A book which will particularly delight advanced students of astronomy and amateurs specializing in the spiral nebulae. For these it will be invaluable, both for wealth of information on the theory and nature of spiral nebulae, their distribution and the shift of light which indicates their rapid recession from the earth, -- and for the manner in which it reveals the working methods of the modern astronomer. Mathematical equations and tabular presentation of material may frighten the average reader away. It is hardly comparable to the usual popular book on astronomy because it is unique in its steadfast concentration on one subject. The author is doing research at the Mt. Wilson Observatory.