IMBEDDING METHODS (PARAFFIN). 107 



The tilt of the knife is regulated by means of mechanical 

 contrivances, of which the most simple are the horseshoe- 

 shaped wedges of NEUMAYER (see Jung's price-list). A pair 

 of these, each ground to the same angle, is taken, and one 

 of them placed under, and the other over, the clamping-arm 

 of the knife-holder. Three pairs, having different degrees 

 of pitch, are supplied, and are sufficient for most work. 

 Other contrivances to the same end consist of knife-holders 

 that permit of rotating the knife on its long axis, and, though 

 more costly, will be found a great convenience where much 

 section-cutting has to be done. For these see Jung's price- 

 list, and various recent papers in the Zeit. f. wis*. Mik., also 

 that of APATHY, in the paper quoted above (very complicated), 

 and especially the description of the two latest of Jung, viz. 

 his model I and model n, by MAYER and SCHOEBEL, in Zeit. 

 f. wiss. Mik., xvi, 1899, p. 29 (see figure of model I in 

 Journ. Ifmj. Mic. Soc., 132, 1899, p. o4b). 



(b) As to the consistency and shape and orientation of 

 the mass to be cut. Solid paraffin varies enormously in hard- 

 ness according to the temperature of its surroundings. It 

 must therefore be taken of a melting-point suitable to the 

 temperature of the laboratory, harder (i. e. of higher melting- 

 point) in summer, softer in winter. On this point see below, 

 141. 



As to the shape and orientation to be given to the block 

 to be cut, these differ accordingly as the cutting is done with 

 a slanting knife or a square- set knife. In the first case, the 

 block is best trimmed to a three-sided 

 prism, and orientated as in Fig. 4, so that 

 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 

 b 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- 



