As astronomy pushed its limits from the solar system to the stars of the Milky Way and to the distant galaxies and clusters of galaxies, the following three problems suggested themselves automatically. Is there matter spread throughout interplanetary space? Is there matter in interstellar space? Are the enormous intergalactic spaces empty or not?
REFERENCES
1.
F. Zwicky, Carnegie Institution of Washington, Year Book, No. 48, 1948–1949, p. 20;
No. 49, 1949–1950, p. 15;
No. 50 1950–1951, p. 21.
2.
3.
F. Zwicky, Theodore von Karman Anniversary Volume, pp. 137–153, California Institute of Technology, 1941.
4.
5.
Keenan in this paper, as far as I know, published the first photograph of a fairly widely separated pair of galaxies, NGC 5216–5218. the two members of which are connected by a luminous band. This system apparently remained an isolated case until the author during the past few years found a great number of similar cases.
6.
H. Schlichting, Grenzschichtttheorie, p. 326. Ed. G. Braun Karlsruhe, 1951.
7.
E. P. Hubble, The Realm of the Nebulae, p. 170. Vale University Press, 1936.
8.
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© 1953 American Institute of Physics.
1953
American Institute of Physics
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