The spectroheliograph in use at the Yerkes Observatory brought
proof of the distribution of elements such as calcium, at
various densities, in layers of the Sun.s atmosphere. Hale named
the billowing clouds of calcium and hydrogen vapor "flocculi."
Hale made his greatest discovery while working at the Mt. Wilson
Observatory. In laboratory experiments he matched the spectral
lines emitted by sunspots, the dark areas of the Sun where the
temperature drops hundreds of degrees below that of the
surrounding photosphere. By applying the Zeeman effect of
magnetic displacement to the spectral data, Hale proved the
shifting magnetic field present in sunspots. Further observation
and experimentation led Hale and his group to discover the Sun's
entire magnetic field and its polarity reversals with the
sunspot cycle. Hale's instrumentation plus the observatories he
designed and built are landmarks in the history of astrophysics
and cosmology.