106 CHAPTER VIII. 



horizon ; any accidental inclination that this may have is a 

 matter of no moment). 



The question of the proper tilt to be given to the knife 

 under different circumstances has . been investigated by 

 APATHY, loc. cit. swpra. He concludes (1) The knife should 

 always be tilted somewhat more than enough to bring 

 the back of the under cutting-facet clear of the object. 

 (2) It should in general be less tilted for hard and brittle 

 objects than for soft ones ; therefore, csetcris paribus, less for 

 paraffin than for celloidin. (3) The extent of useful tilt 

 varies between and 16 or occasionally 20. (4) Exces- 

 sive 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 maybe heard as a deep 

 humming tone. Further, I would add, excessive tilt may 

 cause the knife to act as a scraper, carrying ;iway 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 degrees 

 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 

 fur 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 pre>sed down by the 

 knife, and recovered itself afterwards. This fault is denoted 

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

 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. 



Ribbon section-cutting requires a relatively hard paraffin 

 and less till. With celloidin it is very important to avoid 

 insufficient tilt, as the elastic celloidin yields before an in- 

 sufficiently tilted knife and is not cut. 



