Considering the limiting particle free-stream Reynolds numbers, as \text{Re}_{o} \to 0 then C_D(\text{Re}_{o}) \to 24 / \text{Re}_{o} and therefore \psi \to 1 . Thus as expected there correction factor is unity in the Stokesian drag regime. Wessel & Righi evaluated \psi for C_D(\text{Re}) from the empirical correlation for drag on a sphere from Schiller & Naumann.
