NEBULOSITY IN STAR CLUSTERS.—In a letter to the Observatory for June Dr. Harlow Shapley states that the appearance of nebulosity which has sometimes been noted in visual observations of star clusters has not been confirmed by the Mount Wilson photographs. In the case of the cluster No. 361 of Dreyer's Index Catalogue, the photographs show stars fainter than 18th magnitude but there is no trace of nebulous matter. The cluster N.G.C. 6760, which has also often been observed visually as nebulous, appears purely stellar on the photographs. The actual connection between luminous nebulosity and star clusters seems to be limited to stellar groups of little condensation and richness, where the brighter stars are mainly of type A or B. In these cases the nebulosity becomes visible because of direct reflection of the light of the surrounding stars, and partly on account of selective secondary radiation. Thus the frequent association of diffuse nebulosity with blue stars of high temperature does not necessarily indicate immediate evolutionary relationship. There is at present no certain evidence of luminous nebulosity in globular clusters.