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By scattering light from small particles, their geometrical structure and their state of motion can be measured. An experiment is described for measuring the diffusivity of small particles undergoing Brownian motion using the technique called photon correlation spectroscopy or dynamic light scattering. The necessary experimental apparatus and the related theory are discussed. Photon correlation spectroscopy is a powerful tool for studying the dynamical behavior of fluids near critical points, and a discussion is given of this phenomenon. The same experimental technique also can be used to study laminar or turbulent flows, and the associated theory is introduced to enable such experiments to be interpreted.
W. I. Goldburg (Wed,) studied this question.