  We have developed a technique to measure the absolute frequencies of optical
transitions by using an evacuated Rb-stabilized ring-cavity resonator as a
transfer cavity. We study possible wavelength-dependent errors due to
dispersion at the cavity mirrors by measuring the frequency of the same
transition in the $D_2$ line of Cs at three cavity lengths. We find no
discernable change in values within our error of 30 kHz. Our values are
consistent with measurements using the frequency-comb technique and have
similar accuracy.
