A sequence of absorption or emission lines in the visible part of the spectrum, due to hydrogen; also known as Balmer lines. In order of decreasing wavelength they are Ηα‎ (656.3 nm, red), Ηβ‎ (486.1 nm, blue-green), Ηγ‎ (434.0 nm, blue), and so on, becoming closer together as they approach 364.6 nm (in the ultraviolet), the Balmer limit. Balmer absorption lines are caused by jumps of electrons from the second energy level to higher levels, and emission lines when the electrons drop back to the second energy level. They are named after the Swiss mathematician Johann Jakob Balmer (1825–98). See also hydrogen spectrum.