We describe the design, construction, and performance of a laser system to probe the ultra-narrow (Γ/2π ≈ mHz) clock transition 1S0 → 3P0 in strontium. We present the first reported spectroscopy of this transition in two of the bosonic isotopes, 84Sr and 86Sr. Furthermore, we measure the complete set of isotope shifts between all four stable isotopes on the clock line and the narrow intercombination line 1S0 → 3P1, permitting a King plot analysis of the isotope shifts. Complications arising from the unambiguous determination of a line center in 87Sr 3P1 prevent us from making claims about the King linearity, but we provide a statistical boot- strap analysis of the isotope shifts 88−84Sr and 88−86Sr to compute a field shift ratio F698/F689 = 0.9979, with a 95% confidence interval [0.9952,1.0008]. The intercept term K698 − (F698/F689) K689 is similarly determined to be -2.0 GHz-amu, with a 95% confidence interval [−3.9, −0.3] GHz-amu. Finally, we describe the design of a next-generation apparatus that will enable improvements on the results described

here, as well as other studies that involve coherent manipulation of strontium atoms on the clock line.