IMBEDDING METHODS. 89 



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 

 ixolated 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 thiit it will offer the least re- 

 sistance 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, Amphi- 

 oxus, embryos of vertebrates, 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 fig 4,. 



it be turned so as not to let the whole 



of that diameter be attacked at once by the kuife, but only 

 a corner of it. And as far as possible arrange that the 

 hardest part of air object be the last to be touched by the 

 knife. 



For Noack's simple apparatus for accurately orientating 

 small blocks, see Zeit. wiss. Mile, xv, 1899, p. 438, or Journ. 

 Roy. Mic. 80c., 132, 1899, p. 550. 



Tor Eteenod's macliine for trimming blocks to true cubes, see Zeit. 

 wiss. Mile, XV, p. 421, and for that of ScHAFrBE, iUd., 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 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 



