130 Brown: WooDs FOR MICROSCOPIC STUDY 
plane so that the edge of the blade lay at a definite angle with 
reference to the direction of movement of the carriage on the slide- 
way. For general work, a horizontal angle of inclination of about 
20° was found satisfactory, but to obtain extremely thin sections, 
as in imbedded material, an angle of about 5° proved to be best. 
Since angle 6’ (Fic. 1), which the planes of the average micro- 
tome knife make with one another, is ordinarily more acute than 
at the edge honed to 20°, this difference must be allowed for in 
adjusting the vernier. Thus if 0’ is 13°, the vernier should read- 
7.5° to give an inclination of 4°, an excellent angle for most work. 
A few woods containing gummy or resin-like material cut better 
at a somewhat greater inclination, as 6°. On the other hand, if 
the blade tends to ‘‘pull into”’ the tissue, a more acute inclination 
may be needed. 
It is doubtful if cutting edges with 6 less than 20° should ever 
be used in cutting woody tissues. The more acute edge is too 
easily injured to be used in cutting many of the harder woods, such 
as Pandanus, even when these have been carefully treated. The 
20° edge, on the other hand, has sufficient strength in blades of 
good quality to appear undamaged under the microscope after 
use in cutting any of the treated material, and at the same time 
it is sufficiently acute to meet every requirement. The chief 
objection to using a more obtuse edge is that the section, especially 
at steep inclinations of the blade, is too sharply bent at O, Fic. 1, 
in cutting, so that it tends to curl or even roll up. It is partly for 
this reason, also, that the downward inclination of the knife 
(angle 8) should not be greater than necessary. 
But even if the inclination is correct, the quality of the work 
which the instrument is capable of doing may be seriously im- 
paired by an imperfectly aligned cutting edge. This, of course, 
is a point which does not have to be taken into account at all in 
using a rotary microtome. For the sliding microtome, it is not 
sufficient that the edge shall coincide with a single plane, as OD, 
Fic. 1. Errors of alignment from heel to point may still be present 
by reason of which an otherwise well sharpened edge would not 
coincide with a plane passing through O at right angles to OD. 
Here an error so slight as to escape detection by the eye may be 
the entire cause for poor work. For example, let the error of 
