CHAPTER VIII. 83 



the knife enters it at the angle a and leaves it at the angle c. When 

 the section is cut it will adhere to the knife only by the angle c, and 

 can thus most readily be removed by means of a brush or needle. 

 The object itself should come to lie in the block close to the line 6 c, 

 so that the knife at first cuts only paraffin, and that if the section 

 begins to roll it may be caught and held down by a brush or section- 

 stretcher before the object itself is reached. For the square-set 

 knife the block is best trimmed to a four-sided prism, and orientated 

 as in the first case, so that the knife first touches one angle, if only 

 isolated, sections are to be cut. But if ribbons ( 148) are to be cut, 

 the block must be orientated with one of its sides parallel to the 

 knife-edge, and the opposite side must be strictly parallel to this 

 one. 



An object which is not approximately isodiametrical but gives a 

 section which is wider in one direction than 

 another should be orientated end on, that is, 

 so as to present its narrowest diameter to the 

 knife-edge : for it is in this position that it 

 will offer the least resistance to the blade, 

 and tend the leas't to make the edge bend 

 away or dig into it. This is specially impor- 

 tant with longitudinal sections of worms, 

 Amphioxus, embryos of vertebrates, and the 

 like. Most especially with a square-set knife 

 should the narrowest diameter of the object F IG . 4. 



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 diameter be attacked 

 at once by the knife, but only a corner of it. And as far as possible 

 arrange that the hardest part of an object be the last to be touched 

 by the knife. 



For NOACK'S simple apparatus for accurately orientating small 

 blocks, see Zeit. wiss. Mik., xv, 1899, p. 438, or Journ. Roy. Mic. 

 Soc,, 132, 1899, p. 550. 



For ETERNOD'S machine for trimming blocks to true cubes, sec Zcit. 

 ims. Mik., xv, p. 421, and for that of SCHAFFER, ibid., xvi, 1900, p. 417. 



145. 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 ol the knife is meant the angle that its edge mak s 

 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 



62 



