We consider the behavior of the photon number integral under inversion, concentrating on Euclidean space. The discussion may be framed in terms of an additive differential I which arises under inversions. The quantity \intop \nolimits \intop \nolimits I is an interesting integral invariant whose value characterizes different configurations under inversion.

PACS numbers: 11.15.Kc, 13.40.--f