10 



METHODS IN PLANT HISTOLOGY 



on the market, John Watts, 24 Redcross Street, London, E.G. 1. 

 England, furnished blades so hard that they broke when tightened in 

 the ordinary handles used for shaving. These blades are ideal for his- 



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(D 



Fig. 4. — Clamp for holding safety razor blades in the Spencer rotary microtome 



tological work. In ordering, one should state that he wants blades 

 specially hardened for microtome use. 



Many have trouble with safety razor blades, especially when used 

 with the rotary microtome, where the best holder is of the type shown 

 in Figure 4. The blade is bent into a curve, making 

 the angle of the cutting edge look less than it really 

 is. The holder must stand more nearly vertical than 

 a regular microtome knife to give the cutting edge 

 the same angle. Study Figure 5, which shows cor- 

 rect position. If the blade is too nearly vertical, it 

 will rub the paraffin instead of cutting it ; while there 

 will be scraping instead of cutting, if the blade is 

 too far from the vertical position. The position will 

 vary slightly with hard and soft, and with thick and 

 thin, sections. If the blade projects too far beyond 

 the holder, it will vibrate; if it does not project 

 enough, the paraffin will hit the holder. Figure 5 will make this clear. 

 The stout razors our grandfathers used to shave with are excellent 

 for freehand sectioning and even for cutting sections on the micro- 

 tome. The blade should be sharpened flat on 

 the under side and beveled on the upper, as 

 shown in Figure 6. If sharpened without this 

 chisel bevel, there will be a "wire" edge, and 

 smooth cutting will be impossible. A satisfac- 

 tory bevel can be secured by merely tilting the knife while finishing 

 the sharpening. The heavier microtome knives have a back which is 



Fig. 5. — The correct 

 relative positions of 

 holder, blade, and par- 

 affin, for both sliding 

 and rotary micro- 

 tomes. 



Fig. 6. — A grandfather's razor 

 with chisel-edge sharpening. 



