Chapter III: Peculiarities of the Solar Spectrum

The solar spectrum is densely thronged with unidentified lines. Of these upwards of twelve thousand have been measured and registered, but lack chemical interpretation. They are, however, on the way to receive it. Their recognition will doubtless attend the gradual progress of acquaintance with metallic spectra. Thus cerium, scandium, and other bases of “rare earths” may satisfactorily account for a considerable proportion of them, these substances emitting crowds of rays, as yet only in part recorded. But besides, say, twelve thousand catalogued “unknown” lines, an inestimable number remain unnoticed.[39] They are still, as it were, “in the street”; they have not been admitted even to the antechamber of science; the preliminary steps to their identification have not been taken. They will of course be taken in due time, little by little, as the photography of the Fraunhofer spectrum is brought nearer to perfection; and to many of them chemical meanings full of interest will certainly be assigned. Nevertheless, it can scarcely be expected that the significance of all can ever be made plain. To the very end, probably, a residuum will keep the secret of an origin due to forms or conditions of matter strange to terrestrial experience.

That the _type_ of the sun’s spectrum becomes modified in the course of ages—that it has been, and will again be different from what it now is—may be admitted without hesitation. But this evolutionary change is effected imperceptibly at more than millennial leisure. It might, however, have been expected that transient alterations would manifest themselves—alterations caused by tumultuous movements in the “reversing layer,” or connected, possibly, with periodical outbreaks of spots and prominences. Yet almost none of this definite and obvious character have been noticed. Only a few lines may be set down as somewhat vaguely and indeterminately variable. To take a few examples. In 1891 Father Sidgreaves of Stonyhurst obtained several photographs of the group b (magnesium and iron) in the green part of the spectrum. They showed with excellent definition more faint lines than are contained in Rowland’s or Thollon’s maps; yet one relatively strong in them—“Winlock’s No. 17”—was barely discernible.[40] This indication of change does not seem to have been followed up. Again, Sir Norman Lockyer noted in 1873 the disappearance of a zinc line in the red (Ångström λ 6361·16), which, nevertheless, was seen as usual in 1878, and has not since been missed.[41] The most recent instance of the kind was vouched for from Baltimore. To a faint, slightly nebulous line of unknown origin in the ultra-violet (λ 3719·796) Rowland attached the note, “Variable, though not atmospheric.” Jewell[42] describes it as situated within the shading of a strong iron line, and as “quite distinct upon some plates, while not visible upon others showing lines closer to the iron line, and much weaker than the variable line,” as it originally appeared. It has also been photographed in an intermediate condition, so that its fluctuations of intensity may be said to be ascertained, although their law and cause remain wholly obscure. The only hope of learning anything about these is by continuous and minute observation, which should extend to other suspicious cases of the same kind. Certain interesting questions might thus be answered. For instance, are the alleged alterations connected effects of some general disturbance, or do they occur sporadically, each on its own account? Can they, in any way, be brought into relation with the spot cycle? Are they visible in light taken indiscriminately from all parts of the sun, or are they confined to special localities? These may serve as specimens of the inquiries suggested by phenomena, perhaps none the less significant for being inconspicuous. With the camera at hand the task of daily comparison becomes easy and simple. As Professor Hale wrote in 1896,[43] “Every photograph of the solar spectrum taken with high dispersion must now be regarded as a document of great value, which may ultimately reveal irregular or periodic changes in the condition of the gases and vapours of the solar atmosphere.”

The Fraunhofer lines have of late forfeited their early reputation as “constants of nature.” They are not really “fixed”; their positions in the spectrum are affected by several minutely modifying causes, and they cannot, accordingly, be depended upon as standards for the most refined measurements. It is true that only the extreme accuracy of modern methods has caused them to “step down” from the high level of invariability, for their deviations are very small, and might, superficially regarded, appear negligible. They are of two kinds, physical and kinematical, the former being produced in the very act of emission, the latter in the course of transmission. Pressure-displacements and motion-displacements are, in fact, respectively concerned.

Symptoms of a persistent shift of the Fraunhofer lines towards the red were first detected in 1890 by Professor Lewis E. Jewell of the Johns Hopkins University. Persistent, although unequal. It is not the same for the lines of different elements; it is not even the same for all the lines of the same element. Motion, then, is not its cause. Fortunately, a clue was supplied by laboratory-experiments. Attentive study of the behaviour of metallic lines under varying conditions showed that “with an increase in the amount of material in the arc there was increasing displacement towards the red.” “Considering the subject carefully,” Professor Jewell adds, “there seemed no reason to doubt that the wave-length of a line depended, to a certain extent, upon the conditions under which the material producing the line was present in the electric arc, the vacuum tube, or the solar atmosphere; or, in other words, the vibration period of an atom depends to some extent upon its environment. An increase of the density of the material, and presumably an increase of pressure, seemed to produce a damping effect upon the vibration period.”[44] Confirmatory results were obtained by Messrs. Humphreys and Mohler,[45] and the assumption of a constant vibration-frequency as an essential attribute of the ultimate particles of matter had to be finally abandoned.

The observed changes are clearly distinguishable from ordinary temperature effects. Lines are often broadened; under peculiar circumstances they may be unsymmetrically broadened by thermal influences; but simple displacements are never due to heat. Moreover, they can be produced artificially by condensing the air about an electric arc, so that their immediate cause is not doubtful. “It was often easy,” according to the Baltimore investigators, “to observe a line gradually change its position while the pressure was being let off without alteration in width or other appearance.”

The general upshot of their inquiries[46] was to show that the spectral shifts in question, far from being an isolated phenomenon, stand in close relationship to all the most intimate properties of matter. Their amount, _cœteris paribus_, is proportional to the pressure and to the wave-lengths of the shifted lines. It differs, however, for each series in a given spectrum. For different substances it is usually large or small in the inverse ratio of the absolute temperatures of their melting-points. Again, it is largest for those substances which expand most readily with heat. Finally, and most significantly, line displacements are, in the same group of elements, proportional to the cube roots of their atomic weights. Or, as Mr. Humphreys expresses it, “The shift of similar lines is a periodic function of atomic weight, and consequently may be compared with any other property of the elements which itself is a periodic function of their atomic weights”—that is to say, the measured displacements show recurring maxima and minima in passing from one to the next of Mendeléef’s elemental families. Their gradations thus correspond with those of other physical attributes of material species, and plainly imply that the retarded vibrations are executed by “ultimate” atoms. The confirmatory fact should be noted that band-spectra, universally associated with aggregates of atoms, display no sensitiveness to pressure. And by pressure in this connection is to be understood, not the separate density of the vapour emitting the damped rays, but the total pressure of all the substances promiscuously diffused throughout the stratum or enclosure.

Its complex effects add, in some respects, to the difficulty of interpreting spectral appearances; but they lend to them, on the other hand, new and unlooked-for significance. In solar inquiries more particularly, they have started a fresh lead, sure to be followed up. Thus indications may be gathered from them as to the relative altitudes in the sun’s atmosphere at which different Fraunhofer lines originate, no less than as to the absolute pressures to which they correspond. These are lower than might have been anticipated. They range, according to Professor Jewell, “from little more than zero to only two or three atmospheres, though the shading of the stronger lines may be produced at a greater pressure.”[47] The subject, however, has not got beyond the stage of inception. One important branch of it, the discrimination of lines belonging to the same series, is barely sketched. The phenomena of displacement through pressure evidently involve much more than is yet apparent. They must be present in stars and nebulæ, and may afford curious disclosures regarding their states of density and rarity.

The Fraunhofer lines are, as a rule, narrow and sharp; but minute photographic study reveals, in a certain proportion of them, singular complexities of structure. These are illustrated from Professor Jewell’s observations in Fig. 4, which shows graphically, by four typical examples, the comparative distribution of light in corresponding solar and arc lines.

FIG. 4.—Curves representing the Distribution of Light in Fraunhofer
and Arc Lines (_Astroph. Journ._ vol. iii. p. 100).
]

No. i., a green ray of iron, is bordered in the sun (where it is of course reversed) by a filmy illumination, “the remains of an emission line, either produced at the photosphere, or lower down in the solar atmosphere than the absorption line.”[48] The notch at the summit of the same line gives evidence of radiation at a high level. It is, in fact, an _abortive_ bright iron line, superposed upon a strong absorption line, itself superposed upon a faint effusion of light of identical quality from underlying vapour. Thus this single line is built in three stages, although the foundation and coping are barely discernible as traces of luminosity. In Nos. ii. and iii., “shaded lines” are depicted in the same manner as the “sharp line” in No. i. They belong respectively to iron and magnesium (λ 5183·8 = _b__{1}). Their characteristic feature is the outlying obscurity, which deepens from the edges towards the central shaft. Professor Jewell remarks that the gas producing these shadings “extends through a much greater range of pressure” than that giving rise to the green iron line (No. i.), while the clean-cut line in the middle must be due to absorption “much higher up in the solar atmosphere, where the pressure is very much less.” Similar appearances are conspicuous in the great calcium pair H and K (see Fig. 4, No. iv.). Here the abnormal breadth of their wing-like appendages proves that the “absorption must persist through an extreme range of pressure, or that the amount of calcium gas varies enormously in the solar atmosphere where this absorption is produced.” These lines are obviously twice reversed. A stratum of radiative calcium is apparently interposed, in the sun’s neighbourhood, between two absorptive strata of the same material. That their arrangement is, however, subject to some kind of disturbance is indicated by the irregularity of the diagram; nor is it always disturbed to the same extent. “Upon some plates,” Professor Jewell says, “the central absorption line is almost symmetrical with respect to the emission line, while upon other plates its unsymmetrical character is very marked, the central line being displaced considerably towards the red, and the part of the emission line on the violet side of the central line being much the strongest.” Motion-displacements due to ascending and descending currents are thought to be in question,[49] but there are obstacles to be removed before this explanation can be unreservedly accepted. The extreme difference of velocity suggested by the observed dissymmetry of the calcium lines amounts to no more than 75 miles a minute, but is notably variable. All the “shaded lines” in the spectrum appear to be similarly affected, though in a minor degree. Thus the descending motion corresponding to the narrow central components of the sodium “D” are at the rate of barely one-fifth of a mile per second. Most of the fainter lines, on the contrary, indicate ascending currents over the solar surface at an average speed of about a third of a mile a second.[50] Motion-displacements, besides, due to the earth’s rotation and the eccentricity of its orbit, can be detected, and have been allowed for. These latter minute corrections naturally apply to all the solar lines without distinction.

Enough has been said to give an idea of the manifold considerations which have to be taken into account in estimating the _true_ wave-lengths of the solar absorption rays. They are changed, according to a special and complex law, by the sun’s rotation; they are changed by the movements of approach or recession of the earth as a whole, as well as of each particular spot on the earth; they undergo alteration through the solar atmospheric circulation; they are affected by pressure, perhaps by other undetected influences, and each of these modifying causes acts variably, either in time, or according to locality on the solar surface. Happily, most of them act only to an infinitesimal extent; but their unquestionable, although slight effectiveness illustrates very strikingly the subtlety which every increase in accuracy necessitates in the methods of science.

An embarrassing peculiarity of the Fraunhofer lines is their virtually uniform intensity all over the sun’s disc. Just as the telluric bands develop with the sinking of the sun, they ought to become strengthened near the limb; yet they remain sensibly the same, notwithstanding the greatly augmented depth of the absorbing strata traversed by the light before reaching the eye. This is really a glaring anomaly, and one almost forgotten through sheer hopelessness of getting rid of it.

In concluding this brief chapter we would once more draw attention to the curious individualities of the Fraunhofer lines. They are constructed, in many cases, at successive levels; they are modified by various influences. Some are of hair-like fineness; others, emanating from an identical substance, have nebulous edges. Moreover, the sharp and the diffuse lines respond differently to pressure, so that their characteristic aspects are significant of profound distinctions in their mode of origin. The more closely, in fact, these mysterious rulings are examined, the less trivial or casual their slightest diversities appear. They are charged with meaning, transcending, in part, our actual powers of interpretation, but challenging efforts towards that end, which cannot fail to breach, if they do not wholly raze, the ramparts of ignorance.