  Coexistence curves in mixtures of limited miscibility with the lower critical
consolute temperature (LCT), of 3-picoline with deuterium oxide (D$_2$O), and
D2O/H2O have been determined. They were tested with respect to the order
parameter, the diameter of the binodal, and coordinates of the critical
consolute point (critical temperature Tc and critical concentration fic)
Studies were carried out using the innovative method based on the analysis of
relative volumes occupied by coexisting phases, yielding high-resolution data.
The clear violation of the Cailletet-Mathew law of rectilinear diameter for the
LCT mixtures of limited miscibility is evidenced. For the order parameter, the
new distortion-sensitive analysis method yielded the evidence for the
model-value of the order parameter critical exponent beta = 0.326.up to ca. 1 K
from Tc. Finally, the simple & easy method for determining the critical
concentration by testing relative volumes of coexisting phases (or
alternatively fractional meniscus heights, h) is presented. The significance of
the invariant value hc(Tc, fic) = 1/2 is highlighted. The appearance of the
milky & bluish critical opalescence is also shown
