photographed on a larger
scale, should also reveal smaller details of structure and render
possible higher accuracy of measurement. Finally, the 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