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We have now to make our longest journey of all—to the stars. Even as fast as the telescope will carry us it will take us some years to get there. It is difficult, perhaps, to realize that light seems to go quickly, yet really is taking time to travel; that really it takes time for light to travel, let us say, from me to you—not very long perhaps; but still you do not see me as I really am, but as I was a small fraction of a second ago. It is easier to realize the interval in the case of sound; and several times in his lecture I shall have to remind you of the likeness between light and sound, because it is so much easier to understand in the case of sound things which seem very difficult in the case of light. For instance, we all know that sound takes time to travel, because we have heard echoes. We shout, and presently we hear our shout coming back from some distant point. The time in between is the time taken by the sound to travel. When you look in your mirror in the morning, you really have to wait a little time before the light from your face goes to the mirror and comes back again, so that you get the echo or reflection from the mirror. I shall have something more to say about these light echoes towards the end of the