SEVERAL years ago photographs Profs. E. E. Barnard and Max taken by Wolf rendered the hypothesis highly probable that many of the dark lanes and spaces met with in the Galaxy and in regions of diffused nebulosity were due to the intervention of occulting matter rather than to actual diversity of star distribution. One of the most notable examples occurs in a long strip of nebulosity that runs southward from ζ Orionis. Dr. Isaac Roberts noted that there was “an embayment, free from nebulosity, dividing it into halves.” Prof. Barnard afterwards remarked that this space appeared to be “a dark body, projected against, and breaking the continuity of, the brighter nebulosity.” More recently still the object has been photographed at the Lick and Mount Wilson observatories. The Mount Wilson photograph, taken with the 100-in. Hooker telescope, is here reproduced, and is quite startling from the hard, clear-cut outlines of the marking, which is blacker than the neighbouring sky background. It would seem that this sharp outline must indicate a stratum of dust rather than one of gas; it can be traced for some distance outside the long nebula, and is probably connected with an isolated bright nebula some 15 minutes of arc distant to the northeast. It will be remembered that a great part of Orion is covered with faint nebulosity, first revealed as a large spiral by Prof. Barnard's photo-graphs with a lantern lens. There is a striking falling off of star-density to the east of the long ζ Orionis nebula as compared with that to the west, which presumably indicates a general absorption of light.