MAKING SECTIONS 63 



If the clearance angle is too large, so that the section is not being cut from 

 the block but is being scraped from it, the section will have a wrinkled appear- 

 ance and will usually roll up into a small cylinder. If the clearance angle is 

 too small, so that the lower angle of the facet is scraping the block after the 

 tip is passed, the whole ribbon of sections will be picked up on the top of 

 the block, which will itself crack off when the knife point reaches it. It is 

 obvious that the knife angle will be changed as the angle of the cutting facet 

 is changed, so that it is desirable to maintain the cutting facet of as uniform 

 an angle as possible. This angle is set on the knife in the manner shown in 

 Fig. 35. Notice that the knife has been furnished with a handle and that a 

 small split cylinder of steel has been slipped over the back of the blade. This 

 split cylinder rests flat on the stone as does the edge of the blade, so that 

 when the knife is pushed forward (Fig. 35 shows it at the beginning of the 

 stroke) the cutting facet is produced as the angle between the cutting edge 

 lying on the stone and the enlarged temporary back which has been placed on 

 the knife. Since a much blunter cutting facet is required for hard materials 

 than for soft, it is strongly recommended that either two knives or at least 

 two sharpening backs be secured. It does not matter what kind of stone is 

 used for sharpening, provided that it is of the finest obtainable grit, that it is 

 dead flat, and that under no circumstances whatever is it used for any purpose 

 except the sharpening of microtome knives. It does not matter whether 

 it is a "water stone," lubricated with glycerin, like the "water-of-Ayr" stones 

 generally used in Europe or an "oil stone," lubricated with mineral oil, like 

 the "Pike" stones so commonly employed in the United States. However, it 

 does matter that it should be flooded with lubricant before starting, and that 

 the knife should be drawn with a light pressure (notice that the finger is 

 behind and not on top of the knife in the illustration) the entire length of the 

 stone at each operation. If only the central portion of the stone is used, it 

 soon becomes hollowed out, and it is thus impossible to maintain a uniform 

 angle. About three strokes on each side of the knife are quite enough to pro- 

 duce a perfectly sharp cutting facet, and further strokes will have no effect 

 other than to diminish the length of life of the knife. 



This direction for the use of three strokes in setting applies, of course, only 

 to a knife which has been treated reasonably and not to that which through 

 carelessness has acquired a nick in its edge. Where the nick is large, it is 

 almost impossible to remove it in setting because the continual setting merely 

 grinds away the edge of the knife and ultimately alters the thickness of the 

 blade itself If the knife-edge is nicked to a deeper extent than about a quarter 

 of a millimeter, the only thing to do is either to return the knife to the 



