286 



TECHNICAL MICROSCOPY. 



like the gutta-percha cells, as also those which are composed of 

 thin strips of plate-glass. The latter can be easily prepared by 

 the student himself, by cutting two different kinds of strips, 

 18-20, and 12-15 mm. in length, and cementing them together 

 to form a square framework. A single strip of glass might be 

 bent at right angles over a blow-pipe and the ends fused together, 

 though a practised hand is required for this process. 



The final sealing is a most essential operation. Many cements 

 and varnishes are apt to develope numerous fine cracks through 

 which the air enters; the durability of the preparation depends 

 chiefly upon the proper choice of the method of sealing. For 

 fixing the glass cells, marine glue is best ; it is composed of equal 

 parts of shellac and india-rubber dissolved in benzine. A solution 

 of gutta-percha and shellac in oil of turpentine is recommended 



FIG. 161. 



by Harting. These preparations are heated to the melting point 

 and then spread with a brush on the edges of the cells or strips of 

 glass, which are then applied and pressed into position under heat. 

 For cementing the cover-glasses, however, the asphalt-varnish of 

 commerce a solution of asphalt in linseed-oil and turpentine is 

 usually employed ; this has not proved entirely satisfactory, from 

 its liability to crack and flake in the course of a few months. The 

 asphalt-varnish of Bourgogne, of Paris, is, according to Frey 

 (" Das Mikroskop," 5th ed. p. 133), much better; it dries compara- 

 tively rapidly, and is durable. The same author formerly recom- 

 mended a white cement, invented by Ziegler, a painter of Frankfort, 

 which may be diluted with oil of turpentine as required by slight 

 heat, and which is spread on like asphalt- varnish with the brush. 

 The cement is said to become hard very slowly, and to be adhesive 

 even after some months ; nevertheless, it forms a secure sealing. 

 Gold-size, as used by gilders for leaf-gilding, is also recommended 

 for temporary mounting ; Harting states that he has employed it 



