590 THE MICROSCOPE. 



cutting sections of hoof, horn, claws, quill, &c. ; but the 

 substance, if very hard, requires a previous soaking in 

 warm water. A curious modification of horn is presented 

 in the appendage borne by the Rhinoceros upon its snout, 

 which in many points resembles a bundle of hairs. When 

 a transverse section is made and viewed by polarised light, 

 each cylinder is seen to have a cross diverging from a central 

 spot ; the lights and shadows 

 of this cross are replaced by 

 bands of contrasted comple- 

 mentary colours, if the sele- 

 nite plate is interposed. (Fig. 

 325.) Whalebone is almost 

 identical in structure, and is 

 similarly affected by polarised 



Fig.325. Transverse section of Horn ,. , . %- ,. r j.iT i 



of Rhinoceros, seen by polarised light. bections OI the horny 



li ^- tissues are always mounted in 



Canada balsam. 



It is preferable to mount and preserve specimens of 

 animal tissues in shallow glass cells, which prevents undue 

 pressure on the preparation. Cells may be made of various 

 materials for dry objects ; an efficient cell is readily 

 formed out of a ring of card-board fixed with gum upon the 

 glass slide, or a hole may be punched out of wood, mill- 

 board, or gutta-percha, and cemented to the slip of glass. 

 I however, the cell is intended to contain a preparation 

 immersed in fluid, it must be made of some substance im- 

 pervious to the fluid used. The Brunswick-black cells 

 (directions for making which are given at page 114) answer 

 for many purposes, but the best and most useful are those 

 made with circles of thin glass, and cemented to the glass 

 slide with marine glue, such as we have here represented. 

 (Fig. 326.) The surface of the 

 glass should be slightly roughened 

 by grinding before the cement is 

 applied, as it adheres much more 

 intimately to a roughened surface 

 than to the polished glass. The 

 Fig. 326. Glass-cells for glass must be warmed upon a plate 



Mounting. Q f ' ron Qr oyer a g pi r ft lamp, SO 



that the heat may be applied gradually and equally. It 



