X-Ray, X-Ray Spectra, and Spin
Some people may not know what X-ray is, although the term itself is quite familiar even in ordinary life. An X-ray is a kind of electromagnetic wave with very small wavelength (about 1 nanometer) . It is produced when a metal is bombarded by electrons (cathode ray) . And generally, its wavelength is determined by the amount of energy of those electrons (in the bombardment). However, scientists have noticed that some X-rays do not satisfy this general condition; their wavelength is determined by the kind of metal bombarded. Thus these X-rays, called characteristic X-rays (Tomonaga 1969, 166), played an important role in the research of the atomic structure. Bohr also referred to them in his first paper. When X-ray spectra are talked about, these characteristic X-rays are meant.
Researches based on X-ray spectra revealed difficulties of the Bohr model of atomic structure. An electron has a mass and an electric charge, all right. But eventually it turned out that it has also "spin"--an electron has its own revolution, and its own angular momentum--, and this was the source of the preceding difficulties. Spin is later extended to other elementary particles.
Bohr, N. (1934), Atomic Theory and the Description of Nature, Ox Bow Press (reprint)
Tomonaga, S. (1969) 量子力学 I（第２版）、みすず書房。
Tomonaga, S. (1997) 量子力学 II（第２版）、みすず書房。
See also a useful site on Microphysics, at Kyushu University: http://www2.kutl.kyushu-u.ac.jp/seminar/MicroWorld/MicroWorld.htm
