  Since polycrystals of alkali halides are highly useful as components in
optical devices, a number of mixed crystals of NaBr and KBr have been prepared
from melt by other workers. Among these crystals, it was reported that the
polycrystal Na$_{0.4}$K$_{0.6}$Br exhibits the strongest temperature variation
of the dielectric constant. Here, we show quantitatively that this is due to
the temperature variation of the ionic polarizability.
