THE MICROSCOPE AND MICROSCOPIC METHODS 57 



(c) The xylol should be washed off with absolute alcohol, 

 and then 70 per cent alcohol and finally distilled water. 



{d) The section is stained. 



(e) Dehydrate in absolute alcohol. 



(/") Clear in xylol. 



(g) Mount in balsam. 



Section Cutting. — Cutting is best done with an instrument 

 called a microtome. The tissues may be imbedded in collodion 

 or paraffin; or when they have been hardened with formaldehyde 



Fig. 27. — Automatic laboratory microtome. 



they may be cut after freezing. Bacteria stain admirably in 

 frozen sections. For routine work collodion imbedding will be 

 found' as convenient a process as any. Paraffin imbedding gives 

 the thinnest sections. 



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

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

 the object which is to be cut, any desired distance after each ex- 

 cursion 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 



