88 PARAFFIN METHOD 



tilted for hard and brittle objects than for soft ones ; therefore, 

 cceteris paribus, less for paraffin than for eelloidin. (3) The extent 

 of useful tilt varies between 0° and 16° or occasionally 20°. 

 (4) Excessive tilt causes rifts (longitudinal) in the paraffin, also 

 furrows that in bad cases split up the section into narrow ribbons. 

 It also makes sections roll. Also it may cause the knife not to 

 bite, thus causing sections to be missed. Or it may give an 

 undulatory surface to the sections, owing to vibrations set up in 

 the knife, which may be heard as a deep humming tone. Further, 

 we would add, excessive tilt may cause the knife to act as a 

 scraper, carrying away portions of tissue bodily from their places. 

 Excessive tilt may often 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 hack 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 (§ 174) requires a relatively hard paraffin 

 and less tilt. With eelloidin it is very important to avoid in- 

 sufficient tilt, as the elastic eelloidin 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 Neumayer (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 



