The New Heavens | Page 8

greater theoretical resolving power of the larger aperture, providing
it can be utilized, should permit the separation of the members of close
double stars beyond the range of the smaller instrument.
CRITICAL TESTS
The many tests already made indicate that the advantages expected of
the new telescope will be realized in practice. The increased
light-gathering power will mean the addition of many millions of stars
to those already known. Spectroscopic observations now in regular
progress have carried the range of these investigations far beyond the
possibilities of the 60-inch telescope. A great class of red stars, for
example, almost all the members of which were inaccessible to the
60-inch, are now being made the subject of special study. And in other
fields of research equal advantages have been gained.
The increase in the scale of the images over those given by the 60-inch
telescope is illustrated by two photographs of the Ring Nebula in Lyra,
reproduced in Fig. 18. The Great Nebula in Orion, photographed with
the 100-inch telescope with a comparatively short exposure, sufficient
to bring out the brighter regions, is reproduced in Fig. 2. It is interesting
to compare this picture with the small-scale image of the same nebula
shown in Fig. 1.
[Illustration: Fig. 15. Photograph of the moon made on September 15,
1919, with the 100-inch Hooker telescope (Pease).
The ring-like formations are the so-called craters, most of them far
larger than anything similar on the earth. That in the lower left corner
with an isolated mountain in the centre is Albategnius, sixty-four miles
in diameter. Peaks in the ring rise to a height of fifteen thousand feet
above the central plain. Note the long sunset shadows cast by the
mountains on the left. The level region below on the right is an
extensive plain, the Mare Nubium.]

[Illustration: Fig. 16. Photograph of the moon made on September 15,
1919, with the 100-inch Hooker telescope (Pease).
The mountains above and to the left are the lunar Apennines; those on
the left just below the centre are the Alps. Both ranges include peaks
from fifteen thousand to twenty thousand feet in height. In the upper
right corner is Copernicus, about fifty miles in diameter. The largest of
the conspicuous group of three just below the Apennines is 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 nebulæ
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, but with components too
close together to be distinguished