BROWN: WOoOoDS FOR MICROSCOPIC STUDY 131 
alignment, 7, midway between heel and point be 0.3 mm. with 
reference to a plane meeting the bisecting plane OD at right angles 
and in contact with heel and point. Let angle 6 = 20°, angle 
a = 10°, angle 6 = 4°, and angle a+ 8 = 14°. Distance y = r 
sin (a + 8), representing the error of alignment of the knife edge 
with reference to the plane of section, will then be 72 u. In other 
words, when heel and point of blade are in contact with the plane 
of section, the edge at mid-distance will be 72 » above this plane; 
whence it follows that with each full heel-to-point horizontal stroke 
in cutting, as when revolved horizontally at an acute angle of 
about 4° with reference to the direction of motion, the edge will 
twice traverse a vertical distance of 72 through the tissue, 
destroying the section. With the edge thus imperfectly aligned, 
the knife can be used to advantage in a sliding microtome only 
when placed less obliquely to the direction of motion than is 
essential for the cutting of thin sections, as at 20°-40°, in which 
position sections are cut with fractional strokes considerably less 
than the length of a 170 mm. blade. 
To give the true edge essential to accurate work in cutting 
woody tissues, the use of a special type of hone is necessary. 
Such a hone consists of a piece of plate glass as wide as the knife is 
long, with a true plane surface covered with Diamantine Powder 
and oil or water, so that the knife remains constantly in contact 
with the hone throughout its length during the process of grinding. 
An excellent abrasive may be prepared by grinding two fine 
Belgian or carborundum hones together, and collecting: the fine 
powder thus produced on the surface of the glass. This is not so 
harsh as the No. 1 Diamantine. Having ground the blade on the 
glass hone until the edge at both sides makes perfect contact with 
the surface, a brief grinding on a Belgian hone (2 x 8 in.) wet with 
30 per cent glycerine will often give an edge sufficiently even and 
sharp for general work. The knife should be drawn across the 
hone obliquely, heel and edge forward, alternately upon each side. 
But, for best results, this grinding should be followed by sharpening 
upon a flat leather-surface hone, the blade in this case. being 
pushed obliquely, point and back forward, along the hone. The 
surface of this hone may be kept in condition by the use of any of 
the fine abrasives used for this purpose on razor strops. A strop 
