I have to express my grateful thanks to Mr. W. H. Wesley, Assistant Secretary of the Royal Astronomical Society; Mr. E. Walter Maunder, Professor H. H. Turner, f.r.s Director of the University Observatory of Oxford; Professor E. B. Frost, Director of Yerkes Observatory, Wisconsin; Mr. Harlow Shapley, Of Mount Wilson Observatory, California, and others, for most kind help given in the work of re-writing, by their ready response to inquiries on my part about difficult questions and new developments.