space, are the bright lines which show that the sun is surrounded by a surging sea of incandescent hydrogen, with oftentimes a lower stratum of incandescent magnesium and sodium vapours. I say lower, because the height of the lines indicates to us exactly the height of the stratum, as shown in the figure. For instance, we have a high prominence of hydrogen, the known lines of which correspond to C F, a line near G and h, and a low injection of magnesium and sodium, the lines of which correspond with b and D respectively.

Although the lines vary in height, they never disappear, showing that for some 5,000 miles in height all round the sun there is an envelope of which the prominences are but the waves. This envelope I named the "Chromosphere," because it is the region in which all the variously coloured effects are seen in total eclipses, and because I considered it of importance to distinguish between its discontinuous spectrum and the continuous one of the photosphere. And now another fact came out. The bright line F took the form of an arrow-head,

Chromosphere spectrum.

Solar spectrum.

FIG. 4.-Showing how the F line broadens as the sun is approached.

the dark Fraunhofer line in the ordinary spectrum forming the shaft, the corresponding chromospheric line forming the head; it was broad close to the sun's edge, and tapered off to a fine point, an appearance not observed in the other lines.

Nature is always full of surprises, and here was a surprise and a magnificent help to further inquiry lurking in this line of hydrogen! MM. Plücker and Hittorf had already recorded that, under

certain conditions, the green line of hydrogen widened out; and it at once struck me that the "arrow-head" was nothing but an indication of this widening out as the sun was approached.

I will now for one moment leave the observatory work to say a word on some results recently obtained by Dr. Frankland and myself, in some researches on hydrogen and other gases and vapours, upon which we have been engaged.

First, as to hydrogen, what could laboratory work tell us about the chromosphere and the prominences?

It was obviously of primary importance to determine the cause to which the widening of the F line was due, and to study the hydrogen spectrum very carefully under varying conditions, with a view of detecting whether or not there existed a line in the orange; where, as shown in Fig. 3, there is a line in the spectrum of the prominences which behaves exactly as the known hydrogen lines do.

We soon came to the conclusion that the principal, if not the only cause of the widening of the F line was pressure. This being so, we were in a position to determine the atmospheric pressures in the chromosphere and prominences; that is, whether the hydrogen was dense or

rare.

With regard to the higher prominences, we have found that the gaseous medium of which they are composed exists in a condition of excessive tenuity; and that even at the lower surface of the chromosphere, that is, on the sun itself, in common parlance, the pressure is very far below the pressure of the earth's atmosphere.

Now these again are facts which bear upon the problem of the sun's condition in a very great degree, indeed they lead us necessarily to several important modifications of the received theory of the physical constitution of our central luminary-the theory which we owe to Kirchhoff, who based it upon his examination of the solar spectrum. According to his idea, the photosphere itself is either solid or liquid, and is surrounded by an extensive non-luminous atmosphere, com

posed of gases and the vapours of the substances incandescent in the photosphere. Kirchhoff's idea demands dense vapours far above where we have found hydrogen alone, and that very rare. So that we must consider that the absorption to which the reversal of the spectrum and the Fraunhofer lines are due takes place in the photosphere itself or extremely near to it, instead of in an extensive outer absorbing atmosphere; so that we may say that the photosphere plus the chromosphere is the real atmosphere of the sun, and that the sun itself is in such a state of fervid heat that the actual outer boundary of its atmosphere, i. e. the chromosphere, is in a state of incandescence.

We must next go a stage lower into the bowels, not of the earth, but of the sun.

As a rule, the chromosphere rests "conformably," as geologists would say, on the photosphere, but the atmosphere (as I have just defined it) is tremendously riddled by convection currents; and where these are most powerfully at work, the upper layers of the photosphere are injected into the chromosphere. Thus in Fig. 3 we see the lines due to the vapour of sodium and magnesium, in the spectrum of the chromosphere, appearing there as very short and very thin lines, generally much thinner than the black lines due to their absorption in the solar spectrum.

These injections are nearly always accompanied by the strangest contortions of the hydrogen lines, the latter towering above the rest, of which more presently.

At the same time we have tremendous changes in the prominences themselves, which I have recently been able to see in all their beauty, by merely opening the slit of the spectroscope. By this method the smallest details of the prominences and of the chromosphere itself are rendered perfectly visible and easy of observation, and for the following reason. Let me explain how this result is accomplished. The hydrogen Fraunhofer lines (like all the others) appear dark because the light which would

see.

otherwise paint an image of the slit in the place they occupy is absorbed; but when we have a prominence on the slit, there is light to paint the slit, and as in the case of any one of the hydrogen lines we are working with light of one refrangibility only, on which the prisms have no dispersive power, we may consider the prisms abolished. Further, as we have the prominence image coincident with the slit, we shall see it as we see the slit, and the wider we open the slit the more of the prominence shall we We may use either the red, or yellow, or green light of hydrogen for the purpose of thus seeing the shape and details of the prominences. I have been perfectly enchanted with the sight which my spectroscope has revealed to me. The solar and atmospheric spectra being hidden, and the image of the wide slit and the part of the prominence under observation alone being visible, the telescope or slit is moved slowly, and the strange shadow-forms flit past, and are seen as they are seen in eclipses. Here one is reminded, by the fleecy, infinitely-delicate cloud-films, of an English hedge-row with luxuriant elms; here, of a densely intertwined tropical forest, the intimately interwoven branches threading in all directions, the prominences generally expanding as they mount upwards, and changing slowly, indeed almost imperceptibly.

In one instance I saw a prominence 27,000 miles high change enormously in the space of ten minutes; and lately I have seen prominences much higher born and die in an hour. This will give an idea of the tremendous forces at work.

So much, then, for the chromosphere and the prominences, which I think the recent work has shown to be the last layer of the true atmosphere of the We now come to spots.

sun.

Now, as a rule, precisely those lines which are injected into the photosphere by convection currents are most thickened in the spectrum of a spot, and the thickening increases with the depth of the spot, so that I no longer regard a spot simply as a cavity-an idea which

dates from the last century-but as a place in which we get the absorption of the vapours of sodium, barium, iron, magnesium, &c., from a much lower level than we do when we observe the photosphere.

Fig. 5 is a sketch of the spectrum of a sun spot. We see a black band run

D

FIG. 5.-Spot spectrum, showing how the solar spectrum is dimmed, and how some of the lines widen as they cross it.

ning across the ordinary spectrum; that black band indicates the general absorption which takes place in a sun spot. Now mark the behaviour of the Fraunhofer lines; see how they widen as they cross the spot, putting on a sudden blackness and width in the case of a spot with steep sides, expanding gradually in a shelving one. The behaviour of these lines is due to a greater absorption of the substance to the absorption of which the line is due.

By examining sodium vapour at different pressures in a tube we can see the absorption line due to sodium, in one part as thin as it is in the ordinary solar spectrum; in another almost if not quite as thick as it appears in a spot.

That grand generalization of Kirchhoff's, by which he accounted for the Fraunhofer lines, may be briefly stated as follows:1.

If we have a gas or a vapour less luminous than another light-source, and view that light-source through the gas or vapour, then we shall observe absorption of those particular rays which the

gaseous vapour would emit if incandescent.

Let us confine our attention to the hydrogen Fraunhofer lines.

When I observe the chromosphere on the sun's limb, with no brighter lightsource behind it, I observe its characteristic lines bright. But when I observe them on the sun itself-that is, when the brighter sun is on the other side of the hydrogen envelope, then, as a rule, its function is reduced-the brighter light behind it, showing on both sides the line, makes the line itself appear comparatively dark. But every now and then the hydrogen lines are seen bright upon the sun itself!

Not only are the lines observed bright, but it would appear that the strongly luminous hydrogen is carried up by the tremendous convection currents at different pressures; and under these circumstances the bright line is seen to be expanded on both sides of its usual position. Moreover, at times there is a dim light on both sides the black line, and the line itself is thinned out, showing that, although there is an uprush of strongly luminous material, the column is still surmounted by some less luminous hydrogen, possibly separated from the other portion, which is comparatively dim.

I now come to a new field of discovery opened out by these investigations, a branch of the inquiry more startling than all the rest-I allude to the movements of the hydrogen envelope and prominences of which I have before

hinted.

Any one who has observed the sun with a powerful telescope, especially in a London fog-all too great a rarity unfortunately for such work-will have been struck with the tremendous changes observed in spots. Now, change means movement, and as spot-phenomena occur immediately below the level of the chromosphere we may easily imagine that the chromosphere and its higher waves, the prominences, will also partake of the movements, be they up or down rushes, cyclones, or merely lateral motions.

The spectroscope enables us to determine the velocities of these movements with a considerable approach to accuracy; and at times they are so great that I am almost afraid to state them.

Let me endeavour to show how this result is arrived at.

Imagine a barrack out of which is constantly issuing with measured tread and military precision an infinite number of soldiers in single or Indian file. And suppose yourself in a street seeing these soldiers pass. You stand still, and take out your watch, and find that so many pass you in a second or minute, and that the number of soldiers, as well as the interval between them, are always the same.

You now move slowly towards the barrack, still noting what happens. You find that more soldiers pass you than before in the same time, and, reckoned by time, the interval between each soldier is less.

You now move still slowly from the barrack, and with the soldiers. You find that fewer soldiers now pass you, and that the interval between each is longer.

Now suppose yourself at rest, and suppose the barrack to have a motion now towards, now from you.

In the first case the men will be payed out, so to speak, more rapidly. The motion of the barrack-gate towards you will plant each soldier nearer the preceding one than he would have been if the barrack had remained at rest. The soldiers will really be nearer together.

In the second case it is obvious that the interval will be greater, and the soldiers will really be further apart.

So that, generally, representing the interval between each soldier by an elastic cord, if the barrack and the eye approach each other by the motion of either, the cord will contract; in the case of recession, the cord will stretch.

Now let the barrack represent the hydrogen on the sun, perpetually paying out waves of light, and let the elastic cord represent one of these waves; its length will be changed if the hydrogen and the eye approach each other by the motion of either.

Particular wave-lengths with the usual velocity of light are represented to us by different colours.

The long waves are red.

The short waves are violet.

Now let us fix our attention on the green wave, the refrangibility of which is indicated by the F line of hydrogen. If any change of wave-length is observed in this line, and not in the adjacent ones, it is clear that it is not to the motion of the earth or sun, but to that of the hydrogen itself and alone that the change must be ascribed.

If the hydrogen on the sun is approaching us, the waves will be crushed together; they will therefore be shortened, and the light will incline towards the violet, that is, towards the light with the shortest waves; and if the waves are shortened only by the ten-millionth part of a millimeter, we can detect the motion.

If the hydrogen on the sun is receding from us, the waves will be drawn out; they will therefore be longer, and the green ray will incline towards the red.

Now there are two different circumstances under which the hydrogen may approach or recede from the eye.

Suppose we have a globe to represent the sun. Fix your attention on the centre of this globe: it is evident that an uprush or a downrush is necessary to cause any alteration of wave-length. A cyclone or lateral movement of any kind is powerless; there will be no motion to or from the eye, but only at right angles to the line of sight.

Next, fix your attention on the edge of the globe; here it is evident that an upward or downward movement is as powerless to alter the wave-length as a lateral movement was in the other case, but that, should any lateral or cyclonic movement occur here of sufficient velocity, it might be detected.

So that we have the centre of the globe or sun for studying upward and downward movements, and the limb for studying lateral or cyclonic movements, if they exist.

Fig. 6 shows the strange contortions which the F hydrogen line undergoes at the centre of the sun's disc. Not only