| 摘要 |
An electron lens having three magnetic pole pieces (7, 8, 9) between which two gaps (S1, S2) are formed, in which magnetic fields are generated having opposing polarity, the magnetomotive force in the upper gap (S1) being fixed at a value which is close to the value for which the focal length is a minimum when the magnetomotive forces are equal in both gaps (S1, S2), and the magnetomotive force in the lower gap (S2) being larger than in the upper gap (S1). <IMAGE>
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