 Sommerfeld retained Bohr's planetary model, but added mildly elliptical orbits (characterized by additional quantum numbers "" and "m") to explain the fine spectroscopic structure of some elements.The multiple electrons with the same principal quantum number ("n") had close orbits that formed a "shell" of positive thickness instead of the infinitely thin circular orbit of Bohr's model.The existence of electron shells was first observed experimentally in Charles Barkla's and Henry Moseley's X-ray absorption studies.