h send it toward the
100-inch concave mirror (M5) at the bottom of the telescope tube.
After this the course of the light is exactly as it would be if
the mirrors M2, M3 were replaced by two holes over the 100-inch
mirror. It is reflected to the convex mirror (M6), then back in
a less rapidly convergent beam toward the large mirror. Before
reaching it the light is caught by the plane mirror (M7) and reflected
through an opening at the side of the telescope tube to the eye-piece
E. Here the fringes are observed with a magnification ranging from
1,500 to 3,000 diameters.

[Illustration: Fig. 24. Twenty-foot Michelson interferometer for
measuring star diameters, attached to upper end of the skeleton
tube of the 100-inch Hooker telescope.

The path of the two pencils of light from the star is shown in
Fig. 23. For a photograph of the entire telescope, see Fig. 4.]

In the practical application of this method to the measurement of
star diameters, the chief problem was whether the atmosphere would
be quiet enough to permit sharp interference fringes to be produced
with light-pencils more than 100 inches apart. After successful
preliminary tests with the 40-inch refracting telescope of the
Yerkes Observatory, Professor Michelson made the first attempt
to see the fringes with the 60-inch and 100-inch reflectors on
Mount Wilson in September, 1919. He was surprised and delighted to
find that the fringes were perfectly sharp and distinct with the
full aperture of both these instruments. 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. 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.

The difficult and delicate problem of adjusting the mirrors of
this instrument with the necessary extreme accuracy was solved by
Professor Michelson during his visit to Mount Wilson in the summer
of 1920, and with the assistance of Mr. Pease, of the observatory
staff, interference fringes were observed in the case of certain
stars when the mirrors were as much as 18 feet apart. All was thus
in readiness for a decisive test as soon as a sui