Tuning the Atoms 247
the spectra induced by electrical sparks in tubes containing various elemental gases, begin-
ning with hydrogen in 1853, he observed discrete bright lines ( figure 16.2 ). To understand
the relation between the absorption and diffusion of these dark and bright lines, Å ngstr ö m
relied on Euler ’ s theory of light and color, itself (as we have seen) constructed in analogy
to the wave theory of sound. Å ngstr ö m began with “ the fundamental principle of Euler ”
that “ the color of a body is produced by the resonance of the oscillations, which can be
assumed by the particles themselves, ” so that “ the same body, when heated so as to become
luminous, must emit the precise rays which, at its ordinary temperature, is absorbed. ”^2
Using this principle of resonance, Å ngstr ö m was able to correlate the dark lines in the
solar spectrum, considered as formed by absorption, with bright lines he observed in
the emission spectra of individual elements. Thus, the solar dark lines are evidence for
the presence of the corresponding elements in the sun (as emerged in subsequentABFigure 16.2
(a) Å ngstr ö m ’ s 1855 comparison of the spectrum of an electric spark passing through air with spectra of various
elements and compounds. (b) Å ngstr ö m ’ s 1868 atlas of spectral lines from the sun (above), identifying them
with various elements (below), in the range 5,400 – 5,700 Å.