Think for a moment what a grand power this gives you of reading as in a book the different gases which are glowing in the sky even billions of miles away. You would never mistake the lines of hydrogen for the line of sodium, but when looking at a nebula or any mass of glowing gas you could say at once "sodium is glowing there," or "that cloud must be composed of hydrogen."
Now, opening the shutters, look at the sunlight through your spectroscopes. Here you have something different from either the continuous spectrum of solids, or the bright separate lines of gases, for while you have a bright-coloured band you have also some dark lines crossing it (No. 2, Plate I.) It is those dark lines which enable us to guess what is going on in the sun before the light comes to us. In 1859 Professor Kirchhoff made an experiment which explained those dark lines, and we will repeat it now. Take a good look at the sunlight spectrum, to fix the lines in your memory, and then close the shutters again.
Fig. 48.
Kirchhoff's experiment, explaining the dark lines in sunlight.
A, Limelight dispersed through a prism. s, Slit through which the beam of light comes. l, Lens bringing it to a focus on the prism p. sp, Continuous spectrum thrown on the wall. B, The same light, with the flame f containing glowing sodium placed in front of it. D, Dark sodium line appearing in the spectrum.
I have here our magic-lantern with its lime-light, in which the solid lime glows with a white heat, in consequence of the jets of oxygen and hydrogen burning round it. This was the light Kirchhoff used, and you know it will give a continuous bright band in the spectroscope. I put a cap with a narrow slit in it over the lantern tube, so as to get a narrow beam of light; in front of this I put a lens l, and in front of this again the prism p. The slit and the prism act exactly like your spectroscopes, and you can all see the continuous spectrum on the screen (sp, A, Fig. 48). Next I put a lighted lamp of very weak spirit in front of the slit, and find that it makes no difference, for whatever light it gives only strengthens the spectrum. But now notice carefully. I am going to put a little salt into the flame, and you would expect that the sodium in it, when turned to glowing vapour, causing it to look yellow, would strengthen the yellow part of the spectrum and give a bright line. This is what Kirchhoff expected, but to his intense surprise he saw as you do now a dark line D start out where the bright line should have been.
What can have happened? It is this. The oxyhydrogen light is very hot indeed, the spirit flame with the sodium is comparatively weak and cool. So when those special coloured waves of the oxyhydrogen light which agree with those of the sodium light reached the flame, they spent all their energy in heating up those waves to their own temperature, and while all the other coloured rays travelled on and reached the screen, these waves were stopped or absorbed on the way, and consequently there was a blank, black space in the spectrum where they should have been. If I could put a hydrogen flame cooler than the original light in the road, then there would be three dark lines where the bright hydrogen lines should be, and so with every other gas. The cool vapour in front of the hot light cuts off from the white ray exactly those waves which it gives out itself when burning.
Thus each black line of the sun-spectrum (No. 2, Plate I.), tells us that some particular ray of sunlight has been absorbed by a cooler vapour of its own kind somewhere between the sun and us, and it must be in the sun itself, for when we examine other stars we often find dark lines in their spectrum different from those in the sun, and this shows that the missing rays must have been stopped close at home, for if they were stopped in our atmosphere they would all be alike.
There are, by the bye, some lines which we know are caused by our atmosphere, especially when it is full of invisible water vapour, and these we easily detect, because they show more distinctly when the sun is low and shines through a thicker layer of air than when he is high up and shines through less.