284 



MANIPULATION. 



IS 



i 



effected by dropping the cover gently upon the fluid, and 

 jjressing it lightly to exclude all the excess, and to leave only 

 a thin stratum intervening between the two 

 lasses ; the excess may be removed either by 

 (rawing it away by the sucking tube, fig. 175, 

 r by small slips of blotting-paper. After this 

 peration is finished, a thin layer of cement is to 

 I le placed where the edges of the cover come in 

 ontact with the bottom glass ; when this is dry, 

 .nother thin layer may be put on, until the angle 

 I letween the two glasses is nearly filled up. The 

 ise of anointing the edges of the cover with the 

 lement before laying it on the fluid is to prevent 

 ts getting wet, and by that means hindering the 

 lement from sticking. Care must be taken to 

 sxclude all air bubbles from between the cover 

 md bottom glass, otherwise the cement wiU run 

 n, especially when the bubbles are near the 

 sdge; should too much of the fluid have been 

 Irawn out from between the glasses, and an air 

 bubble be left, it is then necessary to add a little 

 more of the fluid at the edge where the bubble 

 is, and it will then run in and occupy the place 

 of the bubble, and the excess of fluid may be 

 again taken away in the manner before described. 

 Objects mounted in this way seldom keep very 

 long, and when once an air bubble has made its 

 appearance, it becomes necessary that a watch 

 should be kept upon it, lest the cement also run 

 in and spoil the specimen. When an object possesses any 

 appreciable thickness, it is by far the best plan not to mount 

 it in this manner, but to adopt one or other of the following 

 methods, in all of which a small reservoir is employed to 

 contain both the fluid and the preparation ; this reservoir is 

 termed a cell; various forms of these, which wiU be particu- 

 larized as the thin glass cell, the concave, tubular, and drilled 

 cells, with many others, here require separate mention. 



In order to prevent the cement employed in the flat cell 



Fig. 175. 



