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fainter and fainter objects) all the time, whereas the eye gains nothing by continuing to gaze—it rather loses by getting tired. Another advantage is that when the plate has been developed, it can be copied and the copies sent all over the world: or it can be preserved for many years: or it may be measured at leisure; and so on. The catalogue of advantages is quite considerable, so that the introduction of photography into astronomy has effected nothing less than a revolution.

But scarcely less of a revolution has been effected by the spectroscope, which tells us what the stars are made of, by means of that very colour which was at first simply a nuisance. A great man once said that it was quite certain that we should never know what the stars were made of: but it is now quite certain that we do. You remember the experiment with the prism which showed us the different colours of the rainbow? I now want to show you that these colours are not always the same, We will first throw them on the screen with ordinary light, when they have the familiar appearance; but now we will put a salt of sodium in the source of light and immediately you see that the yellow becomes very bright—much brighter than the rest. (See spectrum No. 4, plate facing p. 273). That effect can be produced by common table salt, which is a salt of sodium; and we know when there is some sodium in the light by this brightening in the yellow. But we can improve the apparatus until we find that the brightening is very local—sodium does not brighten all the yellow, but only a particular part or line of it. If we improve the apparatus still