  Spectroscopic observations of Halpha and Hbeta emission lines of 128
star-forming galaxies in the redshift range 0.75<z<1.5 are presented. These
data were taken with slitless spectroscopy using the G102 and G141 grisms of
the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of
the WFC3 Infrared Spectroscopic Parallel (WISP) survey. Interstellar dust
extinction is measured from stacked spectra that cover the Balmer decrement
(Halpha/Hbeta). We present dust extinction as a function of Halpha luminosity
(down to 3 x 10^{41} erg/s), galaxy stellar mass (reaching 4 x 10^{8} Msun),
and rest-frame Halpha equivalent width. The faintest galaxies are two times
fainter in Halpha luminosity than galaxies previously studied at z~1.5. An
evolution is observed where galaxies of the same Halpha luminosity have lower
extinction at higher redshifts, whereas no evolution is found within our error
bars with stellar mass. The lower Halpha luminosity galaxies in our sample are
found to be consistent with no dust extinction. We find an anti-correlation of
the [OIII]5007/Halpha flux ratio as a function of luminosity where galaxies
with L_{Halpha}<5 x 10^{41} erg/s are brighter in [OIII]5007 than Halpha. This
trend is evident even after extinction correction, suggesting that the
increased [OIII]5007/Halpha ratio in low luminosity galaxies is likely due to
lower metallicity and/or higher ionization parameters.
