THE PARAFFIN METHOD 125 



the under side of the knife with a gentle stroke of the finger, shghtly 

 moistened with xylol. For the best results, the knife should be wiped 

 in this way after every section. The sliding microtome is more con- 

 venient for wiping than the rotary. Ribbons 1 n and even 0.5/1 thick 

 and nearly transparent can be cut on the sliding microtome shown in 

 Figure 2. Such a thin ribbon is not hkely to be more than 10 cm. long; 

 but at 10 n, ribbons 15 or 20 cm. long can be cut on a shding micro- 

 tome. It is a good plan to put a piece of ledger linen paper on the 

 holder or knife. The paper, long enough to hold a short ribbon, will 

 probably stick for a minute or two if merely moistened. If it comes off, 

 a little mucilage or a little piece of gummed paper, overlapping a small 

 part of the holder and paper, will keep the paper in position. 



Sometimes hard paraffin does not ribbon well. This difficulty may 

 be remedied by dipping a hot needle in soft paraffin and applying it to 

 the opposite edges of the block to be cut. Often the mere warming of 

 the opposite edges of the block with a hot needle is sufficient. 



Another method, suggested by Dr. Land to facilitate the cutting of 

 difficult material, has come into general use, and is very effective. 



Paraffin absorbs a small amount of water, or water penetrates be- 

 tween the crystals of paraffin. At any rate, water reaches cell walls 

 and, perhaps, other structures which have not been completely infil- 

 trated, and thus softens them. The paraffin cakes may be left for 

 weeks in water. Cakes of class material may be put in water in a fruit 

 can and kept until ready for use. After such treatment, smooth rib- 

 bons may be cut from material which would hardly cut at all without it. 



If the failure to ribbon well is due to electricity, a very small drop of 

 water on the knife will hold the sections and not prevent them from 

 slipping along in a ribbon. 



After cutting, the ribbons may be kept for a few weeks, on filter 

 paper, in a closed box; but no time is better for mounting than imme- 

 diately after cutting. 



A ribbon carrier is very convenient. A good carrier can be made by 



mounting a couple of spools 20 or 30 cm. apart, with a strong piece of 



cloth for a band. More elaborate carriers may be made if one has 



tools. 



FIXING SECTIONS TO THE SLIDE 



Mayer's fixative. — Sections must be firmly fixed to the slide, or they 

 wiU be washed off during the processes involved in staining. Mayer's 

 albumen fixative is excellent for this purpose. Formula : 



