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require no filtering, as all the solid, particles will settle to the bottom, and the clean solution can be poured off for dilution when required. Should the varnish thus prepared be too limpid or thin, it can be brought to the proper consistency by distilling off the excess of spirit. To do this an ordinary glass retort can be used ; or place the varnish in a vessel, which stands in a large saucepan of boiling water, and keep the water boiling for an hour or two. The peculiar contradictory properties—hardness com bined with elasticity—which photographic varnishes must possess render their fabrication not always a certainty, and it is scarcely possible to give equal satisfaction with respect to both of them; for, besides the above qualities, they must be entirely colourless, adhere tightly to the glass, and yet be so constituted as to allow the plate to be retouched with a lead pencil; and yet the varnish must not crack when the plate is laid away, as this would mean the ruin of the photographic negative. Varnishes made according to the following formul can, however, be depended on, as they have stood the test of many years’ experience. Formulae for Varnishes. Varnish for Photographic Negatives. Photographers' Lacepier. Mastic ... Shellac (bleached) ... ... ... Oil of turpentine ... ... ... Spirits of wine ... ... ... Another Formula. Amber resin ... ... ... Copal ,, ... ... ... Benzole ... ... ... ... Alcohol ... ... ... ... Another Formula. ... 2 parts ... 10 ,, ... 2 „ ... 60 „ ... 1 part ... 1 » ... 2 parts ... 15 „ Amber ... ... 2 parts Copal ... 2 ,, Mastic ... 1 part Petroleum naphtha ... 10 parts Spirits of wine ... 20 „ Great care should be exercised in choosing the raw materials for photographic lacquers, as it is absolutely necessary that they should be entirely colourless. There are many other formula), but the above will be found efficacious enough to need no addition. THE OPTICS OF THE LANTERN* BY J. TRAILL TAYLOR. Sandarac resin ... 4 parts Spirits of wine ... ... ... ... 20 „ Chloroform... 0’5 part Oil of lavender 3 parts To use this varnish, pour it when filtered on to the glass plate, and dry by applying heat. A perfectly colourless form, which does not crack even if the negative be stored away for a long time. Hard Lacquer for Photographic Negatives. Sandarac resin ... ... ... 7 ounces Venetian resin 0-7 ounce Oil of lavender 0-875 ,, Ether 0-875 ,, Absolute alcohol ... ... ... 17'5 ounces Retouching Varnish for Photographs. Shellac ... ... ... ... 0-035 ounce Sandarac 0-21 „ Mastic ... ... ... ... 0'21 ,, Ether... ... ... ... ... 2'7 fl. dr. 2-7 fluid drachms of pure benzole are added to the mixture after the resins have dissolved in the ether. Monclhoven's Retouching Varnish for Negatives.—“ Shellac is placed for twenty-four hours in a saturated solution of carbonate of ammonia in water. The solution is then poured off, and replaced by an equal quantity of pure water. The fluid is boiled under constant stirring until a complete dissolution has taken place. The pro portion between shellac and water should be as 1:8. This is poured twice in succession over the negative, which must be thoroughly dry. Retouching can be done more quickly and finer upon this coating than upon any other.” Elastic Dammar PMcquer for Photographs. Dammar... ... ... ... ... 1-4 ounce Acetone (wood spirit) ... ... ... 6'3 ounces The resin will be almost completely dissolved in a fortnight if the well-closed bottle containing the substance is allowed to stand in a moderately warm place. The solution should then be carefully poured off from the residuum. To apply the lacquer use a soft brush, and apply it several times. Let us now direct our attention to the condensing element of this optical system. W e have seen that the two elements of the collecting portions must be fixed and inseparable. This, on the contrary, should be variable, and selected to suit the special end in view. Its form may be plano-convex, more especially if for use with long- focus objectives; but if the latter is to be short-focus, and the condenser of crown glass, then is the crossed form, in which the curves are as one to six, or two to thirteen, to be preferred. But dealing, as we now are, with immergent parallel rays, it were folly to imagine that a condenser properly adapted for an objective of 12 in. focus will answer equally well for one of 6 in. Bearing in mind Kepler’s law—which, however, applies only to one kind of glass, and must not be held as applicable equally to the flint glasses, especially those of the denser sort procurable at the present day—I would say that, for long-projection lenses of 12 to 15 in. focus, a plano-convex having a radius of curvature of 7 in. will serve every purpose; for an objective of 8 to 10 in., the radius may be 4} in., while for one of 6 to 8 in., 4 in. will suffice. But, as I have said, this latter may, with advantage, be a crossed lens, in which case the radius of the more convex side will be longer. One word more before dismissing the condensers. Treat them with the most scrupulous care, both in heating and cooling, and avoid allowing a current of cold air to play upon them during the cooling. With this precau tion, superadded to having them set loosely in their cells, a fracture will rarely, if ever, occur. Turn we now to the objective, the image-former. First, its diameter, especially that of its posterior combination, must be sufficiently large to take in not merely the whole of the cone of rays emerging from the condenser, but by preference a little more. This permits of the utilisa tion of a small portion of light radiated from the substance of the image itself. A large back lens also permits it to be brought nearer to the picture, and this is advantageous, especially with * Concluded from page 443,