A Voyage in Space/Lecture V

by Herbert Hall Turner

 

LECTURE V
OUR SUN

To-day we have to make perhaps the most important visit of all—a visit, as near as our telescope will allow us, to our Sun. Now, there are three special reasons why the Sun is so very important. In the first place it is the source, as you know, of all the light by which we see, very nearly all of the heat that warms us, and almost of the life that we live. We know how the plants live on sunlight—if there were no sunlight they would die; we know how animals live their lives when the sun is shining: during the night they sleep. On the half of our Earth not illuminated by the Sun the inhabitants are chiefly asleep: certainly most of the animals are asleep; and even human beings, especially children, are mostly asleep; if they do lie awake, they think the night is rather a dreadful time. Perhaps you ave read a poem called "The City of Dreadful Night"? Those who cannot sleep generally long for the daylight. As the Earth turns round they are carried towards the place where the Sun rises; the birds begin to sing, the animals get up and begin to feed; boys and girls get ready for their breakfast and go to school and enjoy themselves, and have dinner about noon and go on with the other pleasures of the day: and finally, when bedtime comes, they plead for just another ten minutes because they know that going to bed means getting into the dark. They like to keep in the light as long as possible. Even if it is not merely a question of no Sun and full Sun (a question of night and day), when it is a question of much Sun and little Sun (summer and winter), you know very well which you prefer. When summer is coming the spring brings leaves to the trees, and little birds make their nests, and boys and girls begin to think of the summer holidays when they can go to the seaside. But when it comes near the winter—"the winter of our discontent," as Shakespeare puts it—then it is all cold; and many animals go into their winter quarters, perhaps to sleep. Human beings have by this time found many ways of alleviating the winter, especially by means of fires and lights, but we must remember that we owe even these to the Sun. Without the Sun in times gone by, those forests would not have grown which to-day give us our coal: the fires which we light in the winter are in many respects the work of the Sun. Hence it does not surprise us that in the old days they used to worship the Sun as a god. In our own country of Britain, the ancient Britons have left monuments, such as those at Stonehenge, showing the way in which the Sun came into their religion. There is one great stone standing erect at Stonehenge in a line with the sacrificial stone, so that the Sun at sunrise on one particular day in the year (June 21) just shines in a line over these two stones, and for a moment it appears from the sacrificial stone as though the Sun stood on the top of the big one; and at that moment they used to make their sacrifice. We can still see to-day how impressive a sight that is, if we go to Stonehenge just at sunrise on Midsummer Day. I have not seen it myself, but I am told that hundreds of people assemble in their motorcars to see this great sight.

Of course Stonehenge was not the only place where the Sun came into religious ceremonies. In ancient Egypt they worshipped the Sungod (Ra); and the Sun was used by the priests to consecrate the King in a very striking way, if we may accept the conclusions of some writers. Knowing how it would shine on a particular day, the priests built a special passage in one of the pyramids or temples down which the Sun would only shine just that once in the year, for a moment or two; and they used that knowledge to impress the people when they were going to appoint a new King. They would take the people into the Temple when the Sun was not shining; it was all dark. (At this point the lights in the lecture-room were extinguished.) Then they arranged so as to have the King in the right position, and at the proper moment the Sun would rise and the people saw their new King in the glorious blaze of sunlight! (At this point a beam of light was thrown from a special lantern to illuminate a small boy, dressed in kingly attire, who had been placed on the lecture table in the few moments darkness; his regal bearing was deservedly applauded.)

The first reason for the Sun's importance, then, is that he is the source of our light and heat, and almost of our life itself. The second reason is because he is in a sense the father of us all. The Earth and planets are "chips of the old block" in the ordinary phrase. One of the regular photographs of the Sun taken at Greenwich. They are parts of the Sun detached from him in the manner we illustrated in the last lecture in talking of satellites. You remember that they were detached as the rate of spin increased. Now I want you to realize that the Sun rotates. You know how we showed in the first lecture that the Earth was rotating, and last Saturday we showed that Jupiter and the other planets were all rotating; and now we have to realize that the great Sun is rotating on his axis also. That was first found out by Galileo when he observed sunspots. Here is a picture of the Sun as it is taken at the Royal Observatory, Greenwich, every fine day in the year. The two lines crossing the picture are the spider lines of the telescope; they tell us which is North and South: but all the rest of the bright part is the Sun. You notice that the edge is darker than the middle; and that tells us that the Sun has an atmosphere, of which we shall have more to say presently. You see also the spots, and you will please note their position on the first picture, which was taken on February 18; and now you will see how they have changed when we come to February 19, and again to February 20, and following dates. They do not move straight across horizontally because the Sun's axis is tilted; but careful measurement shows that there is a real axis, which has remained so far as we can tell in the same position since it has been investigated.

The path of the sunspot can be seen better if we make a "composite photograph" of all the days, by photographing them all on to the same plate. We can then see how the spots are travelling. From such a composite photograph I have made (see Fig. 57) a drawing showing the position of the big spot only and leaving out the others. The positions for the different days are numbered accordingly. On February 23, and again on February 25 and 26, the weather at Greenwich did not allow a photograph to be taken.

Before we go on to consider how the Sun's rotation leads to the formation of planets let us say a word or two more about these spots. They were discovered, so far as Western peoples are concerned, by Galileo in 1610. Fig. 57.—Composite drawing of a Sunspot from several Greenwich photographs. The Chinese had noticed some of the large ones which can be seen with the naked eye from A.D. 188 onwards, but we had no knowledge of them at all till Galileo's time. Undoubtedly they are regions of fierce disturbance. Even in the old days, when Galileo first found them, the notion of disturbance or conflagration suggested itself, as old drawings show. Later Herschel thought that they were holes in the bright surface of the Sun through which his dark interior could be seen; but the idea of the sun being a dark body surrounded by a bright envelope has long been given up. We scarcely know as yet what the spots are, or how they are caused; but there is no doubt at all that they are regions of fierce disturbance where terrible tornadoes are raging.

On one famous occasion (represented in Fig. 58) two independent observers, Carrington and Hodgson, saw an especially noteworthy disturbance. Two intensely bright spots appeared at the positions marked A and B and then travelled in about five minutes to the new positions C and D. The distances do not look very large on the picture, perhaps, but on the Sun himself they represent nearly 100,000 miles, so that the rate of travel was about 300 miles a second! That will give you some idea of the fury of the storms that must be raging in the Sun.

A curious thing about these sunspots is that they wax and wane about every eleven years. At the present time (i. e. Christmas 1913–14, when these lectures were delivered) the Sun has been nearly free from spots for at least two years. It is what is called a time of "minimum." But we are expecting spots to begin again soon, and they will increase in number till they reach a "maximum," and then they will die away again to a minimum about eleven years from now: say about 1924, since we are already probably past the minimum. We have records which show us that this waxing and waning has gone on at least during the last century and a half. Before that the records are too scanty to give us full or trustworthy information: people did not pay enough attention to the Sun. Fig. 58.—A violent disturbance on the Sun. We must not be too ready to blame them because we ourselves do not pay nearly enough attention to him even now, considering his immense importance to us. Up till a year or two ago, no one had ever taken the trouble to make sure whether light and heat came to us from the Sun with strict regularity, or whether they vary in amount as the spots do. It has been calmly assumed that we can depend on a strictly regular supply; but Mr. C. G. Abbot has now found out for us that this is not so: the light and heat do vary, and it is most important for us to watch the variations, considering that the future history of the world may depend on them.

But let us return to the spots and let us look a little more closely at the diagram (Fig. 59) showing their fluctuations. When the curve rises to a peak or maximum there were numerous spots: when it falls to a valley or minimum there were very few. The dates are shown along the bottom line every ten years. You will see that the interval between two minima is not always exactly eleven years: thus there is only about nine years between the minima of 1775–6 and 1784–5; but as much as thirteen years between the minima of 1811 and 1824. The variation is not regular, and there must be some reason for the want of regularity. I have been studying this matter specially for the last year and have found what I think is the key to the puzzle: I think there is a swarm of meteors revolving round the Sun, not in a nearly circular track like our Earth, but in an elongated track like that of a comet. I hope you remember the way in which a comet moves—loitering along slowly when it is far from the Sun, quickening up as it comes nearer, and whizzing round the sharp turn when it is closest to the Sun—what is called perihelion. Now I think this meteor swarm whizzes round so close to the Sun's surface that some of the meteors actually graze the surface and make the sunspots. The swarm is collected mostly at one part of the track, like a lot of people running a race; but if it is a long race, such as a mile, we have seen some of the runners get far behind the leaders, sometimes a whole lap behind. If you stood inside the track close enough for the runners to graze you as they went by, you would be touched a good many times as the leaders went by, but only occasionally by the stragglers: and I suppose that the Sun is close up to the meteor track in this way, Fig. 59.—Waxing and waning of Sunspots. so that he shows numerous spots when the leaders are going by and only a few as the stragglers tail off. But how does this notion help us to explain why the leaders should come round the track sometimes in eight years and sometimes in thirteen years, sticking a sort of average of eleven years? That is the really important point, and supplies the reason for putting forward this notion at all. Let me go back for a moment to Halley's comet. His attention was first drawn to the probability that it would return by the figures for the years 1531, 1607 and 1682 being so much alike: he inferred that it must be the same comet running regularly round a track. Then he noticed that the returns to the Sun were not quite regular: there was an interval of more than seventy-six years between the first pair and of less than seventy-five years between the second pair. This was against him unless he could give a reason for the difference. With great acuteness he assigned the reason in general terms: he said that when the comet was far away from the Sun, loitering slowly along, any of the planets which happened to come by might attract it out of its course a little, upsetting the regularity of the return. It will do us no harm to look at his actual words—

The motion of Saturn is so disturbed by the other planets, and especially by Jupiter, that his periodic time is uncertain, to the extent of several days. How much more liable to such perturbations is a comet which recedes to a distance nearly four times greater than Saturn, and a slight increase in whose velocity could change its orbit from an ellipse to a parabola?… I may, therefore, with confidence predict its return in the year 1758. If this prediction be fulfilled, there is no reason to doubt that the other comets will return.

Now, the meteor swarm which I consider responsible for sunspots can be pulled out of its course in the same kind of way as Halley's comet: and if we look for occasions when it was so attracted we find plain indications of disturbance near the years 1766, 1799, 1833, 1866, 1899, which are a set of occasions already well known to astronomers from the great showers of meteors which were seen then. It has been shown that these showers are all due to a great meteor swarm called the Leonids, travelling round the Sun in about thirty-three years. My notion is that the sunspot swarm is disturbed by these Leonids every time they come round: I think that, as already stated, one end of the track grazes the Sun, but the other end is close to the track of the Leonids, so that it is particularly easy for the Leonids to attract the sunspot swarm, and to upset its regularity. Every time the Leonids come round we open a new chapter of sunspot history. Why should the end of the one track lie so close to the other? Of course we can say that there is no reason why it shouldn't, but it would be much more satisfactory if we could mention a reason why it should; and a very good reason seems to me to be that the sunspot swarm was broken off from the other near the point where they now approach so closely. Here again it is desirable to give a reason for the breakage, and here again we find one in the planet Saturn, which passes close to this same spot in its journey round the Sun (Fig. 60). It does not always meet the Leonids there, because when Saturn comes to the critical spot the Leonids may be in quite a different part of their track: indeed, they generally are. But about every 265½ years Saturn and the Leonids hit off the same moment for being at the meeting-place: and when this happens we find (by looking at the Chinese records which go back nearly 2000 years, as I have already mentioned) that there is a new crop of sunspots. You will see, therefore, how I have come to suggest that a collision between Saturn and the Leonids causes meteors to fall into the Sun and make sunspots. Fig 60.—Suggestion for the explanation of Sunspots by a Meteor swarm. The idea is a new one and has attracted some attention: a clever draughtsman has illustrated it in the Illustrated London News (Fig. 61); I should myself have drawn the details rather differently, but his picture is certainly interesting. Let me give you another illustration in words. On a ship when there is a head wind, the bows will sometimes hit a wave with a resounding smack, and a shower of spray is tossed into the air; the wind catches the spray, carries it down the boat, and dashes it in your face perhaps though you may be far away from the bows. Fig 61. In some such way I suppose Saturn and his Ring to hit the wave of Leonids: a shower of spray from the Leonids, or from Saturn's Ring, or from both, is tossed up; so to speak; is caught, not by the wind, but by the Sun's attraction, and is carried down to hit the Sun in the face, though he may be a long way from the scene of collision.

Let me tell you frankly that other astronomers have not as yet looked with a very friendly eye on this idea: I think they will come round in time, though at present they have not done so. On the other hand, I may be mistaken, and some other origin of sunspots may be found. But in any case I hope you may have been interested to follow the course of the story, seeing how one idea leads to another. Unless we can get some chain of reasoning of this kind, which can be checked at various points, we cannot advance our knowledge: when, on the other hand, we can see our way to some check and find that it works, it gives us confidence that we are on the right road. It was a great pleasure to me when, having seen that collisions between Saturn and the Leonids ought to recur in about 265½ years, I looked at the Chinese observations for the check, and found it very completely shown.

Although the particular idea I have just sketched is new, it is by no means a new idea that meteors should fall into the Sun. At one time it was thought that his heat and light were kept up in this way. If a shot is fired into a target, both shot and target are warmed. The heat comes from the stoppage of the motion of the shot. Do you remember our experiment to illustrate why meteors shine when they strike our air? We whirled an electric junction through the air and it became warm, because the air was resisting the motion and stopping it partly. If a bullet went right through the target and continued its course, nevertheless it would be partly stopped—it would scarcely continue its course so quickly as before: and we should get some heat: when the motion is wholly stopped we get more. Thus it was thought that meteors attracted to the Sun and falling into him with tremendous speed, would develop enough heat to keep him going. You see, the Sun is giving out enormous quantities of heat; we get a good deal on our Earth, and some goes to Mercury and Venus and the other planets. But vastly more goes straight out into space because there is no planet in the direct line to catch it. From the amount our Earth receives we can calculate the total output, and it is truly terrific, enough to melt in a single second a solid column of ice two miles thick stretching from the Earth all the ninety-three million miles to the Sun. How is this outpouring kept up? One idea used to be that meteors fall in; but it was calculated that so many would be required that the Sun would grow visibly larger, which is not the case. Hence, though meteors may account for a part of the heat, there must be some other supply too. I must mention here that the Sun is not burning like a fire or a gas jet: it is glowing like an electric glowlamp. You know the difference between the two cases: a gas jet or candle gets quickly used up; but the filament of an electric light bulb, thin though it is, lasts for hundreds of hours without being consumed. The light comes, not from the consumption of the filament, but from the energy supplied to it from the electric power-house. The question we are considering is—what is the Sun's power-house, what is the source of the energy supplied which makes it glow?

The explanation considered sufficient until recently is almost the opposite of that we have just considered. Meteors falling into the Sun would make it grow bigger: we believe, on the contrary, that it is getting smaller—"shrinking with the cold." That notion was mentioned in the last lecture in connection with the planets, and we saw how it led to the further notion that they would spin more quickly and ultimately throw off satellites. If, as we suppose, the planets themselves are satellites of the Sun thrown off in the same way, then at any rate he must have been shrinking in time past, and we know of no reason why the shrinkage should have stopped. But it seems extraordinary that we should try to explain his generous output of heat by saying that he is "shrinking with the cold": it looks as though we are explaining a thing by its opposite. We can, however, see how this may happen. Supposing your father were to come home one day and say to your mother: "I have been balancing accounts, and I find we have spent a great deal of money this year; we must reduce our establishment; let us give up our motor-car"; it might then happen that giving up the motor-car saved more money than was needed to bring the expenses down to the right figure, so that in other ways the family would be richer than before. It is rather like that with the Sun: losing a lot of heat makes him reduce his establishment—makes him shrink, but the very shrinking causes a development of heat—more heat, perhaps, than he lost, so that he is actually hotter after the shrinkage than before.

This way of getting heat by shrinkage does not appeal to our imaginations so readily as the idea of getting it from fierce blows from meteors; but it is known to be just as real a way, and it can be calculated how much contraction is required to give out the heat we now experience. You might scarcely credit it, but the amount is so small that we could not have noticed it in the 200 years for which we have been measuring the Sun's size accurately. Until we had really good telescopes, (that is to say until the beginning of the eighteenth century), no measures could be made of the Sun's size sufficiently accurate to be worth considering. Hence we practically began measuring the Sun 200 years ago, and all the heat he has sent out during these 200 years could have been produced by his whole body shrinking ten miles inwards. Now, ten miles seems a long distance when we have to walk it, but it is quite imperceptible on the Sun, owing to his great distance. An inch is easy to see when close to us; but put an inch 135 miles away and how big would it look? Just as big as ten miles on the Sun. The Sun might have shrunk since astronomers watched him closely, not merely ten miles but even 100 miles without our being able to detect it.

Can this shrinking go on for ever? It is a hard question to answer. If it goes on steadily there must come a time when the Sun is as compact and solid as our Earth. At present we find that he is not solid, and not nearly so dense as the Earth. But even our solid Earth is still shrinking, as we know from the occurrence of earthquakes and the existence of mountains and valleys. When an orange dries and shrinks, its skin goes into folds: and so does the skin of our Earth as it shrinks. The mountains and valleys are the crinkles: the shrinkage is always trying to crumple them closer together with a stronger and stronger grip: they may hold out for a time, but at last they give way suddenly and then we have an earthquake. If we knew how much our Earth shrinks each year, we might be able to look forward and calculate how much smaller it will be in a million years from now; but we do not really know enough as yet to make the calculation. Or we could look backward and calculate how much bigger it was a million years ago, or two million or fifty million: and similarly with the Sun. The case of the Sun is easier because we have something to go upon; we can measure the amount of heat he is now giving out, and that tells us how fast he must be shrinking, whereas we have no such information about the Earth. In that way Lord Kelvin calculated backwards and thought he could find how many million years ago the Sun could have started being a Sun at all. You see as you go backwards you must suppose him bigger and bigger until he is spread out so much that he would no longer be like a Sun at all, and Lord Kelvin thought this was about 100,000,000 years ago. But he was assuming all the time that the heat we receive all comes from the shrinkage: that wonderful substance radium had not then been discovered, and there seemed to be no escape from Lord Kelvin's conclusion. Since the discovery of radium, however, the matter has assumed an entirely different aspect; we have learnt of the existence of a new kind of power-house for supplying energy, namely the breaking up of an atom, of which no suspicion had entered any one's head till recently. If, as we seem entitled to think, there is this kind of power-house available in the Sun, he need not be shrinking so rapidly—indeed he need not be shrinking at all. The calculations of Lord Kelvin are, in fact, rendered so much waste-paper, since they start with an assumption which may not be correct at all.

I am sure you would all like to see just one experiment with radium, as you have heard so much about it. We will charge up this electroscope so that the gold leaves stand apart from each other. If now I bring a little radium near it, they close together, showing that the electric charge has been removed, and yet you see I have not actually touched anything. The fact is that little particles are shooting out from the radium: they hit the gold leaves and carry off the electricity.

I must now come to the third reason for the Sun's great importance. The first is that he gives us light, heat and life in such profusion as led to its worship in old days. The second reason is that our Earth and the other planets are actually parts of the Sun, who must be regarded as our parent (as the Moon is our child), and the third reason is that he is the nearest star to us. The Sun is really a star: other stars are a very long way off; but the Sun is comparatively near. The actual distance is 93 millions of miles, and you may think that that is not very near; and indeed it would not be very near if we were to go by an express train. Suppose we travelled in a train at the rate of sixty miles an hour we should take 175 years to get to the Sun. I suppose a return ticket at the ordinary British rates would cost us something under a million pounds. It is curious to think that the Earth is taking a journey three times as long as that every year without charging us a penny. We go round the Sun in the greatest comfort, Drawing of part of the Sun's surface by Sir W. Huggins. with sleeping compartments at night and restaurant cars during the day, moving so quietly along that we can play our games too. The big Atlantic steamers have broad decks on which children can play some few games; but the Earth has a far broader deck on which we can play every kind of game. We ought to be very grateful indeed for all these comforts and opportunities, and yet we scarcely ever think of them.

But there is something that takes us much quicker than the Earth, and that is Light. If we agree to travel by the telescope, then instead of taking 175 years to get to the Sun, we need only spend eight minutes. Eight minutes is not a long time on a journey. Even when the whole journey is only a few hours long, Nasmyth's drawing of a Sunspot and the neighbouring surface. when we are within eight minutes of the end your mother will probably say, "Now, children, get your wraps and things together: we are just there." And so every one collects their wraps and rugs. Why! I have actually seen people stand up for more than eight minutes at the end of the journey, with bags and umbrellas in their hands, so little do they reckon of eight minutes. But even travelling by telescope, at the enormous speed of light, we should take years to get to the stars—very different from the eight minutes to the Sun. Hence you see how close the Sun is compared with any other star. He is so close that we can see details on his surface, whereas even in our most powerful telescope the stars are mere points of light.

Let us look again at some of the representations of the Sun's surface made at different times. Sir William Huggins, a great English astronomer, whom we lost recently, drew the surface as a kind of mosaic pattern, while Mr. James Nasmyth (the inventor of the steam hammer, of which there is a fine working model in the machinery museum at South Kensington—I hope you all know that museum) drew a pattern of what he called willow leaves. There is a good deal of difference between their pictures, but they agree in claiming that the surface is made up of numerous bright grains—another observer compared them to rice grains—packed fairly close together excepting near a spot. Small as they appear in the pictures, these rice grains or willow-leaves must be of enormous size in the sun, say 1000 miles long by 500 wide. That these observers were not deceived has been amply proved by photography, especially the photographic enlargements taken by a Russian astronomer, M. Hansky. By a tragic accident he was drowned while bathing, and no one else has paid the same attention to photographing these rice grains on the Sun; but he obtained a sufficient series of pictures to show us at what a great rate they are moving about. Even in a few seconds the pattern becomes quite differently arranged, as you can see by comparing one of M. Hansky's pictures with another. Hansky's photographs of a small portion of the Sun's surface, June 25, 1905. They must be moving with great speed, some of them perhaps at 100 miles per second. We saw an instance of a great solar storm observed by Carrington and Hodgson; but we now begin to realize that the whole surface of the Sun is in a state of constant turmoil. We get another proof of this fact if we look at the edges of the Sun, where we see great red flames shooting up to enormous heights. Perhaps we ought not to call them flames, because in our earthly fire-grates a flame means that something is being burnt, generally the gas from the coal which is being burnt up in the air. The solar flames do not represent anything burning; they glow like the filament of an electric lamp, but we have no better word to describe them than "flames."

We cannot see these flames in the ordinary way; but because they are of a special red colour we can see them by means of a trick. You remember our experiments with colours two lectures ago: we found that a red ribbon would show bright in the red, but in green light it appeared black; and do you remember how we lighted the room with the yellow light made by burning common salt, and then only yellow things showed bright? All other colours in a picture we were looking at—the reds and blues and violets—all disappeared. If we had taken a photograph in this yellow light, the yellow parts of the picture would have shown up, while the others would have been quite faint or altogether absent. And I then told you that this trick was used to take photographs, with an instrument called the spectro-heliograph, and with this instrument we can photograph the red flames round the Sun's edge; of course we use red light in order to show them up. The general plan is simply this: if we pass the Sun's light through a glass prism we have seen that it is spread out into all the different colours. Now let us block out all the other colours except the special red we want; for this we need only a screen with a slit in it at the right place; all other colours will be stopped by the screen, but the special red will shine through the slit.

The "Red Flames" round the edge of the Sun.
(The whole of the ordinary Sun is blocked out.)

This special colour need not, of course, always be red; we can alter the place of the slit so as to use any other colour we please, which is lucky, because red is not at all a good colour for photography. You probably know that you can have red light in the dark room without spoiling your negative. And, further, we need not only photograph the red flames at the Sun's edge, we can use the instrument to photograph the details of the surface, which Huggins and Nasmyth tried to draw and Hansky succeeded in photographing in ordinary light. When we make pictures of them in special colours with this new instrument we get a different picture for every new colour. Let me show you a couple of such pictures of a small portion of the Sun's surface near a spot. Fig. 62.—A Sunspot photographed in Calcium light (Oct. 9, 1903). The sunspot is a great convenience in this case, because the pictures of the surroundings differ so completely that unless you had the spot to go by, you would scarcely believe that you were looking at the same part of the Sun; and yet it is exactly the same part, and the pictures were taken within a few minutes of each other; but in one the slit of the instrument was set for calcium light, and in the other for a special light made by hydrogen. You see how different they are; and I hope you understand the reason. Suppose we made a letter picture of those last three words—

UNDERSTAND THE REASON	UNDERSTAND THE REASON	UNDERSTAND THE REASON
Full Light.	Yellow Light.	Red Light.

colouring all the letters E with yellow, all the letters A with red, and the other letters green. Then if we took a photograph in yellow light we should get only the E's as in the second diagram; if we took a photograph in red light we should get only the A's. The two photographs would be quite different although we had actually photographed the same picture. Perhaps some of you have already taken colour photographs by the three-colour process; if so you will know about this principle without this explanation. The point is that just as we find out in one photograph where the letters E are distributed and in the other photograph the letters A, so in the case of the Sun we find in one photograph where the calcium is distributed and in the other photograph the hydrogen. Why should these two substances be arranged so differently? We get a hint of one probable reason from Fig. 63, in which we see a number of curved lines. There is no such regularity in the calcium picture; but the hydrogen picture shows these curves, which remind us at once of curves made by iron filings when near a magnet. Let us throw a picture of them on the screen. We first place a bar magnet on this sheet of glass and then pepper some iron filings all over the glass. They do not go into a pattern at once, because even the smooth glass is not quite smooth; but if we tap the glass gently so as to make the filings jump up and down, then while they are in the air they feel the pull of the magnet and get a chance to obey it. He soon arranges them in the curved lines (see Fig. 64). Fig. 63.—The same Spot as in Fig. 62, but photographed in Hydrogen light. Now if we were to scatter among these iron filings some filings of silver or other non-magnetic substance, the silver would not feel the pull of the magnet. Mr. W. S. Gilbert made a pretty song about the "silver churn" if you remember, in his opera Patience—

"A magnet never by any endeavour
Can attract a silver churn;"

and it is the same with silver filings; if they were scattered on the glass along with the iron filings, you might tap the glass as long as you like and they would never get into curved lines; they would remain higgledy-piggledy. In the same way the hydrogen on the Sun gets into curved lines because it feels the magnetic attraction, while the calcium remains higgledy-piggledy.

Fig. 64.—Iron filings near a Magnet.

But, you will ask, what is it on the Sun that correspends to the little magnet? and the answer has been given us in a beautiful manner quite recently, by Professor Hale, the great American astronomer, who has erected the Mount Wilson Observatory in California. His proof depends on experiments with polarized light which we cannot stop to explain just now, though I will show you some pretty experiments with it at the end of the lecture; but the gist of the matter is that a sunspot is an electric whirlpool, which is as good as a magnet. Many of you know already what an electro-magnet is; if we take a coil of wire and send a current round and round it, the coil will attract iron filings in the same way as our little bar magnet. The wire may be made of copper and need not have any iron in it at all, though usually an iron "core" is added to make it stronger; but it would still act as a magnet if there were only the copper wire and the electricity circling round in it. Similarly, if we can get an electric whirlpool in any other way we shall have something which behaves like a magnet, and Professor Hale has shown that the sunspots behave in this way. Some of them are whirling in one direction and some in the other—he can detect the direction by the magnetic action. You know there are two ends to a magnet, often called the north and south poles, a whirlpool in one direction acts like a north pole and in the other like a south pole.

We shall not be able to describe Professor Hale's experiments, but I can show you some beautiful pictures taken by his assistant, Mr. St. John, with the spectro-heliograph, which show that a spot has the sucking action of a whirlpool. Have you ever read about the Maelstrom, the great Norwegian whirlpool? A boat which gets caught in it circles slowly round and round at first in the outer parts, but is always being drawn nearer and nearer to the centre, in a spiral. It is whirled more and more rapidly and ultimately sucked down at the centre, as though by some great octopus which waved its long arms out and gradually drew the poor boat to its greedy maw. The pictures taken by Mr. St. John show us something of this kind. You see the sunspot, which is the centre of the whirlpool, about the middle of each picture, and near the bottom is an object which in Fig. 68 resembles a fish. At first this object is peacefully at rest, there being little change between May 29 and June 2, though the tail of the fish has turned towards the vortex: on June 3 this turning rapidly develops within a few minutes: it becomes clear that the fish is caught by its tail, and in Fig. 69 we see him being swallowed. Next day there is no trace of him (Fig. 70).

These actual proofs that a spot is a magnetic vortex are quite recent; but for a long time we have suspected some kind of magnetism in the Sun; indeed, it has been much more than a suspicion. Our magnets on the Earth are disturbed in a regular manner which corresponds closely to the ups and downs of sunspots; that was noticed half a century ago. But instead of calling your attention to these more or less regular changes, I will show you how beautifully Mr. Maunder proved that magnetic "storms" on Earth originate in some way in the Sun. These "storms" are quite irregular (like our storms in the weather), we should never notice them in ordinary life, but a telegraph clerk finds them a great nuisance; if they are violent and persistent they may make it impossible for him to send or receive his messages. At the time when Carrington and Hodgson noticed that great disturbance on the Sun, of which I spoke early in this lecture, there was a furious magnetic storm on Earth, which was naturally attributed in some way to the solar disturbance. But now there is this difficulty: other disturbances in the Sun have been noticed without any corresponding magnetic storm on Earth; while on the other hand we have had storms which have driven the poor telegraphists nearly frantic while the surface of the Sun has betrayed no emotion whatever, so far as we could see. Hence, the question was in a very unsatisfactory state until Mr. Maunder made his ingenious suggestion. To explain the nature of it, perhaps you will let me first give an illustration of a more familiar kind. You know that when you are at the seaside the best time for bathing alters during the day, according to the tide. The time of high-tide falls later and later by about fifty minutes each day, because the tides are caused by the Moon and they follow it round. When the Moon has made a complete turn round the Earth, that is to say, in a month—the tides have changed by twenty-four hours, which comes to about fifty minutes each day. Let us make a diagram (Fig. 71) in which the twenty-four hours go from left to right, putting the days below one another; and let us mark the high tides, or the best bathing times on the diagram: there will be two of them each twenty-four hours, though one of them may come in the middle of the night when not many people want to bathe; but put them all down. Then the fifty minutes alteration each day will cause the marks to slope to the right. If we were to mark lunch time, which depends on the Sun, the marks would fall exactly below one another, at the same time every day (that is supposing you are always in time for lunch); but since the tides depend on the Moon, the marks slope; and we could tell from the amount of slope that the marks referred to something depending on the Moon,

Fig. 71.

because in a month they go right through the twenty-four hours and start afresh. When the marks form a long series like this, it is easy to interpret the diagram.

We must notice just one more thing: the line is not quite straight, but rather wavy; this is because the Sun also helps to cause tides, but you see the effect is small, and we will not notice it further.

Fig. 72.

But now suppose the series of marks is, owing to some cause or other, broken up. For instance, suppose that, instead of stopping at one place, you travel about to different places. Then the diagram would be altered as in Fig. 72, which I call the "American Tourist's Bathing Timetable," because Americans have a reputation for never staying long in one place. So long as they stay anywhere, if only for three days, we shall get three marks in a sloping line of the right slope: even two consecutive days will give us the right slope; single days, of course, tell us nothing. The important point is that wherever and whenever we get a few days together, the proper slope shows itself, proving that the marks must have to do with the Moon in some way, though we may not be able to explain why they are so broken up unless we happen to know that they were made by an American.

Now I think we can follow Mr. Maunder's diagram of magnetic storms. Each horizontal line refers, not to the twenty-four hours in which the Earth turns on its axis, but to the twenty-seven days in which the Sun turns on its axis, as we saw at the beginning of the lecture. The marks show when a magnetic storm occurred on the Earth, and you will see at once that they are apt to occur in groups of three or four, one under the other. Whenever they are exactly one under the other (like the marks for lunch time in the other case), it means that when the Sun has rotated exactly once the storm is repeated, just as the striking of a clock is repeated when the minute hand has gone round exactly once. Mr. Maunder claims that the Sun stretches out a long finger like the hand of a clock, rotating as the Sun rotates; and that this finger strikes the Earth and causes a storm; goes round completely once, strikes the Earth again and causes another storm, and so on, until the finger changes its place like the restless American tourist. Some astronomers cannot believe in the possibility of the "finger"; but in any case it seems clear that something on the Sun causes the storm, because the storm is repeated when the Sun has turned round once. But you will no doubt notice that the marks are by no means always exactly one below another, though they may be nearly so; sometimes there is

Fig. 73.

a slope in one direction and sometimes in another, which means that the cause is recurring either a little more or a little less rapidly than the time for the Sun to go round. The real fact is that there is no one special time which we can assign as that in which the Sun rotates, because he is not solid. When you stand on one of these nice new escalators at the tube stations, all the platform moves together because it is made of solids; but a liquid stream moves faster in the middle and slower near the banks, and the Sun is like the stream, rather than the escalator. At any rate the spots move quicker when they are near his equator, and slower near the poles; and the slopes of the groups of marks in Mr. Maunder's diagram correspond to the paces of different spots; there is no difficulty in finding a place on the Sun which would suit any of the slopes in the diagram. Hence he concludes confidently, and we may agree with him, that the magnetic storms on our Earth originate in some way on the Sun; and this alone suggests pretty clearly that the Sun himself must be magnetic, at any rate, in certain parts. Professor Hale has carried the story further and told us where to look for the origins. He has enormously increased our daily work in observing the Sun; for, to take only one instance, he has shown that we must photograph him not once a day only, as is done at Greenwich, but many times and in different colours, since each colour gives us a different picture. But then Professor Hale has also banded astronomers together into a great union for observing the Sun, as Bode banded them a century ago to discover the missing planet. This new company of "astronomical police" is to photograph the Sun in all sorts of ways as often as possible; they are scattered round the world so that when the Sun is hidden at one place it may be shining in another. At present there is a rather wide gap in the ring of observations; but we are hoping that either Japan or Australia or New Zealand, or perhaps all three, will establish solar observatories; indeed, a rich and generous man, Mr. Cawthron, has already promised £30,000 to build a solar observatory in New Zealand; and the Australian Commonwealth have promised one for Australia; so that we hope the Sun will be presently under constant police supervision, and will not be able to have any disturbance of note without its being recorded.

But there is one part of the Sun which cannot be seen in the ordinary way, nor photographed even with Professor Hale's new instrument. The beautiful corona which surrounds the ordinary Sun can only be seen when there is a total eclipse of the Sun,

Fig. 74.—A total Eclipse of the Moon.

and that is a comparatively rare event in our experience. Most people have never seen a total eclipse of the Sun; possibly I am the only one in the room who has; and although I have seen several, I have had to travel many thousands of miles for the special purpose.

Many or all of you may have seen a total eclipse of the Moon, when the Earth comes between the Moon and the Sun and cuts off the Sun's light from it; the Earth is so much bigger than the Moon that it casts a shadow wide enough to envelope the Moon for an hour and more. You remember Jupiter's big shadow and how the little satellites disappeared into and remained in it for a long time before they came out on the other side. The Earth's shadow is much smaller than that of Jupiter, but it is still large enough to swallow the Moon entirely. On the other hand, when the Moon comes between the Earth and the Sun, its shadow cannot swallow the Earth at all, it can only at best make a little dark patch upon it; if you can get within that dark patch you will see a total eclipse of the Sun, but it may not be easy to get there. The patch does not stay in one place all the time because the Moon is continually moving, and the Earth also is turning on its axis; the patch, therefore,

Fig. 75.—A total Eclipse of the Sun.

makes a track on the Earth, and it may interest you to see the way in which these tracks arrange themselves on the Earth as years roll on. The movements of the Earth and Moon round the Sun are of such a kind that after about eighteen years they come back to nearly the same relative positions. If they came back to precisely the same, then, of course, the track would be exactly repeated in the same place; but the repetition is not so exact as this. Moreover, the interval is not eighteen years exactly, but eighteen years ten and one-third days, and the one-third of a day is important, because it shifts the track just one-third of the way round the Earth, You will see how the track of 1824 is followed eighteen years later by the track of 1842 to the left, and that again by the track of 1860, and that by 1878. But now we have made three steps, each one-third of the way round; and we come back after fifty-four years nearly to the same place.

Fig. 76.

We must keep putting in the word "nearly," because none of these things happen exactly; you see that the 1878 track is just above the 1824 track, which has the 1770 track below it in turn. So that these tracks make a regular pattern on the Earth which we can almost draw for ourselves when we have got a few of them.

Since the tracks are edging a little further north each time, they will at last go over the edge of the Earth, at the North pole, and that family of eclipse tracks will be finished. It came in at the South pole 1200 years ago; travelled gradually further and further north, and will disappear after 1932. There are altogether at any one time twelve such "families" of eclipse tracks; six of them are travelling northwards and six are travelling southwards. Every now and then one of them goes out at one pole or the other; but there is always a new family born about the same time to keep the number of twelve families complete. A few years ago, in 1909, a new family was born at the North pole which is of special interest to us. At its next return, in 1927, the track will come much further south and will cross the north of England (Fig. 77). So in thirteen years time you will have a chance of seeing a total eclipse without leaving England; you are very fortunate young people, for your parents and grandparents and greatgrandparents have had no such chance since 1724, nearly 200 years. You must be careful to be ready for it; I think you will only have twenty-five seconds of total eclipse; but still that will be long enough to give you a good view of the beautiful corona.

When the time is so short (twenty-five seconds is specially short, but even the longest total eclipse only lasts a few minutes) it is naturally important to make the most of it. Photography has helped us considerably in doing so, because we can arrange our cameras beforehand in exactly the right positions, and we can drill those who are to take the photographs until they can make the exposures quickly and without a mistake. A delay or a mistake would waste the precious seconds terribly; but the force of habit is so strong that after going through the operations several times in rehearsal, the short time available is used to the very best advantage. The operations are as a rule quite simple, nothing more than opening a shutter at the right moment, and closing it; but even simple operations require a little practice to get them just right. Fig. 77. We know the difference between a company of recruits and the same people after they have been drilled a little; simple operations like "shouldering arms" are done clumsily at first, but smartly after drill. And after a man has learnt one kind of drill it is easier for him to learn another kind. For this reason the help of His Majesty's forces, naval or military, is specially welcome in eclipse work, and I am glad to say that it has on many occasions been very freely accorded. Eclipse tracks often lie in such remote parts of the Earth that we cannot afford to send many astronomers there; but there are few parts of the Earth so remote that the help of British soldiers or sailors cannot be obtained; and with such help even a single astronomer may arrange his work so as to secure a number of valuable records.

To bring home to you the exciting nature of work of this kind, we will pretend to have an eclipse in this room. Here is the astronomer come to choose his site for observation (one of the "juvenile audience," previously instructed, here selected a site on the lecture room floor); he calls in the aid of His Majesty's forces (here several boy scouts entered the room) and directs them to erect his piers and instruments (the boy scouts now brought in a rough wooden model of a coelostat and telescope-camera and set them up on boxes representing piers). You see the astronomer proposes to use a coelostat—this mirror is to reflect the Sun's light into the camera. There is the Sun on our lantern-screen, and you see that the eclipse has already begun, for the edge of the Moon has taken a small bite out of the Sun's disc. It takes about an hour to cover the whole disc; we will not keep you so long as that, but while it is moving slowly across I will tell you what these gentlemen propose to do. The camera is pointed to the coelostat mirror in exactly the right position; and the astronomer is to take five photographs with different exposures. The first exposure is to be only for one second; this will suffice to show the brightest parts of the corona though not the faint portions; but even if the eclipse only lasted one second before (say) clouds came up, the astronomer would have got something to take away. Hence, he puts that short exposure first. Next he ventures on five seconds; and then when he has got those two he ventures on a long exposure of forty seconds, so as to photograph the faint parts of the corona. The coelostat and telescope used by the Astronomer Royal in Japan in 1896.
(By a Japanese Artist.) A ten-second and two-second exposure complete the programme of five. Altogether, therefore, the camera will be open for 1 + 5 + 40 + 10 + 2 = 58 seconds, and since the whole eclipse is to last 100 seconds it looks as though we might take more time than this; but you must remember that it requires some seconds to change the plate for the next photograph. There are four changes to be made, and if every change took ten seconds we should be running it rather fine. Fortunately the changes can be made more quickly than this. Special plate-holders are arranged which open like a hinged door instead of like a sliding door as usual; and to ensure rapidity the astronomer has two assistants, one to hand him each plateholder when he wants it, the other to take it from him when exposed. In this way he can change plates in six or seven seconds. There are nevertheless a good many things to do in this time. After giving word to the man at the lens to put on the cap (which for eclipse work may be a light Japanese fan held in front of the lens without touching it) he must close the door of the slide, take it out of position and hand it to one assistant; receive the new slide from the other assistant, put it in place in the camera, open the door and (after, perhaps, allowing a second for the instrument to settle) give the word for the cap to be removed. With a little practice this can be done in six seconds, and these gentlemen have very kindly been practising this morning so that they may not fail to carry through the programme smartly this afternoon. One of them I have not yet mentioned the time-caller. It is his business to look at a watch and call out seconds in a loud voice from the moment the eclipse commences, so that the astronomer may make the exposures of the right length, and know how time is passing. Thus suppose that when the astronomer has put in the holder for the fourth exposure of ten seconds, the time-caller is calling seventy-nine; the astronomer will know that he must close the cap at eighty-nine; which leaves him eleven seconds in which to get the last exposure of two seconds. If, however, there has been an unfortunate delay anywhere, and the time-caller is calling eighty-five instead of seventy-nine, the astronomer realizes that if he gives the full ten seconds up to ninety-five, it will be practically impossible for him to get the last exposure of two seconds unless the eclipse lasts unexpectedly long (as may happen); he can thus shorten the exposure from ten seconds to five seconds if he likes, so as to make sure of the last plate. He has probably thought over what to do in such a case beforehand, for it is desirable not to leave such decisions to be made in the exciting moments of the eclipse.

Now I think I have explained the details, and the time of total eclipse is drawing very near, as you see. The daylight is perceptibly less (a few lamps were turned out), but it is by no means quite dark at a total eclipse; one can read the figures on a watch-face, though the time-caller may prefer to use a metronome which gives him the seconds by ear, and leaves his eyes free to watch the eclipse. By this time we ought to be feeling distinctly chilly; it has been said that a cold wind springs up about now, though others say the feeling is merely due to the fall of temperature. You know well enough the way in which it seems cooler even when the Sun only goes behind a cloud; at a total eclipse this effect is much more noticeable; and it must be admitted that there is a very solemn feeling about it all. Natives are often very much frightened; if they have not been told anything beforehand it naturally takes them very much by surprise; while if they have been told, the tale has often come from the mouths of their superstitious priests, who tell it in their own way and for their own purposes. In India an eclipse is a specially good time during which to bathe in sacred waters; certain sacred pools used to receive such masses of bathers at an eclipse that hundreds of them were suffocated; but they died in ecstasy, believing the manner of death to be such as would ensure eternal bliss. Our careful English rule has changed all this by passing the bathers with great rapidity as through a turnstile, down into the water, and out again the other side quick, so that the next person may come—but even our tyranny has never contemplated stopping the bathing. Animals have no one to instruct them beforehand and are taken by surprise. In the next lecture we will have the kinematograph, and you will see how the hens go to roost when totality comes on and come running out after it is over. Even astronomers, who know exactly what to expect, feel the strain, especially in these last few minutes when there is nothing to do but wait, thinking over for the hundredth time whether everything is in exact readiness. Some of them may have dreamt the night before that the eclipse is beginning and they have nothing ready for it, and I can assure you that is a very distressing form of nightmare. But here you see we are all ready, and in a few seconds more the total eclipse will begin. I will ask you to keep silence, please, so that the time-caller and the astronomer's instructions may be clearly heard. Now!

[At this signal the picture of the round Sun on the screen, over which the dark Moon had been slowly creeping, was covered entirely up, while a representation of the corona was flashed out by means of a mechanical device in the slide. A few more lamps were lowered to represent the comparatively sudden decrease of daylight as totality comes on.

The total Eclipse of January 22, 1898

The time-caller counted seconds: one, two, three ... in an even tone up to the 100 seconds agreed upon; the astronomer received the plate-holders (which were imitated in wood and cardboard), put them into the camera, called for an exposure of the required length and its close handed over the finished plate and put in the new one, all without a hitch or mistake of any kind. As a mere piece of drill the performance was warmly praised by a military man present in the audience. During the long (forty-second) exposure, the party of observers duly took the opportunity of regarding the corona, according to instructions; but for the rest of the time their "eyes were in the boat." At the hundredth second the corona disappeared with the reappearing crescent of the Sun, and the lamps previously lowered were relit; the eclipse was over; but all the photographs had been secured, with several seconds to spare, an achievement which was duly appreciated by the audience.]

Now the eclipse is over, and if our photographs are not properly taken we must wait till the next eclipse; we cannot repeat this one. Fortunately we have had fine weather sometimes poor astronomers travel thousands of miles, get everything ready, and then it is cloudy or raining!—but I feel sure that when we develop our plates we shall find good pictures on them. And now what is the use of these pictures when we have them? Well, I cannot bother you with all the details of eclipse work, but I will try to give one or two illustrations of the way in which knowledge may be gained by the study of these pictures; and I will take the illustrations from my own work at eclipses, not because it is more important than that of other people, but because I know more about it.

I have been measuring the brightness of the corona at different distances from the Sun. Near the Sun it is very bright, as bright as the full Moon or brighter. But it fades away so rapidly that at about a diameter from the Sun it is very difficult to see it or photograph it. When it is photographed you get dense blackness on the negative for the bright parts and hardly any effect for the faint parts. My work was to measure just how dense the different parts were; and the measurements are best made when the grades are not either too dense or too faint, but intermediate. Hence, you see why it is desirable to take photographs with different exposures; for by giving a long exposure you can make the fainter portions affect the plate more; it is as though they were not so faint. On the other hand, the short exposure of one second will suit the dense parts better.

But supposing these measures made, and the brightness of different parts compared, what then? What use will that be?

Well, we want to find out what the corona is made of; is it simply an atmosphere of gases like our own round the Earth, or has it solid particles in it, as our own atmosphere sometimes has in a fog? And again, is the atmosphere at rest or moving? If there are solid particles in it are they floating quietly, or are they being shot up out of the Sun with great velocity? Or are they, perhaps, falling continually into the Sun? If an eclipse only lasted long enough, we might perhaps answer some of these questions by watching the changes; but in the few minutes of a total eclipse the changes are too small to be noticed, and, by the time the next eclipse comes, everything has been altered past recognition. Hence, we have to work in other ways. Suppose the particles are being shot up out of the Sun; then they will scatter as they go, like the sparks out of a rocket; and being more diffused they will send us less light per unit area. This will explain in a general way why the corona should be fainter as we recede from the Sun; but in science we must not be content with general explanations, we must see whether they fit particularly. We can calculate not only that the corona ought to be fainter but how much fainter it ought to be; and we can measure on the photographs how much fainter it is; and see whether the two "how much's" agree: if not, then our idea of the particles being shot up won't work and we must try some other idea, perhaps that they are falling continually down. When I tried these different ideas I concluded that the particles were behaving as a fountain behaves—both being shot up and falling down again. It would take us too long to explain why—all I want you to see is the kind of way in which we may learn about the corona from photographs at an eclipse.

But I have not explained why there need be solid particles in it at all. Why should it not be merely a mass of gas like our own air? That again can be answered from the photographs if we take them in what is called polarized light. I am afraid I am worrying you with rather hard notions this afternoon, or at any rate hard names; there is nothing very hard about the notion of polarized light, if you will think of a ray of light as a flat thing like a strip of cardboard which will bend quite easily sideways but not edgeways. I ought, however, to compare a ray of light not to a single strip of card, but to a bundle of such strips with their edges in all directions, not a tight bundle, but a loose bundle in which one strip does not interfere with another. Now what would happen if we tried to bend the whole bundle in any particular direction? Some of them would have their edges turned in that direction and would refuse to bend at all;

Fig. 78.

others would have their flat sides and would bend easily; and we may say that after bending we should have only strips with their flat sides more or less in the right direction. So it is with a ray of light which is bent by being reflected from a solid particle. If there are solid particles in the corona, then we see them because the sunlight which shines on them is bent or diverted to our eyes. In Fig. 78 I have tried to draw four rays of light like strips of cards; starting from the Sun in the middle and going out sideways from him, so that if they are not bent or reflected, they will never reach the Earth, which is supposed to be on the right of the picture. But I have supposed them to be reflected (by solid particles in the surrounding corona) at the places shown, and to come earthwards. Rays which are edgewise and cannot thus be reflected are left out of the picture. Now, on reaching the Earth, if we try to "polarize" these four rays by bending them downwards, the top and bottom rays will bend, but those at the sides are edgewise to this direction of bending and refuse. Hence in our "polarized" picture we should get light from the corona at the top and bottom of the picture, but not from the sides. Another picture with sideways bending would show the opposite.

The sunlight is originally a bundle of rays with their edges in all directions; but in the encounter with the solid particle those rays which are edgewise refuse to bend, leaving only the others, so that the ray is now "polarized" as it is called. If the corona is merely a mass of gas, the light will not be polarized. If the corona is partly gaseous and partly solid particles, we shall get some light from the solid particles which is polarized and some from the gas which is not. How much is there of each kind? Here again we come to the question of measurement, of finding out how much of a thing there is, and by arranging the particular photographs to be taken we can find it out by measurements of the same kind as before, viz. of the density of the negative at different points. In Fig. 79 the

Fig. 79.

light of the corona is supposed to be equal in all directions as in the top picture. If on taking a pair of pictures, one polarized for bending right and left, and the other up and down, we got the results shown in the middle, we should say that the polarization is complete. If we get a pair as shown at the bottom, we should say that it is only partial. The actual pair I got in Egypt in 1905 is shown in Fig. 80. My conclusion is that there are a large number of solid particles in the corona, because there is a great deal of polarized light;

Fig. 80.—Two pictures of the 1905 Corona, polarized in perpendicular planes.

and other astronomers have found the same thing. And now, since you have listened so patiently to this tiresome explanation, I should like to show you that polarized light can give us some very pretty colours, which I am sure you will enjoy seeing.

[The lecture concluded with an exhibition of polariscopic effects from the magnificent collection of apparatus at the Royal Institution.]