| 摘要 |
PURPOSE:To drastically improve the lens performance by providing between two opposing electrodes of a main lens member having means for substantially enlarging the lens aperture an auxiliary electrode having apertures larger than those of these electrodes. CONSTITUTION:The cup-shaped electrode 23b of a third grid 13 opposite to a fourth grid 14 and the cup-shaped electrode 24a of the fourth grid 14 opposite to the third grid 13 have at the respective bottoms large apertures 43, 34 composed of three interconnected beam-passing holes and at portions slightly inside of the bottoms flat electrodes 101, 102, 103, 104. An auxiliary electrode 16 is composed of two cup-shaped electrodes having track-like apertures 36, 46 larger than the large apertures 43, 34 of the third grid 13 and fourth grid 14, respectively, and has the substantially intermediate potential between those of the third grid 13 and fourth grid 14. |
