How Does Rosin Affect Sound?

The effects of rosin properties on the bowed-string sound are frequently discussed among string players and luthiers. The following comments and questions are common: Rosin is rosin. Should I use different rosins for summer and winter? Does anyone actually have a method for quantitatively measuring the physical behavior of different rosins? What are the effects of hair scales and hair flexibility? In this article the author, who has been studying these effects over a number of years both as a professional player and as an acoustician, presents some of the underlying physical aspects relevant for further discussion.