recognised dark line in the spectrum of a star-say, for instance, the F line-is found not to agree exactly in position with the corresponding line in the spectrum of a fixed source of light—as a hydrogen flame, for instance -then the difference of position must be ascribed to a motion of recess or approach on the part of the star, and the rate of such motion may be determined by noticing the amount by which the line is displaced.

Now, this method admits of being applied under exceptionally favourable conditions to the examination of solar cyclonic motions-if only these motions are sufficiently rapid to fall within the province of this

FIG. 39.

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special mode of research. For we have in the lines appertaining to parts of the Sun which are relatively at rest the means of determining very surely, and measuring somewhat exactly, the displacement due to such motions as we are considering.

Let us take as an instance the case represented in fig. 39. Here s s', as before, represents the portion of the prominence P P' which is under examination. Only a small part of the spectrum is shown, that namely, near the F line; and the bright line of the prominence which under normal conditions coincides with the dark line F of the solar spectrum is seen to be displaced towards the violet. It thus appears that

owing to a motion of approach affecting the portion of the prominence included within the narrow space s s', the light-waves producing the F line seem shortened. The general fact of a motion of approach is thus ascertained. But the rate of approach can also be measured; for we know the length of the light-waves corresponding to the line F, Van der Willingen and Ängström having independently determined the wave-length corresponding to the principal lines in the solar spectrum. So that if we measured in any way the distance of the bright F line of the prominence from the dark F line of the solar spectrum, we should be able to calculate the amount of the apparent change. A better plan, however, is available. For the bright solar spectrum (fig. 39), as also the atmospheric spectrum above, is crossed by other dark lines besides the F line, and these enable us to see at once how far the bright line has shifted. Suppose 1, for example, to be another dark line of the solar spectrum, and that the bright prominence-line has moved half-way from its proper place towards 7, then we know that its wavelength is changed to a value midway between the wave-length corresponding to the lines F and 1.* The change of value thus indicated gives us at once the

This is true for such small displacements as are here considered. For greater differences of refrangibility, no such simple proportions exist, partly because the actual change of wave-length (for given differences of refrangibility) diminishes towards the violet end, and partly on account of the irrationality of dispersion for all known media, when the spectra they give is compared with what Ängström has called the normal solar spectrum.

rate of the motion of approach which affects the portion of the prominence-matter corresponding to the space s s'*—because, though the wave-length corresponding to the line will not be indicated in the tables of Ängström or of Van der Willingen (which only include the principal lines) yet it is readily determinable, and indeed may be regarded as a known quantity. And, in a similar way, if the line is shifted towards the red end, the velocity of recession of the prominence-matter can readily be determined.

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But as a general rule the whole line would not be shifted bodily as in fig. 39; since indeed this would imply that the whole of that portion of the prominence which is seen within the space s s' was travelling bodily towards the observer; whereas obviously such a motion could very seldom be expected to occur. narily, then, we may expect to find a configuration of the bright line indicating varieties of motion. The same holds also in the case of portions of the solar photosphere, or spots, lying near the edge of the Sun's disc (so that ordinary cyclonic motions within them may be capable of being recognised by the method we are considering), or in the case of more central portions of the Sun's disc where ascending and descending

* The general rules on which the calculation proceeds are sufficiently simple. Suppose the wave-length corresponding to the line to be 485-89 millionths of a millimeter, that corresponding to the line F being 486-39 such millionths. Then, since the prominence F-line appears half-way between F and 7, the wave-length has been reduced to 486 19 millionths of a millimeter, or diminished by 0-2 such millionths. Hence the velocity of approach of the prominence matter is 20ths of the velocity of light, or some 80 miles per second.

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motions are taking place, resulting in motions of recess or approach with reference to the terrestrial observer. In all such cases we may expect to find peculiarities in the affected lines, corresponding to varieties in the motion or rates of motion of the parts examined.

I give a few examples, illustrating the way in which such peculiarities are to be interpreted. I consider, for convenience, motions taking place in that coloured envelope (whence the solar prominences seem to spring) which has been called the chromosphere :

Suppose ss to represent the portion of the Sun and chromosphere under examination, s s' being the Sun's

FIG. 40.

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limb, c c' the (invisible) outline of the chromosphere. Now, if the F line appeared as in I., we should conclude that the hydrogen in the part of the chromosphere under examination was quiescent near the Sun's surface, as far as motions of approach or recess are concerned (though it might be moving very rapidly in a direction square to the line of sight), but that at some distance from the Sun's surface it was moving very rapidly towards the eye, the rate of motion increasing with the vertical height above the Sun. If, on the other hand, the spectrum appeared as at II. (fig. 40), we should come to a similar conclusion, substituting only a motion

of recession for one of approach. If the spectrum appeared as at III. we should conclude that to a certain level above the Sun's limb there was a gradually increasing motion of approach, but that at and above that level there was a motion of recession tolerably uniform in rate to a considerable height. The case would resemble those instances in our own atmosphere where an upper air current blows in a different direction than the air nearer the sea-level. If, lastly, the spectrum appeared as at IV., we should conclude that to a considerable height above the Sun's surface there was no motion of recess or approach; but that in higher regions of the chromosphere there were masses (within the long range of chromospheric matter really included in the direction of the visual line) moving both from and towards the eye at an enormously rapid rate. The greater or less width of different parts of the bright line would indicate the greater or less pressure at which the hydrogen existed at the corresponding levels during the time of observation. Hence, a bulb on any part of the bright line would indicate a corresponding layer of relatively compressed hydrogen, while a marked narrowing would indicate a layer of hydrogen existing for the time at relatively low pressure.

Similar considerations apply to the spectroscopic analysis of solar spots, or of faculous regions of the Sun's surface, or generally of any regions where disturbances may produce solar atmospheric currents of approach or recess. Combining the observed shifting of portions of a spectral line with its observed thickness