layer had already been discovered, Mr. Lockyer devised a name for it, entitling it the chromosphere.

Whether this envelope is to be regarded as a true solar atmosphere, in the sense in which that term is usually understood, may be seriously questioned. As seen in the telescope it presents a well-defined and very uneven limit, giving the idea rather of a region of flames or clouds than of an envelope of the nature of an atmosphere. That the substance producing the coloured light of the chromosphere is gaseous admits indeed of no question; but so also the prominences are gaseous. Yet we do not regard these as of the nature of an atmosphere. Now, if the surface of the Sun be covered at all times with small prominences, bearing somewhat the same relation to the gigantic horns and boomerangs' seen during eclipses that the bushes covering certain forest regions bear to the trees, then there can be no doubt that the chromosphere could not rightly be regarded as an atmosphere. We have

Leverrier and Secchi believed in the existence of this envelope because they had seen it; and Secchi had the further evidence (though no further evidence could be needed) derived from his photographs. Under these circumstances no subsequent observer can claim to have been the first to give experimental proof of the existence of the chromosphere.' Or if,--then Galileo might have claimed to have been the first to give experimental proof of the existence of the Moon or of Jupiter, since undoubtedly he first studied these bodies telescopically. That Mr. Lockyer would have discovered the chromosphere had it not been discovered before (unknown to him), may be well believed, precisely as we may be assured that had not the eclipse observers in 1868 discovered the nature of the prominence-spectrum, Mr. Lockyer would have done so. That he did not do either of these things is no discredit to his abilities, since no man that ever lived has succeeded in discovering the discovered; and it may be surmised that no man ever will.

no evidence that it even extends downwards as far as the visible limits of the photosphere, since undoubtedly if a distance of forty or fifty, nay, even of two or three hundred miles, separated the lower limit of the chromosphere from the photosphere, no telescopes we possess could suffice (when supplied with suitable spectroscopic appliances) to reveal any trace of this space. A width of two hundred miles at the Sun's distance subtends an arc of less than half a second: and telescopists who know the difficulty of separating a double star whose components lie so close as this will readily understand that a corresponding arc upon the Sun would be altogether unrecognisable.

I am very far, therefore, from accepting with confidence the view that the chromosphere is the true solar atmosphere. Rather I believe it consists of mattergaseous, no doubt, but-suspended in the true solar atmosphere. I think that not only is this view confirmed by the appearance of the chromosphere as depicted in Plate VI., further on, but that the general resemblance as regards structure and appearance between the chromosphere and the prominences suffice to render it at least highly probable that the former can no more be described as the solar atmosphere than the latter. But even more striking is the evidence deduced from Plate V. For here we see in the first figure a prominence which has been obviously erupted. We see it as it springs upward, expanding with the diminishing atmospheric pressure, and we see in the second figure how the erupted matter has slowly begun

to sink, with expansion, indicating low pressure in the upper regions, but with a figure indicating a real resistance. It is then this invisible atmosphere, through which the upper portion of the prominence has begun to subside, that would seem to constitute the true solar atmosphere, while the chromosphere should be regarded as consisting of a multitude of smaller prominences, either freshly erupted or the remains of

Illustrating the widening of the F-line of hydrogen near the
base of the chromosphere.

former eruptions, floating in the lower regions of the solar atmosphere, much as the second prominence of Plate V, is floating (for a brief while) in the upper regions.

The inferences which have been deduced respecting the pressure exerted by the solar atmosphere at and near the level of the photosphere remain unchanged, how

ever, whatever view we adopt on this point. Fig. 72 illustrates the evidence on this point. Here 1 is a part of the spectrum of the Sun's limb, while 2 represents the line F in the spectrum of the chromosphere. The widening of this line close by the Sun's limb may be regarded as unquestionably indicating an increase of pressure, because the researches of Plücker, Hittorf, Huggins, and Frankland have demonstrated that the F-line of hydrogen does actually increase in this way in width when the pressure at which the hydrogen subsists is increased. Temperature, also, has an effect on the hydrogen lines; nor is it quite easy to separate the effects due to pressure from those due to temperature.* But, on the whole, it seems probable that pressure is chiefly in question, while it may be regarded as absolutely certain that temperature alone is insufficient to account for the observed change. Now, whether we regard the glowing hydrogen of the chromosphere as forming a true constituent of the solar atmosphere or as bearing a somewhat similar relation to that atmo

* In a paper by Dr. Zöllner (Über die Temperatur und physische Beschaffenheit der Sonne), an abstract of which, by the present writer, appeared in the English Mechanic for September 1870, it is suggested that means might be devised for distinguishing between the effects of pressure and temperature by causing a discharge of gas to take place at the moment when the induction spark is passed through the hydrogen. It is certainly a problem of the utmost importance thus to distinguish between the two effects which play the chief part in solar phenomena.

The disappearance of one of the magnesium lines at a certain height above the spectrum of the limb (see fig. 71) in which the lines в belong to magnesium, is significant of a very low degree of pressure at the level where the shortest line ceases to be visible.

sphere that the aqueous vapour in our own air bears to the permanent constituents, it is yet certain that the pressure of the hydrogen near the solar photosphere is a measure of the atmospheric pressure there. And from the observed width of the F-line near the Sun's limb (see fig. 72) it has been estimated by Wüllner that the pressure on the base of the chromosphere, or at the surface of the photosphere, is below the pressure of the Earth's atmosphere.* He has even assigned the limits of pressure at the level of the solar photosphere as lying certainly 'between 50 and 500 millimetres (or between 2 inches and 20) of a mercurial barometer at the Earth's surface.'

It is within the chromosphere and certain of the prominences that spectroscopes of high dispersive power exhibit those signs of cyclonic motions taking

* It must not be forgotten, however, that the width of the hydrogenline where it actually reaches the spectrum of the limb is not known. The observed width on which Wüllner founded his researches may be that corresponding to a height of 50, 100, or even 200 miles above the photosphere; and within these 50, or 100, or 200 miles an increase of pressure may take place by which the actual density of the atmosphere close by the photosphere may be enormously increased. There may be an atmosphere including the vapours of iron, sodium, magnesium, &c. (of all the elements, in fine, whose dark lines appear in the solar spectrum) extending, say, 100 miles above the photosphere; and yet no instruments we possess could suffice to reveal any trace of its existence, unless the dark lines in the solar spectrum be thought to demonstrate the fact that such an atmosphere actually does exist. The fact that on some occasions Mr. Lockyer has seen hundreds of the Fraunhofer lines (as bright lines) in the spectrum of the chromosphere renders this far from improbable. The arguments on the strength of which it has been assumed that the absorption to which the dark lines are due takes place below the visible photosphere, appear, to say the least, far from demonstrative.