Excerpt: ...A photograph of the interferometer is shown in Fig. 24. THE 20-FOOT INSTRUMENT The 20-foot interferometer designed by Messrs. Michelson and Pease, and constructed in the Mount Wilson Observatory instrument-shop, is shown in the diagram (Fig. 23) and in a photograph of the upper end of the skeleton tube of the telescope (Fig. 24). The light from the star is received by two flat mirrors (Ml, M4) which project beyond the tube and can be moved apart along the supporting arm. These take the place of the two holes ...
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Excerpt: ...A photograph of the interferometer is shown in Fig. 24. THE 20-FOOT INSTRUMENT The 20-foot interferometer designed by Messrs. Michelson and Pease, and constructed in the Mount Wilson Observatory instrument-shop, is shown in the diagram (Fig. 23) and in a photograph of the upper end of the skeleton tube of the telescope (Fig. 24). The light from the star is received by two flat mirrors (Ml, M4) which project beyond the tube and can be moved apart along the supporting arm. These take the place of the two holes over the object-glass in our experiment. From these mirrors the light is reflected to a second pair of flat mirrors (M2, M3), which send it toward the 100-inch concave mirror (M5) at the bottom of the telescope tube. After this the course of the light is exactly as it would be if the mirrors M2, M3 were replaced by two holes over the 100-inch mirror. It is reflected to the convex mirror (M6), then back in a less rapidly convergent beam toward the large mirror. Before reaching it the light is caught by the plane mirror (M7) and reflected through an opening at the side of the telescope tube to the eye-piece E. Here the fringes are observed with a magnification ranging from 1,500 to 3,000 diameters. Fig. 24. Twenty-foot Michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the 100-inch Hooker telescope. The path of the two pencils of light from the star is shown in Fig. 23. For a photograph of the entire telescope, see Fig. 4. In the practical application of this method to the measurement of star diameters, the chief problem was whether the atmosphere would be quiet enough to permit sharp interference fringes to be produced with light-pencils more than 100 inches apart. 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, ..
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