AS a subject progresses the attractive simplicity of the early researches gives place to laborious elaboration. In the last three years, Dr. Chandrasekhar has been very active in the mathematical development of stellar dynamics. The trend of his work may be judged from the fact that one contribution alone contains more than 1,800 numbered formulae. There is no denying that this heavy method of attack can be justified ; but it leaves us with the depressing feeling that the subject which began thirty years ago as a joyous adventure has reached a stage of uninspiring ugliness. We are the more grateful to Chandrasekhar that in his new book he has not allowed the subject to be crushed by an overweight of mathematical formulae. The treatment is, of course, mathematical throughout ; but it is mathematics of the methodological kind which the reader tolerates, not heavy algebra which he skips. The book fills an obvious gap in astronomical literature, and it should give a valuable stimulus to research. There are some omissions to which reference will be made later ; but, if judged by what it contains rather than by what it leaves out, it is a very helpful synthesis. Principles of Stellar Dynamics By S. Chandrasekhar. (Astrophysical Monographs sponsored by the Astrophysical Journal.) Pp. x + 251 + 6 plates. (Chicago: University of Chicago Press; London: Cambridge University Press, 1942.) 5 dollars.