An Etymological Dictionary of Astronomy and Astrophysics

The → neutral → helium  → spectral line at 5876 Å.

See also: D3, because of confusion with the sodium → D lines.
When Joseph N. Lockyer first observed this line in the solar spectrum at the eclipse of 1868, helium was not yet isolated on Earth. Initially,
this line was thought to be the third member of the D1 and D2 line family
of sodium which lie in the same yellow part of the spectrum; → line.