Chapter 6

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 nebulæ 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 nebulæ, we see that the size of the whole material universe is indeed beyond our comprehension.

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 passed 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 constitutes 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.

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 accumulating. As hour after hour passes, 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 _transit_ instruments, 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 passage 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 constitution of the stars, has added a vast new domain to astronomy. In the simplest form of this instrument the analysing 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 analytical 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 glass 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 constituents 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 spaced 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 analysis 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 Canals_. 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 institutions. 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.

Our solar system was once represented by a nebula of some sort, and we may speak of the evolution of the sun and the planets. But since it has been _the same material throughout_ that has changed in its distribution and forms, it might be clearer to use some word like genesis. Similarly, our human institutions were once very different from what they are now, and we may speak of the evolution of government or of cities. But Man works with a purpose, with ideas and ideals in some measure controlling his actions and guiding his achievements, so that it is probably clearer to keep the good old word history for all processes of social becoming in which man has been a conscious agent. Now between the genesis of the solar system and the history of civilisation there comes the vast process of organic evolution. The word development should be kept for the becoming of the individual, the chick out of the egg, for instance.

Organic evolution is a continuous natural process of racial change, by successive steps in a definite direction, whereby distinctively new individualities arise, take root, and flourish, sometimes alongside of, and sometimes, sooner or later, in place of, the originative stock. Our domesticated breeds of pigeons and poultry are the results of evolutionary change whose origins are still with us in the Rock Dove and the Jungle Fowl; but in most cases in Wild Nature the ancestral stocks of present-day forms are long since extinct, and in many cases they are unknown. Evolution is a long process of coming and going, appearing and disappearing, a long-drawn-out sublime process like a great piece of music.

§ 2

The Beginning of the Earth

When we speak the language of science we cannot say "In the beginning," for we do not know of and cannot think of any condition of things that did not arise from something that went before. But we may qualify the phrase, and legitimately inquire into the beginning of the earth within the solar system. If the result of this inquiry is to trace the sun and the planets back to a nebula we reach only a relative beginning. The nebula has to be accounted for. And even before matter there may have been a pre-material world. If we say, as was said long ago, "In the beginning was Mind," we may be expressing or trying to express a great truth, but we have gone BEYOND SCIENCE.

The Nebular Hypothesis

One of the grandest pictures that the scientific mind has ever thrown upon the screen is that of the Nebular Hypothesis. According to Laplace's famous form of this theory (1796), the solar system was once a gigantic glowing mass, spinning slowly and uniformly around its centre. As the incandescent world-cloud of gas cooled and its speed of rotation increased the shrinking mass gave off a separate whirling ring, which broke up and gathered together again as the first and most distant planet. The main mass gave off another ring and another till all the planets, including the earth, were formed. The central mass persisted as the sun.

Laplace spoke of his theory, which Kant had anticipated forty-one years before, with scientific caution: "conjectures which I present with all the distrust which everything not the result of observation or of calculation ought to inspire." Subsequent research justified his distrust, for it has been shown that the original nebula need not have been hot and need not have been gaseous. Moreover, there are great difficulties in Laplace's theory of the separation of successive rings from the main mass, and of the condensation of a whirling gaseous ring into a planet.