the constitution of matter and the transmutation of the elements.
Radium and other radioactive substances, such as uranium, spontaneously
emit negatively charged particles of extremely small mass (electrons),
and also positively charged particles of much greater mass, known
as alpha particles. Rutherford and Geiger actually succeeded in
counting the number of alpha particles emitted per second by a
known mass of radium, and showed that these were charged helium
atoms.

To discuss more at length the extraordinary characteristics of
helium, which plays so large a part in celestial affairs, would
take us too far afield. Let us therefore pass to another case in
which a fundamental discovery, this time in physics, was first
foreshadowed by astronomical observation.

SUN-SPOTS AS MAGNETS

No archaeologist, whether Young or Champollion deciphering the Rosetta
Stone, or Rawlinson copying the cuneiform inscription on the cliff
of Behistun, was ever faced by a more fascinating problem than that
which confronts the solar physicist engaged in the interpretation
of the hieroglyphic lines of sun-spot spectra. The colossal whirling
storms that constitute sun-spots, so vast that the earth would make
but a moment's scant mouthful for them, differ materially from
the general light of the sun when examined with the spectroscope.
Observing them visually many years ago, the late Professor Young,
of Princeton, found among their complex features a number of double
lines which he naturally attributed, in harmony with the physical
knowledge of the time, to the effect of "reversal" by superposed
layers of vapors of different density and temperature. What he
actually saw, however, as was proved at the Mount Wilson Observatory
in 1908, was the effect of a powerful magnetic field on radiation,
now known as the Zeeman effect.

[Illustration: Fig. 31. The 150-foot tower telescope of the Mount
Wilson Observatory.

An image of the sun about 16 inches in diameter is formed in the
laboratory at the base of the tower. Below this, in a well extending
80 feet into the earth, is the powerful spectroscope with which
the magnetic fields in sun-spots and the general magnetic field
of the sun are studied.]

Faraday was the first to detect the influence of magnetism on light.
Between the poles of a large electromagnet, powerful for those
days (1845), he placed a block of very dense glass. The plane of
polarization of a beam of light, which passed unaffec