If you will fix another sheet of white cardboard in a _vertical_ position so that the colors will shine on it in a band, the colors, beginning with red at the bottom, next orange, then yellow, green, blue, indigo and violet, will follow each other to the top bright and beautiful. These rainbow colors, spread out in a band on the screen, form what is called the _solar spectrum_, that is, the spectrum which is produced by sunlight. This beautifully colored spectrum is really made up of a great many images of the slit in the cardboard, one for each wave-length of the light. If the slit is made very narrow, the images of the slit will not overlap much and you can tell whether any of the wave-lengths are missing by looking for fine black lines crossing the spectrum. Such dark lines are very important in finding out what the Sun and Stars are made of. [Illustration: FIG. 182.--BOY LOOKING THROUGH PRISM AT SLIT IN CARDBOARD.] 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. [Illustration: 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.