Introduction

Let me preface this paper to say how honoured I am to have been given this opportunity to pay tribute simultaneously to two of the principal sources of scientific inspiration of my career. In my attempts over the past twenty years to understand and to analyse the optical and UV spectra of shock-excited plasmas, Don's books (1974, 1988) have been invaluable to both myself and to my students. In Australia we used to refer to the Physics of Gaseous Nebulae somewhat irreverently as “the new testament” to distinguish it from the earlier work by Aller! The famous diagnostic diagrams of Baldwin, Phillips & Terlevich (1981), of Veilleux & Osterbrock (1987), and of Osterbrock, Tran & Veilleux (1992) provide both an inspiration, and a powerful means of distinguishing between various excitation mechanisms.