These ideas led to the development of the quantum theory.

Spectrum Lines
When a substance is vaporized, and the vapour is heated until it emits light, a single colour may predominate, as in the yellow colour of sodium-vapour lamps, the red colour of neon lamps, and the blue-green colour of mercury-vapour lamps. The spectrum in such cases consists of several lines of specific wavelength, separated by regions of absolute darkness. In the case of sodium vapour, two lines of approximate wavelength 589.0 and 589.6 mm produce the yellow colour. The difference in colour between these two lines is not detectable by the human eye, but the lines may be readily resolved, or separated and distinguished, by a good spectroscope. These two lines are called D2 and D1. Their wavelengths may be more accurately measured; thus, the D2 line has a wavelength of 588.9977 mm. Even more accurate measurements have been made of the wavelengths of certain lines in the spectrum of isotopically pure mercury. A spectrograph of great resolving power produces a spectrum in which the lines occupy only a very small percentage of the area, the overwhelming majority of the spectrum often being completely blank.
Although most of the energy of the spectrum of sodium vapour is concentrated in the two D lines, numerous other faint lines are present in the spectrum. At the higher temperatures such as those of the electric arc, or at the higher temperature and ionising conditions of the electric spark, an enormous number of other lines are present in the spectrum of sodium. Historically, the first spectrum to be satisfactorily explained was that of the hydrogen atom, which is the simplest atom and which produces the simplest spectrum. In the early 1880s, the Swiss mathematician and physicist Johann Jakob Balmer discovered four lines, of wavelengths 656.3, 486.1, 434.0, and 410.2 mµ, in the visible spectrum of the hydrogen atom. These lines are designated Ha, Hb, Hg, and Hd, respectively. Balmer also showed that these four wavelengths form a series, now called the Balmer series, and can all be expressed by means of one single simple formula:
In which N has the value 3, 4, 5, or 6. Shortly thereafter the British astronomer Sir William Huggins discovered in the ultraviolet region a number of other spectrum lines produced by hydrogen that have wavelengths determined by the same formula, except that N has successively higher values. At very large values of N, the lines are closer, merging at the limiting value 364.6 mµ.

Work of Niels Bohr
In 1913 the Danish physicist Niels Bohr rejected the concept of the emission of radiation by electrically charged particles moving in orbits inside an atom, as developed from the electromagnetic theory of the British physicist James Clerk Maxwell.