  We present measurements of the hyperfine coefficients and isotope shifts of
the Dy I $683.731 $nm transition, using saturated absorption spectroscopy on an
atomic beam. A King Plot is drawn resulting in an updated value for the
specific mass shift $\delta \nu_\mathrm{684,sms}^\mathrm{164-162}=-534 \pm 17
MHz$. Using fluorescence spectroscopy we measure the excited state lifetime
$\tau_{684}=1.68(5) \mu$s, yielding a linewidth of $\gamma_\mathrm{684} = 95
\pm 3 kHz$. We give an upper limit to the branching ratio between the two decay
channels from the excited state showing that this transition is useable for
optical pumping into a dark state and demagnetization cooling.
