I'm having a hard time following one part of Bethe "Theory of diffraction by small holes" paper, which can be found here: https://web.stanford.edu/class/ee349/Handouts/Bethe_PR1944.pdf
At the end of page 167 he uses the fact that an uniformly polarized ellipsoid creates a constant field inside, which is the desired solution for the boundary conditions in the apperture.
He then says that the surface density of dipoles is proportional to "the ordinate", which is then said to be proportional to $\mu=(a^2-r'^2)^{1/2}$. Why is that?
Finally, he states that the surface charge density is proportional to $H_0· \nabla \mu$; again, why?
Thanks in advance.