MOUNTING AND PRESERVING OBJECTS. 77 



necessary to have the same structure prepared in different 

 ways. 



Opaque Objects have sometimes been attached by thick 

 gum to small disks of paper, etc , or to the bottom and 

 sides of small pill-boxes, or in cavities in slides of bone or 

 wood, but they are better preserved on glass slides, as 

 hereafter described. 



The most convenient form of slide for microscopic pur- 

 poses is made of flattened crown or flint glass, cut into 

 slips of three inches by one inch, and ground at the edges. 

 Some preparations are mounted on smaller slips, but they 

 are less convenient than the above, which is regarded as 

 the standard size. 



On such slides objects are fixed, and covered by a square 

 or round piece of thin glass, varying from o'oth to oio tn 

 of an inch in thickness. Both slides and thin glass can 

 be procured at opticians' stores. Laminae of mica or talc 

 are sometimes used for lack of better material, but are too 

 soft. For object-glasses of the shortest focal length, how- 

 ever, it is necessary at times to resort to this sort of cov- 

 ering. 



Great care should be taken to have both slide and cover 

 clean. With thin glass this is difficult, owing to its brit- 

 tleness. Practice will teach much, but for the thinnest 

 glass two flat pieces of wood covered with chamois leather, 

 between which the cover may lie flat as it is rubbed, will 

 be serviceable. 



Very thin specimens may be mounted in balsam, glyc- 

 erin, etc., covered with the thin glass cover, and then 

 secured by a careful application of cement to the edges of 

 the cover. If, however, the pressure of the thin glass be 

 objectionable, or the object be of moderate thickness, some 

 sort of cell should be constructed on the slide. 



The thinnest cells are made with cement, as gold size, 

 Brunswick black, etc., painted on with a camel's-hair pen- 

 cil. For preparing these with elegance, Shadbolt's turn- 



