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(as described by Mr. Lockyer) when the visual ray passes through a depth forty times as great, seems to me wholly inadmissible. We have no evidence that the portions of the chromosphere giving the three displacements shown in fig. 73 may not have lain 20,000, 30,000, or even 40,000 miles apart, in the direction of the line of view; in other words, that the observed solar storms, though undoubtedly raging with amazing fierceness, were necessarily cyclonic in character. This remark does not apply with equal force to the evidence deduced from the appearance of the prominence F-line depicted in fig. 74,† because we cannot consider it likely that two prominences of the enormous height indicated by the extension of the shattered F lines from the spectrum of the limb, would lie along the same visual line at the moment of observation.

Professor Young, of America, has, however, noted a remarkable circumstance which seems in some sense to throw doubt on the inferences which have been deduced from the displacement of the F-line.

* Throughout Mr. Lockyer's papers there is to be noted a mode of explaining the observed phenomena which seems to imply that the spectroscope exhibits to us the condition of a mere slice of the solar envelopes. The effect of the Sun's solidity in giving an enormous extension to the visual lines through the chromosphere seems wholly neglected, even where a reference to the breadth of the region really included within the slit would appear to invite a reference to the other form of extension.

In this figure the dots below the spectra indicate the amount of displacement on either side of the normal position of the F-line, corresponding to a velocity of 8, 16, and 28 German geopraphical miles per second (a German geographical mile being equal in length to a fifteenth part of a degree of the Earth's equator-i. e., to about 43 English miles).

He observed on one occasion that while the F-line of hydrogen in a prominence was absolutely shattered, the c-line of the same element presented its ordinary appearance. It seems absolutely essential to the interpretation hitherto placed on the displacement of one line of hydrogen that the other line should exhibit a similar displacement, differing only to a slight degree as regards extent. If the observation should be con

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Illustrating the spectroscopic indications of rapid motions in tall coloured prominences.

firmed that the c and F lines of hydrogen in the same prominence-spectrum behave differently, a most perplexing problem will be presented not only to astronomers, but to physicists.

We have now, however, to turn to a yet more interesting and valuable series of researches, by which astronomers have been able to observe the exact figure and changes of figure of the prominences. The principle on which these researches, first successfully pursued by Dr. Huggins, have been based, have been

already explained towards the close of Chapter III. The visibility of the prominence-lines depends on the dispersive power of the spectroscope; and, clearly, the narrower the opening of the slit the more effective will this dispersion be. The lines of the prominences are thus, indeed, narrowed, but they do not diminish in brightness; whereas the spectrum of the illuminated atmosphere diminishes in brightness in precise proportion to the narrowness of the slit. But if we widen the slit the forms of the prominences may be seen, if only the brightness of the atmospheric spectrum is not too greatly increased. In order, therefore, that this feat may be achieved, we must have a spectroscope of great dispersive power.

Now, I have said that Dr. Huggins's spectroscope was not particularly well suited for the kind of observation we are considering. It had not, in fact, sufficient dispersive power; so that when the idea occurred to him of seeking for the prominences with an open slit, he had no great hope of succeeding. As a matter of fact, he failed. The increased light blotted

out the prominences altogether.

But he was not so to be foiled. He who had shown astronomical spectroscopists the way to success in the beginning, was now-with inadequate means-to show his pupils (if one may so speak) how to study the prominences to new purpose. Supplementing the powers of his spectroscope by the use of coloured glass, he was able to solve the great problem which was the true end of all the observations hitherto made.

Others

were aiming at the solution of the problem. Lieut. Herschel had suggested the use of coloured media admitting only such rays as the prominences emit. Mr. Lockyer was trying I know not what device of rotating or vibrating slits,

And one had aimed an arrow fair,

But sent it slackly from the string;
And one had pierced an outer ring
And one an inner here and there;
But last the master bowman, he
Had cleft the mark.

Fig. 75 represents the picture-rough, but instructive-of the first solar prominence ever seen when the Sun was not eclipsed.

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The first prominence seen by means of the spectroscope. (Huggins.)

So soon as the open-slit method had been suggested by Huggins many other observers adopted it. Mr. Lockyer, availing himself of the great dispersive power of the instrument Mr. Browning had made for bim, found that he could dispense with the use of coloured glasses. The prominences were rendered distinctly visible with

the open slit alone; and he could readily watch the changing figures of these mysterious objects. The accompanying drawings (figs. 76 and 77) exhibit two views of a wild and fantastic group of prominences, drawn by Mr. Lockyer, the second only ten minutes after the first was completed.

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A group of solar prominences.-March 14, 1869, 11h. 5m. (Lockyer.) Dr. Zöllner, the eminent German photometrician, applied the same method in a systematic manner. Some of the pictures he has published are singularly interesting.

It is to be noticed, in the first place, that Dr. Zöllner

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