  Singly-ionized ytterbium, with ultra-narrow optical clock transitions at 467
nm and 436 nm, is a convenient system for the realization of optical atomic
clocks and tests of present-day variation of fundamental constants. We present
the first direct measurement of the frequency ratio of these two clock
transitions, without reference to a cesium primary standard, and using the same
single ion of 171Yb+. The absolute frequencies of both transitions are also
presented, each with a relative standard uncertainty of $6\times 10^{-16}$.
Combining our results with those from other experiments, we report a three-fold
improvement in the constraint on the time-variation of the proton-to-electron
mass ratio, $\dot{\mu}/{\mu} = 0.2(1.1)\times 10^{-16}$ year$^{-1}$, along with
an improved constraint on time-variation of the fine structure constant,
$\dot{\alpha}/{\alpha} = -0.7(2.1)\times 10^{-17}$ year$^{-1}$.
