tube length
P 5 load, kgf;
Ap 5 projected area of the indentation, mm2;
d 5 length of the long diagonal of the indentation, mm;
Cp 5 1⁄2 (cot A/2 3 tan B/2);
A 5 included longitudinal edge angle (see Fig. 1); and
B 5 included transverse edge angle (see Fig. 1).
However, in the microscope only part of this diagonal is seen
due to the finite resolving power of the light microscope