edge of the sun is continually moving towards us with an
equatorial speed of about a mile and a quarter per second, the western
edge retreating at the same rate. The displacements--towards the violet
on the east, towards the red on the west--corresponding to this velocity
are very small; so small that it seems hardly credible that they should
have been laid bare to perception. They amount to but 1/150th part of
the interval between the two constituents of the D-line of sodium; and
the D-line of sodium itself can be separated into a pair only by a
powerful spectroscope. Nevertheless, Professor Young[632] was able to
show quite satisfactorily, in 1876, not only deviations in the solar
lines from their proper places indicating a velocity of rotation (1.42
miles per second) slightly in excess of that given by observations of
spots, but the exemption of terrestrial lines (those produced by
absorption in the earth's atmosphere) from the general push upwards or
downwards. Shortly afterwards, Professor Langley, then director of the
Allegheny Observatory, having devised a means of comparing with great
accuracy light from different portions of the sun's disc, found that
while the obscure rays in two juxtaposed spectra derived from the solar
poles were absolutely continuous, no sooner was the instrument rotated
through 90 deg., so as to bring its luminous supplies from opposite
extremities of the equator, than the same rays became perceptibly
"notched." The telluric lines, meanwhile, remained unaffected, so as to
be "virtually mapped" by the process.[633] This rapid and unfailing mode
of distinction was used by Cornu with perfect ease during his
investigation of atmospheric absorption near Loiret in August and
September, 1883.[634]

A beautiful experiment of the same kind was performed by M. Thollon, of
M. Bischoffsheim's observatory at Nice, in the summer of 1880.[635] He
confined his attention to one delicately defined group of four lines in
the orange, of which the inner pair are solar (iron) and the outer
terrestrial. At the centre of the sun the intervals separating them were
sensibly equal; but when the light was taken alternately from the right
and left limbs, a relative shift in alternate directions of the solar,
towards and from the stationary telluric rays became apparent. A
parallel observation was made at Dunecht, December 14, 1883, when it was
noticed that a strong iron-line in the yellow part of the solar spectrum
is