Mapping low-density intergalactic gas: A third helium Ly alpha forest

We present a new HST/STIS spectrum of the z = 3.18 quasar PKS 1935-692 and summarize the spectral features shortward of 304 Angstrom in the rest frame likely to be caused by foreground He II Ly alpha absorption. In accord wit h the previous results on two other quasars at similar redshifts, we demons trate a correlation with the H I Ly alpha forest absorption and show that m uch of the helium absorption is caused by a comparable quantity of more dif fuse gas with Omega(g) - 10(-2), which is not detected in H I. The helium i onization zone around the quasar is detected, and a void is seen in both H I and He II. The properties of the absorption are in broad agreement with t hose of the other quasars and with models of the protogalactic gas distribu tion and ionization at this redshift.