92 OHAPTEIt VIII. 



be recognised by the knife giving out a short metallic sound 

 just as it leaves the object. For knives with plane under- 

 surfaces it is seldom advisable to give less than 10 tilt. 

 Knives with concave under-surfaces, on the contrary, may 

 require to be placed almost horizontal. Jung's knife-holders 

 give mostly a tilt of about 9, which is only enough for 

 cutting ribbons with hard paraffin. 



A knife with too little tilt will often cut a second section, 

 or fragments of one, without the object being raised, showing 

 that during the first cut the object was pressed down by the 

 knife, and recovered itself afterwards. This fault is denoted 

 by the ringing tone given out by the knife on passing lack 

 over the object before the latter is raised. Such a knife 

 gives out a dull rattling sound whilst cutting. Too little 

 tilt causes folding or puckering of sections, and does not 

 allow of the cutting of the thinnest possible sections, as the 

 edge does not bite enough. It is thus frequently a cause 

 of sections being missed, or coming off thicker at one end 

 than the other. 



A slanting knife should have more tilt given to it than a 

 square-set one. 



Ribbon section-cutting ( 148) requires a relatively Lard 

 paraffin and less tilt. With celloidin it is very important 

 to avoid insufficient tilt, as the elastic celloidin yields before 

 an insufficiently tilted knife and is not cut. 



The tilt of the knife is given to a certain extent by the 

 knife-holder sold with the microtome. With plane-concave 

 knives it can be regulated to a certain extent by simply 

 turning the blade over. It is more accurately regulated by 

 means of mechanical contrivances, of which the most simple 

 are the horseshoe-shaped wedges of NKUMAYER (see Jung's 

 price-list). A pair of these, each ground to the same angle, 

 is taken, and one of them placed (thin end towards the 

 operator) under, and the other (thick end towards the 

 operator) 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 



