[509] [P. 141].

[510] M. N. R. A. S., 83, 403, 1923.

[511] Chapter II, [p. 31].

[512] Adams and Joy; Mt. W. Contr. 142, 1917.

[513] A. N., 219, 361, 1923.

[514] Lundmark, Pub. A. S. P., 34, 147, 1922.

[515] Shapley, H. B. 796, 1924.

[516] Shapley and Cannon, Proc. Am. Ac. Sci., 59, 217, 1924.

[517] Shapley and Cannon, ibid., 59, 230,1924.

CHAPTER XV
ON THE FUTURE OF THE PROBLEM