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Television has been rendered possible by the discovery of lubstances like selenium which change their electrical proper- ties when light falls upon them. Suppose you train a spot of light on a sheet of paper and allow the reflection to fall Upon a " selenium cell " ; the response of the cell to the light reaching it will develop an electric current which may be made to light up a distant lamp. Each time the spot of light is trained on to some black mark on the paper then the reflected light will be smaller and the distant lamp will shine less brightly. In 'order to use this principle for tele- vision it is further necessary to devise means whereby any movements of the spot of light at the sending end are accur- ately reproduced by movements of the lamp at the distant end. Such means have been found and are well described in Practical Television, by Mr. E. T. Lamer (Benn, 10s. Od.). Suppose, for instance, that we wish to transmit a photograph. The transmitting spot of light rapidly traverses the photograph and the reflected light acts on the selenium cell, which again in turn acts on the distant moving lamp. This lamp, therefore, lights up the paper it shines on more or less brightly according to the impulse it receives, and therefore reproduces the lights and shades which constitute the original photograph. By means such as these, people in New York have been enabled to recognize friends in London. Mr. Lamer's book deals fully with the whole of this fascinating subject, starting with the elementary theory and finishing with a description of the latest instrument, which is the size of a large suit case and is worked in much the same way as an ordinary wireless receiver. There is a preface by Mr. Baird, of whom we wrote in the Spectator in our issue of July 81st, 1926, This book can be recommended to anyone wishing to under- stand this latest marvel of science.
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