Galactic Star Clusters.

The principal methods for determining intragalactic distances greater than ~oo par- secs are discussed and their application to galactic star clusters is outlined. A compari- son of distance moduli derived from photometric spectroscopic data with distances based on angular diameters yields an average coefficient of photographic absorption of o'~ per kiloparsec. Preliminary results for the radial velocities of ~ clusters lead to a value A = +o.oi~ km/sec parsec for the coefficient of differential galactic ro- tation. The good agreement of this result with other determinations confirms the cor- rectness of the adopted distance scale of galactic star clusters. The radial velocities also indicate a negative k term increasing in amount with distance. The appearance of galactic star clusters seen at great distances projected on the background of faint stars is studied. At distances of more than 5000 parsecs even the most favorable objects would have been missed by the surveys of the past, while the poorer and less luminous objects may have been overlooked at much smaller distances. This explains why the space distribution of the known star clusters shows little relation to the structure of our galaxy. Attention is drawn to the promise of a search for more distant clusters on photographs taken in red light with instruments of large aperture and large field