DRY OPAKE OBJECTS. 11 



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 be- 

 comes 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. 



