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The possibilities of determining nuclear spin, moments and variation in charge volume by optical pumping of on line masseparated isotopes are described for the example of the isotopic chain lY1Hg-205Hg. The variation in nuclear charge volume (6 ) along this chain and in particular its abrupt change for the lightest Hg-isotopes, indicating a phase transition in nuclear structure, are discussed. 1. Introduction. - This talk is based on expe- riments (I), (2), (3) performed recently by a visiting team (*) at the ISOLDE facility at CERN with the members : J. Bonn, G. Huber, H.-J. Kluge, U. Kopf, L. Kugler, J. Rodriguez, and E. W. Otten. The investigation of the hyperfine structure (hfs) and isotopic shift (IS) of optical spectra allows the determination of the basic nuclear ground state properties : spin (I), magnetic moment (p,), electric quadrupole moment (Q,), and change of the mean squared radius of the nuclear charge distribution between different isotopes (6 ). The subject has therefore widely been explored in the past for stable and long lived isotopes (4), (5) and contributed substantially to the systematic topography of nuclear structure in the valley of stability which could be established within the last two decades. Still basical and quantitative questions are open in understanding nuclear matter. In this situation it makes from first principles is missing. In this situation it makes sense to enlarge the scope of investigations principally. In particular we should be in a position to choose and change systematically the parameters N and Z of a nucleus independently and free from the boun- dary condition of being close to the stable valley. With other words, a systematic investigation of nuclear properties along lines of constant Z or N across the full array of nuclei which we can reach in our days, would give us a much wider and more reliable view on the topography of nuclear structure (*) In the course of the experiments the team changed its home university from Heidelberg to Mainz.