Archimedes and at the lower end of the Alps is Plato. Note the
long sunset shadows cast by the isolated peaks on the left. The
central portion of the picture is a vast plain, the Mare Imbrium.]

The sharpness of the images given by the new telescope may be
illustrated by some recent photographs of the moon, obtained with
an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged
region of the moon, containing many ring-like mountains or craters.
Fig. 16 shows the great arc of the lunar Apennines (above) and the
Alps (below), to the left of the broad plain of the Mare Imbrium.
The starlike points along the moon's terminator, which separates
the dark area from the region upon which the sun (on the right)
shines, are the mountain peaks, about to disappear at sunset. The
long shadows cast by the mountains just within the illuminated
area are plainly seen. Some of the peaks of the lunar Apennines
attain a height of 20,000 feet.

In less powerful telescopes the stars at the centre of the great
globular clusters are so closely crowded together that they cannot
be studied separately with the spectrograph. Moreover, most of
them are much too faint for examination with this instrument. At
the 134-foot focus the 100-inch telescope gives a large-scale image
of such clusters, and permits the spectra of stars as faint as
the fifteenth magnitude to be separately photographed.

[Illustration: Fig. 17. Hubble's Variable Nebula. One of the few
nebulae known to vary in brightness and form.

Photographed with the 100-inch telescope (Hubble).]

CLOSE DOUBLE STARS

A remarkable use of the 100-inch telescope, which permits its full
theoretical resolving power to be not merely attained but to be
doubled, has been made possible by the first application of Michelson's
interference method to the measurement of very close double stars.
When employing this, the 100-inch mirror is completely covered,
except for two slits. Beams of light from a star, entering by the
slits, unite at the focus of the telescope, where the image is
examined by an eyepiece magnifying about five thousand diameters.
Across the enlarged star image a series of fine, sharp fringes is
seen, even when the atmospheric conditions are poor. If the star is
single the fringes remain visible, whatever the distance between the
slits. But in the case of a star like Capella, previously inferred
to be double from the periodic displacement of the lines in its
spectrum, b