288 MICROTOMES AND SECTION KNIVES \_CH. X 



The microtome knives shown on the various instruments figured in this 

 chapter illustrate well the proper form of section knives. (Figs. 227, 238.) 



FIG. 227. Section knife with the honing back in position (Cut loaned by 

 the Spencer Lens Co. ) 



435- Sharpening Section Knives ; Hones and Strops. 

 Perhaps it should be taken for granted that any one would appre- 

 ciate the impossibility of making good sections with a dull section 

 knife, but experience teaches the contrary. Students are prone to 

 believe that with one of the elaborate automatic microtomes, good 

 sections may be made with any kind of an edge on the knife. It is 

 forgotten that the knife is the most important part, all the other 

 mechanism is simply its servant. 



For sharpening, select a fine, yellow Belgian hone, and a very 

 fine Arkansas hone. As a rule hones from the factory are not suffi- 

 ciently plane. They may be flattened by rubbing them on a piece 

 of plate glass covered with moderately fine emory or carborundum 

 wet with water. Round the corners and edges of the hones on the 

 plate glass or- on a grindstone. In using the Belgian hone for 

 sharpening knives, wet the surface well with a moderately thick 

 solution of soap. With the Arkansas stone use some thin oil 

 xylene or kerosene mixed with a little olive oil or machine oil. 



Honing. Before honing a section knife, make sure that the 

 edge is smooth, that is that it is free from nicks. Test this by 

 shaving off the surface of a block of paraffin. If nicks are present 

 the cut surface will show scratches. It is advisable also to look at 

 the edge of the knife with a magnifier and with a low power (50 

 mm. ) objective. If nicks are are present remove them by draw- 

 ing the edge along a very fine Arkansas hone. 



