544 SUMMARY OF CURRENT RESEARCHES RELATING TO 



at such a sharp angle as they did originally, and the edge is, in con- 

 sequence, not so sharp. The harder the object which is to be cut 

 through, the thicker the iron, and the blunter the angle of the edge, 

 the more frequently must the cutting surface be sliariiened and ground. 

 In fine knives the transformation of the triangular into a pentagonal 

 wedge must have a very prejudicial influence on the capacity of the 

 edge, but careful grinding of the whole surface takes up much time, 

 so this means has been devised of grinding the knife hollow, as it 

 must always be kept hair-sharj). We distinguish between whole 

 and half-hollow ground knives. It is a great error to lay the hollow 

 ground knife obliquely, and not perfectly flat on the sharpening sur- 

 face. Half-hollow ground knives should be laid on the strop as 

 shown in fig. 123 in cross-section. In whole-hollow ground razors the 



Fig. 122. Fig. 123. 



fMf a mM,. Sir. 



Fig. 124. 



anterior margin of the edge lies flat on the surface of the stone or 

 strop, and the ground surfaces consequently converge at a very acute 

 angle (fig. 124). But this advantage is combined with a serious dis- 

 advantage for our purpose, that the edge, which is almost as thin as 

 paper, is very unresisting, and easily bends and gives way before the 

 object to be cut. After all, one should not select such a knife as the 

 latter for a microtome, and it is only useful for quite soft and imrc- 

 sisting preparations. Two foims of knife have proved themselves to 

 be sufficient in practice. The one is only slightly hollow ground, and 

 is used foi hard objects ; the other, -which is the most useful, has the 

 side which is directed upwards whole-hollow ground, and the lower side 

 either quite plane or only slightly hollow. If it is plane a thin wire 

 is i)]aced at the back during sharpening, so that onlj- that part which 

 is next to the edge, and not the whole surface, is sharpened (fig. 125). 



Fig. 125. Fig. 126. 



StK ^^« Sir. 



Of all razor strops for knives, those are to be rejected in which 

 two leather straps, which can be stretched tight at jileasure, furnish 

 the rubbing surface. An inflexible knife must, if a fine edge is 

 to be obtained, be moved backwards on a perfectly plane and hard 

 surface. This assertion is best established by reference to fig, 123 



