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a time, but it can easily be removed with a little acid, and then a new bright film can be put on so that the mirror is as good as new. A good many mirrors two or three feet wide have been made in this way; but the first really big one was made by Dr. Common at Ealing. It was five feet wide, not so wide as Lord The great Five-foot Telescope being taken up Mount Wilson. Rosse's (six feet), but still a great advance on any previous " silver-on-glass " mirror. With it he took some wonderful pictures of nebulae and other objects of the heavens, but he did not live long after he had made it, and the telescope was sold at his death to the Harvard Observatory in the United States.

Since then a five-foot mirror even better than Dr. Common's has been made and set up at Mount