  Optical frequency measurements of the intercombination line
$(6s^{2})\,^{1}S_{0} -(6s6p)\,^{3}P_{1}$ in the isotopes of ytterbium are
carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic
beam. A dispersive signal is generated to which a master laser is locked, while
frequency counting of an auxiliary beat signal is performed via a frequency
comb referenced to a hydrogen maser. The relative separations between the lines
are used to evaluate the $^{3}P_{1}$-level magnetic dipole and electric
quadrupole constants for the fermionic isotopes. The center of gravity for the
$^3P_1$ levels in $^{171}$Yb and $^{173}$Yb are also evaluated, where we find
significant disagreement with previously reported values. These hyperfine
constants provide a valuable litmus test for atomic many-body computations in
ytterbium.
