  Isotope shift spectroscopy with narrow optical transitions provides a
benchmark for atomic structure calculations and has also been proposed as a way
to constrain theories predicting physics beyond the Standard Model. Here, we
have measured frequency shifts of the ${^1}S_0 \rightarrow {^3}P_1$ and
${^1}S_0 \rightarrow {^3}P_0$ transitions between all stable isotopes of
strontium relative to ${^{88}}$Sr. This includes the first reported
measurements of the ${^1}S_0 \rightarrow {^3}P_0$ isotope shift of
${^{88}}$Sr-${^{86}}$Sr and ${^{88}}$Sr-${^{84}}$Sr. Using the isotope shift
measurements of the two transitions, a King plot analysis is performed. These
results, combined with other recent isotope shift measurements in other atomic
systems, will help refine atomic structure calculations and theoretical
predictions for new physics.
