Complex sentence: At first it was hypothesized that the line might be due to an unknown element, which was named "nebulium"--a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.
Split into simpler sentences: At first it was hypothesized that the line might be due to an unknown element, which was named "nebulium". A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.