The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz.
The Hercules cluster is of this form; another example is Centauri, in which over 6000 stars have been counted, comprised within a circle of about 40' diameter.
These clusters present many unsolved problems. Thus Perrine, from an examination of ten globular clusters (including Messier 13 and Centauri), has found in each case that the stars can be separated into two classes of magnitudes.
Henderson at the Cape of Good Hope measured the parallax of a Centauri, but his resulting value 1" was considerably too high.
For one or two of the more famous stars such as a Centauri the probable error is less than so oi"; but for others in the list it ranges up to X0.05".
