80 IMBEDDING METHODS. 



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 corning 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 hard 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 

 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 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 Zeit. wiss. 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. wiss. Mik., xvi, 1899, p. 29 (see figure of model / 

 in Journ. Roy. Mic. Soc., 132, 1899, p. 546). Also CARPENTER'S 

 The Microscope, p. 463. 



146. Cutting and Section-stretching. Paraffin sections are cut 

 dry, that is, with a knife not moistened with alcohol or other 

 liquid. By this means better sections are obtained, but a difficulty 

 generally arises owing to the tendency of sections so cut to curl up 

 on the blade of the knife. It is sometimes difficult by any means to 

 unroll a thin section that has curled. To prevent sections from 

 rolling, the following points should be attended to. 



First and foremost, the paraffin must not be too hard, see 151. 



If, after cutting has begun, the paraffin be found to be too hard, it 

 may be softened by placing a lamp near the imbedded object. But 

 then, the paraffin being warmed most on the side nearest the lamp. 



