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Page:Popular Science Monthly Volume 89.djvu/925

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Popular Science Mo)if/il!/

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���Above: The build- ing complete, up to the covering of the dome. This is fur- nished with a sys- tem of shutters which with the double wall permit an even tempera- ture to be main- tained in the interior of the building

��Above: The iron framework of the walls. Horizontal ribs are attached in pairs both to the outer and inner edges of the up- right beams, thus forming a double wall with an inter- mediate air-space

��The permanent concrete pier at Vic- toria. The ends of the polar axis are supported on steel castings which are bolted to the heads of the piers

��having already been captured. It is also extremely valuable for spectroscopic work. A long exposure is required even with the great forty-inch Yerkes refractor to olilain the spectrogram of a star of the fourth magnitude. This is much reduced at Mt. Wilson by using the short focus sixty-inch mirror, not only on account of the larger size, but also because the loss of light caused by reflection is much less than that

��suffered by a ray of light in passing through the thick lenses of a large refractor.

Recently, a great deal of attention has been paid to the study of the spectra of nebulae, and some extraordinary results have been obtained. It has been found that some of them show evidences of rota- tion, a most important fact in its bearing on the e\olu(ion of star s\\stcms, if it can be established by photography.

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