THE PRESERVATION OF MICROSCOPIC SPECIMENS. 285 



paper; the edge of the cover-glass is then painted over with a 

 somewhat thick varnish of the same kind. Instead of an enclosed 

 ridge for the base, Schacht recommends two parallel lines of 

 varnish upon which the cover-glass rests. The formation of air- 

 bubbles is thus more easily avoided, but, on the other hand, the 

 difficulty of securing perfect hermetic sealing is increased. 



Square cover-glasses are more difficult to cement than circular 

 ones. The latter are most easily cemented by the use of Frey's 

 improved revolving-stage, shown in Fig. 160, the action of which 



FIG. 160. 



is briefly as follows : The circular brass plate has a horse-shoe 

 spring-clip to grip the slides. Concentric circles of different sizes 

 are cut on the brass plate, corresponding with the cover-glasses, and 

 indicate where the brush is to be applied perpendicularly to make 

 the rings. Further, the brass plate is rotated rapidly by applying 

 the finger to the milled edge beneath. A fine brush is dipped in 

 the cement, and applied very lightly at first, increasing the pressure 

 as the rotation slackens. A little practice will soon enable the 

 operator to form regular rings. For circles the cement should be 

 more liquid than that used for square cover-glasses. 



If the preparations are so thick that the ridges of cement, 

 though laid on several times, are not satisfactory, cells or troughs 

 of gutta-percha, india-rubber, or glass, may be cemented on to the 

 slide. The gutta-percha or india-rubber cells may be made from 

 the thin sheets of commerce by cutting out square pieces, some- 

 what narrower than the object-plate, and boring them through 

 with a punch (Fig. 161). 



Glass cells of circular form are obtained by sawing off 1 rings 

 of suitable thickness from glass tubing, and grinding the surfaces 

 flat ; they require, of course, circular cover-glasses. We prefer 

 small rectangular plates of glass with circular or oval aperture, 



