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The absorption around the Balmer lines disappeared as the opacity of the gas reduced and consequently the emission lines became more visible.
Until that time it was known that a number of diffuse nebulae emitted an emission spectrum of strong Balmer lines, arising from n = 2 transitions of the hydrogen atom.
In Stromgren's 1939 paper, he attributed to Struve and Elvey the discovery of extended areas in the Milky Way in which the Balmer lines were observed.