  Exoplanetary transmission spectroscopy in the near-infrared using
Hubble/NICMOS is currently ambiguous because different observational groups
claim different results from the same data, depending on their analysis
methodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to
resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets
HD209458b and XO-1b in transit, using spatial scanning mode for maximum
photon-collecting efficiency. We introduce an analysis technique that derives
the exoplanetary transmission spectrum without the necessity of explicitly
decorrelating instrumental effects, and achieves nearly photon-limited
precision even at the high flux levels collected in spatial scan mode. Our
errors are within 6-percent (XO-1) and 26-percent (HD209458b) of the
photon-limit at a spectral resolving power of 70, and are better than
0.01-percent per spectral channel. Both planets exhibit water absorption of
approximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b
contradicts the much larger absorption derived from NICMOS spectroscopy. The
weak water absorption we measure for HD209458b is reminiscent of the weakness
of sodium absorption in the first transmission spectroscopy of an exoplanet
atmosphere by Charbonneau et al. (2002). Model atmospheres having
uniformly-distributed extra opacity of 0.012 cm^2 per gram account
approximately for both our water measurement and the sodium absorption in this
planet. Our results for HD209458b support the picture advocated by Pont et al.
(2013) in which weak molecular absorptions are superposed on a transmission
spectrum that is dominated by continuous opacity due to haze and/or dust.
However, the extra opacity needed for HD209458b is grayer than for HD189733b,
with a weaker Rayleigh component.
