86 



PARAFFIN METHOD 



An object, which is not approximately isodiametrical but gives 

 a section which is wider in one direction than another should be 

 orientated e«cZ on, that is, so as to present its narrowest diameter 

 to the icnife-edge ; for it is in this position that it will offer the 

 least resistance to the blade, and tend the least to make the edge 

 bend away or dig into it. This is specially important with 

 Longitudinal sections of worms, Amphioxus, embryos of verte- 

 brates, and the like. Most especially with a square-set knife 



should the narrowest diameter of the 



object be presented to the knife; and 



only when the object is particularly hard, 



or otherwise difficult to cut, should it be 



^ turned so as not to let the whole of that 



I diameter be attacked at once by the knife, 



j but only a corner of it. And as far as 



I possible arrange that the hardest part 



^ K of an object be the last to be touched bv 



W the knife. 



Fje 4 For Noack's simple apparatus for 



accurately orientating small blocks, see 



Zeit. zviss. Mik., XV, 1899, p. 438. or Journ. Ron. Mic Soc 132 



1899, p. 550. J ■ ', io^, 



For Eternod's machine for trimming blocks to true cubes, see Zeit 

 wiss. Mik., XV, p. 421, and for that of Schaffer, ibid., xvi, 1900, p. 417.' 



165. Knife Position. The position to be given to the knife 

 may be considered under two heads, viz., its slant and its tilt. 



By the slant of the knife is meant the angle that its edge makes 

 with the line of section : that is, with the line along which it is 

 drawn through the object (or along which the object moves across 

 It in the case of microtomes with fixed knives). The position is 

 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 trans- 

 verse 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 



* 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 

 m Behrens, Kossel und Schiefferdecker, Das Mikroskop., pp 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. 



