We have seen how, when little was known about astronomy, it was natural to imagine that the stars went on for ever and ever, so that, however far one probed into space, one merely came to more stars. It was only like the town-bred child imagining that the lamp-posts go on for ever and ever. Yet we now know that if we go far enough out into space, we come to regions where the stars first begin to thin out, and then disappear altogether: we are now out in the depths of space beyond the Milky Way. The stars are like the lights of a vast city, but no city, however great, extends for ever, and if we go far enough, we get out of the city, and come at last to the dark open country beyond.

Yet this is not the whole story. We now know that the wheel-shaped system of stars bounded by the Milky Way is not the only system of stars in space. Far beyond the Milky Way are other cities, each with its own system of lights. The dark open country which surrounds our own city is not the end of everything; if we persevere through it for long enough, we shall come in time to another city whose lights are stars similar to those surrounding our sun. Let me explain the evidence for this statement.