These two papers by Fritz Zwicky constitute  among the earliest announcements on the use of gravitational lensing, and the first mention of the possibility of using galaxies as gravitational lenses. The  two letters to the editor followed the very brief notice that Einstein sent to Science magazine in 1936 (and which was a follow-up on some earlier thinking (24 years 

earlier) he had done on the gravitational lens, this publishing as a result of “insistent prodding”¹ by Rudi Mandl, a Czech engineer. Einstein all but disowned the idea, noting “of course, there is not much hope  of observing this phenomenon directly”² The papers also follow a popular mention of the idea by Henry Norris Russell in Scientific American in 1936.  Aside from the appearance of the idea in Zwicky’s 1957 Morpholigical Astronomy, it is noted in Singularity Theory and Gravitational Lensing (see footnotes below) that there is a period of virtual silence on the subject for the next 25 years, and that from 1960 to 1980 there seem to have been fewer than two dozen additional papers. [ZWICKY, Fritz. “Nebulae as Gravitational Lenses”, 1937, in the Physical Review, 51(4), a one-page letter to the editor on page 290, and  ZWICKY,  Fritz. “On the Probability of Detecting Nebulae Which Act as Gravitational Lenses.”  American Physical Society,  1937, in  The Physical Review, 51 (8), April 15, 1937.]   

                                Footnote 3, –Long quote from Gravitational Lenses, by P. Schneider, J. Ehlers, E.E. Falco