  A two-step, two-color laser spectroscopy technique has been used to measure
the hyperfine splitting of the 7p$_{1/2}$ excited state in $^{203}$Tl and
$^{205}$Tl, as well as the isotope shift within the 7s$_{1/2}$ - 7p$_{1/2}$
transition. Our measured values for the hyperfine splittings, 2153.2(7) MHz (in
$^{203}$Tl) and 2173.3(8) MHz (in $^{205}$Tl), each differ by 20 MHz from
previously published values which quoted comparable precision. The transition
isotope shift of $^{203}$Tl relative to $^{205}$Tl was measured to be 534.4(9)
MHz. In our experiment, one laser was locked to the thallium ground-state
6p$_{1/2}$ - 7s$_{1/2}$ 378 nm transition, while the second, spatially
overlapping laser was scanned across the 7s$_{1/2}$(F=1) - 7p$_{1/2}$(F=0,1)
hyperfine transitions. To facilitate accurate frequency calibration,
radio-frequency modulation of the laser was used to create sidebands in the
absorption spectrum.
