In the same year, Welsbach brought out an improvement on his incandescent gas-mantle that was valuable for cases in which a brilliant illumination was desired, that leaped almost immediately into public favor. In the following year of 1888, Sprague made the first installation of street electric railways in the United States, and the first in the world in which the conditions of operating were difficult. The success of Sprague's system was largely due to the excellence of Sprague's electric motor, which had the curious property of being designed on principles which the scientific men of those days declared to be wholly wrong. Sprague's reputation rests mainly on his electric railway; but, from the standpoint of the inventor, Sprague's invention of his electric motor was of a higher order than that of his electric railway. In 1888, Harvey invented his process of making armor-plate. In the same year, Eastman and Walker invented the kodak camera, in which the novelty consisted mainly of a continuous roll of sensitized film, on which photographs could be successively made; and De Chardonnet invented his process of manufacturing artificial silk from threads that were made by forcing collodion through very small holes. These were important in fact; but in comparison with the discoveries in the realm of the actual ether made in the same year by Hertz, they were quite trifling. These discoveries resulted from experiments with electric apparatus of the simplest and most inexpensive character, in a space near which sparks were passing between the two terminals of a Rhumkorff coil. It had been known before that each spark accompanied and therefore represented an establishment of equilibrium between the two oppositely charged terminals, and that each discharge was of an oscillatory character--as any readjustment of equilibrium always is. By means of a mere single wire, curved into a circle, except that the two ends were not quite joined, Hertz discovered that the space was filled with electric waves that were propagated in straight lines from the source (as light is) and accompanied with vibrations at right angles to the direction of propagation (also as light is); and also that the electric rays were refracted, reflected and polarized, as light rays are. Subsequent experiments with modified apparatus measured the velocity of the propagation of electric waves, and found that it was virtually the same as that of light.