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. 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 confirmed 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. As he had shown astronomical spectroscopists the way to success in the beginning, he now-with inadequate means-showed 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. One had suggested the use of coloured media admitting only such rays as the prominences emit. Another 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.

(Huggins.)

Mr.

So soon as the open-slit method had been suggested by Huggins many other observers adopted it. Lockyer, availing himself of the great dispersive power of the instrument Mr. Browning had made for him, 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.

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 observed the same protuberance in three different

colours, corresponding to the three lines of its spectrum. He found a material difference between the red and the blue image on the one hand and the yellow on the other. The latter is very intense only in close proximity to the Sun's limb,* and corresponds there to

It will be observed that Zöllner's results as regards the coloured images correspond with those obtained by Secchi in observing the coloured lines.

the other images; but the more delicate details disappear at a greater distance. Zöllner suggests that this may be explained on the hypothesis that the rays which give rise to the yellow image emanate from a

FIG 77.

gas having a greater specific gravity than hydrogen, and therefore existing at a lower level.'

The most interesting of Zöllner's drawings are exhibited in Plates IV. and V. It must be conceived by the reader that these views are only the red images. Zöllner saw perfectly similar blue images, and nearly