EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 39 



called a microtome. The tissues may be imbedded in collodion 

 or parafiin; or when they have been hardened with formalde- 

 hyde they may be cut after freezing. Bacteria stain admirably 

 in such frozen sections. For routine work collodion imbedding 

 will be found as convenient a process as any. Parafhn im- 

 bedding gives the thinnest sections. 



A microtome consists of a heavy, sliding knife-carrier, which 

 moves with great precision on a level, and of a device for elevat- 

 ing the object which is to be cut any desired distance after each 

 excursion of the knife. The thickness of the section will be the 

 distance which the object is elevated. The knife is kept wet 

 with alcohol during the cutting of collodion sections, otherwise 

 it is left dry. The microtome is usually provided with a special 

 form of knife. A razor will serve nearly as well, after having 

 had the lower side ground flat. If a razor is used, a special 

 form of razor-holder must be attached to the microtome to 

 receive the razor. Above all, it is necessary that the knives 

 should be kept in good condition. Only occasionally will they 

 need honing, using a fine water-stone or Belgian hone. Prefer- 

 ably the knife should not be honed directly on the stone itself 

 but on a piece of clean plate glass, on which the stone is first 

 rubbed with water. By this means the entire cutting edge is 

 sharpened in one plane. The movement in honing should be 

 from heel to toe, and toward the cutting edge, always placing 

 the back of the knife next the hone when turning. The knife 

 should be stropped frequently. The leather of the strop should 

 be glued to a strip of wood to make a flat surface. The move- 

 ment in stropping should be from toe to heel. Sections should 

 be cut to a thickness of not more than 25 /i. Thinner sections 

 (5 to 10 /i) are to be desired. 



Staining of Sections. — A watery solution of one of the 

 aniline dyes is used — fuchsin, gentian-violet or methylene-blue 

 — made by adding a few drops of the alcoholic solution to a dish 



