The nature of Arago's neutral point of atmospheric polarization and its underlying physical principles are explained. Various criteria of atmospheric turbidity that can be derived from observations of this neutral point are discussed and their relative merits compared. Examples of the usefulness and potentialities of these criteria are cited. In particular, the influence of the size of the particles polluting the atmosphere and of meteorological factors on spectral observations of Arago's neutral point is shown. A selection of problems connected with the interpretation of measurements of Arago's point is presented.

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