderable, especially with some of the Diatomaceæ, often termed “test-objects.” The wooden slide is cut with the central opening as above, and two pieces of thin glass are laid upon it, betwixt which the diatoms or other objects are placed, and kept in their proper position by a paper cover. This arrangement is a good one, insomuch as the very small portion of glass through which the light passes on its way to the microscope from the reflector causes the refraction or interference to be reduced to the lowest point.
A novice would naturally think the appearance of some of the slides above mentioned very slovenly and unfinished; but they are often covered with ornamental papers, which may be procured at almost every optician’s, at a cost little more than nominal, and of innumerable patterns and colours. How to use these will be described in another place.