The summits of these filaments of fire are commonly inclined, as if by a wind sweeping over them, when the sun's activity is near its height, but erect during his phase of tranquillity. Spörer, in 1871, inferred the influence of permanent 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 at a target at fixed intervals of time. If the marksman advances, say twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals. The result will of course be the same whether the target or the marksman be in movement.
So far Doppler was altogether right. As regards sound, anyone can convince himself that the effect he predicted is a real one, by listening to the alternate shrilling and sinking of the steam-whistle when an express train rushes through a station. But in applying this principle to the colours of stars he went widely astray; for he omitted from consideration the double range of invisible vibrations which partake of, and to the eye exactly compensate, changes of refrangibility in the visible rays. There is, then, no possibility of finding a criterion of velocity in the hue of bodies shining, like the sun and stars, with continuous light. The entire spectrum is slightly shifted up or down in the scale of refrangibility; certain rays normally visible become exalted or degraded (as the case may be) into invisibility, and certain other rays at the opposite end undergo the converse process; but the sum total of impressions on the retina continues the same.
We are not, however, without the means of measuring this sub-sensible transportation of the light-gamut. Once more the wonderful Fraunhofer lines came to the rescue. They were called by the earlier physicists "fixed lines;" but it is just because they are not fixed that, in this instance, we find them useful. They share, and in sharing betray, the general shift of the spectrum. This aspect of Doppler's principle was adverted to by Fizeau in 1848,[628] and the first tangible results in the estimation of movements of approach and recession between the earth and the stars, were communicated by Sir William Huggins to the Royal Society, April 23, 1868. Eighteen months later, Zöllner devised his "reversion-spectroscope"[629] for doubling the measurable effects of line-displacements; aided by which ingenious instrument, and following a suggestion of its inventor, Professor H. C. Vogel succeeded at Bothkamp, June 9, 1871,[630] in detecting effects of that nature due to the solar rotation. This application constitutes at once the test and the triumph of the method.[631]
The eastern 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°, 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 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. Dunér to test and extend Carrington's and Spörer'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° 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 Dunér'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, faculæ 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 faculæ, 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 surprising discovery of line-shiftings through end-on motion to the study of prominences, the discontinuous light of which affords precisely the same means of detecting movement without seeming change of place, as do lines of absorption in a continuous spectrum. Indeed, his observations at the sun's edge almost compelled recourse to an explanation made available just when the need of it began to be felt. He saw bright lines, not merely pushed aside from their normal places by a barely perceptible amount, but bent, torn, broken, as if by the stress of some tremendous violence. These remarkable appearances were quite simply interpreted as the effects of movements varying in amount and direction in the different parts of the extensive mass of incandescent vapours falling within a single field of view. Very commonly they are of a cyclonic character. The opposite distortions of the same coloured rays betray the fury of "counter-gales" rushing along at the rate of 120 miles a second; while their undisturbed sections prove the persistence of a "heart of peace" in the midst of that unimaginable fiery whirlwind. Velocities up to 250 miles a second, or 15,000 times that of an express train at the top of its speed, were thus observed by Young during his trip to Mount Sherman, August 2, 1872; and these were actually doubled in an extraordinary outburst observed by Father Jules Fényi, on June 17, 1891, at the Haynald Observatory in Hungary, as well as by M. Trouvelot at Meudon.[642]