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250 lecture. Light, then, takes time to travel, but the time taken is very, very small. I wonder how small a fraction of a second you think it takes to travel from me to you? Well, you know how long a year is; and how long a second is. Have you any idea how many seconds there are in a year? About 30 millions. Now suppose you pretend that a second is itself like a year; divide it into its seconds, that is into 30 million parts; one of these tiny parts will be about the time that what we may call a "wireless" wave of electricity takes to vibrate. I do not mean the time that the spark takes, but the time a single wave takes to be transmitted its whole length. And then suppose you take one of these "wireless" wave times, and divide that up into 30 million parts, you get about the time that a light wave takes to vibrate. And the time that it takes you to see me is about one wireless vibration, a very small fraction of a second, but still containing 30 million light vibrations.

We might make a new set of tables for you to learn—

30 million light vibrations make one wireless vibration.

30 million wireless vibrations make one second.

30 million seconds make one year.

And 30 million years—what do they make? Well, I shall have something more to say about that towards the end of the lecture.

For the present we will start with a year or two. It takes a year or two to get to the very nearest star, and many years to get, for instance, to most of the bright stars of the zodiac. "The Ram, the Bull,