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A 100-foot interferometer might be designed in many different forms, and one of these may ultimately be found to be within the range of possibility. Meanwhile the 20-foot interferometer has been improved so materially that it now promises to yield approximate measures of stars at first supposed to be beyond its capacity.

"If you want an opinion from me," I said, with a laugh, "you'll have to tell me first what I am looking at." "That," he explained, as I continued to gaze, "is one of the latest forms of the spectroscope, known as the interferometer, with delicately ruled gratings in which power to resolve the straight, close lines in the spectrum is carried to the limit of possibility. A small watch is delicate.

In 1890 Michelson, unaware of the earlier work, published in the Philosophical Magazine a complete description of an interferometer capable of determining with surprising accuracy the distance between the components of double stars so close together that no telescope can separate them.

Furthermore, a very large interferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double stars could have been easily studied by this device with several of the large telescopes of the early nineties. But whatever the reasons, a powerful method of research lay unused.

A different form of interferometer has more recently enabled him to measure the minute tides within the solid body of the earth not the great tides of the ocean, but the slight deformations of the earth's body, which is as rigid as steel, that are caused by the varying attractions of the sun and moon.

The success of this work strongly encouraged the more ambitious project of measuring the diameter of a star, and the 20-foot interferometer was built for this purpose.

This determination shows Betelgeuse to be 160 light-years from the earth. On December 13, 1920, Mr. Pease successfully measured the diameter of Betelgeuse with the 20-foot interferometer.

As the outer mirrors were separated the interference fringes gradually became less distinct, as theory requires, and as Doctor Merrill had previously seen when observing Betelgeuse with the interferometer used for Capella. At a separation of 10 feet the fringes disappeared completely, giving the data required for calculating the diameter of the star.

Unfortunately, only a few giant stars are certain to fall within the range of our present instrument. An interferometer of 70-feet aperture would be needed to measure Sirius accurately, and one of twice this size to deal with less brilliant white stars.

Doctor Anderson, of the observatory staff, then devised a special form of interferometer for the measurement of close double stars, and applied it with the 100-inch telescope to the measurement of the orbital motion of the close components of Capella, with results of extraordinary accuracy, far beyond anything attainable by previous methods.