454 THE ANATOMY OF WOODY PLANTS 



Luther Company of Milwaukee are excellent, particularly the 

 latter. 



After the edges have been sharpened on the grinding wheel 

 and hone the knives are finished by means of a strop. The best 

 type has four sides and a solid wooden center. The four sides are 

 covered with leather surfaced in various manners. The first face 

 provides a coarse polish and the two following ones successively 

 finer degrees. The last face is of smooth leather and imparts the 

 final smoothness to the edge. The knife should be carefully wiped 

 with a cloth after being passed on the successive surfaces to avoid 

 the impairment of the finer ones by material from the coarser 

 grades. In stropping, the edge should be drawn backward and 

 not pushed forward as in honing, since failure to observe this pre- 

 caution results in injury to the leather surfaces. Strops consisting 

 of unsupported leather are not desirable for use with microtome 

 knives. 



The knives are held in place on the microtome by the knife- 

 holder, which in turn is fastened to a heavy block of phosphor- 

 bronze running in a channel on the right side of the microtome. 

 The holder may be altered in position on the block, forward and 

 backward, inward and outward, by means of appropriate screw 

 holes. It may be raised by plates provided for insertion beneath 

 its shaft. The best type of holder is provided with a tilting mech- 

 anism which makes it possible to vary the horizontal angle of the 

 knife to the object. In cutting all hard tissues but coal the micro- 

 tome knife should be in an oblique position to the object, and in 

 general the inclination should be somewhat acute. In the case of 

 large objects and of those which are likely to curl in cutting, a 

 more oblique position is sometimes advantageous. In making 

 sections the surface of the knife should be kept wet with alcohol 

 of 95 per cent strength. This is ordinarily effected by means of a 

 camel's-hair brush. This is also used for preventing the curling 

 of the section as it comes up on the knife. The proper touch for 

 flattening the section without either rolling or dragging it is acquired 

 by experience only. Sections may be cut from two or three micra 

 to many times that thickness, depending on the particular condi- 

 tions involved. If the sections curl after being placed in alcohol 



