10 THE ELEMENTS OF VEGETABLE HISTOLOGY 



divided into those in which the object-clamp is 

 fixed and the knife is movable (Fig. 4), and those 

 in which the object-clamp is movable and the 

 knife is fixed (Fig. 6). The type with sliding knife 

 is less liable to injure delicate specimens in cutting, 

 because the block does not come into as sudden con- 

 tact with the knife as in those machines in which the 

 knife is rigid. Rotary microtomes are usually of the 

 fixed-knife type and are so constructed that the object 

 is fed toward and brought into contact with the knife 

 at each revolution of a flywheel. Thickness of sec- 

 tion is adjusted by means of a graduated cam, which 

 limits the movement of a pawl in contact with a large 

 toothed wheel, the shaft of which is threaded and 

 inserted in the object-clamp. As the object strikes 

 squarely against the knife edge, this type of micro- 

 tome can only be used for sectioning fairly soft materi- 

 als and those which are of even texture throughout. 

 It is the most rapid hi operation, as one need only 

 turn the flywheel to obtain sections. All types of 

 microtomes should be kept free of dust and all moving 

 parts should be well greased or oiled. Microtome 

 knives must be kept well sharpened and should show 

 a straight edge without overhang when examined 

 under low magnification. Different types of knives 

 are required for objects of various textures, and each 

 should be reserved for its particular purpose. In 

 general, a heavy knife for wood and a second knife 

 for materials of medium texture will answer all pur- 

 poses and cover a fairly wide range. Knives having 

 a flat side should be sharpened only on the concave 

 surface, with perhaps a few finishing strokes on the 

 at side. 



