I have been learning a little about two-electron atoms, and there are some things that I do not fully comprehend. Some context:
In the two books I have been reading (Physics of atoms and molecules, by Bransden & Joachain, and Quantum Mechanics of one- and two-electron atoms, by Bethe and Salpeter), ortho and para eigenstates are mentioned when discussing the Pauli principle. Since the global wavefunction of the two-electron atom needs to be anti-symmetric due to the exclusion principle, eigenstates of the Hamiltonian that are symmetric (para states) can only have a spin wavefunction from the spin singlet (anti-symmetric). Anti-symmetric eigenstates (ortho states) can only haven an associated spin wavefunction from the spin triplet (symmetric). This makes perfect sense to me in an abstract way, but then two things are mentioned: ortho helium and para helium, two types of helium, each with its own energy level diagram. Parahelium is different because the lower energy state allowed is the 1s orbital, which has an associated orbital harmonic function which is symmetric. Each energy level (defined by two quantum numbers, $n$ and $\ell$) can be inhabited by a single electron. Orthohelium does not have this energy level at all, and each energy level can be inhabited (if I understood correctly) by three electrons, because $M = 2S + 1 = 2\times 1 +1 = 3$. This is what I gathered from reading the two relevant sections in the aforementioned books.
My question is: what are these different helium varieties? I would imagine, even if there are two possibilities (orthohelium with spin triplet electrons and parahelium with spin singlet electrons), real helium gas would be comprised of a mix of them. Therefore, measuring its spectrum would yield a mix of the two different spectra. How could early 20th century spectroscopists tell they where two superimposed spectra, instead of considering the mix of lines one single spectrum?
I think I'm failing to see the connection between theory and experiment.