As it has been shown in the first publication under this title (Pasha 2002, hereinafter Paper I), by the 1960s understanding the spiral structure of galaxies entered a new stage of unusually vigorous activity, not always very united or monothematic, but broadly grouped under the umbrella marked "density-wave theory". Its foremost enthusiast and proponent was C.C. Lin. His papers with Frank Shu (Lin and Shu 1964, 1966) 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. Already at the time, however, Lin's optimism for spirals as quasi-steady waves was not entirely shared by other experts, and toward the 1960s 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.
We start this second part of our narrative with the events that occurred and developed right in the period of Lin and Shu's initial semi-empirical explorations of 1963-66 on the alternative, dynamical front of sheared-wave research. It was those early analyses that first taught us, then in a local approximation, that massive shearing disks tend to be wonderful amplifiers and to respond strongly, though always in a trailing-spiral manner, to several quite plausible forms of forcing. After that, we will use Chapter II to describe most engaging topics like neutral tightly wound modes, spiral shocks and star migration that Lin and Shu plus several associates continued to explore from about 1966 onward, whereas in Chapter III we will turn to a fascinating and very serious difficulty with the group velocity that emerged only near the end of that decade. We will try to wrap it all up in Chapter IV which will focus mainly on a remarkable conference on spiral structure that took place in Basel, Switzerland in August 1969. Though its coverage may have been a little too slanted a priori toward praising mostly just the Lin-Shu ideas as major steps forward in this subject, that meeting also attracted nearly all of the other main players, and it appears interesting now to examine in retrospect which points they themselves chose to emphasize there.
2 Throughout the paper, the italicized names in parentheses refer to private communications as identified in the note to the list of references. Back.