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LEFT ROEBLING 90 ROENTGEN suspension bridge across the chasm of the Niagara river to unite the New York Central and Great Western (Canada) railroads. He began the Cincinnati sus- pension bridge in 1856 and completed it in 1867. His greatest work was the first bridge over the East river, connecting New York and Brooklyn. He died while the construction was in progress, in Brooklyn, July 22, 1869, and the bridge was completed by his son. ROEBLING, WASHINGTON AUGUS- TUS, an American civil engineer; born in Saxonburg, Pa., May 26, 1837; son of the preceding. He was graduated at the Rensselaer Polytechnic Institute, Troy, N. Y., in 1857; was an engineer officer during the Civil War and attained the rank of colonel of volunteers. In 1865 he resigned from military service to become assistant to his father in con- structing the suspension bridges at Cin- cinnati and Pittsburgh. In 1869 he was assistant engineer under his father in the construction of the first suspension bridge over the East River between New York and Brooklyn; and on his father's death became chief engineer, which post he held till the completion of the bridge in 1883. He then became president of the large wire manufactory at Trenton, N. J. He published many valuable engineering reports, notably several relating to the construction of the East river bridge. ROENTGEN, WILLIAM CONRAD, VON, a German scientist; born in Prus- WILLIAM CONRAD VON ROENTGEN sia in 1845; was graduated in medicine at the University of Zurich in 1869, and accompanied Professor Kundt, his teacher, to Wiirzburg, where he engaged in prac- tice. He went to Strasburg, in 1873, as assistant professor, and for 20 years was conspicuous as Professor of Mathematics and Physics, and also as a scientist. In November, 1895, Roentgen made the dis- covery of what has since been known as the Roentgen, or X-rays. The German emperor bestowed the Order of the Royal Crown on the discoverer, who afterward also was ennobled. He received the Nobel Prize for physics in 1901. ROENTGEN, or RONTGEN, RAYS, certain invisible non-refractable rays emanating from the surface of an elec- trically excited vacuum tube opposite the cathode electrode, having power (1) of permeating objects impervious to light or heat rays, (2) of discharging electri- fied bodies or surfaces exposed to them, (3) of exciting fluorescence in fluorescent salts, and (4) of affecting sensitized pho- tographic plates in a manner similar to light rays. They were discovered by William Conrad Roentgen, Professor of Physics at the Royal University of Wiirz- burg, in Germany, toward the close of the year 1895. Not being certain as to the nature of the rays, Professor Roent- gen provisionally termed them the X-rays, and they are still commonly known by that name, though the name Roentgen rays is also common. At the beginning of 1894, Prof. P. E. A. Lenard, at Bonn, announced the discovery that by using a Crookes tube in which the cathode rays were made to impinge on a thin sheet of aluminum a screen covered with a phosphorescent substance outside the tube could be made to phosphoresce by their action. That, further, it was possible by means of these cathode rays, as he supposed, to obtain "shadows" of objects through optically opaque substances and to produce an impress of these "shadows" on photographic plates, which could after- ward be developed and fixed by ordinary photographic processes. Working on this line of investigation Professor Roentgen inclosed an excited vacuum tube in black- ened cardboard treated with barium platino-cyanide, and discovered that the cathode beam is accompanied by certain rays not before known, which, though of phosphorescent and photographic quality, differ from any known form of light in not being susceptible of refraction. These were the wonderful X-rays> which have opened up to the world a new region of scientific exploration. Besides obtaining radiographs of the bones in the living human hand, Professor Roent- gen radiographed a compass card com- pletely inclosed in a metallic box. From these and similar experiments he inferred