Density-Wave Spiral Theories in the 1960s. I
  With the arrival of computers, plasma physics and several fresh investigators by the early 1960s, understanding the spiral structure of galaxies entered a new stage of unusually vigorous activity broadly grouped under the umbrella marked "density-wave theory". Paper I starts with acknowledging B. Lindblad, rightly regarded the main father of this whole subject, and then describes the early contributions by Lynden-Bell, Toomre, Hunter and Kalnajs, who had formulated and applied such notions as the stability of flat galaxies, the regenerative spiral phenomenon, the shearing density waves and the global spiral modes. But the foremost enthusiast and proponent of the density-wave picture was undoubtedly C.C. Lin whose 1964 and 1966 papers with Shu, written in support of his working hypothesis of the quasi-stationary wave-mode spiral structure, had a big and immediate impact upon astronomers, at least as a welcome sign that genuine understanding of the spiral phenomenon seemed in some sense to be just around the corner.   We find that even Lin occasionally let himself get carried away with too much enthusiasm as for instance when he wrote that his relatively exploratory work with Shu had already led to a "theory free from the kinematical difficulty of differential rotation", or that it "enables us to provide a mechanism to explain the existence of a spiral pattern over the whole disk while allowing the individual spiral arms to be broken and fragmentary" (Lin 1967b, p.462). Already at the time such optimism was not entirely shared by other experts, and by the late 1960s - as we shall see in Paper II - it had become very clear to everyone that much hard work still remained to explain even the persistence, much less the dynamical origins, of the variety of spirals that we observe. 
I.I. Pasha