is the retardation potential in volts on encountering filter l1 the direction of the electron 39 s motion is reversed in the z axis and the electron emerges with an angle of 180 u00b0 u03b8 degrees we now consider the fate of these selected electrons when they pass into regions with higher or lower magnetic fields according to equation 2 the angle which an electron 39 s momentum makes to the axis is increased on going to higher field strength and decreased on going to lower note however that the transformation is not linear different starting angles are changed to varying extents on leaving filter l1 the electrons are transported to filter l2 experiencing an increase in magnetic field which according to equation 2 results in an increase of the axial angle fig5 ii note that lower energy electrons will be mapped into different cone angles than higher ones since their starting angles were different any electrons which had near 90 u00b0 axial angle would be reflected in the magnetic mirror before reaching filter l2 in reality the magnetic collimation from the sample to filter l1 has already oriented the electrons to lie close to the axis and magnetic mirror reflection could only take place in the impractical situation of the magnetic field at filter l2 being greater than that at the sample we however continue the analysis considering a complete initial population of axial angles the retardation field at filter l2.