Balmer, Johann Jakob (1825-1898), Swiss mathematician and physicist, born in Lausanne, Switzerland. In 1885 Balmer discovered a simple mathematical formula that generated the wavelength values for a certain series of spectral lines of the element hydrogen. This series of spectral lines is now called the Balmer series. The reason that Balmer's formula generated the correct wavelength values was not understood until the development of quantum theory in the early 1900s.

, in physics and physical chemistry, the study of spectra. The basis of spectroscopy is that each chemical element has its own characteristic spectrum. The German scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen recognized this fact in 1859. They developed the prism spectroscope in its modern form and applied it to chemical analysis. One of two principal spectroscope types, this instrument consists of a slit for admitting light from an external source, a group of lenses, a prism, and an eyepiece. Light that is to be analysed passes through a collimating lens, which makes the light rays parallel, and the prism; then the image of the slit is focused at the eyepiece. One actually sees a series of images of the slit, each a different colour, because the light has been separated into its component colours by the prism. The German scientists were the first to recognize that characteristic colours of light, or the spectra, are emitted and absorbed by particular elements.

Spectrograph
In a spectrograph, a camera replaces the eyepiece. Colour photography is not necessary to identify the images of the slit, known as the spectrum lines; their wavelengths can be calculated from their positions on the film. Spectrographs are useful throughout the ultraviolet and visible regions of the spectrum, and as far as 1200 mm (0.000048 in) in the infrared region. Spectroscopy in the extreme ultraviolet and infrared regions is similar to that in the visible region, except that glass does not transmit such radiations; lenses and prisms are made of quartz, fluorite, sylvine, or rock salt. Concave mirrors can also be substituted for lenses. Special photographic emulsions are used.