There are several spectral features that can be used to characterise galax- ies, including the Hydrogen-o emission at 656.3 nm and the Hydrogen-B emission at 486.8 nm. For the Hydrogen-o emission, the spectrograph on the FET just gives sufficient resolution to resolve the emission line, which has a width of 100 km s . Calculate the ruling density necessary to observe the Hydrogen- B emission line at the same resolution, if the observations are taken in first order and in 1-arcsecond seeing