CH. VII.] 



MOUNTING AND LABELING. 



149 



cell until a shining ring appears, showing that there is an intimate con- 

 tact. In doing this use the convex part of the fine forceps or some 

 other blunt, smooth object ; it is also necessary to avoid pressing on the 

 cover except immediately over the wall of the cell for fear of breaking 

 the cover. When the cover is in contact with the wall of cement all 

 around, the slide should be placed on the turn-table and carefully ar- 

 ranged so that the cover-glass and cell wall will be concentric with the 

 guide rings of the turn-table. Then the turn-table is whirled and a 

 ring of fresh cement is painted, half on the cover and half on the cell 

 wall (Fig. 140) . If the cover-glass is not in contact with the cell wall at 

 any point and the cell is shallow, there will be great danger of the fresh 

 cement running into the cell and injuring or spoiling the preparation. 



When the cover-glass is properly sealed, the preparation is put in a 

 safe place for the drying of the cement. It is advisable to add a fresh 

 coat of cement occasionally. 



§ 235. Mounting Objects in Media Miscible with Water. — 

 Many objects are so greatly modified by drying that they must be 

 mounted in some medium other than air. In some cases water with 

 something in solution is used. Glycerin of various strengths, and 

 glycerin jelly are also much employed. All these media keep the ob- 

 ject moist and therefore in a condition resembling the natural one. 

 The object is usually and properly treated with gradually increasing 

 strengths of glycerin or fixed by some fixing agent before being per- 

 manently mounted in strong glycerin or either of the other media. 



In all of these different methods, unless glycerin of increasing 

 strengths has been used to prepare the tissue, the fixing agent is 

 washed away with water before the object is finally and permanently 

 mounted in either of the media. 



For glycerin jelly no cell is necessary unless the object has a con- 

 siderable thickness. 



Fig. 126. Centering Card. A card 

 with stops for the slide and circles in 

 the position occupied by the center of the 

 slide. If the slide is put upon such a 

 card it is very easy to arrange the 

 object so that it will be approximately in 

 the center of the slide. (From the Mi- 

 croscope, Dec, 1886.) 



§ 236. Order of Procedure in Mounting Objects in Glycerin. 



1. A cell must be prepared on the slide if the object is of considerable 

 thickness (§ 233, 234). 



