276 



PRACTICAL MICROSCOPY. 



the piece d. The wedge is then closed until the motion is 

 stayed by the glass, when the number of the divisions indi- 

 cating the thickness can be read off. 



None but the very thinnest glass can be used with the 

 dry A- and -^ objectives ; it is much more expensive and 

 difficult to cut than ordinary cover-glass. 



Cells and Cell-making. Many objects requiring to be 

 permanently mounted are of such a thickness that the thin 

 cover needs some support at its edges, or again others are 

 better preserved when mounted in fluids, and for these pur- 



FIG. 228. 



poses cells are employed. These are of several kinds, but 

 may be conveniently divided into three classes, varnish 

 cells, solid, and built-up cells. 



The cell-making machine was originally devised by Mr. 

 Shadbolt, as a simple brass circular plate about three inches 

 in diameter, upon the surface of which was placed a pair of 



