84 IMBEDDING METHODS. 



transverse when the edge makes an angle of 90 with the line of 

 section, or the knife in that case is said to be set square. It is 

 oblique or slanting when it makes a smaller angle with that line. 

 The difference between the effect of the two positions is that the 

 oblique position affords a more acute-angled wedge 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 between 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 the object, and the point of the 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 e'dge 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 practically a much more acute- 

 angled wedge than in the first case ; and so on, of course, for inter- 

 mediate positions. (See the stereometrical constructions of these 

 relations by SCHIEFFERDECKER, op. cit., p. 115 ; and also with more 

 instructive figures, APATHY, <; Ueber die Bedeutung des Messer- 

 halters in der Mikrotomie," in Sitzber. med.-naturw. Section d. 

 Siebenbiirgischen Museumvereins, Bd. xix. Heft 7. p. 1 (Kolozsvar, 

 1897, A. K. Ajtai). 



For honing knives see SSOBOLEW, Zeit. wiss. Nik., xxvi, 1909, p. 65 ; 

 LENDVAI, ibid., p. 203 ; FUNCK, ibid., xxvii, 1910, p. 75. 



Very large objects are best cut with the slanting knife, and so 

 are all objects of very heterogeneous consistency, such as tissues 



* 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 BEHRENS, KOSSEL und SCHIEFFERDECKER, Das Mikroskop., p. 115, 

 ft seq. ; and in APATHY'S paper quoted below). It will be seen that the 

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



