14 PREPARATION AND MOUNTING 



slides. Two pieces of hard wood of the usual size (3 in. by 1 

 in.), not exceeding one-sixteenth of an inch in thickness, are 

 taken, and a hole is then drilled in the middle of one of these of 

 the size required. The two pieces are then united by glue or 

 other cement, and left under pressure until thoroughly dry, when 

 the cell is fit for use. Others substitute cardboard for the lower 

 piece of wood, which is less tedious, and is strong enough for 

 every purpose. This class of " cell " is, of course, fitted for 

 opaque objects only where no light is required from below ; and 

 as almost all such are better seen when on a dark background, 

 it is usual to fix a small piece of black paper at the bottom of the 

 cell upon which to place them. For very small objects the 

 " grain " which all such paper has when magnified detracts a 

 little from the merit of this background ; a,nd lately I have used 

 a small piece of thin glass covered on the back with black var- 

 nish, and placed the object upon the smooth untouched side. 



Another method of making these cells is as follows: Two 

 "punches," similar to those used for cutting "gun-wads," are pro- 

 cured, of such sizes that with the smaller may be cut out the 

 centre of the larger, leaving a ring whose side is not less than one- 

 eighth of an inch wide. These rings may be readily made, the 

 only difficulty being to keep the sides parallel ; but a little care 

 will make this easy enough. For this purpose close-grained 

 cardboard may be conveniently used. It must have a well-glazed 

 surface, else the varnish or cement used in affixing the thin glass 

 cover sinks into the substance, and the adherence is very imper- 

 fect. When this takes place it is easily remedied by brushing 

 over the surface of the cardboard a strong solution of gum or isin- 

 glass ; and this application, perhaps, closes also the pores of the- 

 card, and so serves a double purpose. But, of course, the gum 

 must be perfectly dried before the ring is used. 



For cardboard, gutta-percha has been substituted, but cannot 

 be recommended, as it always becomes brittle after a certain time, 

 never adheres to the glass with the required firmness, and its shape 

 is altered when worked with even a little heat. Leather is often 

 used, and is very convenient ; it should be chosen, however, of a 

 close texture, and free from oil, grease, and all those substances 

 which are laid upon it by the " dressers." 



