CHAPTER XXIII
ASTRONOMY

THE advance in observational astronomy is due in no small part to the introduction of photographic methods which were rapidly applied to all branches of the science and greatly increased the wealth of available data. Special types of lenses were devised for photographic telescopes, and new forms of measuring machines were designed which much improved the precision attainable. The direct photography of large areas of the sky, such as the Milky Way, brought out astonishing revelations of the immense number of stars and their aggregations in great star clouds. Prolonged exposures (giving an accumulation of the chemical action of light on the plate) bring out faint objects and details that can never be seen with the eye using a telescope with a lens of the same size as that employed for photography.

The application of the photographic method with large telescopes for determining the distances of the stars was initiated by Dr. Frank Schlesinger with the forty-inch Yerkes refractor. At the beginning of this period the distances of hardly thirty stars were known with any considerable precision; now after thirty years, the distance of more than two thousand stars has been determined by trigonometric methods. By the remarkable application of these results in connection with photographs of stellar spectra, the approximate distances of many thousands of additional stars have been determined. Thus the structure of our stellar universe has finally been studied on the basis of definite knowledge of its scale. The result has been that our conception of the size of the universe has been in forty years magnified many thousand fold, perhaps a million fold in the minds of most astronomers.

The revival of the use of reflecting telescopes with which Sir William Herschel had made his remarkable discoveries also came at this time and contributed much to the advance of knowledge. Reflectors have some advantages over refracting telescopes, as they may be made in much larger dimensions, and they have but one surface

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