We observe the electric-dipole forbidden $7srightarrow8s$ transition in the francium isotopes $^{208-211}$Fr and $^{213}$Fr using a two-photon excitation scheme. We collect the atoms online from an accelerator and confine them in a magneto optical trap for the measurements. In combination with previous measurements of the $7srightarrow7p_{1/2}$ transition we perform a King Plot analysis. We compare the thus determined ratio of the field shift constants (1.230 $pm$ 0.019) to results obtained from new ab initio calculations (1.234 $pm$ 0.010) and find excellent agreement.
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