Astronomical Discovery

Perhaps I may add a few remarks on one or two features of these bodies. Firstly, let us note that Professor Pickering of Harvard is now able to make a most important contribution to the former history of these objects—that is to say, their history preceding their actual detection. We remember that, after Uranus had been discovered, it was found that several observers had long before recorded its place unknowingly; and similarly Professor Pickering and his staff have usually photographed other new objects unknowingly. There are on the shelves at Harvard vast stores of photographs, so many that they are unable to examine them when they have been taken; but once any object of interest has been discovered, it is easy to turn over the store and examine the particular plates which may possibly show it at an earlier date. In this way it was found that Dr. Anderson’s new star had been visible only for a few days before its discovery, there being no trace of it on earlier plates. Similarly, in the case of the new star found at Oxford, plates taken on March 1st and 6th, fifteen days and ten days respectively before the discovery-plate of March 16th, showed the star. But, in this particular instance, greater interest attaches to two still earlier plates taken elsewhere, and with exposures much longer than any available at Harvard. One had been obtained at Heidelberg by Dr. Max Wolf, and another at the Yerkes Observatory of Chicago University, by Mr. Parkhurst; and on both there appeared to be a faint star of about the fourteenth or fifteenth magnitude, in the place subsequently occupied by the Nova; and the question naturally arose,Was Nova Geminorum previously shining faintly? Was this the object which ultimately blazed up and became the new star? To settle this point, it was necessary to measure its position, with reference to neighbouring stars, with extreme precision; and here it was unfortunate that the photographs did not help us as much as they might, for they were scarcely capable of being measured with the requisite precision. The point was an important one, because if the identity of the Nova with this faint star could be established, it would be the second instance of the kind; but so far as they went, measurements of the photographs were distinctly against the identity. Such was the conclusion of Mr. Parkhurst from his photograph alone; and it was confirmed by measures made at Oxford on copies of both plates, which were kindly sent there for the purpose. The conclusion seemed to be that there was a faint star very near, but not at, the place of the new star; and it was therefore probable that, although this faint star was temporarily invisible from the brightness of the adjacent Nova, as the latter became fainter (in the way with which we have become familiar in the case of new stars), it might be possible to see the two stars alongside each other. The suspicion negatived.This critical observation was ultimately made by the sharp eyes of Professor Barnard, aided by the giant telescope of the Yerkes Observatory; and it seems clear therefore that the object which blazed up to become the Nova of 1903 could not have previously been so bright as a faint star of the fourteenth magnitude. Although this is merely a negative conclusion, it is an important one in the history of these bodies.

The second point to which I will draw your attention is from the history of the other Nova just mentioned—Dr. Anderson’s New Star of 1901. In this instance it is not the history previous to discovery, but what followed many months after discovery, that was of engrossing interest; and again Yerkes Observatory, with its magnificent equipment, played an important part in the drama.Nebula round Nova Persei.

Its changes. When, with its giant reflecting telescope, photographs were taken of the region of Nova Persei after it had become comparatively faint, it was found that there was an extraordinarily faint nebulosity surrounding the star. Repeating the photographs at intervals, it was found that this nebulosity was rapidly changing in shape. “Rapidly” is, of course, a relative term, and a casual inspection of two of the photographs might not convey any impression of rapidity; it is only when we come to consider the enormous distance at which the movements, or apparent movements, of the nebulæ must be taking place that it becomes clear how rapid the changes must be. It was not possible to determine this distance with any exactness, but limits to it could be set, and it seemed probable that the velocity of the movement was comparable with that of light.Due to travelling illumination. The conclusion suggested itself that the velocity might actually be identical with that of light, in which case what we saw was not the movement of actual matter, but merely that of illumination, travelling from point to point of matter already existing.

SEPT. 20, 1901

NOV. 13, 1901

IX—Nebulosity round Nova Persei
(From photographs taken at the Yerkes Observatory by G. W. Ritchey.)

An analogy from the familiar case of sound may make clearer what is meant. If a loud noise is made in a large hall, we hear echoes from the walls. The sound travels with a velocity of about 1100 feet per second, reaches the walls, is reflected back from them, and returns to us with the same velocity. From the interval occupied in going and returning we could calculate the distance of the walls. The velocity of light is so enormous compared with that of sound that we are usually quite unable to observe any similar phenomenon in the case of light. If we strike a match in the largest hall, all parts of it are illuminated so immediately that we cannot possibly realise that there was really an interval between the striking of the match, the travelling of the light to the walls, and its return to our eyes. The scale of our terrestrial phenomenon is far too small to render this interval perceptible. But those who accept the theory above mentioned regarding the appearances round Nova Persei (although there are some who discredit it) believe that we have in this case an illustration of just this phenomenon of light echoes, on a scale large enough to be easily visible. They think that, surrounding the central star which blazed up so brightly in February 1901, there was a vast dark nebula, of which we had no previous knowledge, because it was not shining with any light of its own. When the star blazed up, the illumination travelled from point to point of this dark nebula and lighted it up; but the size of the nebula was so vast that, although the light was travelling with the enormous velocity of 200,000 miles per second, it was not until months afterwards that it reached different portions of this nebula; and we accordingly got news of the existence of this nebula some months after the news reached us of the central conflagration, whatever it was.When did it all happen? Remark that all we can say is that the news of the nebula reached us some months later than that of the outburst. The actual date when either of the actual things happened, we have as yet no means of knowing; it may have been hundreds or even thousands of years ago that the conflagration actually occurred of which we got news in February 1901, the light having taken all that time to reach us from that distant part of space; and the light reflected from the nebula was following it on its way to us all these years at that same interval of a few months.

Now, let me refer before leaving this point to the chief objection which has been urged against this theory. It has been maintained that the illumination would necessarily appear to travel outwards from the centre with an approach to uniformity, whereas the observed rate of travel is not uniform, and has been even towards the centre instead of away from it; which would seem as though portions of the nebula more distant from the centre were lighted up sooner than those closer to it. By a simple illustration from our solar system, we shall see that these curious anomalies may easily be explained. Let us consider for simplicity two planets only, say the Earth and Saturn. We know that Saturn travels round the sun in an orbit which is ten times larger than the orbit of the earth. Suppose now that the sun were suddenly to be extinguished; light takes about eight minutes to travel from the sun to the earth, and consequently we should not get news of the extinction for some eight minutes; the sun would appear to us to still go on shining for eight minutes after he had really been extinguished. Saturn being about ten times as far away from the sun, the news would take eighty minutes to reach Saturn; and from the earth we should see Saturn shining more[3] than eighty minutes after the sun had been extinguished, although we ourselves should have lost the sun’s light after eight minutes. I think we already begin to see possibilities of curious anomalies; but they can be made clearer than this. Instead of imagining an observer on the earth, let us suppose him removed to a great distance away in the plane of the two orbits; and let us suppose that the sun is now lighted up again as suddenly as the new star blazed up in February 1901. Then such an observer would first see this blaze in the centre; eight minutes afterwards the illumination would reach the earth, a little speck of light near the sun would be illuminated, just as we saw a portion of the dark nebula round Nova Persei illuminated; eighty minutes later another speck, namely, Saturn, would begin to shine. But now, would Saturn necessarily appear to the distant observer to be farther away from the sun than the earth was? Looking at the diagram, we can see that if Saturn were at S₁ then it would present this natural appearance of being farther away from the sun than the earth; but it might be at S₂ or S₃, in which case it would seem to be nearer the sun, and the illumination would seem to travel inwards towards the central body instead of outwards. Without considering other cases in detail, it will be tolerably clear that almost any anomalous appearance might be explained by choosing a suitable arrangement of the nebulous matter which we suppose lighted up by the explosion of Nova Persei. Another objection urged against the theory I have sketched is that the light reflected from such a nebula would be so feeble that it would not affect our photographic plates. This depends upon various assumptions which we have no time to notice here; but I think we may say that there is certainly room for the acceptance of the theory.

Fig. 6.

Now, if this dark nebula was previously existing in this way all round the star which blazed up, the question naturally arises whether the nebula had anything to do with the conflagration. Was there previously a star, either so cold or so distant as not to be shining with appreciable light, which, travelling through space, encountered this vast nebula, and by the friction of the encounter was suddenly rendered so luminous as to outshine a star of the first magnitude? The case of meteoric stones striking our own atmosphere seems to suggest such a possibility. These little stones are previously quite cold and invisible, and are travelling in some way through space, many of them probably circling round our sun. If they happen in their journey to encounter our earth, even the extremely tenuous atmosphere, so thin that it will scarcely bend the rays of light appreciably, even this is sufficient by its friction to raise the stones to a white heat, so that they blaze up into the falling stars with which we are familiar. This analogy is suggested, but we must be cautious in accepting it; for we know so very little of the nature of nebulæ such as that of which we have been speaking. But in any case, a totally new series of phenomena have been laid open to our study by those wonderful photographs taken at the Yerkes Observatory and the Lick Observatory in the few years which the present century has as yet run.

One thing is quite certain: we must lose no opportunity of studying such stars as may appear, and no diligence spent in discovering them at the earliest possible moment is thrown away. We have only known up to the present, as already stated, less than a score of them, and of these many have told us but little; partly because they were only discovered too late (after they had become faint), and partly because the earlier ones could not be observed with the spectroscope, which had not then been invented. It seems clear that in the future we must not allow accident to play so large a part in the discovery of these objects; more must be done in the way of deliberate search. Although we know beforehand that this will involve a vast amount of apparently useless labour, that months and years must be spent in comparing photographic plates, or portions of the sky itself, with one another without detecting anything remarkable, it will not be the first time that years have been cheerfully spent in such searches without result. We need only recall Hencke’s fifteen years of fruitless search, before finding a minor planet, to realise this fact.

One thing of importance may be done; we may improve our methods of making the search, so as to economise labour, and several successful attempts have already been made in this direction.Superposition of plates. The simplest plan is to superpose two photographs taken at different dates, so that the stars on one lie very close to those on the other; then if an image is seen to be unpaired we may have found a new star, though of course the object may be merely a planet or a variable. The superposition of the plates may be either actual or virtual. A beautiful instrument has been devised on the principle of the stereoscope for examining two plates placed side by side, one with each eye. We know that in this way two photographs of the same object from different points of view will appear to coalesce, and at the same time to give an appearance of solidity to the object or landscape, portions of which will seem to stand out in front of the background. The stereo-comparator.Applying this principle to two photographs of stars, what happens is this: if the stars have all remained in the same positions exactly, the two pictures will seem to us to coalesce, and the images all to lie on a flat background; but if in the interval between the exposures of the two plates one of the stars has appreciably moved or disappeared, it will seem, when looked at with this instrument, to stand out in front of this background, and is accordingly detected with comparatively little trouble. This new instrument, to which the name Stereo-comparator has been given, promises to be of immense value in dredging the sky for strange bodies in the future. I am glad to say that a generous friend has kindly presented the University Observatory at Oxford with one of these beautiful instruments, which have been constructed by Messrs. Zeiss of Jena after the skilful designs of Dr. Pulfrich. Whether we shall be able to repeat by deliberate search the success which mere accident threw in our way remains to be seen.

CHAPTER V

In the next chapter we shall come across a very striking instance of this type; but even in the discovery that there was a periodicity in the solar spots, with which I propose to deal now, Herr Schwabe began his work in the face of deterrent opinions from eminent men. His definite announcement was first made in 1843, though he had himself been convinced some years earlier. In 1857 the Royal Astronomical Society awarded him their gold medal for the discovery; and in the address delivered on the occasion the President commenced by drawing attention to this very fact,Nothing expected from spots. that astronomers who had expressed any opinions on the subject had been uniformly and decidedly against the likelihood of there being anything profitable in the study of the solar spots. I will quote the exact words of the President, Mr. Manuel Johnson, then Radcliffe Observer at Oxford.

It will thus be evident that Herr Schwabe had the courage to enter upon a line of investigation which others had practically condemned as likely to lead nowhere, and that his discovery was quite contrary to expectation. It is a lesson to us that not even the most unlikely line of work is to be despised; for the outcome of Schwabe’s work was the first step in the whole series of discoveries which have gradually built up the modern science of Solar Physics, which occupies so deservedly large a part of the energies of, for instance, the great observatory attached to the University of Chicago.

It has been our practice to recall the actual words in which the discoverer himself stated his discovery, and I will give the original modest announcement of Schwabe, though for convenience of those who do not read German I will attempt a rough translation. He had communicated year by year the results of his daily counting of the solar spots to the Astronomische Nachrichten, and after he had given ten years’ results in this way he collected them together, but he made no remark on the curious sequence which they undoubtedly showed at that time. Waiting patiently six years for further material, in 1843 he ventured to make his definite announcement as follows:—“From my earlier observations, which I have communicated annually to this journal, there was manifest already a certain periodicity of sun-spots; and the probability of this being really the case is confirmed by this year’s results. Although I gave in volume 15 the total numbers of groups for the years 1826-1837, nevertheless I will repeat here a complete series of all my observations of sun-spots, giving not only the number of groups, but also the number of days of observation, and further the days when the sun was free from spots. The number of groups alone will not in itself give sufficient accuracy for determination of a period, since I have convinced myself that when there are a large number of sun-spots the number will be reckoned somewhat too small, and when few sun-spots, the number somewhat too large; in the first case several groups are often counted together in one, and in the second it is easy to divide a group made up of two component parts into two separate groups. This must be my excuse for repeating the early catalogue, as follows:—

“If we now compare together the number of groups, and the days free from spots, we find that the sun-spots have a period of about ten years, and that for about five years they are so numerous that during this period few days, if any, are free from spots. The sequel must show whether this period is constant, whether the minimum activity of the sun in producing spots lasts for one or two years, and whether this activity increases more quickly than it decreases.”

Still the subject attracted little attention. Turning over the leaves of the journal at random, I came across the annual report of the Astronomer Royal of England, printed at length. But in it there is no reference to this discovery, which opened up a line of work now strongly represented in the annual programme of the Royal Observatory at Greenwich. Mr. Johnson remarks that the only person who had taken it up was Julius Schmidt, who then resided near Hamburg.until eight years later. But Schwabe went on patiently accumulating facts; and in 1851 the great Von Humboldt in the third volume of his Cosmos, drew attention to the discovery, which was accordingly for the first time brought into general notice. It will be seen that there are not many facts of general interest relating to the actual discovery beyond the courage with which the work was commenced in a totally unpromising direction, and the scant attention it received after being made for us. We may admit that interest centres chiefly in the tremendous consequences which flowed from it. We now recognise that many other phenomena are bound up with this waxing and waning of the solar spots.Other phenomena sympathetic We might be prepared for a sympathy in phenomena obviously connected with the sun itself; but it was an unexpected and startling discovery that magnetic phenomena on the earth had also a sympathetic relation with the changes in sun-spots, and it is perhaps not surprising that when once this connection of solar and terrestrial phenomena was realised, various attempts have been made to extend it into regions where we cannot as yet allow that it has earned a legitimate right of entry. We have heard of the weather and of Indian famines occurring in cycles identical with the sun-spot cycle; and it is obvious how tremendously important it would be for us if this were found to be actually the case. For we might in this way predict years of possible famine and guard against them; or if we could even partially foretell the kind of weather likely to occur some years hence, we might take agricultural measures accordingly. The importance of the connection, if only it could be established, is no doubt the reason which has misled investigators into laying undue stress on evidence which will not bear close scrutiny.and others not. For the present we must say decidedly that no case has been made out for paying serious attention to the influence of sun-spots on weather. Nevertheless, putting all this aside, there is quite enough of first-rate importance in the sequel to Schwabe’s discovery.

Let us review the facts in order. Most of us, though we may not have had the advantage of seeing an actual sun-spot through a telescope, have seen drawings or photographs of spots. There is a famous drawing made by James Nasmyth (of steam-hammer fame), in July, 1864, which is of particular interest, because at that time Nasmyth was convinced—and he convinced many others with him—that the solar surface was made up of a miscellaneous heap of solid bodies in shape like willow leaves, or grains of rice, thrown together almost at random, and the drawing was made by him to illustrate this idea. Comparing a modern photograph with it, we see that there is something to be said for Nasmyth’s view, which attracted much attention at the time and occasioned a somewhat heated controversy. But since the invention of the spectroscope it has become quite obsolete; it probably does not correspond in any way to the real facts.Greenwich sun records. But instead of looking at pictures which have been enlarged to show the detailed structure in and near a spot, we will look at a series of pictures of the whole sun taken on successive days at Greenwich in which the spots are necessarily much smaller, but which show the behaviour of the spots from day to day. (See Plates X. and XI.) From the date at the foot of each it will be seen that they gradually cross the disc of the sun (a fact first discovered by Galileo in 1610),The sun’s rotation. showing that the sun rotates on an axis once in about every twenty-five days. There are many interesting facts connected with this rotation; especially that the sun does not rotate as a solid body, the parts near the (Sun’s) Equator flowing quicker than those nearer the Poles; but for the present we cannot stop to dwell upon them. What interests us particularly is the history, not from day to day, but from year to year, as Schwabe has already given it for a series of years.

When it became generally established that this periodicity existed, Rudolf Wolf of Zurich collected the facts about sun-spots from the earliest possible date, and represented this history by a series of numbers which are still called Wolf’s Sun-Spot Numbers. You will see from the diagram the obvious rise and fall for eleven years,—not ten years, as Schwabe thought, but just a little over eleven years. The facts are, however, given more completely by the work done at the Royal Observatory at Greenwich. It is part of the regular daily work of that Observatory to photograph the sun at least twice. Many days are of course cloudy or wet, so that photographs cannot be obtained; but there are available photographs similarly taken in India or in Mauritius, where the weather is more favourable, and from these the gaps are so well filled up that very few days, if any, during the whole year are left without some photograph of the sun’s surface.Greenwich areas. On these photographs the positions and the areas of the spots are carefully measured under a microscope, and the results when submitted to certain necessary calculations are published year by year. It is clearly a more accurate estimate of the spottedness of the sun to take the total area of all the spots rather than their mere number, for in the latter case a large spot and a small one count equally. Hence the Greenwich records will perhaps give us an even better idea of the periodicity than Wolf’s numbers. Now, at the same observatory magnetic observations are also made continuously. If a magnet be suspended freely we are accustomed to say that it will point to the North Pole; but this is only very roughly true. In the first place, it is probably well known to you that there is a considerable deviation from due north owing to the fact that the magnetic North Pole is not the same as the geographical North Pole; but this for the present need not concern us.Magnetic fluctuations. What does concern us is, that if the needle is hung up and left long enough to come to rest, it does not then remain steadily at rest, but executes slow and small oscillations backwards and forwards, up and down, throughout the day; repeating nearly the same oscillations on the following day, but at the same time gradually changing its behaviour so as to oscillate differently in summer and winter. These changes are very small, and would pass unnoticed by the naked eye; but when carefully watched through a telescope, or better still, when photographed by some apparatus which will at the same time magnify them, they can be rendered easily visible. When the history of these changes is traced it is seen at once that there is a manifest connection with the cycle of sun-spot changes; for instance, if we measure the range of swing backwards and forwards during the day and take the average for all the days in the year, and then compare this with the average number of sun-spots, we shall see that the averages rise and fall together. Similarly we may take the up and down swing, find the average amount of it throughout the year, and again we shall find that this corresponds very closely with the average number of sun-spots.

Plate XII.

But perhaps the most striking way to exhibit the sympathy is to combine different variations of the needle into one picture. And first we must remark that there is another important variation of the earth’s magnetic action which we have not yet considered. We have mentioned the swing of the needle to and fro, and the swing up and down, and these correspond to changes in the direction of the force of attraction on the needle. But there may be also changes in intensity of this action; the pull may be a little stronger or a little weaker than before, and these variations are not represented by any actual movement of the needle, though they can be measured by proper experiments. We can, however, imagine them represented by a movement of the end of the needle if we suppose it made of elastic material,Daily curves. so that it would lengthen when the force was greater and contract slightly when the force was less. If a pencil were attached to the end of such an elastic needle so as to make a mark on a sheet of paper, and if for a moment we exclude the up and down movements, the pencil would describe during the day a curve on the paper, as the end of the needle swung backwards and forwards with the change in direction, and moved across the direction of swing with the change in intensity. Now when curves of this kind are described for a day in each month of the year, there is a striking difference between the forms of them.Difference between summer and winter, and between sun-spot maximum and minimum.

Cause unknown. During the summer months they are, generally speaking, comparatively large and open, and during the winter months they are small and close. This change in form is seen by a glance at Plate XIII., which gives the curves throughout the whole of one year. Let us now, instead of forming a curve of this kind for each month, form a single average curve for the whole year; and let us further do this for a series of years. (Plate XIV.) We see that the curves change from year to year in a manner very similar to that in which they change from month to month in any particular year, and the law of change is such that in years when there are many sun-spots we get a large open curve similar to those found in the summer, while for years when there are few sun-spots we get small close curves very like those in the winter. Hence we have two definite conclusions suggested: firstly, that the changes of force are sympathetic with the changes in the sun-spots; and secondly, that times of maximum sun-spots correspond to summer, and times of minimum to winter. And here I must admit that this is about as far as we have got at present in the investigation of this relationship. Why the needle behaves in this way we have as yet only the very vaguest ideas; suggestions of different kinds have certainly been put forward, but none of them as yet can be said to have much evidence in favour of its being the true one. For our present purpose, however, it is sufficient to note that there is this very real connection, and that consequently Schwabe’s sun-spot period may have a very real importance with regard to changes in our earth itself.

But I may perhaps repeat the word of caution already uttered against extending without sufficient evidence this notion of the influence of sun-spots to other phenomena, such as weather. A simple illustration will perhaps serve better than a long argument to show both the way in which mistakes have been made and the way in which they can be seen to be mistakes. There is at the Royal Observatory at Greenwich an instrument for noting the direction of the wind, the essential part being an ordinary wind-vane, the movements of which are automatically recorded on a sheet of paper.Illustration of spurious connection. As the wind shifts from north to east the pencil moves in one direction, and when it shifts back again towards the north the pencil moves in the reverse way. But sometimes the wind shifts continuously from north to east, south, west, and back to north again, the vane making a complete revolution; and this causes the pencil to move continuously in one direction, until when the vane has come to north again, the pencil is far away from the convenient place of record; on such occasions it is often necessary to replace it by hand. Then again, the vane may turn in the opposite direction, sending the pencil inconveniently to the other side of the record. During the year it is easy to count the number of complete changes of wind in either direction, and subtracting one number from the other, we get the excess of complete revolutions of the vane in one direction over that in the other. Now if these rather arbitrary numbers are set down year by year, or plotted in the shape of a diagram, we get a curve which may be compared with the sun-spot curve, and during a period of no less than sixteen years—from 1858 to 1874—there was a remarkable similarity between the two diagrams. From this evidence alone it might fairly be inferred that the sun-spots had some curious effect upon the weather at Greenwich, traceable in this extraordinary way in the changes of the wind. But the particular way in which these changes are recorded is so arbitrary that we should naturally feel surprise if there was a real connection between the two phenomena; and fortunately there were other records preceding these years and following them which enabled us to test the connection further, and it was found, as we might naturally expect, that it was not confirmed. On looking at diagrams (Plate XV.) for the periods before and after, no similarity can be traced between the sun-spot curve and the wind-vane curve, and we infer that the similarity during the period first mentioned was entirely accidental. This shows that we must be cautious in accepting, from a limited amount of evidence, a connection between two phenomena as real and established; for it may be purely fortuitous. We may particularly remark that it is desirable to have repetitions through several complete periods instead of one alone. It is possible to reduce to mathematical laws the rules for caution in this matter; and much useful work has already been done in this direction by Professor Schuster of Manchester and others, though as yet too little attention has been paid to their rules by investigators naturally eager to discover some hitherto unthought-of connection between phenomena.

With this example of the need for caution, we may return to phenomena of which we can certainly say that they vary sympathetically with the sun-spots. Roughly speaking, the whole history of the sun seems to be bound up with them. Besides these dark patches which we call spots (which, by the way, are not really dark but only less bright than the surrounding part of the disc), there are patches brighter than the rest which have been called faculæ. With ordinary telescopes, either visual or photographic, these can generally only be detected near the edge of the sun’s disc; but even with this limitation it can easily be established that the faculæ vary in number and size from year to year much in the same way as the spots, and this conclusion is amply confirmed by the beautiful method of observing the faculæ with the new instrument designed by Professor Hale of the Yerkes Observatory. With this instrument, called a spectroheliograph, it is possible to photograph the faculæ in all parts of the sun’s disc, and thus to obtain a much more complete history of them, and there is no doubt whatever of their variation sympathetically with the spots.and the chromosphere. Nor is there any doubt about similar variations in other parts of the sun which we cannot see at all with ordinary telescopes, except on the occasions when the sun is totally eclipsed. Roughly speaking, these outlying portions of the sun consist of two kinds, the chromosphere and the corona, the former looking like an irregular close coating of the ordinary sun, and the latter like a pearly halo of light extending to many diameters of the sun’s disc, but not with any very regular form.