Early schemes for television

More than a dozen schemes for sending visual images by electricity appeared from 1877 to 1884. Some used multiwire lines and mosaic arrays; others used single lines and a scanning method¿autographic, spiral, linear. Selenium cells and incandescent filaments were common elements. Basic ideas on scanning speed, repetition frequency, synchronism, picture elements, and beam modulation evolved during these years. Some schemes employed magnetooptic effects, others used polarized light, and an optical equivalent of the cathode-ray tube was proposed for one receiver. Mechanical problems were finally solved by the scanning disk, which, 40 years later, with the aid of electronic techniques, became the foundation for practical mechanical television. Both facsimile and television proposals were covered, partly because they were inseparable during this era and partly to show the continuity of developments.