资料题：A doctor, in return, keeps others healthy by（　）。

The word "television" is made up of two parts, the Greek "tele"and the Latin "video".Television means seeing at a distance, and it has taken many years to develop.It was not until 1936 that the BBC (the British Broadcasting Company) started the first regular television service in the world.The viewer, looking at his television screen, does not see a moving picture.He sees a series of still photographs, each a little different.These pass before his eyes so quickly that he has an impression of continuous movement.The picture on the screen is composed of a series of lines which run from left to right and from top to bottom.These lines are lines of various shades of light.They travel across the screen so quickly that they give the impression of one steady continuous picture.The viewer sees twenty- five pictures per second, with a fixed number of lines making up each picture.Depending on which system is used, the number of lines is either four hundred and five or six hundred and twenty - five.Unlike films, which are taken at one time and then shown at another, a television transmission is usually instantaneous.Events are seen at the same time as they take place.In sound broadcasting, sounds are changed into small electric currents by a microphone.These small currents are then carried by a cable to a transmitter.The transmitter sends them out into space as radio waves.They can be picked up by a sound receiver which changes them back into the original sounds.A similar method is used in television.The picture is seen by an electric"eye"-the television camera-which changes it into small electric currents.As with sound radio, the electric currents are carried by a cable to a transmitter which radiates them into space in the from of electrical waves.The waves carrying the sound and the picture arrive at the antenna of the receiver.it changes them back into the pictures that the viewer sees on his screen.