  We investigate the importance of the far red wing of the Lyman $ \alpha$ line
of hydrogen in the atmospheres of cool white dwarfs of pure hydrogen
composition. We find that this absorption process dominates all important
sources of opacity in the blue part of the optical spectrum of these stars. Our
successful fits to the spectra of cool DA/DC white dwarfs indicate that the far
red wing of the $\rm Ly \alpha$ line is the source of opacity that had been
missing in the models. The observed sequence of cool white dwarfs in
color-color diagrams is very well reproduced by our new pure hydrogen
atmosphere models, suggesting that the atmospheric composition of the coolest
DC white dwarfs must be revisited.
