IMBEDDING METHODS (PARAFFIN). 105 



merely that the oblique position affords a more acute-angled wedge than 

 the transverse one. 



It does so for the following reason: 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 dis- 

 tance 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 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 practically a much more acute-angled wedge 

 than in the first case ; and so on, of course, for intermediate 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 Messerhalters in der Mikrotomie," in Sitzberg. med.- 

 natunv. Section d. Siebenbiirgischen Museumvereins, Bd. xix, Heft 7, 

 p. 1 (Kolozsvar, 1897, A. K. Ajtai). 



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

 objects of very heterogeneous consistency, such as tissues that contain much 

 chitin or much muscular tissue. In general all very difficult objects are 

 better cut with the slanting knife than the transverse one (and better with 

 a slowly-working sliding microtome than with a quick-working Rocker or 

 the like). Soft masses, such as gelatin or celloidin cut wet, canjmly be 

 cut with the slanting knife. The slanting position causes less compression 

 of sections than the transverse one. It has the defect of producing rolling 

 in paraffin sections more easily than the transverse position. The letter is 

 the proper position for cutting ribbons of sections from paraffin. 



By the tilt of the knife is meant the angle that a plane 

 passing through its back and edge makes with the plane of 

 section : or, practically, the greater or less degree of eleva- 

 tion of the back above the edge (it is not to be confounded 

 with the inclination of the long axis of the knife to the 



* 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 BEHBENS, 

 KOSSEL und SCHIEFFERDECKER, Das Mikroskop., p. 115, et 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. 



