permanently double on the sun's eastern, but single on his western
limb;[636] opposite motion-displacements bringing about this curious
effect of coincidence with, and separation from, an adjacent stationary
line of our own atmosphere's production, according as the spectrum is
derived from the retreating or advancing margin of the solar globe.
Statements of fact so precise and authoritative amount to a
demonstration that results of this kind are worthy of confidence; and
they already occupy an important place among astronomical data.

The subtle method of which they served to assure the validity was
employed in 1887-9 by M. Duner to test and extend Carrington's and
Spoerer's conclusions as to the anomalous nature of the sun's axial
movement.[637] His observations for the purpose, made with a fine
diffraction-spectroscope, just then mounted at the observatory of
Upsala, were published in 1891.[638] Their upshot was to confirm and
widen the law of retardation with increasing latitude derived from the
progressive motions of spots. Determinations made within 15 deg. of
the pole, consequently far beyond the region of spots, gave a
rotation-period of 38-1/2, that of the equatorial belt being of 25-1/2
days. Spots near the equator indeed complete their rounds in a period
shorter by at least half a day; and proportionate differences were found
to exist elsewhere in corresponding latitudes; but Duner's observations,
it must be remembered, apply to a distinct part of the complex solar
machine from the disturbed photospheric surface. It is amply possible
that the absorptive strata producing the Fraunhofer lines, significant,
by their varying displacements at either limb, of the inferred varying
rates of rotation, may gyrate more slowly than the spot-generating
level. Moreover, faculae appear to move at a quicker pace than
either;[639] so that we have, for three solar formations, three
different periods of average rotation, the shortest of which belongs to
the faculae, one of intermediate length to the spots, and the most
protracted to the reversing layer. All, however, agree in lengthening
progressively from the equator towards the poles. Professor Holden aptly
compared the sun to "a vast whirlpool where the velocities of rotation
depend not only on the situation of the rotating masses as to latitude,
but also as to depth beneath the exterior surface."[640]

Sir Norman Lockyer[641] promptly perceived the applicability of the
sur