Astronomy for Everybody/Part 2/Chapter 4

The spectroscope is an instrument for analysing light. It is a much more recent instrument than the telescope, having first been applied to astronomical observation about 1864. To convey an intelligent idea of its use we must say something about the heat and light radiated by the heavenly bodies.

We know that the sun, a gas light, or other bright body gives us heat as well as light. A very simple observation will show that the rays of heat proceed in straight lines like those of light, and that they can pass through air and other transparent bodies without warming them, just as light does. If we make a large fire on the hearth in a perfectly cold room, we shall feel the heat on our faces although the air may be frosty. A striking experiment is that of making a lens out of ice and using it as a burning glass. The rays of the sun passing through the ice may be concentrated so as to burn the hand, and that without the ice melting.

It was formerly supposed that heat and light were two distinct agents; now it is known that such is not the case. As emitted by a hot body both may be called by the general name of radiance. All radiance, when it falls on a surface, produces heat, just as the blaze of the fire produces heat on the walls of a room. But not all radi​ance affects the optic nerve of the eye so as to produce a sensation of light and enable us to see bodies.

It is now know that radiance consists of something in the nature of waves in an ethereal medium which fills all space, even to the most distant star. These waves are exceedingly short. To form an idea of their length we must measure by the micron, which is one thousandth of a millimetre. Those which produce the sensation of light on the optic nerve mostly range between four and seven tenths of a micron.
Fig. 14.—Wave Length of Light. This allows between forty and eighty thousand waves to the inch. We represent these waves by the little wave line in the figure. The distance between the dotted lines is the wave lengths. The peculiar feature of the radiance emitted by the sun, or any other body that is not transparent, is that it is not all of the same wave length, but of a very wide range of wave lengths all mixed together. We must imagine that between the rays which we represent in the figure there are an infinity of others, all varying in their wave lengths. In this respect radiance is like the waves of the ocean, which range in length from several hundred yards to a few inches, all piled upon each other.

When the radiance passes through a glass prism it is refracted from its course. Different wave lengths are refracted differently, but waves of the same length are always refracted by the same amount. This is shown by the familiar experiment of forming a spectrum of the ​sun with a triangular prism. Arranging the light to be thrown on a screen, we see red light at the bottom then yellow above it, then in succession, green, blue, and violet.
Fig. 15.—Arrangement of the Colours of the Spectrum, with the Dark Lines A, B, C, D, etc., of the Spectrum. This arrangement of colours on a surface is called a spectrum. The colour of the light in the spectrum depends on the wave length. If the wave length is greater than about seventy-five one-hundredths of a micron, that is, one forty-four-thousandth of an inch, the eye does not see it, and, for us, it passes simply as heat. From this length to one fifty-thousandth it looks red, when a little shorter it looks scarlet, then yellow, and so on. Shorter than forty-three one-hundredths of a micron it is difficult to see it at all. But the violet light affects the photographic plate even more strongly than the light which looks brightest to the eye. The light which is most easily photographed is the blue and violet, and as we go toward the red the photographic effect diminishes.

All bodies emit radiance, but, at ordinary temperatures, the wave lengths of this radiance are too long to be visible to the eye. Not until we heat a body red hot does it emit radiance of wave length short enough to ​form light. As we make it hotter it still emits more and more waves of long wave lengths, and also waves of shorter and shorter wave lengths. Thus as we heat up a piece of iron, it appears first as red hot, and afterward as white hot.

The possibility of reaching conclusions about the constitution of a hot body from the light which it emits arises from the fact that different bodies emit light of different wave lengths. If the body is solid, it emits light of all wave lengths, and we cannot tell much about it. But if it is a mass of transparent gas, it only emits light of certain wave lengths, depending on the nature of the gas.

The easiest way of making a gas emit its peculiar light is by passing an electric spark or current through it. Then, if we analyse the light produced by the spark with a prism, we find that the spectrum is composed of one or more bright lines, varying in position according to the nature of the gas. Thus we have a spectrum of hydrogen, another of oxygen, and others of almost all the bodies which we know. Solid bodies, including all the metals, can be made to give their spectrum by being heated so intensely by the electric spark that a small quantity of the body is changed into a gas. Thus we may even form a spectrum of iron, which the practised observer can immediately detect as iron by the position and arrangement of the lines of the spectrum.

The fundamental principle of spectrum analysis is that if the light of an incandescent body passes through ​a gas which is cooler than the body, the latter will cull out and absorb from the light those wave lengths which it would emit if it were itself incandescent. The result is that the spectrum from the solid body will be seen crossed by certain dark lines, depending on the nature of the gas through which the light has passed. Thus, if we observe an electric light through a prism in its immediate neighbourhood, the spectrum will be unbroken from one end to the other. But if the light is at a great distance, we shall see it crossed by a great number of dark lines. These lines are produced by the air through which the light has passed culling out the light which has certain wave lengths. It is of interest that the aqueous vapour in the air is the most powerful agent in this, and culls out great groups of lines, by which its presence in the air can be immediately detected. The darkest of the lines found in the spectrum of the sun are designated by the letters A, B, C, etc., as shown in the preceding figure.

We may describe the spectroscope in the most comprehensive way by saying that it is an instrument for studying the spectra of bodies, whether in the heavens or on the earth.

The studies of the heavenly bodies with the spectroscope have two objects. One is to determine the nature of the bodies; the other their motions to or from us. The possibility of the latter is one of the most wonderful achievements of modern science. If a star is coming toward us, the wave length of the light which it emits is slightly shorter in consequence of the motion; if it is ​going away from us, it is longer. Thus, by measuring the positions of its lines in the spectrum, it is possible to determine whether a star is approaching us or moving away from us.

In recent years the studies of the spectra of stars have been made almost entirely by photography. It is found that, as in other cases, the sensitive plates now used in that art will take impressions of objects which the eye cannot see in the telescope. So the astronomer photographs the spectrum of a star, which will show all the lines he can see with the naked eye, and perhaps a great many more. The positions of these lines are measured and studied, and the astronomer's conclusions are drawn from these studies.