e two
light-pencils.

[Footnote *: For an explanation of the phenomena of interference,
see any encyclopaeedia or book on physics.]

Now suppose the two holes over the object-glass to be in movable
plates, so that their distance apart can be varied. As they are
gradually separated the narrow vertical fringes become less and
less distinct, and finally vanish completely. Measure the distance
between the holes and divide this by the wavelength of light, which
we may call 1/50000 of an inch. The result is the angular width
of the distant slit. Knowing the distance of the slit, we can at
once calculate its linear width. If for the slit we substitute a
minute circular hole, the method of measurement remains the same,
but the angular diameter as calculated above must be multiplied
by 1.22.[*]

[Footnote *: More complete details may be found in Michelson's Lowell
Lectures on "Light-Waves and Their Uses," University of Chicago
Press, 1907.]

To measure the diameter of a star we proceed in a similar way,
but, as the angle it subtends is so small, we must use a very large
telescope, for the smaller the angle the farther apart must be the
two holes over the object-glass (or the mirror, in case a reflecting
telescope is employed). In fact, when the holes are moved apart to
the full aperture of the 100-inch Hooker telescope, the interference
fringes are still visible even with the star Betelgeuse, though its
angular diameter is perhaps as great as that of any other star.
Thus, we must build an attachment for the telescope, so arranged
as to permit us to move the openings still farther apart.

[Illustration: Fig. 23. Diagram showing outline of the 100-inch
Hooker telescope, and path of the two pencils of light from a star
when under observation with the 20-foot Michelson interferometer.

A photograph of the interferometer is shown in Fig. 24.]

THE 20-FOOT INSTRUMENT

The 20-foot interferometer designed by Messrs. Michelson and Pease,
and constructed in the Mount Wilson Observatory instrument-shop,
is shown in the diagram (Fig. 23) and in a photograph of the upper
end of the skeleton tube of the telescope (Fig. 24). The light from
the star is received by two flat mirrors (Ml, M4) which project
beyond the tube and can be moved apart along the supporting arm.
These take the place of the two holes over the object-glass in
our experiment. From these mirrors the light is reflected to a
second pair of flat mirrors (M2, M3), whic