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(书外文章) Television was not invented by any one person. Nor did it spring into being overnight. It developed gradually, over a long period, from the ideas of many people - each one building on the work of their predecessors’. The process began in 1873, when it was accidentally discovered that the electrical resistance of the element selenium (硒) varied in proportion to the intensity of the light shining on it. Scientists quickly recognized that this provided a way of transforming light variations into electrical signals. Almost immediately a number of schemes were proposed for sending pictures by wire (it was, of course, before radio). One of the earliest of these schemes was patterned on the human eye. Suggested by G. R. Carey in 1875. It envisioned (想像，展望) a mosaic (感光嵌镶幕) of selenium cells on which the picture to be transmitted would be focused by a lens system. At the receiving end there would be a similarly arranged mosaic made up of electric lights. Each selenium cell would be connected by an individual wire to the similarly placed light in the receiving mosaic. Light falling on the selenium cell would cause the associated electric light to shine in proportion. Thus the mosaic of lights would reproduce the original pictures, Had the necessary amplifiers(放大器)and the right kind of lights been available this system would have worked. But it also would have required an impractical number of connecting wires. Carey recognized this and in a second scheme proposed to “scan” the cells - transmitting the signal from each cell to its associated light, in turn, over a single wire. If this were done fast enough the retaining power of the eye would cause the resultant image to be seen as a complete picture. An important discovery in early television was the electrical resistance of___