A Wonderful Instrument and what it Reveals
The spectroscope is an instrument for analysing light. So important is it in the revelations it has given us that it will be best to describe it fully. Every substance to be examined must first be made to glow, made luminous; and as nearly everything in the heavens is luminous the instrument has a great range in Astronomy. And when we speak of analysing light, we mean that the light may be broken up into waves of different lengths. What we call light is a series of minute waves in ether, and these waves are—measuring them from crest to crest, so to say—of various lengths. Each wave-length corresponds to a colour of the rainbow. The shortest waves give us a sensation of violet colour, and the largest waves cause a sensation of red. The rainbow, in fact, is a sort of natural spectrum. (The meaning of the rainbow is that the moisture-laden air has sorted out these waves, in the sun's light, according to their length.) Now the simplest form of spectroscope is a glass prism—a triangular-shaped piece of glass. If white light (sunlight, for example) passes through a glass prism, we see a series of rainbow-tinted colours. Anyone can notice this effect when sunlight is shining through any kind of cut glass—the stopper of a wine decanter, for instance. If, instead of catching with the eye the coloured lights as they emerge from the glass prism, we allow them to fall on a screen, we shall find that they pass, by continuous gradations, from red at the one end of the screen, through orange, yellow, green, blue, and indigo, to violet at the other end. In other words, what we call white light is composed of rays of these several colours. They go to make up the effect which we call white. And now just as water can be split up into its two elements, oxygen and hydrogen, so sunlight can be broken up into its primary colours, which are those we have just mentioned.
This range of colours, produced by the spectroscope, we call the solar spectrum, and these are, from the spectroscopic point of view, primary colours. Each shade of colour has its definite position in the spectrum. That is to say, the light of each shade of colour (corresponding to its wave-length) is reflected through a certain fixed angle on passing through the glass prism. Every possible kind of light has its definite position, and is denoted by a number which gives the wave-length of the vibrations constituting that particular kind of light.
Now, other kinds of light besides sunlight can be analysed. Light from any substance which has been made incandescent may be observed with the spectroscope in the same way, and each element can be thus separated. It is found that each substance (in the same conditions of pressure, etc.) gives a constant spectrum of its own. Each metal displays its own distinctive colour. It is obvious, therefore, that the spectrum provides the means for identifying a particular substance. It was by this method that we discovered in the sun the presence of such well-known elements as sodium, iron, copper, zinc, and magnesium.
Every chemical element known, then, has a distinctive spectrum of its own when it is raised to incandescence, and this distinctive spectrum is as reliable a means of identification for the element as a human face is for its owner. Whether it is a substance glowing in the laboratory or in a remote star makes no difference to the spectroscope; if the light of any substance reaches it, that substance will be recognised and identified by the characteristic set of waves.
The spectrum of a glowing mass of gas will consist in a number of bright lines of various colours, and at various intervals; corresponding to each kind of gas, there will be a peculiar and distinctive arrangement of bright lines. But if the light from such a mass of glowing gas be made to pass through a cool mass of the same gas it will be found that dark lines replace the bright lines in the spectrum, the reason for this being that the cool gas absorbs the rays of light emitted by the hot gas. Experiments of this kind enable us to reach the important general statement that every gas, when cold, absorbs the same rays of light which it emits when hot.
Crossing the solar spectrum are hundreds and hundreds of dark lines. These could not at first be explained, because this fact of discriminative absorption was not known. We understand now. The sun's white light comes from the photosphere, but between us and the photosphere there is, as we have seen, another solar envelope of relatively cooler vapours—the reversing layer. Each constituent element in this outer envelope stops its own kind of light, that is, the kind of light made by incandescent atoms of the same element in the photosphere. The "stoppages" register themselves in the solar spectrum as dark lines placed exactly where the corresponding bright lines would have been. The explanation once attained, dark lines became as significant as bright lines. The secret of the sun's composition was out. We have found practically every element in the sun that we know to be in the earth. We have identified an element in the sun before we were able to isolate it on the earth. We have been able even to point to the coolest places on the sun, the centres of sun-spots, where alone the temperature seems to have fallen sufficiently low to allow chemical compounds to form.
It is thus we have been able to determine what the stars, comets, or nebulæ are made of.
In 1868 Sir Norman Lockyer detected a light coming from the prominences of the sun which was not given by any substance known on earth, and attributed this to an unknown gas which he called helium, from the Greek helios, the sun. In 1895 Sir William Ramsay discovered in certain minerals the same gas identified by the spectroscope. We can say, therefore, that this gas was discovered in the sun nearly thirty years before it was found on earth; this discovery of the long-lost heir is as thrilling a chapter in the detective story of science as any in the sensational stories of the day, and makes us feel quite certain that our methods really tell us of what elements sun and stars are built up. The light from the corona of the sun, as we have mentioned indicates a gas still unknown on earth, which has been christened Coronium.
Measuring the Speed of Light
But this is not all; soon a new use was found for the spectroscope. We found that we could measure with it the most difficult of all speeds to measure, speed in the line of sight. Movement at right angles to the direction in which one is looking is, if there is sufficient of it, easy to detect, and, if the distance of the moving body is known, easy to measure. But movement in the line of vision is both difficult to detect and difficult to measure. Yet, even at the enormous distances with which astronomers have to deal, the spectroscope can detect such movement and furnish data for its measurement. If a luminous body containing, say, sodium is moving rapidly towards the spectroscope, it will be found that the sodium lines in the spectrum have moved slightly from their usual definite positions towards the violet end of the spectrum, the amount of the change of position increasing with the speed of the luminous body. If the body is moving away from the spectroscope the shifting of the spectral lines will be in the opposite direction, towards the red end of the spectrum. In this way we have discovered and measured movements that otherwise would probably not have revealed themselves unmistakably to us for thousands of years. In the same way we have watched, and measured the speed of, tremendous movements on the sun, and so gained proof that the vast disturbances we should expect there actually do occur.