We may now consider that we have properly said good-bye to our old friend the Earth, who attracts us in more ways than one; we have also looked at the station or port, and it is really time to be starting. We must seriously choose our carriage and get into it. What kind of a carriage is it to be?

We saw last time that an airship or aeroplane can only take us a very little way—six or seven miles at the most—and we want to go much farther than that. Jules Verne chose to shoot a great projectile holding three people, which went as far as the Moon; but then he had to start it with great velocity, and it was only his enthusiastic imagination which enabled the travellers to survive the shock. Mr. H. G. Wells had the happy idea of screening out gravity by a newly discovered substance called Cavorite (in honour of its inventor). For that purpose blinds were fitted to the windows of the "sphere" that he mentions in his fascinating book; and so this "sphere" was enabled to float, or move in some indescribable way (because we are so accustomed to gravity we do not quite know what to call movement without it) as far as the Moon. It might have gone even much further, without difficulty or inconvenience, except that it would almost certainly have taken a long time; and we want something to go much quicker than that sphere. It accordingly occurred to me that we could not do better than adopt a hint from our old friend Mark Twain in his Tramp Abroad (a delightful book which can be bought for sevenpence nowadays). Perhaps you have read it, and you remember that after he had braved the perils of the ascent of the Riffelberg (which had taken him seven days!), he was contemplating going up Mont Blanc. But the perils of the actual ascent seemed so great that he decided to make it by telescope. He found in the village street at Chamonix a man with a telescope directed on a party going up the mountain; he followed them up by means of that telescope, feeling all the time just as though he were along with them; so that when they had got to the top, he quite felt that he had got there himself, and cheered so loudly that he disturbed the people round and they made remarks which called him back to Chamonix.

The advantage of going by telescope is, first of all, as Mark Twain hints, that it is very much safer. I am afraid I have not got a licence to drive an aeroplane, and if I had I might even then have an accident. But besides being safer, the telescope takes us much more quickly. Supposing you wanted to get to the Sun, even in two years, how fast do you think you would have to go? We said the Sun is 93 million miles away; and there are about a million minutes in two years; so that you would have to go about 93 miles a minute which is pretty quick! But we can go much quicker than that by telescope. We can get to the stars, I was going to say, in no time; but it is really less than no time because we shall get there before to-day; we shall see things happening not to-day, but many years ago; just as when abroad you get a newspaper not of to-day, but of yesterday or perhaps a week before; and thus you learn what your friends were doing yesterday or a week before. Now, although light travels very quickly, it takes time to travel; and the light that comes to us from the distant stars tells us what happened years ago, perhaps thousands of years ago; and this gives one a very curious feeling when we reflect upon it. When we were little babies and went out for our first walk, and saw the sky for the first time, some of the light which fell upon us went back towards the sky and part of it enabled our nurse to see us. Unless the light did come back from the baby in that way, the nurse would not see it. But she did not use up all the light from the baby; another part of it went on past the nurse, perhaps into the telescope of an angel (if angels have telescopes), and then the angel saw the baby; but not till some time after the nurse had seen it, because the light would take time to get from nurse to angel. And another part of the light which missed the nurse and missed the angel would perhaps go on to a star; but it would only get there after some years. Now if you could be put in one of those stars immediately, and if you had a powerful enough telescope, you might be able to see yourself as a baby going out for the first time; because the light would not have got there immediately, but would have taken all the years you have been alive to get there. If you went on to more distant stars you might (if you had a strong enough telescope; it is a large IF), you might see your grandmother going out as a baby for the first time.

Well, as I say, that means that if we use the telescope, we shall get to the heavenly bodies in really less than no time, which would be very convenient; Fig. 22. and so I propose to say a word or two about this wonderful car we are going in the Telescope; for it is just as well to understand your car. Any one who gets a motor-car looks carefully at all the parts; and to-day I want you to learn about the telescope, and what has been its history. Perhaps I ought not to say telescope, but camera, for most of the telescopes astronomers use nowadays are just photographic cameras on a large scale. So we might say that we will journey by camera. But in its early history the telescope was not a camera, and so we will for the present use the name telescope.

The earliest telescopes of any importance were very long and thin. Perhaps you will hardly recognize that object in Fig. 22 as a telescope; one end is high up in the air and the other is down on the ground. Sometimes these telescopes were 200 feet long, and they must have been extraordinarily difficult to manage. The astronomers who used them, and really got very accurate results with them, must have been men of great skill. Why did they make them so terribly long when we in modern days have been satisfied with much shorter telescopes? The reason was that they were bothered by colour. We shall see how colour comes into the question in a few moments when we do some experiments; and we shall also see how colour is now a positive advantage to astronomers, when properly used; but in those early days it was only recognized as a disadvantage which drove astronomers to make their telescopes so long and thin that they became difficult to manage. In Fig. 23 is a way of making a long telescope, suggested by the great astronomer Hevelius; he was very proud of his invention and wrote a book about it; and the whole invention was that you need not have an actual tube for the telescope (made of four planks, like a long thin box; that was the way some of them were made), but that a single plank would suffice, if only a lot of circular diaphragms were attached to the plank at intervals. His whole invention was to save three of the four planks and use one only! Perhaps I am not quite fair, for he was also rather proud of his invention of the lifting gear and of the floor under which the telescope could be stowed when out of use.

Fig. 23.

You will readily see the great difficulties of using these machines, so that astronomers cast about for some other way of getting over the difficulty of colour. It occurred to several people independently that if a mirror could be used instead of a lens to bring the light to focus there would then be no colour and therefore no difficulty. We will explain this fact and illustrate it by experiment in a few minutes; but first let me say a word or two about the consequences of this new move in the making of telescopes. They were made with mirrors—reflecting telescopes, as they are called—and they ceased to be so very thin. Indeed, on the contrary, they became quite fat, for there is a great advantage in having the mirror as wide as possible: W. Herschel at his great Telescope. you can see much more clearly with a wide telescope than with a narrow one, always assuming that the wide one is just as well made. But it is much harder to make a big wide mirror than a little one, just as it is harder to get 100 runs at cricket than to get ten; the batsman may play several overs successfully, but he has to be terribly careful not to make a mistake if he wants to make 100. So with mirrors; it may be easy to make a little one without any mistake, but it is awfully hard to make a large one without some flaw in it which prevents it being a good mirror. The first man to get a century, so to speak, by making a really large mirror without flaw, was William Herschel, who did not begin to try until he was 37 years old. As a boy he was in the band of a Hanoverian regiment, like his father and several of his brothers. It is sad that we cannot claim Herschel as a born Englishman, but he settled in England and got a position as organist at Bath. He showed some cleverness in getting that position, but scarcely of a kind to suggest the wonderful work he was to do in astronomy afterwards. The story is that when the different candidates for the post of organist were being tried, one of them played so skilfully that a friend remarked to Herschel that it was scarcely possible to play any better. "I don't think fingers can do it," agreed Herschel. Nevertheless he went up into the organ loft and produced such astonishing effects that the judges awarded him the post. The same friend, who had shared in the general astonishment, could not help asking how in the world he had managed to play as he did; whereupon, with a twinkle in his eye, Herschel produced from his pocket two leaden discs, and said, "You remember I told you fingers alone could not beat the playing we had heard; but I put these discs, one on a low note and the other on a high note, leaving my hands free for the middle, and in that way I managed to surprise you." After all, though the trick may seem a simple one, it must have required great skill to use it to full advantage; I expect only a remarkable man would have thought of it. And presently Herschel showed much more clearly how remarkable he was. He became interested in the making of mirrors almost by accident; but once started, he went on from one success to another. I have compared making a big mirror to getting 100 runs; and it required quite as much watchful care on Herschel's part. Caroline Herschel. His innings at polishing went on for long hours sometimes twelve hours at a stretch, with no intervals for meals. All the food he got was put into his mouth by his sister Caroline, while he still went on polishing. She was a splendid sister to him took down notes of what he saw through his telescopes when he had made them, and wrote them out neatly for publication; and when he was not using his telescopes, Caroline would use them herself, and she found several comets in that way. William Herschel worked very hard, but Caroline worked just as hard in helping him; and whenever the brother is mentioned with honour, I think the sister ought to be mentioned also. The greatest thing William Herschel did was to discover the planet which we now call Uranus, though he himself wished it called Georgium Sidus, after King George III, who had given him much encouragement. Other people wanted it called Herschel, in honour of the discoverer, and both these names were used for many years. But still other people said that for anybody to have his name attached to a planet was far too great an honour, whether he were discoverer or king; and ultimately these got their way by establishing the name Uranus.

Herschel's largest mirror was four feet wide; but afterwards Lord Rosse made one six feet wide, which is, up to the present, the largest telescope ever made. A man could stand in the mouth of it. The making of the mirror itself was a great achievement, but that was by no means all; to fit it into a telescope that could be easily handled was a triumph of engineering skill and could only have been accomplished by a great engineer such as Lord Rosse was. One of his sons is also a great engineer; he is the Mr. Parsons who invented the steam turbine, and made a success of it in spite of enormous discouragements; and he told us, you remember, about the diamonds in meteorites.

Herschel's four-foot mirror and Lord Rosse's six-foot are still in existence, and the latter is actually in use in the telescope, though the former has been dismounted. But the surfaces of both have become tarnished, so that they no longer show things clearly. They are made of a peculiar kind of metal called speculum metal, which remains bright for some time, but gradually tarnishes in the course of years in a manner difficult to remedy. To avoid this gradual deterioration, mirrors have been made recently of glass which is then silvered with a thin film on the front surface. This film also tarnishes after a time, but it can easily be removed with a little acid, and then a new bright film can be put on so that the mirror is as good as new. A good many mirrors two or three feet wide have been made in this way; but the first really big one was made by Dr. Common at Ealing. It was five feet wide, not so wide as Lord The great Five-foot Telescope being taken up Mount Wilson. Rosse's (six feet), but still a great advance on any previous " silver-on-glass " mirror. With it he took some wonderful pictures of nebulae and other objects of the heavens, but he did not live long after he had made it, and the telescope was sold at his death to the Harvard Observatory in the United States.

Since then a five-foot mirror even better than Dr. Common's has been made and set up at Mount Wilson in California; and it has been so successful that they are attempting a much bigger one still, 100 inches wide, or 8 feet 4 inches. If they succeed in making it, they will have the first telescope to surpass in size that of Lord Rosse. In this case, besides the great difficulties of making the telescopes, An accident to the traffic up Mount Wilson. they had the extra ones of getting them to the top of a mountain 6000 feet high, which are by no means inconsiderable. When I first visited Mount Wilson in 1904, the path up the mountain was only three feet wide, with a steep face on one side and a sudden drop on the other; the path was just about wide enough for one mule; but the mules did not think so; they kept trying to crowd past one another, which is not pleasant for the rider, especially as they try to pass on the outside, where a slip means tumbling down the precipice. Professor Hale had this path widened to five feet all the way before he could venture to send up the big five-foot mirror and its mounting; and I believe it is now being widened further still to twelve feet, so that they can carry up the loo-inch mirror in comfort when it is made. The five-foot has been at work on the top of the mountain for several years, and has taken some most beautiful photographs of objects far away in the depths of space. We will presently put some of them on the screen, and in that way we shall be practically allowing this magnificent telescope to carry us in imagination for a long "voyage in space" such as was not possible a few years ago. But it is a curious thing that when you get to the top of Mount Wilson, the Earth seems almost more striking than the heavens. Down below you in the plains are the two cities of Los Angeles and Pasadena; and when these are lit up at night they make glittering constellations of which the stars are brighter than those in the sky above. (See illustration facing p. 126.)

Having told you something about the history of these big reflecting telescopes—the biggest telescopes in the world—I want to go back and talk a little about the reason for making mirrors instead of lenses: so that the trouble about colour is avoided.

When we make a telescope with a lens we use a property of light called refraction, a word from the Latin which means simply breaking back. You know how a cricket ball can be bowled so as to go straight for some distance, and then when it hits the surface of the ground it "breaks back" owing to the spin the bowler has given it? Well, a beam of light behaves in much the same way when it is refracted; after going straight for some distance, it strikes a surface and turns off at an angle.

In Fig. 24 the beam of light AB goes along straight from A to B, and then strikes SF, the surface of glass or water; thereupon instead of continuing straight along BC it is "broken back," or is refracted along BD. Fig. 24. Now when we use a lens, we have two surfaces to deal with, one where the light goes in and one where it comes out, and both of these are curved, which is a new complication. To keep matters as simple as possible I will first use a prism; that is, a piece of glass with two flat surfaces. When the ray AB strikes the first surface SF (Fig. 25), it is broken back as we have said along BD; and when it comes to the second surface to get out of the glass again it is broken back again along DE. You may think that I have drawn DE the wrong way and that it should have been in some such direction as DX; but when you try with a prism you will find the ray behaves as drawn; it gets bent in the same direction both times, Fig. 25. and comes out in a line quite turned aside from its original direction AB. Fig. 26. But at the same time it becomes coloured, with all the colours of the rainbow. Whenever you get this refraction or "breaking back" you also get colour. When I put this prism in the path of a straight beam of light it is bent aside so that it no longer falls on the screen in front, but on this other screen to the side; and at the same time it becomes beautifully coloured (Fig. 26). The fact is that blue colours are more bendable than yellow, and yellow than red; Fig 27. so that though all the colours start together in the same ray out of the lantern, when the prism bends them, some bend more than others and we get them separated. Now those colours have become lately of the greatest value to astronomers, but at first they were merely a nuisance; they interfered with the use of lenses in telescopes. A lens is a piece of glass with curved surfaces; when rays of light strike those surfaces they are bent to a focus (Fig. 27), as is wanted for making a telescope. But the bending introduces colour which is not wanted; and yet they could not in early days get rid of the colours without also getting rid of the bending to focus. All they could do was to keep the colour effects as small as possible by making the bending also small, which means that the rays have to travel a long way before they come to focus. That is why the early telescopes made with lenses were so long and thin; if they had been shorter and wider the colour effects would have been so great that the observations would have been useless. Newton and Gregory and others therefore turned aside from the making of lenses and proposed to use mirrors, which can also collect rays of light to a focus if they are hollowed out to a particular shape. You can try this for yourselves with the reflector of a common oil-lamp; it will bring rays to a focus just as a burning glass will (Fig. 28).

Fig 28.

But now let me show you another prism. When this is put in the path of the rays you see they are not broken back at all; they still pass straight to the screen; but at the same time they are wonderfully coloured. How is this? We learnt that colour was a result of refraction, and yet here is colour without refraction. The fact is, this prism is really made up of two; one bends the rays aside and colours them, the other bends them back again without undoing the colour entirely. That is a new and valuable idea, that one bending can balance another and yet the colour effects do not balance, because it suggests to us at once that we ought to be able to do just the opposite, that is to balance the colour effects and leave the bendings unbalanced; and in this way we can make a lens which will not show colour. When this was once realized astronomers went back to lenses for their large telescopes, and they have succeeded in making lenses larger and larger until the largest is now 40 inches wide—the great lens at the Yerkes Observatory near Chicago. That is a long way short of the largest mirror telescope (you remember that Lord Rosse's was six feet wide); but it is far easier to make a mirror than a lens. You see the mirror has only one surface to be made; you must be careful to get that surface as nearly perfect as possible, but when that is done your work is over. But a lens has at least four surfaces; for to get rid of colour we must put two lenses together, balancing their colour effects, but leaving the focussing effects unbalanced. Each one of these four surfaces must be as carefully made as the single surface of the mirror; and besides, we have to be very careful that the glass inside is as perfect as possible, since a slight flaw in it will make trouble and confusion at the focus of the telescope. You may ask why, then, do astronomers bother to make lenses at all, since mirrors are so much easier? Well, there are several reasons, but I will mention only one: a lens can be made to show more of the sky at once. When we look through a mirror-telescope (or, let us say, when we photograph with it, for that makes the explanation simpler), then we see that the objects in the middle of the picture are in good focus, but not those at the edges. Perhaps many of you have got cameras of your own, kodaks or brownies, or some other sort, and you may know the difference between good and bad lenses—with a good lens all the picture is in focus, with a poor one the edges show up fuzzy. Well, with a mirror we cannot help the edges being fuzzy, because we have nothing we can alter. If we try to alter the surface, we shall put the centre of the picture out of focus, which leaves us no better off than before. But with a lens we have four surfaces to deal with and can set one against another, Photograph showing very bad edges, though good in centre. much in the same way that has been already done for colour; a little alteration in one can be accompanied by such a corresponding alteration in another that the edges of the field are improved while the centre is no worse off. This way of setting one thing against another becomes even more easy when we have three or four lenses instead of two only; we can then arrange to get quite a large picture with no fuzziness anywhere.

Let me now tell you a little about some of the largest lenses in the world. They have nearly all been made by an American firm called Alvan Clark. It would have been nicer for us to say that they were made in England: we have, however, the satisfaction of knowing that an Englishman, the Rev. W. R. Dawes, was chiefly instrumental in drawing attention to the excellent workmanship of the Clarks: so that when a big telescope was to be made, the order was placed with them. One of their first successes was a 26-inch lens built for the Washington Observatory; and almost directly it was pointed to the heavens, two tiny moons of the planet Mars were discovered with it. Up to that time Mars was believed to have no satellite: When in 1832 Tennyson was writing the "Palace of Art," he put in some astronomical verses, including the line—

"She saw the snowy poles of moonless Mars,"

but these verses were left out of the poem, and not published until 1898, when two moons had been discovered, so that the line was altered to

" She saw the snowy poles and moons of Mars."

Another interesting thing about these moons is that Dean Swift jokingly predicted them in his Gulliver's Travels. He speaks of the astronomers of Laputa as having

"discovered 2 lesser stars or satellites which revolve about Mars."

Of course he was only writing in fun, but an old proverb says that "There's many a true word spoken in jest," and so it proved in this instance.

Soon after this the Czar of all the Russias wished to have the largest lens in the world for his observatory at Pulkovo, near Petrograd. The Alvan Clarks were given the order for a 30-inch lens, and at the same time the machinery for the telescope was to be made by the brothers Repsold at Hamburg. As a young man beginning astronomy, I had the privilege of seeing this great telescope in the workshop at Hamburg in 1884, and I well remember my astonishment, not only at the size of the telescope, but at the number of beautiful devices for working it, especially at the eye end. When an astronomer is looking through a telescope, it is a great convenience to him to be able to make notes and measures of what he sees without taking his eye away: William Herschel had his sister Caroline to make the notes for him, but other astronomers have not had such devoted sisters. The Repsolds had accordingly arranged for the observer to do all manner of things by pressing this button or turning that screw, while he was still looking continuously at the star or planet under observation. But no arrangements are perfect, and an eminent German astronomer who was inspecting the telescope when I was there pointed out that this complicated apparatus would infallibly collect much cigar ash!

The Czar was not allowed to remain long in possession of the largest lens: one of 36 inches was ordered for the Lick Observatory and again the Alvan Clarks succeeded in making it. James Lick was a rather eccentric millionaire in San Francisco, who would probably have spent his money in a very different way but for the persuasiveness of an astronomer, Mr. George Davidson. There are many stories about James Lick: perhaps you would like to hear one of them. A great many people applied to him for work, and he had a curious way of deciding whether to give them employment or not. He attached great importance to their obeying orders, however stupid the orders might seem. So when a man came asking for work, he would set him to plant trees upside down, with their roots in the air and their branches in the ground! Those who set to work without protest he kept in his service; but if a man objected or asked questions, he was sent away. You can well imagine that a man like this had queer ideas about what to do with his money, and he was chiefly anxious to have some great memorial to himself. Mr. Davidson persuaded him that a large telescope would be a very good form of memorial; and the bones of James Lick now rest under the great 36-inch telescope on the top of Mount Hamilton. He died, indeed, before it was completed, and was temporarily buried elsewhere: but when the great telescope was at last in place, his bones were dug up and deposited under his chosen memorial. Only one possibility seems to threaten their peaceful rest: California is rather a region for earthquakes, and already the Observatory has been twice seriously shaken. San Francisco was, as you know, wrecked by an earthquake some few years ago. The Lick Observatory is some distance away from San Francisco and only suffered slightly, but a telescope is a delicate instrument that cannot stand much shaking without being damaged. We must hope that good luck will attend the future.

One achievement of the Lick telescope was to discover a new satellite of Jupiter in 1892. Of course the telescope could not have done it without the sharp eyes of Professor Barnard behind it; but we must give the telescope its share of credit, for very few others have managed to show the satellite even now we know it is there, and it is much easier to see something after it has been discovered than to find it in the first place.

It was a condition attached by Mr. James Lick to his gift that the public shall be allowed to use the telescope one night a week, and a great number of visitors go up the mountain on Saturday night just to get a few minutes looking through this great instrument. I am afraid many of them are disappointed, for they expect to see all that they have read about in books or seen pictures of. Now what is set down in books is often the outcome of very careful watching by skilled observers: it cannot be seen every moment, but only on favourable occasions: and without a "seeing eye" it cannot be seen at all. But some of the visitors perhaps only want to say they have looked through the telescope, without caring much what they see.

The largest lens in the world at present is that at the Yerkes Observatory near Chicago. Perhaps you would not call it very near, for it is about 70 miles away; but it was a millionaire of Chicago, Mr. Yerkes, who gave the money for it and wished Chicago to have the credit of the largest telescope, so we must regard it as belonging to Chicago. The Observatory is on the shores of a lake which is now called Lake Geneva, but which had a more beautiful Indian name, Kish-wau-ke-toc, meaning Big Foot, from its shape. The lake is a kind of summer resort for the rich people of Chicago, and when the train comes in, you can see crowds of steam yachts waiting to carry them to their various houses round the shores of the lake, just as we might see a number of motor-cars at a big terminus in England.

The great Yerkes telescope is 60 feet long and has all sorts of beautiful and powerful mechanism to work it. The floor on which the observer stands can be raised or lowered, so as to give him a comfortable position for any object he wants to look at; and this is worked electrically; so that pulling a lever is sufficient to start the mechanism. Pulling another lever starts a motor which turns the great dome round: and generally, by pulling one lever or another all the things can be done which in smaller observatories are done by hand. To get enough electric power for all these things there is a "power-house" placed at some little distance from the telescope. One curious visitor enquired if they put it at that distance because they were afraid it might explode and damage the 100,000 dollar telescope; but the astronomer said no, that was not the reason; they did not fear an explosion, as might be seen from the fact that the power-house was close to his own dwelling-house in which there was a million-dollar baby!

You probably want to know how much these huge telescopes will magnify, and you may be surprised at my answer, "As much as ever you like;" but that is the actual fact. Fig. 29. Any telescope can be made to magnify a thousand times, say, but it does not follow that you will be pleased with the result. The magnifying depends partly on the big lens or mirror about which we have been talking and partly on the "eyepiece," the little lens you put your eye to. If the object is not magnified enough you can take out this little eyepiece and put in another which magnifies more. Perhaps you have had your photograph taken and "enlarged": something of the same kind happens with a telescope, the big lens or mirror makes a little image and the eyepiece enlarges it. Fig. 30. Now you may have noticed that when your photograph is enlarged, various defects appear which were not noticeable in the original small photograph: and the more you enlarge it the more these defects are emphasized, so that to enlarge it beyond a certain point is no advantage. Fig. 29 is a picture of a horse and jockey from the Daily Graphic: and an enlargement of the head of the jockey. The enlargement (Fig. 30) shows that the head is all composed of little dots, and though these do not trouble us on the small scale, they become obtrusive in the enlargement. In the same way if you try to enlarge the image made in a telescope beyond a certain point the defects become glaring and you get no advantage. These defects are due partly to fault in the making of the lens or mirror (because no workmanship can be quite perfect) and partly to the tremulousness of our atmosphere. All kinds of cross currents are continually passing in the air through which we are compelled to look at the heavens; and they all tend to blur and confuse the image. When we do not magnify it much we do not see these blurs; but a high magnification makes them obvious.

This illustration of the jockey reminds us of another fact about a large telescope: we cannot see much at once. In the original picture we see not only the jockey but the horse as well, but when we enlarge it, we must be content with the jockey's head if we are to have the patch of the same size. Perhaps you will ask why we need have this limited patch at all? Why not make the whole picture bigger? I am trying to show you what you actually see when you look through a big telescope: and you will always find your vision bounded by a ring, which limits the "field of view" as it is called. In using a telescope you have to put your eye to a small hole in the eyepiece. Fig. 31 is a picture of the great nebula in Andromeda, and the little hole in the covering screen enables us to see just about as much of it as we should see at one time in a large telescope. If we move the telescope about, we can change the bit we look at. If you cut the screen loose, you can move the hole about over the picture and get an idea of the whole in that way. It is a tedious process, but it was the only way astronomers had of examining a large object like this when they used their eyes and had not learnt to take photographs: and you cannot wonder that they got imperfect ideas of them. The eminent astronomer Trouvelot drew a picture of this same nebula, before the days of photography: and he made those dark rifts practically straight lines, which misses the whole point of the structure.

When, however, we take a photograph, the eye-piece is taken away and the light shines all over the plate at once, so that we are no longer confined to a small ring, but can photograph the whole object at once, faithfully: we see that these rifts are not straight but delicately curved, in a way suggesting that there are several rings whirling round the central bright nucleus. This has a very important bearing on our ideas of the formation of stars from nebulæ: but the drawing made by Trouvelot was meaningless.

Let us take another point about a large telescope. We have chosen a telescope as our travelling car, and we must learn all about it, not only how it is used, but how it set up. You can well imagine that it is rather a business to construct a telescope like Lord Rosse's, big enough for a man to stand up in. Fig. 32 shows one in which a man is not standing up, but lying down, to do some part of the fixing. This is the great Victoria telescope which Dr. McClean presented to the Royal Observatory at the Cape of Good Hope in honour of good Queen Victoria, whom many of you may not remember. We must hope that some one will give a big telescope in honour of King George V, mustn't we?

Now not only is such a telescope awkward to set up, but it is rather awkward to work after it has been set up, unless special arrangements are made. One modern comfort is the rising and falling floor which I mentioned in connection with the Yerkes Observatory. It was designed originally by Sir Howard Grubb, for use with the great Lick telescope; and has been generally adopted for all large telescopes of that pattern. Presently we will notice a different method of working a large telescope,

but before doing so I want to remind you that since our Earth is steadily turning round on its axis, a telescope would soon lose a star unless there is clockwork to counteract the Earth's motion. You know how the various constellations change during the year: there is a little rhyme, made by the good Dr. Watts, which enables us to remember some of them, perhaps you know it already—

The Ram, the Bull, the Heavenly Twins,
And next the Crab, the Lion shines,
The Virgin and the Scales.
The Scorpion, Archer, and Sea-Goat,
The Man that bears the Watering-Pot,
The Fish with shining tails.

That is the order in which the constellations in the Zodiac are arranged, and the Sun appears to visit them all in turn once a year, owing really to the fact that the Earth travels round the Sun. But the Earth also rotates on its axis, as we learnt in the first lecture: and in consequence of this rotation all these constellations cross the sky every day. Some of them cannot be seen to do so, because they are too near the Sun: they cross in the daytime w r hen we cannot see them; but the others cross at night, and if you care to notice at this time of year (early January) you can see the Twins rise in the east soon after sunset and cross the sky much in the same way that the Sun does, setting in the west long after you have gone to bed. If, then, we had a fixed telescope it could only catch sight of the Twins once during the night; but if we arrange that it can turn round by clockwork it can keep the constellation, or any part of it, steadily in view, all night if necessary. Sometimes an astronomer wishes to look steadily at the same object all night in this way: he would probably be also taking a photograph at the same time, of some very faint light such as a faint distant nebula, which would not appear on the plate unless he gave it a very long exposure. So he points the telescope at the object, sets his clockwork going, puts a photographic plate to receive the image: and then in another telescope (firmly attached to the photographic one and moved round by the same clockwork), he watches to see that the clockwork is going smoothly and accurately. If it is not, he has the means of correcting it by pressing an electric button. But it is very tiring to watch for long hours like this, and he is generally glad when the first signs of coming daylight warn him to put the cap on his telescope. Sometimes even then he knows that he has not given a long enough exposure; the object may be so faint that even a long winter night of twelve hours is not long enough for it. What is he to do then? Why! he must cover up his telescope most carefully so that no light can get to the plate during the day: and when darkness comes again, he must turn the telescope to the east where the object will rise, set his clockwork going again, take off the cap, and spend another night watching to see that all goes well. Perhaps even two nights are insufficient, so that he must add a third and a fourth: and I need scarcely remind you that all nights are not fine; when there comes a wet or cloudy night, the astronomer must keep his telescope covered up and wait until the weather improves. So that sometimes a plate has been kept in the telescope for many months before the full exposure has been given. You may not have realized what long hours of waiting some of the beautiful pictures of the heavens which you can now see so easily may have cost!

Now we can return to the way of working a big telescope. One of the most recent modern improvements is to attach the clockwork, not to the telescope itself, but to a mirror which reflects the light into it. The mirror is in this case a flat mirror, not concave so as to bring the light to focus, but made perfectly flat so that it simply turns the light in another direction without altering it. All the alteration is to be done by the telescope, A Coelostat being prepared at Oxford for the Eclipse Expedition of 1896. which can now be firmly fixed: the mirror is turned about by the clockwork so as to send the light always into the fixed telescope. A mirror so arranged is called a Coelostat, which means "sky standing still": and it is an actual fact that if you look into such a mirror at any part of the sky, it will no longer appear to move as the real sky does, but remain steady and constant. You may ask whether we can prevent a constellation setting in this way: of course we cannot do that: a constellation will seem to remain steady as long as you can look at it: but the mirror is being turned round by clockwork, The long shed for the Snow Horizontal Telescope on Mount Wilson, California. and in course of time you will find that it will present its edge to you, and then its back! so that you can no longer see the constellation, which takes the opportunity to set below the horizon.

When a coelostat mirror is used in this way we can fix the telescope in any position we like within certain limits. One position which very naturally occurs to us is the horizontal one. If we lay the big telescope on the ground, there is no danger of its falling: and it is in other respects convenient to work with. Hence several large telescopes have already been built in this way. A good example is the Snow telescope on Mount Wilson. You see in the picture the long shed which is really the telescope itself: and at one end there is the coelostat mirror, reflecting the light (in this case usually the light of the Sun, not that of a star) into the telescope.

But there is one great disadvantage about this horizontal position. The ground gets heated during the day and causes air currents to ascend from it. Now currents of heated air blur the image (see p. 143) and are to be avoided as much as possible, whereas the horizontal position of the telescope seems to encourage them. Hence, Professor Hale tried the plan of putting the telescope vertical instead of horizontal. He built, in the first instance, a tower 60 feet high. The coelostat mirror[1] was placed on the top and the rays from the Sun or stars sent down to the foot of the tower, where they could be examined or photographed. This new move was found to be so successful that a more ambitious experiment was tried. A tower 150 feet high has been built to carry the coelostat at the top: by it the rays of the Sun are reflected down to the ground but do not stop even there, because a well 80 feet deep has been dug below the tower to receive them, so that the whole length of travel is 230 feet. We have got back to the length of those long thin telescopes which were found so inconvenient two hundred years ago; but now they are no longer inconvenient because they need not be moved about, but can be fixed upright once for all. The High "Tower" Telescope on Mount Wilson. That is of course not altogether an easy matter, because a telescope must be fixed very steadily; it will not do, for instance, for it to be shaken by the wind. How are we to prevent a tower 150 feet high from being shaken by the wind? Professor Hale thought of a trick as neat and simple as a conjuring trick—one of those dodges that is so easy when once you know and so difficult before-hand. If any of you likes to test his own brains let him try to think for himself how this was done: and to give you a little time to think I will not tell you the answer just yet.[2] We will look at this picture (p. 124) of the ladies who do the calculations at the Mount Wilson Observatory, ready to ascend the 150-foot tower in the bucket which acts as a lift, and then we will leave the tower for a time and go down into the well, where all kinds of apparatus are available which it would take us several courses of lectures to explain fully, but of which we may get some idea without working too hard.

Let us first think of the three essential parts of a telescope—any telescope—we might take the very first that Galileo looked through. There were two lenses joined by a tube. Now these three separate parts of a telescope have each had histories of their own which we have partly reviewed already. The tube went ahead first; you remember how the tube became immensely long, because of colour, without any great alteration in the lenses: then one of the lenses (the object-glass as it is called, because it is turned to the object: the other is called the eye-lens or eyepiece] became a mirror and was made much larger, while the tube shrank back to normal size. We thereupon followed the history of the mirror, seeing how it became larger and larger: and how, when the difficulty of colour was got over, large lenses also began to be made. Meantime the tube had to grow to keep pace with the growth of the object end, and the old difficulties of unwieldiness had to be faced afresh; The Computing Staff about to make their first ascent of the "Tower" Telescope in 1910. and so we came back to the tube and learnt about the coelostat, and ultimately about Professor Hale's great 150-foot tower. But all this time nothing had been said of the poor eye end, which seems to have been left quite out in the cold: and indeed its history did not begin until later, but once begun, it has been far the most eventful of the three, because there has been much more variety in it. The object end and the tube have changed chiefly in size; but the eye end had become a photographic plate, or a spectroscope, or a photometer, or a spectroheliograph, or any one of a number of such things which can be attached at the eye end. One of the practical problems of an astronomer is to change one of these pieces of apparatus for another as easily and rapidly as possible: and Professor Hale has devised the best method up to the present. Perhaps some of you have on your breakfast-tables a dumb-waiter which can be turned round, so that you can get at the butter or the toast or the marmalade when you want it? There is a contrivance of a similar kind down in the 80-foot well below the 150-foot tower. Of course it is much bigger and stronger than your breakfast-table apparatus: but in just the same way, and almost as easily, as you help yourself to toast first and then jam, Professor Hale can put on the spectroheliogaph first and then the polarimeter, say, or whatever he wants to use.

Up to the present, the only one of these things that we have said much about is the photographic plate. We can take away the eyepiece of a telescope (and also the eye that looks through it) and substitute a photographic plate, and some of the advantages of doing this have already been mentioned, especially that the plate can look at much more of the sky at one time. We also mentioned that the plate could be exposed for many hours, and in this it has another advantage over the eye, because it is steadily photographing more and more (that is, fainter and fainter objects) all the time, whereas the eye gains nothing by continuing to gaze—it rather loses by getting tired. Another advantage is that when the plate has been developed, it can be copied and the copies sent all over the world: or it can be preserved for many years: or it may be measured at leisure; and so on. The catalogue of advantages is quite considerable, so that the introduction of photography into astronomy has effected nothing less than a revolution.

But scarcely less of a revolution has been effected by the spectroscope, which tells us what the stars are made of, by means of that very colour which was at first simply a nuisance. A great man once said that it was quite certain that we should never know what the stars were made of: but it is now quite certain that we do. You remember the experiment with the prism which showed us the different colours of the rainbow? I now want to show you that these colours are not always the same, We will first throw them on the screen with ordinary light, when they have the familiar appearance; but now we will put a salt of sodium in the source of light and immediately you see that the yellow becomes very bright—much brighter than the rest. (See spectrum No. 4, plate facing p. 273). That effect can be produced by common table salt, which is a salt of sodium; and we know when there is some sodium in the light by this brightening in the yellow. But we can improve the apparatus until we find that the brightening is very local—sodium does not brighten all the yellow, but only a particular part or line of it. If we improve the apparatus still The Cities of Pasadena and Los Angeles seen at night from the Mount Wilson Observatory. further the line splits up into two: and with further and further progress these lines are so clearly separated that hundreds of other lines can be seen between them. We might spend a lifetime examining these lines: but at present I want us to be content with the very simplest facts which will give us a general idea of the spectroscope.

Now let us try another experiment. We will again put in some sodium and you see the bright line in the yellow: but soon you see it changes to a dark line—instead of the yellow being brighter than the other colours, it is much darker. (Look at spectrum No. 10, "Sirius" plate facing p. 273) The reason is that the sodium has now been put in in such a way that it gives off clouds of its own vapour, and these clouds stop the yellow light though they do not stop other light. The discovery of that fact is one of the greatest discoveries ever made, for it enabled us to read the heavens like a book. Until it was made nobody could understand the meaning of the dark lines which cross the spectrum of the Sun: they were thought at one time to be boundary marks of the different colours. Now we know that they vouch for the presence of sodium and other chemical elements in a state of vapour, stopping the light of particular colours and letting other light pass on. It is as though a French policeman were stationed at the gangway of a steamer, stopping all Frenchmen from disembarking: all other nations might pass—English, Germans, Portuguese, Chinamen, Hottentots and all others—because the French policeman would have no authority over them; but he would stop every Frenchman, and so there would be none in the miscellaneous crowd issuing from the gangway: and if we could notice this absence of Frenchmen, we might say "Why! there must have been some French officials stopping them at the gangway." So in the same way when we see the absence of a sodium colour (or an iron colour), from the light of the Sun, we say, "There must have been some sodium vapour (or some iron vapour) stopping the sodium light (or iron light) at the gangway (that is, just where the light was leaving the Sun)."

It may seem strange that we notice the light which is stopped rather than that which comes to us. Why should we not rather look for a bright light sent out by sodium, like that I showed you first? Well! we often do: but the dark lines, shown in the second experiment, are much commoner, and have told us on the whole far more than the bright lines. The fact is that the stars are mostly very very hot, so that there are masses of vapour surrounding them through which the light must pass. The stoppages are therefore far the most conspicuous features of their light. You will understand from this brief description what a spectroscope is. The chief part of it is the prism[3] which spreads out the light into a band of colour: but we must be careful to limit the light to a narrow line or "slit," otherwise we shall not see the stoppages distinctly. We can either examine the result with the eye, or we can photograph it: and there are much the same advantages in photographing here as in the case of the direct light.

But we can also do something else. When the light is spread out in this way we can throw most of it away and keep only one colour, and then nothing will shine unless it has that colour. Let us do another experiment or two with our prism. Here is the light spread out into colours as usual, and now I will take a red ribbon and put it in the red light: you see it looks red as usual, but in the green or blue it looks black—it gives us no light. Here is a yellow ribbon which shines yellow in the yellow light, but when I put it in the red or blue it looks black: and so on.

We will try another experiment. Instead of spreading out our light into colours, let us take light of only one colour, to begin with. We can do that by putting some common table salt into the flame of a spirit-lamp, when it will blaze with that brilliant yellow colour which we have seen is due to sodium. Now here is our bunch of ribbons of all colours, which perhaps one of my audience will kindly hold: when we burn our table salt only the yellow ribbon will show—the others will all appear black but we are not quite ready yet. Here are some letters of different colours pasted on this board, which perhaps two others of the audience will hold: and here again is a picture in colours for some one else. You see all these coloured objects in ordinary light: now we will darken the room and put in the table salt, and in the weird yellow light you see only the yellows—all the other colours appear black, but we see at a glance where there is any yellow.

On this principle is constructed one of the instruments that replace the eye end of the telescope. It has a terribly long name—the spectro-heliograph: spectro from the colours, helio from the Sun, which is usually the picture experimented upon; graph because the records are written by photography. The thing it does is to make a picture of the Sun in one colour, just as we used only yellow light to look at this picture. Then all the parts which have any yellow (or red or whatever colour we choose) show up, while the rest remain black. We can tell at a glance whereabouts there is any sodium (or hydrogen or whatever gives that colour) in the Sun. Suppose we could apply this process to the Earth and take a picture of it showing just where any gold was, would that be a good plan, do you think? London would be a bright spot, and New York and other big cities; places like Alaska would show up, if the gold is not yet all dug out; and then there would be other spots which nobody knows of yet, where the gold is still in the ground,—what a rush there would be to them! The pictures of the Sun do not show us the distribution of anything so exciting as gold, but what they show us is nevertheless of great interest to astronomers, and we will say more of it when we visit the Sun.

I will tell you about just one more of the things we can put on the end of a telescope—something rather simple? this time because perhaps you may think we are getting too complicated for comfort. The point of this instrument is merely its great sensitiveness—it detects variations in light which the eye cannot notice. There is a star called Algol which we may translate "The Wonderful" ("Al" is the Arabic for "the," and "gol" is the word used for genii or fairies in the Arabian Nights, for instance: though it is mixed up with the word "ghoul," which is not so pleasant in association). Fig. 33. This star behaves like the light of some lighthouses, it shines steadily for a time and then goes faint. The steady shining lasts for about three days and then comes the dip. Now the eye is able to see this without any telescope at all; and we can interpret the dip to mean that something comes in front of the light. Imagine two bodies, one of them bright, the other dark, revolving round each other (Fig. 33). So long as the dark body is clear of the bright one, or behind it, the bright one shines steadily: but if the dark one gets in front we have an "eclipse" and the light dips. Punch's Representation of Algol, "An Astronomical Reprobate."
(See Notes to Illustrations.)
This imagination or suspicion has been confirmed in a remarkable way by the spectroscope; and from being only a hypothesis has become a serious addition to our knowledge so serious that it has been honoured with an illustration in Punch by that clever artist Mr. E. T. Reed. By kind permission of the proprietors I am able to show you the picture.

But what I now want to tell you about is the new knowledge we got simply by watching more carefully with a new kind of photometer invented by Mr. Joel Stebbins in America. The principle of it can easily be illustrated by an experiment. There is a substance called selenium, which is sensitive to light, so that when light falls upon it the resistance alters, and that spot of light on the screen will move. The selenium is shut up in this box, so that light shall not disturb it, otherwise the spot would be moving about instead of remaining steady. But now we will darken the room and open the box: so long as the box remains still in the dark the spot remains steady; but now we shine a taper on the selenium and you see the spot move off at once. You will understand that by using this principle, Mr. Stebbins was able to tell when a star was shining on his selenium and also how bright the star was: and though he had many difficulties at first, he got over them so successfully that in the end the instrument could tell the brightness of the star much better than the human eye could. I will mention just one of his difficulties. He found that the instrument behaved differently in warm weather and cold, so that to get consistent results he determined to keep it always at the freezing temperature by packing it in ice. In America there is always plenty of ice to be got, but it is generally used for cooling drinks: and when Mr. Stebbins sent in his bill for all the ice wanted to keep his photometer cool, the authorities pretended to think that he must have required a great many drinks. But Mr. Stebbins was so determined to make his photometer work that he bore even practical jokes in the good cause. And now I want to show you what beautiful results he obtained. We may represent what the eye saw by the diagram of Fig. 34. The time when the light is steady is represented by the horizontal lines like CD and FG: the eclipses by the sudden drops ABC and DEF. But the first thing found by the selenium photometer was that in the middle of the straight portions (or what had been thought to be straight portions) there was another little drop so small that the eye had overlooked it. What does this mean? Fig. 34. The big drop ABC means that the dark body comes in front of the bright one: the little drop means that the bright one comes in front of the dark. If it were entirely dark, that would of course make no difference: and since there is a noticeable difference, it follows that the second body cannot be entirely dark, but must be giving off some light which is cut off when the brighter body comes in front. This, then, was the first thing found out; but there is more to come. Not only is there a little dip in the middle of each supposed straight portion, but the remaining parts of those portions were found to slope slightly in opposite directions, as shown in Fig. 35. This means that even when the two bodies are clear of one another, and no eclipse is taking place, the light is still changing in some way. Mr. Stebbins was able to give the explanation very easily. He pointed out that the bright body must be shining on the dark one, just as the Sun shines on our Earth and our Moon: it will illuminate half of it, and this half will be turned sometimes towards us and sometimes away from us, just as the Moon is sometimes full and sometimes new. This is seen easily enough in the case of the Moon, but in Algol we cannot see the two bodies separate from one another, we only see their combined light; Fig. 35. nevertheless by these delicate observations we can say when one of them is full and when it is new just as easily as we can put the times of full Moon on the almanac. Is that not a wonderful result of patient work? And there is still more wonder to come. Mr. Stebbins found that he was able to calculate how large these two bodies were and how far apart (Fig. 36), and he found that the bright one must be 240 times as bright as our Sun, and even the fainter one, which has hitherto been called "dark," is 16 times as bright as our Sun! I think you will agree with me that this is a wonderful addition to our knowledge; we have been able to find out that a star which always appears just as a speck of light is really made up of two at a certain distance apart, one very bright and the other faint, partly shining of itself and partly being illuminated by the other; and that even the dark side of the fainter body is 16 times as bright as our Sun. And the chief part of this knowledge comes, not from using a very large telescope, but from putting a sensitive apparatus in place of the eye end.

There is one more thing I must tell you in concluding this lecture. I must answer the question about Professor Hale's tower how it is kept from shaking. We could keep a reed from being shaken by the wind if we enclosed it in a tube: the wind would blow on the tube and perhaps shake it, but could not get at the reed. Fig. 36.—Mr. Stebbins's Representation of Algol. Professor Hale's idea is to build a tower of rods and surround each rod by a tube, joining all the tubes together to form an outer tower or casing, on which the wind may blow and which perhaps may shake, but which keeps the wind off the inner tower altogether. The coelostat is of course attached to the inner tower, so that it may not be shaken. "The proof of the pudding is in the eating," and such an idea can only be justified by trying it. It has been tried and found completely successful. Professor Hale finds that even in high winds the image of the Sun in his big tower telescope does not shake at all!

  1. There are really two plane mirrors, one moved by clockwork as already described, the other fixed. If we had only the first mirror, then any part of the sky would indeed remain stationary, but the telescope must be pointed to it in a particular direction. The second mirror makes it unnecessary to point the telescope.
  2. The answer is given at the end of the chapter, but I would urge all and sundry to try to guess it for themselves, if they do not happen to have heard it.
  3. Instead of a prism we can use what is called a "grating"; but the general result is the same.