As a first step towards an empirical estimation of the minimum size of interstellar dust formation units (clouds etc.) the correlation between angular separation and difference in colour excess has been studied for a selection of close star couples with coinciding magnitude and spectral type. Principally, the result indicates that there is no palpable correlation between the difference in colour excess and the lateral separation of the stars. This and other properties speak in favour of the conception that obscuring matter in the Milky Way is concentrated in extremely small units of high density.