Page:The American Cyclopædia (1879) Volume XV.djvu/263

 SPECTRUM ANALYSIS 251 self at a distance of 12 ft. or more he viewed this slit through a prism of homogeneous glass held close to the eye. This method of obser- vation shows the spectrum crossed transverse- ly to its length hy dark lines and bands ; and hence the spectrum from a prism of given ma- terial and angle becomes a sort of scale or map, to a fixed position in which every gra- dation of hue and every dark band can be ex- actly referred. Among the observations upon the spectrum, partially anticipating Kirchhoff s principle, were those of Fraunhofer (1815), of Talbot (1826), of Brewster (1832), of Wheat- stone (1835), and of Foucault (1849). In 1855 In , blue violet FIG. 1. Table of Spectra according to Kirchhoff and Bunsen. Prof. A. J. Angstrom of Sweden, applying Euler's principle of the reciprocation and ab- sorption by bodies of the same sorts of undu- lations which they are capable of emitting when themselves originally excited, was led to the view that any body at a glowing heat emits the same rays (refrangibilities) of light and heat as, in the like condition, it will absorb if they im- pinge upon it. The first decisive and general proof in reference to light of the principle as- sumed by Euler and Angstrom was furnished by KirchhofFs experiments in 1859, with flames