The method given for Brownian Molecular movement is to prepare a differential equation for the probability of a given position for the particles. This was done by Einstein for the transpositional movement and recently by Perrin for the treatment of a special problem of the rotation around a fixed point. As should be indicated in the paper, this method permits one to treat quite generally the molecular movement of an arbitrary body. Thus, one can deal with, for example, the simultaneous transpositions and rotations of a triaxial ellipsoid and consequently the special cases of spheres, needles, and discs.
Trans. of Annalen der Physik (Germany) v86 n4 p628-656 1928.