144 The Preparation of Microscope Slides 



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

 maintain the cutting facet at as uniform an angle as possible. This 

 angle is set on the knife in the manner shown in Fig. 109. 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. 109 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 that has been placed on 



Fig. 109. Setting the cutting facet. The razor is drawn gently forward. 



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 obtained. It does not matter what kind of 

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

 grit, that it is absolutely flat, and that under no circumstances 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," lubri- 

 cated 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 produce a perfectly sharp 

 cutting facet, and further strokes will have no effect other than to diminish 

 the length of life of the knife. 



