All you have to do is to stand away from the slit five or six feet and look at the beam of light through a prism just as you did at the flame of the candle. This is the simplest form of a spectroscope, that is an instrument for splitting up the light of any object which is self-luminous, into a spectrum, as shown in [Fig. 182].
When you look at the sunlight streaming in through the slit in the cardboard you will see a number of dark lines crossing the colored band. These dark lines are called Fraunhofer’s lines, because Fraunhofer, who was a German telescope maker, was the first to show how important they are. The chief dark lines of the Sun’s spectrum are known by the letters of the alphabet, and, like the colored images of the slit, they are always in the same place. They are shown in [Fig. 183].
Having found out a little about the spectrum and how it is formed by the Sun, let’s find out next how it tells us what the Sun and the stars are made of.
Fig. 183.—Fraunhofer’s Lines.
Now, in the Sun’s spectrum there are a pair of lines close together and near the middle which are called the D lines—[see Fig. 183]—and when a beam of sunlight is split up by a spectroscope, or rather by the prism of a spectroscope, these D lines always appear in the yellow part of the spectrum.
There was a time, and not so very long ago, when no one had the faintest idea why the Sun’s light did not contain these two wave-lengths, when the spectrum from a candle flame was a continuous band of light without dark lines. Then two experimenters, Kirchhoff and Bunsen, found that various substances when converted into luminous gases in a hot flame always produced spectra which consisted only of certain bright lines. Thus it was found that sodium gas gives two bright lines in the exact positions of the D lines of the Sun’s spectrum. This made it look as if the Sun contained no sodium. But the experimenters did something else: they placed a very brilliant light back of the sodium gas and on looking through their spectroscope they saw a continuous spectrum with black D lines. This meant that the sodium gas had absorbed from the light passing through it the wave-lengths which it had given out when shining by itself. We now know that the Sun is surrounded by a layer of gases which is somewhat cooler than the interior of the Sun and that each substance in this “reversing layer,” as it is called, takes out some of the light which is characteristic of it and thus produces the Fraunhofer lines.
In the same way iron and other metals and sodium and other substances which are heated until they become gases, and hydrogen and other gases which are aflame, produce spectra consisting of certain bright lines, and as the light from the Sun also produces black lines in the same identical places, it is known that the Sun contains these different substances which are heated up in it to a white heat.
Fig. 184.—The Spectroscope.