re would prevent them from
being clearly seen with large telescopes. 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.

The approaching completion of the 100-inch telescope of the Mount
Wilson Observatory led me to suggest to Professor Michelson, before
the United States entered the war, that the method be thoroughly
tested under the favorable atmospheric conditions of Southern
California. He was at that time at work on a special form of
interferometer, designed to determine whether atmospheric disturbances
could be disregarded in planning large-scale experiments. But the
war intervened, and all of our efforts were concentrated for two
years on the solution of war problems.[*] In 1919, as soon as the
100-inch telescope had been completed and tested, the work was
resumed on Mount Wilson.

[Footnote *: Professor Michelson's most important contribution during
the war period was a new and very efficient form of range-finder,
adopted for use by the U. S. Navy.]

A LABORATORY EXPERIMENT

The principle of the method can be most readily seen by the aid
of an experiment which any one can easily perform for himself with
simple apparatus. Make a narrow slit, a few thousandths of an inch
in width, in a sheet of black paper, and support it vertically
before a brilliant source of light. Observe this from a distance of
40 or 50 feet with a small telescope magnifying about 30 diameters.
The object-glass of the telescope should be covered with an opaque
cap, pierced by two circular holes about one-eighth of an inch in
diameter and half an inch apart. The holes should be on opposite
sides of the centre of the object-glass and equidistant from it,
and the line joining the holes should be horizontal. When this
cap is removed the slit appears as a narrow vertical band with
much fainter bands on both sides of it. With the cap in place, the
central bright band appears to be ruled with narrow vertical lines
or fringes produced by the "interference"[*] of the two pencils of
light coming through different parts of the object-glass from the
distant slit. Cover one of the holes, and the fringes instantly
disappear. Their production requires the joint effect of th