Introduction.- A Theoretical Understanding of Nuclear Structure.- Nuclear Characteristics in the Optical Spectrum.- Resonance Ionization Spectroscopy for Nuclear Physics.- ISOLDE and RILIS.- The VADLIS Ion Source.- Ionization Scheme Development for the ISOLDE RILIS.- Mercury Isotopes the playground of the optical spectroscopist.- In-source Resonance Ionization Spectroscopy of Mercury.- Charge Radii of Neutron Deficient Mercury.- Charge Radii of Neutron Rich Mercury.- Conclusion.