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128 further the line splits up into two: and with further and further progress these lines are so clearly separated that hundreds of other lines can be seen between them. We might spend a lifetime examining these lines: but at present I want us to be content with the very simplest facts which will give us a general idea of the spectroscope.

Now let us try another experiment. We will again put in some sodium and you see the bright line in the yellow: but soon you see it changes to a dark line—instead of the yellow being brighter than the other colours, it is much darker. (Look at spectrum No. 10, "Sirius" plate facing p. 273) The reason is that the sodium has now been put in in such a way that it gives off clouds of its own vapour, and these clouds stop the yellow light though they do not stop other light. The discovery of that fact is one of the greatest discoveries ever made, for it enabled us to read the heavens like a book. Until it was made nobody could understand the meaning of the dark lines which cross the spectrum of the Sun: they were thought at one time to be boundary marks of the different colours. Now we know that they vouch for the presence of sodium and other chemical elements in a state of vapour, stopping the light of particular colours and letting other light pass on. It is as though a French policeman were stationed at the gangway of a steamer, stopping all Frenchmen from disembarking: all other nations might pass—English, Germans, Portuguese, Chinamen, Hottentots and all others—because the French policeman would have no authority over them; but he would stop