It will now be necessary to remove any superfluous glycerine which may have collected around and near the cover. The great bulk must be wiped away by means of a camel’s-hair pencil, slightly wetted between the lips, any remaining stickiness being removed with a bit of blotting paper which has been slightly damped. With a very small camel’s-hair pencil, charged with solution of gelatine, a ring must be made round the margin of the cover, of sufficient breadth to take in a small tract of both cover and slide. As this cement is perfectly miscible with glycerine, it readily unites with any of that fluid which may ooze from beneath the cover, and which, in the case of any of the ordinary varnishes, would act as a fatal obstacle to perfect adhesion. To make the cement, take half an ounce of Nelson’s opaque gelatine, put in a small beaker, add sufficient cold water to cover it, and allow the mixture to remain until the gelatine has become thoroughly soaked. The water is now poured off, and heat applied until the gelatine becomes fluid, when three drops of creosote should be well stirred in, and the fluid mixture transferred to a small bottle to solidify. Before use, this compound must be rendered liquid by immersing the bottle containing it in a cup of warm water. When the ring of gelatine has become quite set and dry (which will not take long), every trace of glycerine must be carefully removed from the cover and its neighborhood, by gently swabbing these parts with a large camel’s-hair pencil dipped in methylated spirit. After drying the slide a ring of Bell’s microscopical cement may be applied over the gelatine, and, when this is dry, another coat is to be laid on. If it be desired to give the slide a neat and tasteful appearance, it is a very easy matter, by means of the turn-table, to lay on a final ring of Brunswick black or white zinc cement (§ 24). Every care has now been taken to render our preparation permanent; but, to make assurance doubly sure, it will be well to follow Dr. Carpenter’s advice, and, every year or so, to lay on a thin coating of good gold-size.[12] Footnote 10: The appearance of a slide is vastly improved if the preparation be placed _exactly_ in its centre. This may readily be done in the following manner:—Take some very finely-powdered Prussian blue, and rub it up in a mortar with a little of the weak gum solution (§ 11), so as to form a thin blue pigment. A quantity of this should be made, so as always to be at hand. A slide having been cleaned, the _best surface_ is to be selected, and on the _reverse_ side, by means of the self-centring turn-table, a small circle is to be drawn with a camel’s-hair pencil, charged with the pigment. In the centre of this ring, but on the opposite side of the slide, the section is to be placed, when it of course will occupy a position exactly central. When the slide comes to be finished, the blue ring may easily be removed with a wet rag. Footnote 11: