  A measurement of the hyperfine coefficients and isotope shifts for the Dy I
421.291 nm transition [$4f^{10}6s^2 (J=8)\to4f^{10}6s6p (J=9)$] using atomic
beam laser-induced fluorescence spectroscopy is presented. A King Plot analysis
is performed to determine a specific mass shift of $\delta
\nu_{sms}^{164-162}=11(7)$ Hz for the 421-nm transition, confirming the pure
$4f^{10}6s6p$ configuration of the excited state. This transition is currently
being explored for laser cooling of an atomic beam of dysprosium used in a
search for a temporal variation of the fine-structure constant.
