Oxford University Press, 1937
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At the Clarendon Press
This book contains the Rhodes Memorial Lectures delivered at Oxford in the Autumn of 1936, under the general title, `The Observational Approach to Cosmology'.
The observable region of space, the region that can be explored with existing instruments, is a sample of the universe. If the sample is fair, its observed characteristics should furnish important information concerning the universe at large. The lectures describe the general features now known, and discuss the nature of the inferences to which they lead.
The features, however, include the phenomena of red-shifts whose significance is still uncertain. Alternative interpretations are possible, and, while they introduce only minor differences in the picture of the observable region, they lead to totally different conceptions of the universe itself. One conception, at the moment, seems less plausible than the other, but this dubious world, the expanding universe of relativistic cosmology, is derived from the more likely of the two interpretations of red-shifts. Thus the discussion ends in a dilemma, and the resolution must await improved observations or improved theory or both.
However, the significance of the investigation lies not in the failure to reach a unique solution to the problem of the structure of the universe, but rather in the fact that the venture is now permissible. As late as fifteen years ago the observable region was restricted to our own system of stars, the system of the Milky Way. Since that time great reflectors have identified the nebulae as independent stellar systems, the true in- habitants of space. Explorations, using the nebulae as gigantic landmarks, have swept out beyond the Milky Way to the very limits of existing telescopes. The observable region, our sample of the universe, has been suddenly magnified a million million fold. Now, for the first time, the sample may be fair.
The break through into extra-galactic space and the preliminary reconnaissance of the observable region have been described in The Realm of the Nebulae, recently published by the Yale University Press. The Rhodes Memorial Lectures form a sequel to the story, for they present the results of accurate surveys which followed the reconnaissance and suggest their cosmological significance. Since the new results could not be discussed in complete isolation it has been necessary to include a considerable background derived from the earlier investigations. It is a pleasure to acknowledge the courtesy of the Yale University Press in permitting the generous use of material from The Realm of the Nebulae.
Although the subject is developed from the observers' point of view, it is necessarily permeated with cosmological theory. Fortunately, the writer has had the privilege of association with Richard C. Tolman of the California Institute of Technology, who has presented the theory in a manner especially adapted to the limitations of the observational technique. Any errors in the application of the theory must be attributed to the misuse of his friendly counsel.
The illustrations reproduce photographs made with the telescope chiefly responsible for the recent development of the field of nebular research, namely, the 100-inch reflector of the Mount Wilson Observatory of the Carnegie Institution of Washington.
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