misreading of Berne and Pecora’s excellent monograph, Chapter 5, in which Berne and Pecora discuss the motions of monodisperse probe particles in simple Newtonian fluids by using the Langevin model. The spectra of monodisperse Langevin-model particles are without exception single exponentials. The calculation is correct, but does not refer to non-Newtonian fluids or polydisperse probe systems. Having corrected this error, I turn to several subsidiary points of misinterpretation.

The functional form of $P(\Delta x, t)$ can be inferred, at least approximately, from the angular dependence of $g^{(1s)}(q,t)$, namely as seen in eq \[eq:g1sPDelta\] the correlation function $g^{(1s)}(q,t)$ at fixed $t$ is the spatial Fourier transform of $P(\Delta x, t)$. If $g^{(1s)}(q,t)$ is determined sufficiently accurately over an adequate range of $q,$ an inverse spatial Fourier transform can take the experimenter back from $g^{(1s)}(q,t)$ to $P(\Delta x, t)$. To the author’s knowledge, this inversion has only been done for the ubiquitous polystyrene spheres in distilled water, for which $g^{(1s)}$ is a simple exponential $\exp(- \Gamma t)$, while $\Gamma$ is found to be accurately linear in $q^{2}$, showing that $P(\Delta x, t)$ in this system has the Gaussian form of eq \[eq:gaussianform\]. Measurements of the $q$-dependence of $g^{(1s)}$ for probes in complex 