 Permittivity convention for transmitted image charges

For a real charge $Q$ in medium 1, the potential in medium 2 can be represented by the source $Q_t=2{ϵ}_{2}Q/({ϵ}_1+{ϵ}_2)$ using medium 2's Coulomb prefactor $1/(4\pi {ϵ}_2)$. The same potential uses $2{ϵ}_{1}Q/({ϵ}_1+{ϵ}_2)$ if medium 1's prefactor is retained instead. These different effective source labels do not change the physical field. A decomposition $Q_t=Q+Q^{\prime }$ must state which prefactor it uses.