Came across it when I was studying about Cathode ray oscilloscopes, it says that two sets of parallel plates are kept at a high pd with another one of relatively lower pd between them. Then electrons from an electron gun are shot into it. It says that the electrons are focused on the cro's screen by electrostatic lens effect. I've looked up on the effect but I couldn't find any satisfactory explanations.. If someone can give me a clearer picture, I'd appreciate it.