Sodium D Line Splitting
The Sodium D line splitting lab is extremely similar to the H" Isotope Shift lab. In a sodium atom the outer electron "sees" a magnetic field due to the nucleus, and its energy level is higher or lower depending upon whether or not it is aligned or anti-aligned with the field. The practical result of this is that when you look a the spectral lines coming from sodium one spectral line is split into two, the sodium doublet. The first part of this lab is to use normal spectroscopy to resolve this feature and to determine how far apart the split pair of lines are apart and where they are.
The second part of this lab is to use a piece of equipment called the Fabry-Perot Interferometer to separate out the light coming from the Sodium source and to recombine it later in such a manner that you see either destructive (the light "subtracts" from itself) or constructive (The light "adds" back together) interference. If you look at the resulting light as it slowly moves from constructive to destructive interference you see something like this.
Using that image, believe it or not, you can pretty easily determine how far apart the two lines are and so you have a much more accurate check on your first method. Well, that's the lab.