Thus by partial influences in unending succession the equilibrium state of the two spheres could be approximated to as nearly as may be desired. An infinite trail of electric images within each of the two spheres is thus obtained, and the final state of each conductor can be calculated by summation of the effects of each set of images. If the final potentials, V₁, V₂, say, of the spheres are given the process is somewhat simpler. Let first the charges be supposed to exist uniformly distributed over each sphere, and to be of amount a₁V₁, a₂V₂ in the two cases. The uniform distribution on S₁ will raise the potential of S₂ above V₂, and to bring the potential down to V₂ in presence of this distribution we must place an induced distribution over S₂, represented as regards the external field by the image-charge －a₂a₁V₁⧸f (at the image of C₁ in S₂) where f is the distance between the centres. The charge a₂V₂ on S₂ will similarly have an action on S₁ to be compensated in the same way by an image-charge －a₁a₂V₂⧸f at the image of C₂ in S₁. Now these two image-charges will react on the spheres S₁ and S₂ respectively, and will have to be balanced by induced distributions represented by second image-charges, to be found in the manner just exemplified. These will again react on the spheres and will have to be compensated as before, and so on indefinitely. The charges diminish in amount, and their positions approximate more and more, according to definite laws, and the final state is to be found by summation as before. The force of repulsion is to be found by summing the forces between all the different pairs of charges which can be formed by taking one charge of each system at its proper point: or it can be obtained by calculating the energy of the system. The method of successive influences was given originally by Murphy, but the mode of representing the effects of the successive induced charges by image-charges is due to Thomson. Quite another solution of this problem is, however, possible by Thomson's method of electrical inversion. A similar process to that just explained for two charged and mutually influencing spheres will give the distribution on two concentric conducting spheres, under the influence of a point-charge q at P between the inner surface of the outer and the outer surface of the inner, as shown in Fig. 7. There the influence of q at P, and of the induced distributions on one another, is represented by two series of images, one within the inner sphere and one outside the outer. These charges and positions can be calculated from the result for a single sphere and point-charge.