Aksel Kipper explained the continuous spectrum of planetary nebulae with the two-photon transition in the hydrogen atom.

In the final phase of the development of the stars the planetary nebula is mainly the spherical formation, consisting of the matter thrown out of the stars, around the central star. The mätter in this nebula is very sparse and this is why such processes which cannot take place in the ordinary state of the matter may take place in this nebula. In this state such transitions may appear in the gases which in the ordinary – much more dense – state are prohibited.

Axel Kipper was not only a theoretician. At the museum the objective prism polished by him in 1934 is deposited. It was used in front of the Petzwal astrograph for studying the periodical changes of brightness of pulsating giant stars or Cepheid in different areas of the spectrum. (ÜAM 1146:1-90 AjK)