90 GUAPTER viir. 



knives). The position is transverse whan the edge lual'ces an 

 angle of 9U with tlie line of section^ or the knife in that 

 case is said to be set square. It is ohlique or slautiiii/ when 

 it makes a smaller angle with that line. The difference 

 between the effect of tlie two positions is that the oblique 

 position affords a more acute-angled ivedge than the transverse 

 one. 



It does so for the following reasons : — Neglecting for the 

 moment the distinction between the cutting-facets and the 

 surfaces of the blade (which are distinct usually because they 

 are not ground to the same angle),* it is clear that the knife 

 itself is a wedge, the angle of which depends on the relation 

 befween the height of its base and the distance from the 

 base to the edge. With the same base the angle becomes 

 more acute the greater the distance from edge to base. 

 Now by slanting the knife we can effect what is equivalent 

 to an increase in the distance from edge to base ; for we can 

 thus increase the distance between the point of the edge 

 which first touches tlie object, and the point of tlie back 

 (strictly, of the back edge of the under cutting-facet) which 

 last leaves it. When the knife is set transversely, the line 

 along which any point of it traverses the object is the 

 shortest possible from edge to base of the wedge, and the 

 effective angle of wedge is the least acute obtainable with 

 that knife. But if it is set as obliquely as possible, the line 

 along which any point of it traverses the object traverses the 

 knife from heel to toe, that is, along the greatest possible 

 distance from edge to base, and therefore affords jiracticallij 

 a much more acute-angled wedge than in the first case ; and 

 so on, of course, for intermediate positions. (8ee the stereo- 

 metrical constructions of these relations by Schieffeeueckee, 

 op. cit., p. 115; and also with more instructive figures. 

 Apathy, " Ueber die Bedeutung des Messerhalters in der 

 Mikrotomie," in Sitzher. ined.-naturio. Section d. Siahen- 



* The edge of a microtome knife is composed of two plane surfaces — 

 the upper and lower cutting-facets, which meet one another at an acute 

 angle, the cutting-edge, and posteriorly join on to the upper and lower 

 surfaces of the blade (see some good figures of differently shaped knives 

 in Bbhbens, Kossel und Schiefperdeckee, Bus Milcronkoj]., p. 11.5, 

 et seq. ; and in Apathy's paper quoted below). It will Ije seen that the 

 two facets together form a wedge welded on to the blade by the base. 



