The Outline of Science Part 2

It is estimated that between ten and a hundred million meteorites enter our atmosphere and are cremated, every day. Most of them weigh only an ounce or two, and are invisible. Some of them weigh a ton or more, but even against these large ma.s.ses the air acts as a kind of "torpedo-net." They generally burst into fragments and fall without doing damage.

It is clear that "empty s.p.a.ce" is, at least within the limits of our solar system, full of these things. They swarm like fishes in the seas. Like the fishes, moreover, they may be either solitary or gregarious. The solitary bit of cosmic rubbish is the meteorite, which we have just examined. A "social" group of meteorites is the essential part of a comet. The nucleus, or bright central part, of the head of a comet (Fig. 19) consists of a swarm, sometimes thousands of miles wide, of these pieces of iron or stone. This swarm has come under the sun's gravitational influence, and is forced to travel round it. From some dark region of s.p.a.ce it has moved slowly into our system. It is not then a comet, for it has no tail. But as the crowded meteors approach the sun, the speed increases. They give off fine vapour-like matter and the fierce flood of light from the sun sweeps this vapour out in an ever-lengthening tail. Whatever way the comet is travelling, the tail always points away from the sun.

A Great Comet.

The vapoury tail often grows to an enormous length as the comet approaches the sun. The great comet of 1843 had a tail two hundred million miles long. It is, however, composed of the thinnest vapours imaginable. Twice during the nineteenth century the earth pa.s.sed through the tail of a comet, and nothing was felt. The vapours of the tail are, in fact, so attenuated that we can hardly imagine them to be white-hot. They may be lit by some electrical force. However that may be, the comet dashes round the sun, often at three or four hundred miles a second, then may pa.s.s gradually out of our system once more. It may be a thousand years, or it may be fifty years, before the monarch of the system will summon it again to make its fiery journey round his throne.

[Ill.u.s.tration: Photo: Harvard College Observatory.

FIG. 21.--TYPICAL SPECTRA.

Six main types of stellar spectra. Notice the lines they have in common, showing what elements are met with in different types of stars. Each of these spectra corresponds to a different set of physical and chemical conditions.] [Ill.u.s.tration: Photo: Mount Wilson Observatory.

FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS.

Showing a great projection of "dark matter" cutting off the light from behind.]

[Ill.u.s.tration: Photo: Astrophysical Observatory, Victoria, British Columbia.

FIG. 23.--STAR Cl.u.s.tER IN HERCULES.

A wonderful cl.u.s.ter of stars. It has been estimated that the distance of this cl.u.s.ter is such that it would take light more than 100,000 years to reach us.]

THE STELLAR UNIVERSE.

-- 1.

The immensity of the Stellar Universe, as we have seen, is beyond our apprehension. The sun is nothing more than a very ordinary star, perhaps an insignificant one. There are stars enormously greater than the sun. One such, Betelgeux, has recently been measured, and its diameter is more than 300 times that of the sun.

The Evolution of Stars.

The proof of the similarity between our sun and the stars has come to us through the spectroscope. The elements that we find by its means in the sun are also found in the same way in the stars. Matter, says the spectroscope, is essentially the same everywhere, in the earth and the sun, in the comet that visits us once in a thousand years, in the star whose distance is incalculable, and in the great clouds of "fire-mist" that we call nebul-.

In considering the evolution of the stars let us keep two points clearly in mind. The starting-point, the nebula, is no figment of the scientific imagination. Hundreds of thousands of nebul-, besides even vaster irregular stretches of nebulous matter, exist in the heavens. But the stages of the evolution of this stuff into stars are very largely a matter of speculation. Possibly there is more than one line of evolution, and the various theories may be reconciled. And this applies also to the theories of the various stages through which the stars themselves pa.s.s on their way to extinction.

The light of about a quarter of a million stars has been a.n.a.lysed in the spectroscope, and it is found that they fall into about a dozen cla.s.ses which generally correspond to stages in their evolution (Fig. 21).

The Age of Stars.

In its main lines the spectrum of a star corresponds to its colour, and we may roughly group the stars into red, yellow, and white. This is also the order of increasing temperature, the red stars being the coolest and the white stars the hottest. We might therefore imagine that the white stars are the youngest, and that as they grow older and cooler they become yellowish, then red, and finally become invisible--just as a cooling white-hot iron would do. But a very interesting recent research shows that there are two kinds of red stars; some of them are amongst the oldest stars and some are amongst the youngest. The facts appear to be that when a star is first formed it is not very hot. It is an immense ma.s.s of diffuse gas glowing with a dull-red heat. It contracts under the mutual gravitation of its particles, and as it does so it grows hotter. It acquires a yellowish tinge. As it continues to contract it grows hotter and hotter until its temperature reaches a maximum as a white star. At this point the contraction process does not stop, but the heating process does. Further contraction is now accompanied by cooling, and the star goes through its colour changes again, but this time in the inverse order. It contracts and cools to yellow and finally to red. But when it again becomes a red star it is enormously denser and smaller than when it began as a red star. Consequently the red stars are divided into two cla.s.ses called, appropriately, Giants and Dwarfs. This theory, which we owe to an American astronomer, H. N. Russell, has been successful in explaining a variety of phenomena, and there is consequently good reason to suppose it to be true. But the question as to how the red giant stars were formed has received less satisfactory and precise answers.

The most commonly accepted theory is the nebular theory.

THE NEBULAR THEORY.

-- 2.

Nebul- are dim luminous cloud-like patches in the heavens, more like wisps of smoke in some cases than anything else. Both photography and the telescope show that they are very numerous, hundreds of thousands being already known and the number being continually added to. They are not small. Most of them are immensely large. Actual dimensions cannot be given, because to estimate these we must first know definitely the distance of the nebul- from the earth. The distances of some nebul- are known approximately, and we can therefore form some idea of size in these cases. The results are staggering. The mere visible surface of some nebul- is so large that the whole stretch of the solar system would be too small to form a convenient unit for measuring it. A ray of light would require to travel for years to cross from side to side of such a nebula. Its immensity is inconceivable to the human mind.

There appear to be two types of nebul-, and there is evidence suggesting that the one type is only an earlier form of the other; but this again we do not know.

The more primitive nebul- would seem to be composed of gas in an extremely rarified form. It is difficult to convey an adequate idea of the rarity of nebular gases. The residual gases in a vacuum tube are dense by comparison. A cubic inch of air at ordinary pressure would contain more matter than is contained in millions of cubic inches of the gases of nebul-. The light of even the faintest stars does not seem to be dimmed by pa.s.sing through a gaseous nebula, although we cannot be sure on this point. The most remarkable physical fact about these gases is that they are luminous. Whence they derive their luminosity we do not know. It hardly seems possible to believe that extremely thin gases exposed to the terrific cold of s.p.a.ce can be so hot as to be luminous and can retain their heat and their luminosity indefinitely. A cold luminosity due to electrification, like that of the aurora borealis, would seem to fit the case better.

Now the nebular theory is that out of great "fire-mists," such as we have described, stars are born. We do not know whether gravitation is the only or even the main force at work in a nebula, but it is supposed that under the action of gravity the far-flung "fire-mists" would begin to condense round centres of greatest density, heat being evolved in the process. Of course the condensation would be enormously slow, although the sudden irruption of a swarm of meteors or some solid body might hasten matters greatly by providing large, ready-made centres of condensation.

Spiral Nebul-.

It is then supposed that the contracting ma.s.s of gas would begin to rotate and to throw off gigantic streamers, which would in their turn form centres of condensation. The whole structure would thus form a spiral, having a dense region at its centre and knots or lumps of condensed matter along its spiral arms. Besides the formless gaseous nebul- there are hundreds of thousands of "spiral" nebul- such as we have just mentioned in the heavens. They are at all stages of development, and they are visible to us at all angles--that is to say, some of them face directly towards us, others are edge on, and some are in intermediate positions. It appears, therefore, that we have here a striking confirmation of the nebular hypothesis. But we must not go so fast. There is much controversy as to the nature of these spiral nebul-. Some eminent astronomers think they are other stellar universes, comparable in size with our own. In any case they are vast structures, and if they represent stars in process of condensation, they must be giving birth to huge agglomerations of stars--to star cl.u.s.ters at least. These vast and enigmatic objects do not throw much light on the origin of our own solar system. The nebular hypothesis, which was invented by Laplace to explain the origin of our solar system, has not yet met with universal acceptance. The explanation offers grave difficulties, and it is best while the subject is still being closely investigated, to hold all opinions with reserve. It may be taken as probable, however, that the universe has developed from ma.s.ses of incandescent gas.

[Ill.u.s.tration: Photo: Yerkes Observatory.

FIG. 24.--THE GREAT NEBULA IN ORION.

The most impressive nebula in the heavens. It is inconceivably greater in dimensions than the whole solar system.]

[Ill.u.s.tration: Photo: Lick Observatory.

FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914.

This spiral nebula is seen full on. Notice the central nucleus and the two spiral arms emerging from its opposite directions. Is matter flowing out of the nucleus into the arms or along the arms into the nucleus? In either case we should get two streams in opposite directions within the nucleus.]

THE BIRTH AND DEATH OF STARS.

-- 3.

Variable, New, and Dark Stars: Dying Suns.

Many astronomers believe that in "variable stars" we have another star, following that of the dullest red star, in the dying of suns. The light of these stars varies periodically in so many days, weeks, or years. It is interesting to speculate that they are slowly dying suns, in which the molten interior periodically bursts through the sh.e.l.l of thick vapours that is gathering round them. What we saw about our sun seems to point to some such stage in the future. That is, however, not the received opinion about variable stars. It may be that they are stars which periodically pa.s.s through a great swarm of meteors or a region of s.p.a.ce that is rich in cosmic dust of some sort, when, of course, a great illumination would take place.

One cla.s.s of these variable stars, which takes its name from the star Algol, is of special interest. Every third night Algol has its light reduced for several hours. Modern astronomy has discovered that in this case there are really two stars, circulating round a common centre, and that every third night the fainter of the two comes directly between us and its companion and causes an "eclipse." This was until recently regarded as a most interesting case in which a dead star revealed itself to us by pa.s.sing before the light of another star. But astronomers have in recent years invented something, the "selenium-cell," which is even more sensitive than the photographic plate, and on this the supposed dead star registers itself as very much alive. Algol is, however, interesting in another way. The pair of stars which we have discovered in it are hundreds of trillions of miles away from the earth, yet we know their ma.s.ses and their distances from each other.

The Death and Birth of Stars.

We have no positive knowledge of dead stars; which is not surprising when we reflect that a dead star means an invisible star! But when we see so many individual stars tending toward death, when we behold a vast population of all conceivable ages, we presume that there are many already dead. On the other hand, there is no reason to suppose that the universe as a whole is "running down." Some writers have maintained this, but their argument implies that we know a great deal more about the universe than we actually do. The scientific man does not know whether the universe is finite or infinite, temporal or eternal; and he declines to speculate where there are no facts to guide him. He knows only that the great gaseous nebul- promise myriads of worlds in the future, and he concedes the possibility that new nebul- may be forming in the ether of s.p.a.ce.

The last, and not the least interesting, subject we have to notice is the birth of a "new star." This is an event which astronomers now announce every few years; and it is a far more portentous event than the reader imagines when it is reported in his daily paper. The story is much the same in all cases. We say that the star appeared in 1901, but you begin to realise the magnitude of the event when you learn that the distant "blaze" had really occurred about the time of the death of Luther! The light of the conflagration had been speeding toward us across s.p.a.ce at 186,000 miles a second, yet it has taken nearly three centuries to reach us. To be visible at all to us at that distance the fiery outbreak must have been stupendous. If a ma.s.s of petroleum ten times the size of the earth were suddenly fired it would not be seen at such a distance. The new star had increased its light many hundredfold in a few days.

There is a considerable fascination about the speculation that in such cases we see the resurrection of a dead world, a means of renewing the population of the universe. What happens is that in some region of the sky where no star, or only a very faint star, had been registered on our charts, we almost suddenly perceive a bright star. In a few days it may rise to the highest brilliancy. By the spectroscope we learn that this distant blaze means a prodigious outpour of white-hot hydrogen at hundreds of miles a second. But the star sinks again after a few months, and we then find a nebula round it on every side. It is natural to suppose that a dead or dying sun has somehow been reconverted in whole or in part into a nebula. A few astronomers think that it may have partially collided with another star, or approached too closely to another, with the result we described on an earlier page. The general opinion now is that a faint or dead star had rushed into one of those regions of s.p.a.ce in which there are immense stretches of nebulous matter, and been (at least in part) vaporised by the friction.

But the difficulties are considerable, and some astronomers prefer to think that the blazing star may merely have lit up a dark nebula which already existed. It is one of those problems on which speculation is most tempting but positive knowledge is still very incomplete. We may be content, even proud, that already we can take a conflagration that has occurred more than a thousand trillion miles away and a.n.a.lyse it positively into an outflame of glowing hydrogen gas at so many miles a second.

THE SHAPE OF OUR UNIVERSE.

-- 4.

Our Universe a Spiral Nebula.

What is the shape of our universe, and what are its dimensions? This is a tremendous question to ask. It is like asking an intelligent insect, living on a single leaf in the midst of a great Brazilian forest, to say what is the shape and size of the forest. Yet man's ingenuity has proved equal to giving an answer even to this question, and by a method exactly similar to that which would be adopted by the insect. Suppose, for instance, that the forest was shaped as an elongated oval, and the insect lived on a tree near the centre of the oval. If the trees were approximately equally s.p.a.ced from one another they would appear much denser along the length of the oval than across its width. This is the simple consideration that has guided astronomers in determining the shape of our stellar universe. There is one direction in the heavens along which the stars appear denser than in the directions at right angles to it. That direction is the direction in which we look towards the Milky Way. If we count the number of stars visible all over the heavens, we find they become more and more numerous as we approach the Milky Way. As we go farther and farther from the Milky Way the stars thin out until they reach a maximum spa.r.s.eness in directions at right angles to the plane of the Milky Way. We may consider the Milky Way to form, as it were, the equator of our system, and the line at right angles to point to the north and south poles.

Our system, in fact, is shaped something like a lens, and our sun is situated near the centre of this lens. In the remoter part of this lens, near its edge, or possibly outside it altogether, lies the great series of star clouds which make up the Milky Way. All the stars are in motion within this system, but the very remarkable discovery has been made that these motions are not entirely random. The great majority of the stars whose motions can be measured fall into two groups drifting past one another in opposite directions. The velocity of one stream relative to the other is about twenty-five miles per second. The stars forming these two groups are thoroughly well mixed; it is not a case of an inner stream going one way and an outer stream the other. But there are not quite as many stars going one way as the other. For every two stars in one stream there are three in the other. Now, as we have said, some eminent astronomers hold that the spiral nebul- are universes like our own, and if we look at the two photographs (Figs. 25 and 26) we see that these spirals present features which, in the light of what we have just said about our system, are very remarkable. The nebula in Coma Berenices is a spiral edge-on to us, and we see that it has precisely the lens-shaped middle and the general flattened shape that we have found in our own system. The nebula in Canes Venatici is a spiral facing towards us, and its shape irresistibly suggests motions along the spiral arms. This motion, whether it is towards or away from the central, lens-shaped portion, would cause a double streaming motion in that central portion of the kind we have found in our own system. Again, and altogether apart from these considerations, there are good reasons for supposing our Milky Way to possess a double-armed spiral structure. And the great patches of dark absorbing matter which are known to exist in the Milky Way (see Fig. 22) would give very much the mottled appearance we notice in the arms (which we see edge-on) of the nebula in Coma Berenices. The hypothesis, therefore, that our universe is a spiral nebula has much to be said for it. If it be accepted it greatly increases our estimate of the size of the material universe. For our central, lens-shaped system is calculated to extend towards the Milky Way for more than twenty thousand times a million million miles, and about a third of this distance towards what we have called the poles. If, as we suppose, each spiral nebula is an independent stellar universe comparable in size with our own, then, since there are hundreds of thousands of spiral nebul-, we see that the size of the whole material universe is indeed beyond our comprehension.

[Ill.u.s.tration: Photo: Mount Wilson Observatory.

FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON.

Notice the lens-shaped formation of the nucleus and the arm stretching as a band across it. See reference in the text to the resemblance between this and our stellar universe.]

[Ill.u.s.tration: Photo: H. J. Shepstone.

100-INCH TELESCOPE, MOUNT WILSON.

A reflecting telescope: the largest in the world. The mirror is situated at the base of the telescope.]

[Ill.u.s.tration: ______ | | | THE SOLAR SYSTEM | |________| | | | | | | | | MEAN DISTANCE | PERIOD OF | | | | NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF | | | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES | | | MILES) | (IN YEARS) | | | |_______|________| | | | | | | | MERCURY | 36.0 | 0.24 | 3030 | 0 | | VENUS | 67.2 | 0.62 | 7700 | 0 | | EARTH | 92.9 | 1.00 | 7918 | 1 | | MARS | 141.5 | 1.88 | 4230 | 2 | | JUPITER | 483.3 | 11.86 | 86500 | 9 | | SATURN | 886.0 | 29.46 | 73000 | 10 | | URa.n.u.s | 1781.9 | 84.02 | 31900 | 4 | | NEPTUNE | 2971.6 | 164.78 | 34800 | 1 | | SUN | ------ | ------ | 866400 | -- | | MOON | ------ | ------ | 2163 | -- FIG. 27]

[Ill.u.s.tration: _________| | | STAR DISTANCES | |______| | | | DISTANCE IN | | STAR LIGHT-YEARS | | | | POLARIS 76 | | CAPELLA 49.4 | | RIGEL 466 | | SIRIUS 8.7 | | PROCYON 10.5 | | REGULUS 98.8 | | ARCTURUS 43.4 | | [ALPHA] CENTAURI 4.29 | | VEGA 34.7 | |________| | | | SMALLER MAGELLANIC CLOUD 32,600[A] | | GREAT Cl.u.s.tER IN HERCULES 108,600[A] | |______| [A] ESTIMATED.

FIG. 28.

The above distances are merely approximate and are subject to further revision. A "light-year" is the distance that light, travelling at the rate of 186,000 miles per second, would cover in one year.]

In this simple outline we have not touched on some of the more debatable questions that engage the attention of modern astronomers. Many of these questions have not yet pa.s.sed the controversial stage; out of these will emerge the astronomy of the future. But we have seen enough to convince us that, whatever advances the future holds in store, the science of the heavens const.i.tutes one of the most important stones in the wonderful fabric of human knowledge.

ASTRONOMICAL INSTRUMENTS.

-- 1.

The Telescope.

The instruments used in modern astronomy are amongst the finest triumphs of mechanical skill in the world. In a great modern observatory the different instruments are to be counted by the score, but there are two which stand out pre-eminent as the fundamental instruments of modern astronomy. These instruments are the telescope and the spectroscope, and without them astronomy, as we know it, could not exist.

There is still some dispute as to where and when the first telescope was constructed; as an astronomical instrument, however, it dates from the time of the great Italian scientist Galileo, who, with a very small and imperfect telescope of his own invention, first observed the spots on the sun, the mountains of the moon, and the chief four satellites of Jupiter. A good pair of modern binoculars is superior to this early instrument of Galileo's, and the history of telescope construction, from that primitive instrument to the modern giant recently erected on Mount Wilson, California, is an exciting chapter in human progress. But the early instruments have only an historic interest: the era of modern telescopes begins in the nineteenth century.

During the last century telescope construction underwent an unprecedented development. An immense amount of interest was taken in the construction of large telescopes, and the different countries of the world entered on an exciting race to produce the most powerful possible instruments. Besides this rivalry of different countries there was a rivalry of methods. The telescope developed along two different lines, and each of these two types has its partisans at the present day. These types are known as refractors and reflectors, and it is necessary to mention, briefly, the principles employed in each. The refractor is the ordinary, familiar type of telescope. It consists, essentially, of a large lens at one end of a tube, and a small lens, called the eye-piece, at the other. The function of the large lens is to act as a sort of gigantic eye. It collects a large amount of light, an amount proportional to its size, and brings this light to a focus within the tube of the telescope. It thus produces a small but bright image, and the eye-piece magnifies this image. In the reflector, instead of a large lens at the top of the tube, a large mirror is placed at the bottom. This mirror is so shaped as to reflect the light that falls on it to a focus, whence the light is again led to an eye-piece. Thus the refractor and the reflector differ chiefly in their manner of gathering light. The powerfulness of the telescope depends on the size of the light-gatherer. A telescope with a lens four inches in diameter is four times as powerful as the one with a lens two inches in diameter, for the amount of light gathered obviously depends on the area of the lens, and the area varies as the square of the diameter.

The largest telescopes at present in existence are reflectors. It is much easier to construct a very large mirror than to construct a very large lens; it is also cheaper. A mirror is more likely to get out of order than is a lens, however, and any irregularity in the shape of a mirror produces a greater distorting effect than in a lens. A refractor is also more convenient to handle than is a reflector. For these reasons great refractors are still made, but the largest of them, the great Yerkes' refractor, is much smaller than the greatest reflector, the one on Mount Wilson, California. The lens of the Yerkes' refractor measures three feet four inches in diameter, whereas the Mount Wilson reflector has a diameter of no less than eight feet four inches.

[Ill.u.s.tration: THE YERKES 40-INCH REFRACTOR.

(The largest refracting telescope in the world. Its big lens weighs 1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about 12,000 pounds. The parts to be moved weigh approximately 22 tons.

The great 100-inch reflector of the Mount Wilson reflecting telescope--the largest reflecting instrument in the world--weighs nearly 9,000 pounds and the moving parts of the telescope weigh about 100 tons.

The new 72-inch reflector at the Dominion Astrophysical Observatory, near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts about 35 tons.)]

[Ill.u.s.tration: Photo: H. J. Shepstone.

THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE.

The smaller telescope at the top of the picture acts as a "finder"; the field of view of the large telescope is so restricted that it is difficult to recognise, as it were, the part of the heavens being surveyed. The smaller telescope takes in a larger area and enables the precise object to be examined to be easily selected.]

[Ill.u.s.tration: MODERN DIRECT-READING SPECTROSCOPE.

(By A. Hilger, Ltd.).

The light is brought through one telescope, is split up by the prism, and the resulting spectrum is observed through the other telescope.]

But there is a device whereby the power of these giant instruments, great as it is, can be still further heightened. That device is the simple one of allowing the photographic plate to take the place of the human eye. Nowadays an astronomer seldom spends the night with his eye glued to the great telescope. He puts a photographic plate there. The photographic plate has this advantage over the eye, that it builds up impressions. However long we stare at an object too faint to be seen, we shall never see it. With the photographic plate, however, faint impressions go on acc.u.mulating. As hour after hour pa.s.ses, the star which was too faint to make a perceptible impression on the plate goes on affecting it until finally it makes an impression which can be made visible. In this way the photographic plate reveals to us phenomena in the heavens which cannot be seen even through the most powerful telescopes.

Telescopes of the kind we have been discussing, telescopes for exploring the heavens, are mounted equatorially; that is to say, they are mounted on an inclined pillar parallel to the axis of the earth so that, by rotating round this pillar, the telescope is enabled to follow the apparent motion of a star due to the rotation of the earth. This motion is effected by clock-work, so that, once adjusted on a star, and the clock-work started, the telescope remains adjusted on that star for any length of time that is desired. But a great official observatory, such as Greenwich Observatory or the Observatory at Paris, also has transitinstruments, or telescopes smaller than the equatorials and without the same facility of movement, but which, by a number of exquisite refinements, are more adapted to accurate measurements. It is these instruments which are chiefly used in the compilation of the Nautical Almanac. They do not follow the apparent motions of the stars. Stars are allowed to drift across the field of vision, and as each star crosses a small group of parallel wires in the eye-piece its precise time of pa.s.sage is recorded. Owing to their relative fixity of position these instruments can be constructed to record the positions of stars with much greater accuracy than is possible to the more general and flexible mounting of equatorials. The recording of transit is comparatively dry work; the spectacular element is entirely absent; stars are treated merely as mathematical points. But these observations furnish the very basis of modern mathematical astronomy, and without them such publications as the Nautical Almanac and the Connaissance du Temps would be robbed of the greater part of their importance.

-- 2.

The Spectroscope.

We have already learnt something of the principles of the spectroscope, the instrument which, by making it possible to learn the actual const.i.tution of the stars, has added a vast new domain to astronomy. In the simplest form of this instrument the a.n.a.lysing portion consists of a single prism. Unless the prism is very large, however, only a small degree of dispersion is obtained. It is obviously desirable, for accurate a.n.a.lytical work, that the dispersion--that is, the separation of the different parts of the spectrum--should be as great as possible. The dispersion can be increased by using a large number of prisms, the light emerging from the first prism, entering the second, and so on. In this way each prism produces its own dispersive effect and, when a number of prisms are employed, the final dispersion is considerable. A considerable amount of light is absorbed in this way, however, so that unless our primary source of light is very strong, the final spectrum will be very feeble and hard to decipher.

Another way of obtaining considerable dispersion is by using a diffraction grating instead of a prism. This consists essentially of a piece of gla.s.s on which lines are ruled by a diamond point. When the lines are sufficiently close together they split up light falling on them into its const.i.tuents and produce a spectrum. The modern diffraction grating is a truly wonderful piece of work. It contains several thousands of lines to the inch, and these lines have to be s.p.a.ced with the greatest accuracy. But in this instrument, again, there is a considerable loss of light.

We have said that every substance has its own distinctive spectrum, and it might be thought that, when a list of the spectra of different substances has been prepared, spectrum a.n.a.lysis would become perfectly straightforward. In practice, however, things are not quite so simple. The spectrum emitted by a substance is influenced by a variety of conditions. The pressure, the temperature, the state of motion of the object we are observing, all make a difference, and one of the most laborious tasks of the modern spectroscopist is to disentangle these effects from one another. Simple as it is in its broad outlines, spectroscopy is, in reality, one of the most intricate branches of modern science.

BIBLIOGRAPHY.

(The following list of books may be useful to readers wishing to pursue further the study of Astronomy.) BALL, The Story of the Heavens. BALL, The Story of the Sun. FORBES, History of Astronomy. HINCKS, Astronomy. KIPPAX, Call of the Stars. LOWELL, Mars and Its Ca.n.a.ls. LOWELL, Evolution of Worlds. MCKREADY, A Beginner's Star-Book. NEWCOMB, Popular Astronomy. NEWCOMB, The Stars: A Study of the Universe. OLCOTT, Field Book of the Stars. PRICE, Essence of Astronomy. SERVISS, Curiosities of the Skies. WEBB, Celestial Objects for Common Telescopes. YOUNG, Text-Book of General Astronomy.

II.

THE STORY OF EVOLUTION.

INTRODUCTORY.

THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING CREATURES.

-- 1.

The Evolution-idea is a master-key that opens many doors. It is a luminous interpretation of the world, throwing the light of the past upon the present. Everything is seen to be an antiquity, with a history behind it--a natural history, which enables us to understand in some measure how it has come to be as it is. We cannot say more than "understand in some measure," for while the fact of evolution is certain, we are only beginning to discern the factors that have been at work.

The evolution-idea is very old, going back to some of the Greek philosophers, but it is only in modern times that it has become an essential part of our mental equipment. It is now an everyday intellectual tool. It was applied to the origin of the solar system and to the making of the earth before it was applied to plants and animals; it was extended from these to man himself; it spread to language, to folk-ways, to inst.i.tutions. Within recent years the evolution-idea has been applied to the chemical elements, for it appears that uranium may change into radium, that radium may produce helium, and that lead is the final stable result when the changes of uranium are complete. Perhaps all the elements may be the outcome of an inorganic evolution. Not less important is the extension of the evolution-idea to the world within as well as to the world without. For alongside of the evolution of bodies and brains is the evolution of feelings and emotions, ideas and imagination.

Organic evolution means that the present is the child of the past and the parent of the future. It is not a power or a principle; it is a process--a process of becoming. It means that the present-day animals and plants and all the subtle inter-relations between them have arisen in a natural knowable way from a preceding state of affairs on the whole somewhat simpler, and that again from forms and inter-relations simpler still, and so on backwards and backwards for millions of years till we lose all clues in the thick mist that hangs over life's beginnings.