30 PREPARATION AND MOUNTING 



very small objects the grain which all such paper has 

 when magnified detracts a little from the merit of this 

 background; and lately I have used a small piece of thin 

 glass covered on the hack with black varnish, and placed the 

 object upon the smooth untouched side ; but a solution of 

 the best Egyptian asphalt in benzole of moderate thickness 

 may be painted on v\ith this further advantage, that it will 

 be sufficient to arrange them in the positions they are to 

 occupy, when by slightly warming the slide they will adhere 

 to the asphalt. 



Another method of making these cells is as follow r s : 

 Two punches, similar to those used for cutting gun-wads, 

 are procured, 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 imperfect. 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 process. But, of 

 course, the gum must be perfectly dried before the ring is 

 used. 



For cardboard, gntta-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. 



Rings of cardboard, &c., have been rejected by persons of 



