This paper describes a method for the compensation of the spherical aberration of a polar diatomic molecule in an electrostatic quadrupole field. This perturbation, caused by the higher order terms in the Stark effect, limits the opening angle of the lens and is compensated by an electrostatic hexapole field placed in the middle of the quadrupole field where the influence is the strongest. A detailed mathematical treatment of the motion of a molecule in a composite lens is given and the calculations show that the transmission of the lens increases by a factor 5. Preliminary experiments confirm the theoretical predictions.