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This series, Smithsonian Contributions to Astrophysics, was inaugurated in 1956 to provide a proper communication for the results of research conducted at the Astrophysical Observatory of the Smithsonian Institution. Its purpose is the "increase and diffusion of knowledge" in the field of astrophysics, with particular emphasis on problems of the sun, the earth, and the solar system. Its pages are open to a limited number of papers by other investigators with whom we have common interests.The policy of issuing the Contributions in volumes consisting often of unrelated papers numbered sequentially ended with volume 11. With this issue (No. 12), each Contribution will bear a single series number and will contain either one or several related papers.