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.
A convex lens is fitted in the other end of the collimator, or first tube, so that the light after passing through the slit will then pass in a beam through the prism. The other tube—the one you look through—is called a telescope and in this one is fitted a convex object glass and an eyepiece which magnifies the spectrum made by the prism.
When a camera is used to photograph the spectrum the eyepiece is taken out of the tube of the spectroscope and a little camera is put in its place. In this way the camera and the spectroscope are combined and this helps chemists to compare the spectra of different burning substances far better than they could do with the naked eye alone.
To photograph the spectrum of the Sun the eyepiece of the big telescope is taken out and the end of the tube of the spectroscope with the slit in it is fitted in its place. In making a photograph of the spectrum of a star the slit is not needed, for the light of the star itself is but a mere point.
In photographing the spectrum of the Sun and stars three wonderful instruments are combined, namely, the telescope, the spectroscope and the camera; perhaps I should have said four wonderful instruments, for without the brain of a genius using them the Sun and stars never would have given up their secrets.