the mirrors of the interferometer
were moved apart, the fringes gradually decreased in visibility
until they finally disappeared at a mirror separation of 19.6 feet.
Adopting a mean wave-length of 5600/10000000 of a millimetre for
the light of Arcturus, this gives a value of 0.022 of a second of
arc for the angular diameter of the star. If we use a mean value
of 0.095 of a second for the parallax, the corresponding linear
diameter comes out 21,000,000 miles. The angular diameter, as in
the case of Betelgeuse, is in remarkably close agreement with the
diameter predicted from theory. Antares, the third star measured
by Mr. Pease, is the largest of all. If it is actually a member of
the Scorpius-Centaurus group, as we have strong reason to believe,
it is fully 350 light-years from the earth, and its diameter is
about 400,000,000 miles.

[Illustration: Fig. 28. Diameters of the Sun, Arcturus, Betelgeuse,
and Antares compared with the orbit of Mars.

Sun, diameter, 865,000 miles.

Arcturus, diameter, 21,000,000 miles.

Betelgeuse, diameter, 215,000,000 miles.

Antares, diameter, 400,000,000 miles.]

It now remains to make further measures of Betelgeuse, especially
because its marked changes in brightness suggest possible variations
in diameter. We must also apply the interferometer method to stars
of the various spectral types, in order to afford a sure basis for
future studies of stellar evolution. 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. A 100-foot instrument, if feasible
to build, would permit objects representing most of the chief stages
of stellar development to be measured, thus contributing in the
highest degree to the progress of our knowledge of the life history
of the stars. Fortunately, though the mechanical difficulties are
great, the optical problem is insignificant, and the cost of the
entire apparatus, though necessarily high, would be only a small
fraction of that of a telescope of corresponding aperture, if such
could be built. 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 measur