296 MANIPULATION. 



this purpose, as all inequalities of surface may be ground 

 down on the metal-plate. 



Fig. 198. 



When much deeper cells than these are required, we must 

 employ the glass box, also the invention of Mr. Goadby. 



This consists of four pieces of thick plate-glass, abed, 

 cemented together upon a bottom piece or slide by iheir edges, 

 as seen in fig, 199. The edges are ground flat, and the sides. 



Fig. 199. 



c d, made rectangular; this form of cell is not so easily 

 cemented as any of the preceding, and when the marine-glue 

 is once melted upon the edges, the pieces should all be put 

 together as speedily as possible, so that one part may not be 

 made much hotter than the other, otherwise the glue, when 

 over-heated, is apt to get thick and dry up. 



These cells or boxes may be made of any size to suit par- 

 ticular preparations, but in proportion to the dimensions so 

 ought the thickness of the plate-glass to increase; it must, 

 however, be borne in mind, that the box should not be deeper 



