CHAPTER XXII.

THE SPECTROSCOPE.

A New Department of Science-The Materials of the Heavenly Bodies-Meaning of Elementary Bodies-Chemical Analysis and Spectroscopic Analysis-The Composite Nature of Light-Whence Colours?-The Rainbow-The PrismPassage of Light through a Prism-Identification of Metals by the Rays they emit when Incandescent-The great Discovery of the Identity of the D-Lines with Sodium-The Dark Lines in the Solar Spectrum Interpreted-Metals present in the Sun-Examination of Light from the Moon or the Planets-The Prominences surrounding the Sun-Photographs of Spectra-Measurement of the Motion of the Stars along the Line of Sight.

THE revelations of the spectroscope form an important chapter in modern science. By its aid a mighty stride has been taken in our attempt to comprehend that elaborate system of suns and other bodies which adorn the skies.

The subject is widely dissimilar to those branches of astronomy which had been previously cultivated. The discoveries of the early astronomers had taught us how the earth revolves around the sun. We have been told how the distances of the different bodies were to be measured, and how their sizes were found. More wonderful still, the older astronomy narrated how we could weigh one body in the heavens against another. We knew that the substance of which the sun was made glowed with intense heat; but we could not tell, we had no means of telling, what was the nature of that substance. To this the old astronomy could render no answer whatever; and, indeed, half a century ago it would have been thought incredible that such a question could be answered. If this be the case with regard to the sun, what are we to say of the other and still less known bodies of the heavens? What is the physical nature of the other planets? What are the substances present in the stars, and in those dim nebula which lie on the confines of

the visible universe? have afforded an answer to these questions: not, indeed, with all the fulness that might be desired, and that, perhaps, may yet be expected; but still they have been answered in some degree, and the results are of great interest.

It is the triumph of modern astronomy to

Let us first enunciate the problem in a definite shape; and here we must call in the aid of chemistry to enable us to obtain distinct views on the subject. What are we to understand by the substances of which the sun is composed? What do we even mean by the substances of which the earth is composed? At a first glance we would say that our globe is composed of rocks and clay, of air and water, but the chemist will strive to render our ideas more accurate. He shows us how rocks are composed of certain other substances, which he calls elements, and the elements are substances which he cannot decompose into anything else. will also tell us how the air is composed of two elementary substances, oxygen and nitrogen, mingled together; and how water is composed of oxygen and hydrogen in a state of combination. The chemist pursues his analysis through every solid, liquid, and gas on the globe; he decomposes animal or vegetable substances into their elements, and the outcome of his labours is to demonstrate that every particle of matter cn our globe, or in the atmosphere surrounding it, is composed of one or more elementary bodies, and that the whole number of such elementary bodies is about sixty or rather more. The elements may be solids, like iron, or gold, or carbon; or they may be gases, like oxygen, or hydrogen, or nitrogen. Many of the elements are extremely rare. About twenty of the more ordinary ones are all that need concern us at present.

We are now enabled to state a special problem of modern astronomy. Are these elements of which the earth is composed peculiar to the earth, or are they found on the other bodies which teem throughout space? Take, for example, the most abundant of all metals, iron, which exists in rich profusion near the surface of the earth, and which exists perhaps in no less abundance in the interior of the earth. Is iron a product limited to this, our

globe? or does it enter into the composition of other bodies in the universe? This is one of the questions which modern science has answered. How that answer has been given it is the object of this chapter to unfold.

Quite a new phase of astronomy is here opened up. Great telescopes revealed faint objects, and showed the features clearly which small telescopes only showed dimly, but all the telescopes in the world would not answer the question as to whether iron was found in the sun. A totally distinct branch of inquiry was necessary, which is known as spectrum analysis. We could not expect to actually see iron in the sun. The sun is itself a mighty glowing globe, infinitely hotter than a Bessemer converter or a Siemens furnace; if iron is in the sun, that iron is not only whitehot and molten, but actually driven off into vapour; but vapour of iron is not distinguishable. How would you recognise it? How would you know it if commingled with the vapour of many other metals or other substances? It is, in truth, a delicate piece of analysis to discriminate iron in the glowing atmosphere of the sun. But the spectroscope is adequate to the task, and it renders its analysis with an evidence that is absolutely convincing.

Wide indeed is the gulf which separates this branch of analysis from those to which chemists were formerly restricted. To analyse a body in the old fashion, the chemist must have a sample of that body, and then by his re-agents and his test tubes, he could determine what the body contained. But how is the chemist to procure a sample of the sun? He can never procure a sample of any body external to the earth. No doubt, in the case where a body, which we call a meteorite, falls to the earth, we certainly can submit to chemical analysis an actual fragment derived from external space. It is a matter of significance to observe that when a chemist analyses a meteorite, he finds in it no element with which he is not already familiar; nay, rather, he is struck by the remarkable fact that while nearly all meteorites contain iron, some are almost entirely composed of that widely distributed metal. We cannot, however, for the reasons set forth in Chapter XVII., draw any reliable conclusions from meteorites as to the

constituents of bodies exterior to the earth. In our attempts to analyse the sun, the moon, the planets, or the stars, we are therefore deprived of the ordinary resources of chemistry. To deal with the problem a new branch of science has been created.

Each

What we receive from the sun is warmth and light. The intensely heated mass of the sun radiates forth its beams in all directions with boundless prodigality. Each beam we feel to be warm, and we see to be brilliantly white, but a more subtle analysis than mere feeling or mere vision is required. sunbeam really bears indelible marks of its origin. These marks are not visible until a special process has been applied, but then the sunbeam can be made to tell its story, and it will disclose to us the nature of the sun's constitution.

We speak of the sun's light as colourless, just as we speak of water as tasteless, but both of these expressions relate rather to our own feelings than to anything really characteristic of water or of sunlight. We regard the sunlight as colourless because it forms, as it were, the background on which all other colours are depicted. The fact is, that white is so far from being colourless, that it contains every hue known to us blended together in certain proportions. The sun's light is really extremely composite; Nature herself tells us this if we will but give her the slightest attention. Whence come the beautiful hues with which we are all familiar? Look at the lovely tints of a garden; the red of the rose is not in the rose itself. All the rose does is to grasp the sunbeams which fall upon it, extract from these beams the red which is in them, and radiate that red light to our eyes. Were there not red rays commingled with the other rays in the sunbeam, there could be no red rose to be seen by sunlight. This point is, in fact, well known in ordinary conversation; a lady will often say that a dress which looks very well in the daylight does not answer in the evening. The reason is that the dress is intended to show certain colours which exist in the sunlight; but these colours do not exist to the same degree in gaslight, and consequently the dress has a different hue. The fault is not in the dress, the fault lies in the gas; and when the electric light is used,

it sends forth beams more nearly resembling those from the sun, and the colours appear again with all their intended beauty.

The most glorious natural indication of the nature of the sunlight is seen in the rainbow. Here the sunbeams are refracted and reflected from the tiny globes of water falling as rain-drops from the clouds; these convey to us the sunlight, and in doing so decompose the white beams into the seven primary colours-red, orange, yellow, green, blue, indigo, and violet.

The bow set in the cloud is typical of that great department of modern science which we discuss in this chapter. The globes of water decompose the solar beams; and we follow the course suggested by the rainbow, and analyze the sunlight into its

Fig. 82.--The Prism.

constituents. We are enabled to do this with scientific accuracy when we employ that admirable invention, the spectroscope. The beams of white sunlight really consist of innumerable beams of every hue all commingled together. Every shade of red, of yellow, of blue, and of green can be found in a sunbeam. The magician's wand, with which we can strike the sunbeam and instantly sort out into perfect order the tangled skein, is the simple instrument known as the glass prism. We have represented a prism in its simplest form in the adjoining figure (Fig. 82). It is a piece of pure and homogeneous glass, ground and polished to the shape of a wedge. When a ray of light from the sun or from any source falls upon the prism, it passes through the transparent glass and emerges on the other side; a remarkable change is, however, impressed on the ray by the influence of the glass. It is bent by refraction from the path it originally pursued, and it is compelled to follow a different path. If, however, the prism bent all rays of light