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Rh light into it. The mirror is in this case a flat mirror, not concave so as to bring the light to focus, but made perfectly flat so that it simply turns the light in another direction without altering it. All the alteration is to be done by the telescope,

which can now be firmly fixed: the mirror is turned about by the clockwork so as to send the light always into the fixed telescope. A mirror so arranged is called a Coelostat, which means "sky standing still": and it is an actual fact that if you look into such a mirror at any part of the sky, it will no longer appear to move as the real sky does, but remain steady