The COSMO project, in particular the large coronagraph focusing on the coronal forbidden lines, represents the latest in a long line of research efforts begun in the nineteenth century. The following short article presents a time-line of important developments in this area, with pointers to some of the most important publications.

• Early work: identifications of magnetic dipole lines show that the corona is hot
  
• Advances in the 1940s-early 1960s: more quantitative work
  
• 1965 onwards: linear emission line polarization
  
• Circular polarization: breakthrough measurements
  
• Accelerating along the road to the COSMO project