THE STRUCTURE OF THE UNIVERSE.—The November issue of Scientia contains an article by Prof. J. C. Kapteyn, entitled “On the Structure of the Universe,” which should be read by all those who wish to obtain the most modern view of this most fascinating problem. It was Prof. Kapteyn who, in 1904, first determined the elements of che two star streams, and since then a great advance has been made in extending our knowledge in this direction. In the present article, and, it may be added, it is written in a very clear and concise manner, he places before the reader the general nature ot the problem, and step by step he points out how the various researches of many observers are coordinated and brought to bear in concentrated form on the question of the structure of the universe. The subject being so vast, he confines himself here mainly to that portion concerned with star-streaming, and considers the questions, What has the discovery of star-streaming done, and, What does it promise to do for the solution of the problems (1) that of the distance, and (2) that of the history or evolution of the stellar system. Prof. Kapteyn utilises a modified form of Secchi's stellar classification, and states that there is much evidence to show that this classification is a natural one, and that the order of evolution is as follows:—The helium stars being those of recent birth, while we come to older and older stars in passing from the helium stars to the stars of the first, then to those of the second, and finally to those of the third type.