The two dark lines near the center of this solar spectrum image are due to atomic sodium in the solar atmosphere. Known as D1 (5895.94 Å or 589.594 nm, the weaker one) and D2 (5889.973 Å or 588.9973 nm, the stronger one), this pair is the longest wavelength that unexcited sodium atoms can absorb. There are two lines here because the magnetic moment of the spinning valence electron interacts with the magnetic moment of its orbital motion around the nucleus, producing two possible energy states.
Similarly, excited sodium atoms may emit light of the same wavelength. A simple example of this is to detect sodium in the flame of a match. This time block the light off the mirror from the window by placing a 3x5 card over the mirror. Start a 100 second exposure, but while it is running strike one of the small wood matches (see the drawer under the spectrograph) and pass it back and forth about 2 inches in front of the entrance slit. Light from the flame should enter the slit during the exposure, but other light from the room and windows will be much fainter. When the image displays on ds9 you should see two strong emission (bright) lines where the absorption lines were in the solar spectrum. Since this is also a 100 second exposure the dark frame you took earlier may be used here too.