Max Wolf - Dark Nebulae

One of the many significant contributions Wolf made was in the determination of the nature of dark nebulae. These areas of the sky, thought since William Herschel's time to be "holes in the sky", were a puzzle to astronomers. In these areas no stars could be seen, only featureless black. In collaboration with E. E. Barnard, he proved, by careful photographic analysis, that these dark nebulae were actually huge clouds of fine opaque dust.