308 DAVID H. DOLLEY 
two in this laboratory, will surely cut sections at five micra as 
well tomorrow as today. Variations in section thickness are now 
and again unavoidable of course, but that is a negligible factor 
in median sections of a three dimensional and spheroidal body, 
which, excepting the eccentric nucleus, are the ones we use 
when plasma, nucleus, and nucleolus come into the same optical 
field. For, quoting a mathematical authority (see Dolley, 714) 
‘“‘the diameter of the cross section of a nearly spherical body 
varies very slowly for plane sections nearly median or diametral.” 
Here is the mathematical reason why averages of individual 
xw 
eens 
ng ESR 
z ee 
B B* 
Fig. 1 Diagram of the relation of section frustra to the cell outline in the 
case of extra-diametral sections. 
stages either of areas or diameters are dependable—they are 
from median sections with little variation from that. 
The negligible effect of one micron variations in five micra 
sections may be illustrated very simply from the diagrams of 
figure 1. They represent two cells 20 micra each in diameter, 
which is the average for the transverse diameter of the Purkinje 
cell. The diameter AB is through the axial or median plane of 
each. Each section constitutes a frustrum and it is the edge of 
the maximum base of the section frustrum that we outline from 
the camera lucida. The frustrum in the left hand figure is a 
four micra, and the one to the right a six micra section, both 
being unfavorable possibilities outside of the true median section 
containing AB. The dotted line in each case marked a coin- 
