Patent Number: 
Section: claims

1. A magnetic energy filter having plural magnetic fields and designed to deflect a trajectory of an electron beam extending from an entrance window to an exit slit, said trajectory being in a plane, said magnetic energy filter comprising: said magnetic fields being at least four in number and including first, second, third, and fourth magnetic fields, the trajectory of the electron beam passing through said at least four magnetic fields in the order first, second, third, and fourth;  said plural magnetic fields defining an electron trajectory that has a rotational symmetry axis located midway between said second and third magnetic fields, said axis being perpendicular to said plane; and  said second and third magnetic fields located on opposite sides of a line and being opposite in polarity, said line being in said plane and intersecting said rotational symmetry axis at a point. 2. The magnetic energy filter of  claim 1 , wherein said plural magnetic fields include first and second magnetic fields, and wherein said first and second magnetic fields are opposite in polarity. claim 1 3. The magnetic energy filter of  claim 2 , wherein the sum of the absolute values of the beam deflection angles of said magnetic fields is in excess of 540xc2x0. claim 2 4. The magnetic energy filter of  claim 2 , wherein said magnetic fields are four in number, and wherein the beam deflection angles of said magnetic fields, respectively, are about 110xc2x0, xe2x88x92250xc2x0, 250xc2x0, and xe2x88x92110xc2x0, respectively, in this order from the entrance side. claim 2 5. The magnetic energy filter of  claim 1 , wherein said plural magnetic fields include first and second magnetic fields, and wherein said first and second magnetic fields are identical in polarity. claim 1 6. A magnetic energy filter having plural magnetic fields and designed to deflect a trajectory of an electron beam extending from an entrance window to an exit slit, said trajectory having a first direction in which the electron beam entering said entrance window and a second direction in which the electron beam passes through said exit slit, said second direction being aligned with said first direction, said trajectory being in a plane, said magnetic energy filter comprising: deflecting magnets mounted on opposite sides of a straight line in said plane, said straight line connecting said entrance window and said exit slit. 7. The magnetic energy filter of  claim 5  or  6 , wherein the sum of the absolute values of the beam deflection angles of said magnetic fields is set to about 720xc2x0. claim 5 6