The New Heavens | Page 8

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 nebul? 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 separately, the fringes behave differently. As the slits are moved apart a point is reached where the fringes completely disappear, only to reappear as the separation is continued. This effect is obtained when the slits are at right angles to the line joining the two stars of the pair, found by this method to be 0.0418 of a second of arc apart (on December 30, 1919). Subsequent measures, of far greater precision than those obtainable by other methods in the case of easily separated double stars, show the rapid orbital motion of