We have measured their distances and their
motions, determined their chemical composition, and obtained undeniable
evidence of progressive development, but even in the most powerful
telescopes their images are so minute that they appear as points
rather than as disks. In fact, the larger the telescope and the
more perfect the atmospheric conditions at the observer's command,
the smaller do these images appear. On the photographic plate, it is
true, the stars are recorded as measurable disks, but these are due
to the spreading of the light from their bright point-like images,
and their diameters increase as the exposure time is prolonged.
From the images of the brighter stars rays of light project in
straight lines, but these also are instrumental phenomena, due
to diffraction of light by the steel bars that support the small
mirror in the tube of reflecting telescopes. In a word, the stars
are so remote that the largest and most perfect telescopes show
them only as extremely minute needle-points of light, without any
trace of their true disks.

[Illustration: Fig. 21. Great sun-spot group, August 8, 1917 (Whitney).

The disk in the corner represents the comparative size of the earth.]

How, then, may we hope to measure their diameters? By using, as
the man of science must so often do, indirect means when the direct
attack fails. Most of the remarkable progress of astronomy during
the last quarter-century has resulted from the application of new and
ingenious devices borrowed from the physicist. These have multiplied
to such a degree that some of our observatories are literally physical
laboratories, in which the sun and stars are examined by powerful
spectroscopes and other optical instruments that have recently advanced
our knowledge of physics by leaps and bounds. In the present case
we are indebted for our star-measuring device to the distinguished
physicist Professor Albert A. Michelson, who has contributed a long
array of novel apparatus and methods to physics and astronomy.

THE INTERFEROMETER

The instrument in question, known as the interferometer, had previously
yielded a remarkable series of results when applied in its various
forms to the solution of fundamental problems. To mention only a
few of those that have helped to establish Michelson's fame, we may
recall that our exact knowledge of the length of the international
metre at Sevres, the world's standard of measurement, was obtained
by him with an