The distribution of extragalactic nebulae

. Counts of extragalactic nebulae have been made on plates taken with the o-inch Carnegie astrograph of the Lick Observatory. The area of the sky for which the results are published in the present paper extends in right ascen- sion from 12h to 18h and in declination from -20 to +200. The limiting magnitude of about 18.4 has been made as nearly as possible constant over this entire region through the use of uniform procedures and the application of corrections for systematic errors. The principal data are given in a table containing the counted numbers of nebulae in each square degree together with corrections to reduce these numbers to a uniform basis, and the results are portrayed by means of a chart giving contours of equal numbers of nebulae per square degree. In addition to a very irregular boundary of the zone of avoidance, several more or less detached areas of galactic obscuration are apparent on the chart. In higher latitudes, where the effect of galactic obscuration should be small, the distribution of nebulae is still so far from random as to suggest strongly that clustering is a very general feature of the distribution. The distribution of surface density of nebulae in Coma-type clusters has been studied in a very preliminary way and is found to be well represented by de Vaucouleurs' empirical formula for the distribution of light in elliptical nebulae. The association of clusters to form larger aggregations or clouds of nebulae seems to be a rather general Thature of the distribution.