H \alpha and H \beta spectroscopy with the Wisconsin H-Alpha Mapper ( WHAM ) reveals a strong concentration of high velocity emission in a \approx 5 { { } ^ { \circ } } \times 5 { { } ^ { \circ } } area centered near ( \ell,b ) = ( 27 { { } ^ { \circ } } , -3 { { } ^ { \circ } } ) , known as the Scutum Cloud .
The high velocities imply that we are detecting optical emission from near the plane of the Galaxy out to the tangent point at heliocentric distances of D _ { \odot } \gtrsim 6 kpc , assuming the gas participates in circular Galactic rotation .
The ratio of the H \alpha to H \beta emission as a function of velocity suggests that dust along these lines of sight produces a total visual extinction of A _ { V } \approx 3 at D _ { \odot } \sim 6 kpc .
This makes it possible to use optical emission lines to explore the physical conditions of ionized gas in the inner Galaxy .
At a Galactocentric distance R _ { G } \approx 4 kpc , for example , we find that the H ^ { + } has an rms midplane density of \approx 1 cm ^ { -3 } with a vertical scale height of \approx 300 pc .
We also find evidence for an increase in the flux of Lyman continuum photons and an increase in the ratio of ionized to neutral hydrogen toward the inner Galaxy .
We have extended the measurements of E ( B - V ) in this direction to distances far beyond what has been accessible through stellar photometry and find E ( B - V ) / N _ { H } to be near the local mean of 1.7 \times 10 ^ { -22 } cm ^ { 2 } mag , with evidence for an increase in this ratio at R _ { G } \approx 4 kpc .
Finally , our observations of [ N ii ] ~ { } \lambda 6583 , [ S ii ] ~ { } \lambda 6716 , and [ O iii ] ~ { } \lambda 5007 toward the window reveal that in the inner Galaxy the temperature of the gas and the ionization state of oxygen increase with increasing height from the midplane .