NUCLEAR VOLUME AND MASS EFFECT IN THE OPTICAL ISOTOPE SHIFT OF LIGHT ELEMENTS R. BRUCH, K. HEILIG, D. KALETTA, A. STEUDEL and D. WENDLANDT J. Phys. Colloques, 30 C1 (1969) C1-51-C1-58 DOI: 10.1051/jphyscol:1969118