In my paper on “Green's and other allied theorems” (Trans. R. S. E. 1869–70), I showed that

where P is any scalar function of ρ, and the single integral is extended round any closed curve, while the double integral extends over any surface bounded by the curve, ν being its normal vector.

Writing

this gives at once

of which the scalar and vector parts respectively were, in the paper referred to, shown to be equal.