In 1921, Meitner accepted an invitation from Manne Siegbahn to come to Sweden and give a series of lectures on radioactivity as a visiting professor at Lund University. She found that very little research had been done on radioactivity in Sweden, but she was eager to learn about X-ray spectroscopy, which was Siegbahn's specialty. At his laboratory, she met a Dutch doctoral candidate, Dirk Coster, who was studying X-ray spectroscopy, and his wife Miep, who was working on her doctorate in Indonesian language and culture. Armed with her newly acquired knowledge of X-ray spectroscopy, Meitner took a fresh look at the beta-ray spectra when she returned to Berlin. It was known that some beta emission was primary, with electrons being ejected directly from the nucleus, and some was secondary, in which alpha particles from the nucleus knocked electrons out of orbit. Meitner was sceptical of Chadwick's claim that the spectral lines were entirely due to secondary electrons, while the primary ones formed a continuous spectrum. Using techniques developed by Jean Danysz, she examined the spectra of lead-210, radium-226 and thorium-238. Meitner discovered the cause of the emission of electrons from surfaces of atoms with "signature" energies, now known as the Auger-Meitner effect, in 1922. The effect is co-named for Pierre Victor Auger, who independently discovered it in 1923.