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298 up to the word "," which represents the fiercest outblaze, and then gradually down again to faintness.

Now suppose when my voice had become faint again we were to hear the word "Back," "Back," "Back," repeated several times: we should say at once that those were the echoes of my biggest shout from some distant hills or buildings. And that is what probably happened with light echoes in the case of Nova Persei. A fair explanation seems to be that a faint star in its travels came across a dark nebula, one of the "bunkers" of space; and thereupon blazed up. The blaze lit up the previously dark nebula or bunker for us to see; but as light takes time to travel, it was not until some months after the flare-up that we got the light echoes from the nebula. We identify the illumination as an echo much as we could identify the sound echo: the word "Back" was repeated, not "mouth" or "hell," and this is reasonable because "Back" was the loudest word. Similarly the light of the echo, when analysed with the spectroscope, was found to correspond with the light of the greatest flare-up: the chain of evidence is complete. There is just one alternative supposition in detail which I will mention, but cannot dwell upon. It is possible that the big "flare-up" was caused, not by the entry of a wandering star into the nebula, but by the contraction of a part of the nebula under its own gravitation. The rest of the explanation would then follow as before.

But now I have tried your patience quite enough. Our visits to the stars have been rather more