Hence dry opake objects are usually mounted in such manner as to be enclosed in a cell, the sides being formed by a ring of glass-tube or cork, or a square piece of leather, cardboard, or paper, with a hole cut or punched out of the middle. The glass rings are best; but as they are expensive, some of the other substances are generally used. The size and thickness of the material from which the rings are made must obviously vary according to the size and depth of the object. The rings are cemented to the middle of ordinary slides; and it is best to keep a number of them ready prepared. The cementing material must vary according to the nature of the ring used. If this consists of glass, Canada-balsam or marine glue is best. In using the former, the ring is gently heated over the flame of the spirit-lamp, and a thin layer of the balsam applied to its upper or under surface, by means of an iron wire with a little balsam on its end; it is next warmed over the spirit-lamp, so that the surface is entirely and evenly coated. A clean slide is then slightly heated, the ring laid upon it, and gentle pressure is used to squeeze out the excess of balsam; and the slide is kept at a gentle heat, until on cooling the balsam becomes so hard as not to be indented with the finger-nail. Marine glue is applied in the same way as the balsam, except that prolonged heat is not required to harden it, for it becomes hard on cooling. The balsam may also be replaced by black japan or asphalte. The pieces of cork, leather, or paper are best fastened to the slides with solution of shellac or sealing-wax in methylated alcohol, or with white hard varnish. When the ring or piece has been firmly fixed to the slide by either of the above cementing materials, so as to form the sides of the cell, the bottom is to be covered with a piece of black paper, cut to fit it exactly, and fastened to the surface of the slide with a little gum, or of either of the above varnishes. As soon as this is thoroughly dry, the upper surface of the cell-wall, whether of glass or cork, &c., is thinly covered with varnish, and a clean thin-glass cover laid upon it, and very lightly pressed; the object is then permanently preserved. The main points to be observed are, that the object and varnish are completely dry, and that the cell is thoroughly closed. If the latter be not the case, more varnish must be applied to any little openings which may have been left; and it is better to apply the varnish in very small quantities at a time, the application being renewed as soon as the previous layer is quite dry.