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For the photography of nebulæ and the study of the fainter stars, the reflector has special advantages, illustrated by the work of such instruments as the Crossley and Mills reflectors of the Lick Observatory; the great 72-inch reflector, recently brought into effective service at the Dominion Observatory in Canada; and the 60-inch and 100-inch reflectors of the Mount Wilson Observatory.
Under atmospheric conditions perfect enough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with a given exposure, stars about one magnitude fainter than the faintest stars within reach of the 60-inch.
The large mirror of the 100-inch telescope has an area about 2.8 times that of the 60-inch, and therefore receives nearly three times as much light from a star.
Adams, by his spectroscopic method applied with the 60-inch Mount Wilson reflector, obtained 0.012. Lee's recent value, secured photographically with the 40-inch Yerkes refractor, is 0.022.
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.
But in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the 60-inch reflector. Here is one of the chief sources of interest in the possibilities of a 100-inch reflecting telescope.
The 60-inch reflector, with an area 57,600 times that of the eye, reveals stars of the eighteenth magnitude, while to reach stars of about the twentieth magnitude, photographic exposures of four or five hours suffice with this instrument. Every gain of a magnitude means a great gain in the number of stars rendered visible.
Above it are the "Monastery" and other buildings used as quarters by the astronomers of the Mount Wilson Observatory while at work on the mountain. The dome of the 60-inch reflecting telescope is just below the 150-foot tower, while that of the 100-inch telescope is farther to the right.
After successful preliminary tests with the 40-inch refracting telescope of the Yerkes Observatory, Professor Michelson made the first attempt to see the fringes with the 60-inch and 100-inch reflectors on Mount Wilson in September, 1919. He was surprised and delighted to find that the fringes were perfectly sharp and distinct with the full aperture of both these instruments.
It is therefore important to learn what the telescope will actually accomplish under customary observing conditions. Fortunately we are able to measure the performance of the instrument with certainty. Close beside it on Mount Wilson stands the 60-inch reflector, of similar type, erected in 1908. The two telescopes can thus be rigorously compared under identical atmospheric conditions.
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