  We measure the effective optical depth in the Lyman-alpha (Ly$\alpha$) forest
using 40,035 quasar spectra from the Twelfth Data Release (DR12) of the Baryon
Oscillation Spectroscopic Survey (BOSS) of SDSS-IV. A rigorous selection based
on spectral index and the equivalent width of the C IV emission line is applied
to choose seven uniform samples with minimal intrinsic variations across
redshifts. Modeling the redshift evolution of the effective optical depth with
a power-law, $\tau_\mathrm{eff}=\tau_0 (1 + z)^\gamma$ produces $\tau_0=(5.54
\pm 0.64)\times 10^{-3}$ and $\gamma=3.182 \pm 0.074$. The 2.2% precision
estimate on $\gamma$ is dominated by systematic errors, likely arising from the
bias and uncertainties in spectral index estimates. Even after incorporating
the systematic errors, this work provides the most precise estimates of optical
depth parameters to date. Finally, using the reconstructed Ly$\alpha$ forest
continuum to directly measure the transmitted flux ratio as a function of
redshift, we find deviations of less than 2.5% from the predictions from the
global model and no convincing evidence for signal associated with He II
reionization.
