text: In 1835, Charles Wheatstone reported that different metals could be easily distinguished by the different bright lines in the emission spectra of their sparks, thereby introducing an alternative mechanism to flame spectroscopy. In 1849, J. B. L. Foucault experimentally demonstrated that absorption and emission lines appearing at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source. In 1853, the Swedish physicist Anders Jonas Ångström presented observations and theories about gas spectra in his work Optiska Undersökningar (Optical investigations) to the  [[Royal Swedish Academy of Sciences. Ångström postulated that an incandescent gas emits luminous rays of the same wavelength as those it can absorb. Ångström was unaware of Foucalt's experimental results. At the same time George Stokes and William Thomson (Kelvin) were discussing similar postulates. Ångström also measured the emission spectrum from hydrogen later labeled the Balmer lines. In 1854 and 1855, David Alter published observations on the spectra of metals and gases, including an independent observation of the Balmer lines of hydrogen.
  The systematic attribution of spectra to chemical elements began in the 1860s with the work of German physicists Robert Bunsen and Gustav Kirchhoff, who found that Fraunhofer lines correspond to emission spectral lines observed in laboratory light sources. This laid way for spectrochemical analysis in laboratory and astrophysical science. Bunsen and Kirchhoff applied the optical techniques of Fraunhofer, Bunsens improved flame source and a highly systematic experimental procedure to a detailed examination of the spectra of chemical compounds. They established the linkage between chemical elements and their unique spectral patterns. In the process, they established the technique of analytical spectroscopy. In 1860, they published their findings on the spectra of eight elements and identified these elements presence in several natural compounds. They demonstrated that spectroscopy could be used for trace chemical analysis and several of the chemical elements they discovered were previously unknown. Kirchhoff and Bunsen also definitively established the link between absorption and emission lines, including attributing solar absorption lines to particular elements based on their corresponding spectra. Kirchhoff went on to contribute fundamental research on the nature of spectral absorption and emission, including what is now known as Kirchhoffs law of thermal radiation. Kirchhoffs applications of this law to spectroscopy are captured in three laws of spectroscopy:
  #An incandescent solid, liquid or gas under high pressure emits a continuous spectrum.
  #A hot gas under low pressure emits a "bright-line" or emission-line spectrum.
  #A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.
  In the 1860s the husband-and-wife team of William and Margaret Huggins used spectroscopy to determine that the stars were composed of the same elements as found on earth. They also used the non-relativistic Doppler shift (redshift) equation on the spectrum of the star Sirius in 1868 to determine its axial speed. They were the first to take a spectrum of a planetary nebula when the Cat's Eye Nebula (NGC 6543) was analyzed. Using spectral techniques, they were able to distinguish nebulae from stars.
  August Beer observed a relationship between light absorption and concentration and created the color comparator which was later replaced by a more accurate device called the spectrophotometer.