A small, opaque nebula seen in silhouette against a rich star field or a bright nebula. Globules were first cataloged in the 1920s. In 1947, B. J. Bok called attention to their potential significance for star formation, and since then they have been commonly known as Bok globules. A globule is a region of the interstellar medium containing a high density of interstellar grains that obscure the more distant background stars and cause the region to appear as a dark nebula in optical photographs. Only relatively nearby globules can be identified, because if there are many stars in front of the nebula the contrast with the background is too weak. The distances to such nebulae have been estimated by counting the number of stars photographed in the line of sight and comparing this count with an analogous star count in a clearer comparison field. Comparative star counts have also been used to evaluate the degree of extinction produced by the grains in the nebula; if the optical properties of the grains are known, the total number of grains in a cut through the nebula can be estimated.