~ 
x 
1885.] Microscopy. : 831 
nearly a horizontal position for its lower (plane) surface in sec- 
tioning, while a duller one requires to have its back raised a little 
above the level of its cutting edge. It is safe to say that a knife 
cuts well when thin sections (.005™™ or less) agree in size with the 
cut surface of the paraffine block. It may be possible to cut a 
straight ribbon with a dull knife, but in this case it will probably 
be found that the sections- are shortened in a direction at right 
angles to the edge of the knife, which shows that the knife is 
acting the part of a plough, which crushes more than it cuts. 
The statement that a sharp knife may have a nearly horizontal 
position must be understood to have some limitations. In gen- 
eral it may be said that the larger and harder the object the more 
imperative it becomes to have the under surface of the blade 
slant towards the object, and the necessity for this is greater with 
a transverse than with an oblique knife. For véry hard objects a 
hed thick-edged knife is required as well as a slanting 
positio 
For diny histological or embryological work, the upper 
surface of the blade is ground hollow, the lower surface plane 
(Fig. 1 2), the edge being left very thin so that an extremely 
slight bevel is made in setting. 
bevel there is should be mainly on the 
upper side. The edge when examined 
with a magnifying power of a hundred Fic. 1.—Diagram illustrating 
diameters should be perfectly straight ‘y me A ———— RE 
k, knife; s, oil- 
and smooth. moii wW, wire. í 
Method of Sharpening—Microtome knives can be properly 
sharpened only by those who understand their chief peculiarities,, 
an 
who have trained themselves in this special work. The diffi- 
culties in acquiring the art are not, however, insurmountable ; 
for ‘with the proper means anda little perseverance they can 
mastered in a short time. The first important step is to provide 
oneself either with a good razor strop (those made by Zimmer- 
mann in Berlin are considered eo” or with a long and wide 
In using an oil-stone it is well to cover the surface of the 
Stone with a mixture of glycerine (two parts) and water (one 
part), as recommended by Fol.’ The blade is laid flat on the 
‘Lehrbuch d. Vergl. Mikr. Anat., p. 129, 1884. 
