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. [Illustration: FIG. 184.--THE SPECTROSCOPE.] It’s the same thing with the stars. It makes no difference if the light is made by burning some substance a few inches away from the slit of the spectroscope, or whether it has traveled 90 millions of miles from the Sun--which takes about 8½ minutes, or 45 years for the light to reach it from Aldebaran, it always acts exactly in the same way; and so we know a good deal about the stuff the stars are made of. Let’s see now, how a real spectroscope is made, for it isn’t always convenient to have a pitch dark room, nor is it a very exact way to hold the prism to your eye when examining the light of burning sodium, or hydrogen gas, or other substances. The spectroscope is an instrument made so that the light of a substance burned at one end will pass through a prism and will form a spectrum on the retina of the eye at the other end. It is usually made up of two tubes mounted on a stand at an angle with the prism between them, as shown in Fig. 184. In the end of the first tube, which is called a _collimator_, a very narrow slit is cut, and it is at this end that the substance is placed which is to be burned and whose light is to be split up and examined.