  In a paper on ``the Brownian motion analog of the well-known Milne problem in
radiative transfer theory'' [\textit{J Stat Phys} 25 (1981) 569--82], Burschka
and Titulaer reported: ``The value we find for this `Milne extrapolation
length' is, in the appropriate dimensionless units, approximately twice the
value found in the radiative transfer problem.'' A study by Ziff [\textit{J
Stat Phys} 65 (1991) 1217--33], concerned with the absorption of particles
executing a Rayleigh flight (randomly directed displacements of equal length
$l$) by a black sphere of radius $R$, led to a value for the extrapolation
length $\gamma$ about half as small as the benchmark result (for $R\gg l)$. The
first discrepancy is shown to result from the disparity of the two length
scales; the second, from the zero variance of the jump lengths. Ziff's finding
that $\gamma$ is independent of $R$ for $0<l\leq 2R$, cannot be reconciled with
studies based on the Lorentz-Boltzmann equation and the Klein-Kramers equation.
