  Ornstein and his coauthors, who constructed a dynamical theory of Brownian
motion, taking the equation $mdv/dt =-\zeta v+X$ as their starting point,
usually named the equation after Einstein alone or after both Einstein and
Langevin; furthermore, Ornstein, who was the first to extract from this
equation the correct expression for $\bar{\Delta^2}$, the mean-squared distance
covered by a Brownian particle, credited de Haas-Lorentz, rather than Langevin,
for finding the stationary limit of $\bar{\Delta^2}$. A glance at Einstein's
1907 paper, titled ``Theoretical remarks on Brownian motion'', should suffice
to convince one that it is not unfair to attribute the {\it conception} of the
above equation, now universally known as the Langevin equation, to Einstein.
Langevin's avowed aim in his 1908 article was to recover, through a route that
was `infinitely more simple', Einstein's 1905 expression for the diffusion
coefficient, but a careful reading of Langevin's paper shows that--depending on
how one interprets his description of the statistical behavior of the random
force $X$ appearing in the above equation--his analysis is at best incomplete,
and at worst a mere tautology. Since textbook accounts are based on the
interpretation that renders the proof fallacious, alternative derivations,
which are adaptations of those given by de Haas-Lorentz and Ornstein, are
presented here. Some neglected aspects of the contents of Ornstein's early
papers on Brownian motion are also brought to light.
