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Rh every Frenchman, and so there would be none in the miscellaneous crowd issuing from the gangway: and if we could notice this absence of Frenchmen, we might say "Why! there must have been some French officials stopping them at the gangway." So in the same way when we see the absence of a sodium colour (or an iron colour), from the light of the Sun, we say, "There must have been some sodium vapour (or some iron vapour) stopping the sodium light (or iron light) at the gangway (that is, just where the light was leaving the Sun)."

It may seem strange that we notice the light which is stopped rather than that which comes to us. Why should we not rather look for a bright light sent out by sodium, like that I showed you first? Well! we often do: but the dark lines, shown in the second experiment, are much commoner, and have told us on the whole far more than the bright lines. The fact is that the stars are mostly very very hot, so that there are masses of vapour surrounding them through which the light must pass. The stoppages are therefore far the most conspicuous features of their light. You will understand from this brief description what a spectroscope is. The chief part of it is the prism which spreads out the light into a band of colour: but we must be careful to limit the light to a narrow line or "slit," otherwise we shall not see the stoppages distinctly. We can either examine the result with the eye, or we can photograph it: