of a deflector voltage v x v y and so operable to deflect the charged particles passing between the electrodes of the deflector package in one coordinate direction each such electrode pair hence forms a deflector for deflecting the charged particles in one coordinate direction given a three dimensional cartesian coordinate system with its z axis along the optical axis 15 of the lens system 13 and with the hemispheres 5 symmetrical with respect to the y z plane one electrode pair 33 a 33 c 33 a u2032 33 c u2032 of each deflector package 29 29 u2032 is arranged to deflect the charged particles in the x direction and the other electrode pair 33 b 33 d 33 b u2032 33 d u2032 of each deflector package 29 29 u2032 is arranged to deflect the charged particles in the y direction an electrode pair arranged to deflect the charged particles in the x direction will hereinafter sometimes be referred to as an x deflector and an electrode pair arranged to deflect the charged particles in the y direction will hereinafter sometimes be referred to as the y deflector as illustrated in fig6 showing a more detailed view of parts of the analyser arrangement the deflector voltages applied to.