ent polar currents,[620] but Tacchini showed in 1876
that the deflections upon which this inference was based ceased to be
visible as the spot-minimum drew near.[621]

Another peculiarity of the chromosphere, denoting the remoteness of its
character from that of a true atmosphere,[622] is the irregularity of
its distribution over the sun's surface. There are no signs of its
bulging out at the equator, as the laws of fluid equilibrium in a
rotating mass would require; but there are some that the fluctuations in
its depth are connected with the phases of solar agitation. At times of
minimum it seems to accumulate and concentrate its activity at the
poles; while maxima probably bring a more equable general distribution,
with local depressions at the base of great prominences and above spots.

A low-lying stratum of carbon-vapour was, in 1897, detected in the
chromosphere by Professor Hale with a grating-spectroscope attached to
the 40-inch Yerkes refractor.[623] The eclipse-photographs of 1893
disclosed to Hartley's examination the presence there of gallium;[624]
and those taken by Evershed in 1898 were found by Jewell[625] to be
crowded with ultra-violet lines of the equally rare metal scandium. The
general rule had been laid down by Sir Norman Lockyer that the metallic
radiations from the chromosphere are those "enhanced" in the electric
spark.[626] Hence, the comparative study of conditions prevalent in the
arc and the spark has acquired great importance in solar physics.

The reality of the appearance of violent disturbance presented by the
"flaming" kind of prominence can be tested in a very remarkable manner.
Christian Doppler,[627] professor of mathematics at Prague, enounced in
1842 the theorm that the colour of a luminous body, like the pitch of a
sonorous body, must be changed by movements of approach or recession.
The reason is this. Both colour and pitch are physiological effects,
depending, not upon absolute wave-length, but upon the number of waves
entering the eye or ear in a given interval of time. And this number, it
is easy to see, must be increased if the source of light or sound is
diminishing its distance, and diminished if it is decreasing it. In the
one case, the vibrating body _pursues_ and crowds together the waves
emanating from it; in the other, it _retreats_ from them, and so
lengthens out the space covered by an identical number. The principle
may be thus illustrated. Suppose shots to be fired