were obstructing. That I did not do so is only to be explained by the absence of mind as regarded all else, produced by the concentration of my attention on the problem before me.' Resuming the narrative of his observations, Lieut. Herschel proceeds: A rapid turn of the declination-screw covered the prominence with the needle point, and in another instant I was at the spectroscope. A single glance, and the problem was solved. Three vivid lines-red, orange, and blue! I think I was a little excited about this time, for I shouted quite unnecessarily to my recorder, “Red! green! yellow!" quite conscious of the fact that I meant orange and blue.'

The problem was indeed in a general sense solved. Among all the ideas that had been put forward respecting the actual constitution of the prominences one, and one only, now remained available. Had the prominences been self-luminous solar mountains or clouds (properly so called) a continuous spectrum would have been seen; had they been bodies which shine by reflecting solar light the rainbow-tinted streak, crossed by dark lines, which forms the solar spectrum would have made its appearance. The sole remaining theory was that the prominences consist of glowing gaseous matter. But it remained now to determine what the nature of this matter might be. To this work Captain Herschel turned his attention, and to this work also all the observers who, like him, had succeeded in solving the general problem, devoted themselves during the remaining minutes of totality. The results they

obtained may be thus summed up :-Herschel noted three lines-a red one, which he regarded as possibly C, the red line of hydrogen; an orange one, which he regarded as almost certainly D, the orange (double line) of sodium; and a blue one, which he thought might possibly be F, the blue-green line of hydrogen. Major Tennant saw five lines, which he associated with the lines C, D, b, F, (probably) and G. M. Janssen saw six -red, yellow, green (two), blue, and violet, and established the coincidence of the red and blue lines with the lines C and F. M. Rayet saw no less than nine lines, five of which he recognised as brighter than the rest; these he associated with the lines B, D, E, b, and F.

But we need not consider the evidence on which these various determinations rest, since a new and far more effective mode of observation was to place beyond all possibility of question the true position of these several lines, and of others as yet undetected.

M. Janssen had been struck during the progress of the eclipse by the exceeding brilliancy of some of the bright lines which formed the prominence spectrum. 'Immédiatement après la totalité,' he writes, deux magnifiques protubérances ont apparu: l'une d'elles, de plus de trois minutes de hauteur, brillait d'une splendeur qu'il est difficile d'imaginer.' As he looked, the idea seized him that these lines might be seen when the Sun is not eclipsed. He cried out, he tells us, 'Je reverrai ces lignes-là !'

The principle by which he hoped to effect this will be understood by the reader who has carefully studied the

latter portion of Chapter III. Briefly re-stated it is this: The light of the prominences when dispersed by the spectroscope forms a few lines; that of the illuminated terrestrial atmosphere is spread out into the rainbow-tinted solar spectrum. Therefore, if we only use adequate dispersive power, we can cause the prominence-lines to show conspicuously on the background of dispersed atmospheric light.

Clouds which gathered over the face of the Sun soon after totality prevented M. Janssen that day from pursuing this idea. But on the morrow he applied it with complete success. 'I have experienced,' he said, 'to-day a continuous eclipse.'

We have seen how it becomes possible by this method not only to recognise the spectrum of a prominence during full daylight, but by a series of sectional views, so to speak, to determine the figure of a prominence. To this work Janssen applied himself, and he was presently able to forward to Europe news of his complete success in this new branch of research.

The news of this important discovery was nearly two months in reaching Europe, and a few days before it arrived Mr. Lockyer had independently obtained a similar result. The history of his work and of the way in which it was suggested is not without interest. In May, 1866, Dr. Huggins had examined the spectrum of the star which blazed out suddenly in that year in the constellation Corona. He found that this starnow known as T Corona-gave a spectrum which

* In Mr. Roscoe's valuable work on spectroscopic analysis, this star

differed altogether from any he had previously examined. There was the rainbow-tinted streak crossed by dark lines which ordinarily forms the spectrum of a star; but over this, and obviously corresponding to a large proportion of the star's light, there were bright lines. These bright lines corresponded in position with the lines belonging to hydrogen; so that Dr. Huggins was able to pronounce that the great increase in the star's light was due to an outburst of glowing hydrogen. Afterwards he found that other stars, and notably the middle star in the conspicuous W group of Cassiopeia, show bright lines, superposed on the rainbow-tinted background, though relatively far less bright than those seen in the spectrum of T Coronæ. In these cases, also, the lines were those of glowing hydrogen.

Here, then, was a perfectly apt illustration of the principle dwelt on in the concluding part of Chapter III. Here were the lines of a certain element rendered separately visible as bright lines, though the total amount of light received from the glowing hydrogen was not necessarily (in the case at least of the star in Cassiopeia) equivalent, or nearly so, to the remaining portion of the star's light. The concentration of the light into three definite lines enabled it to

is always referred to as Coronæ. The star Coronæ, however, is a well-known fifth-magnitude star towards the north of the constellation. The real variable (ordinarily a tenth-magnitude star) lies to the south of all the conspicuous stars of Corona. It is called T in accordance with the rule by which the variables successively discovered in a constellation are named from the last letters of the alphabet, beginning with R.

prevail, as far as intrinsic brightness was concerned, over the diffused light, absolutely greater in amount, which formed the rainbow-tinted streak.

We cannot wonder, therefore, that the idea should at once have been suggested that if any portions of the Sun-as, for example, the coloured prominences-consist of glowing gas, the spectrum of such portions might be recognisable even amidst the spectrum of the far more intense light (so far as absolute brightness is concerned) of the solar photosphere.

Mr. Lockyer was preparing about this time to undertake a careful scrutiny of the photosphere with more powerful spectroscopic appliances than had yet been employed. Amongst the expectations which he formed at that time, there is a reference to the problem presented by the prominences. It may be interesting to give at full length the concluding words of the paper he addressed to the Royal Society:—

Seeing that spectrum analysis has already been applied to the stars with such success, it is not too much to think that an attentive and detailed spectroscopic examination of the Sun's surface may bring us much knowledge bearing on the physical constitution of that luminary. For instance, if the theory of absorption be true (!) we may suppose that in a deep spot rays might be absorbed which would escape absorption in the higher strata of the atmosphere; hence, also, the darkness of a line may depend somewhat on the depth of the absorbing atmosphere. May not also some of the variable lines visible in the solar spectrum be due