At relativistic electron velocity the geometrical electron optical equations rely on an index of refraction that includes both the ratio of electron velocity to light v / c = β β and A ⋅ ⋅ s , the component of the magnetic vector potential along the electron direction: n ( s ) = m v 1 − − β β 2 + e c A ⋅ ⋅ s where m , e , and c are the electron mass, electron charge, and the speed of light. The first term is controlled by electrostatic lens while the second one by magnetic lens.